I have other things to spend my money on. I hear many retired folks complaining about socal security, I always respond that my parents were not old enough to vote when they sent their socal security money to the moon, so don't blame me for the mess we are in. (Yes that situation is complex than that) I'd like to keep my tax money. Selfish perhaps, but if you won't let me keep it, at least spend it on something that is of use, not waste it on space.
Here's the hint: humans are all selfish - it's in our nature. However, humanity can't afford humans being selfish - and thankfully, people in our government managed to realize that.
The problem is that humans are lazy . You all want flying cars, and cures for cancer, and a cure for aging, but you don't really have a damned clue how to get there.
The destination doesn't have to be the goal - it's the trip that matters . This is the real reason for "pure science" research. Pure science is more valuable than anything else you can possibly spend your money on. Anything.
That isn't to say we should stand still. Lets develope something of use here. We can catch up to the Chinese anytime. (if only because the spys mean they can't keep the technology secert for very long...)
If we don't keep pushing the envelope of what we can do, we might as well roll over and die as a country - at least as a technology leader. We pretty much already are as far as space goes - in five to ten years, I think people'll be talking about the "heyday of the US", and talking about the amazing new fighter and spy plane designs coming out of Europe, Russia, and China.
Very little scientific research needs to be done in space.
Ah, the voice of ignorance. You mean, very little research like, oh, astronomy (SNAP: Supernova Acceleration Probe, which requires the IR, which you can't see through the atmosphere), basic physics (Gravity Probe B), solar flare monitoring (SOHO), searches for terrestrial planets (the Terrestrial Planet Finder), etc.? Good call. No scientific research needs to be done in space. Sure. Can you do us a favor and let scientists decide what needs to be done in space? Most of us think that there's a metric crapload left to do.
And then if you've just got objections to human space exploration, the point, as always, is that the technology required to have human space exploration massively increases your ability to do things in space. It makes you far better at doing things, and more importantly, it means that you have more flexibility, more capability, and more possibilities than if it's purely robotic.
Hubble'd be half-blind without human intervention. What, you want us to stick around on the Earth until the need arises to leave? Infrastructure can't be built out of nothing. The smart man plans for contingencies.
I mean seriously - where to start. Fusion, alternate propulsion mechanisms, pure material generation, exotic material searches, and then in time, asteroid mining and resource searching.
Look at it this way. If you're one of those nutballs that thinks that Earth has enough of everything it needs from now till eternity, get a clue. Do a Web search on how much helium we have - by the end of this century, it's likely to be gone . Yah. Helium. You know. The stuff they put in balloons. No kidding. Gone.
That's not to say anything of the more exotic heavy metals. We will run out , eventually, and want more. Now, the smart man says: does he look for more materials before he runs out, or after? What you're suggesting is "let's wait a bit, we've got plenty of time!"
The problem with that argument is that it's always valid. Until, of course, we run out of time. And then, we're screwed.
overseas American military target, they'll get nuked off the moon.
How? What missiles do we have that could reach the Moon? Oh wait - we don't have any. And even if we did, they'd take more time than the "rocks" launched from the Moon, and would be relatively trivial to intercept/divert/destroy. Plus with the lack of atmosphere on the moon, you'd pretty much have to have a nearly-direct strike in order for it to physically destroy whatever's there.
domestic military target, their military will get nuked off the face of the planet, the US military will probably invade China and they'll get nuked off of the moon.
How would this fix the basic problem of "they have a base on the moon, and can drop stuff on us"? China might be gone, but, hell, that'd make whoever still controlled the Moon base even MORE powerful. "They just invaded our country! Oh well, hope they can defend every major city, all at once." I'm sure NORAD would love to see "50,000 incoming projectiles".
I'm even ignoring the basic physics of "how do you divert something that's huge and has a ton of kinetic energy".
If China mounts a full-scale attack of the US, then it will be then end of the world.
That, I agree with you on.
But, you might say: "The moon is strategically superior to the Earth, so the US military would be at a disadvantage." The US military will never be at a disadvantage when it can launch a thermonuclear warhead anywhere on the globe with mere minutes of notice. The US military has the hardware and the will to do this.
That's like saying that a dying man isn't at a disadvantage just because he can bludgeon a healthy man to death. If the Chinese would set up a base on the moon, with the capability to launch projectiles at the Earth, the US has two options: armageddon, or talk treaty. A base on the moon, with military goals, is a massive advantage, simply because all of the weaponry, upkeep, etc. are virtually free (in the long term), whereas the Earth-based defense is expensive. You'd win by attrition.
Then again, the US military tends to be nuts, so they'd probably choose armageddon.
The fact is that if China did set up a moon base, then the US, the EU, Russia and India would have telescopes in lunar orbit before China even got their Porta-Potties set up. Not to mention the other 500 tracking systems that would be deployed. So, no surprise attack, no secret construction, no military reason to build a mass driver.
Sure, set up telescopes. Set up tracking systems. What're you going to do? Oh, no, you knocked down one of their projectiles. Big deal. Now prevent the other several hundred that are coming your way as well. Go ahead. Keep preventing them. Knocking them down takes fuel, money, and accuracy. Sending down projectiles, though. That's virtually free.
The point of the mass driver in The Moon is a Harsh Mistress wasn't that they couldn't be blocked. It was that the relative cost for blocking one was much higher than the cost for sending it down.
I'm not arguing that it wouldn't be a bad thing. Not at all. It would be another arms race, and another chance for humans to kill themselves. Not good. I'm however talking from the pure militaristic viewpoint, and there is a pure advantage to a military Moon base there.
There are things more militarily powerful than a nuclear weapon . This shouldn't come as a shock.
I was having real trouble thinking about this until you mentioned that spacetime itself is moving/accelerating
I don't remember saying that... that's definitely not true. The Schwarzschild solution has an event horizon even if it's static.
The problem is that you've got to worry about reference frames quite a bit here. For the observer at infinity, space and time seem infinitely stretched out as you approach the event horizon.
For the observer falling INTO the black hole, nothing weird happens at all near the event horizon - it still takes finite (small!) time to reach the singularity.
(note: I guess in some sense you *could* consider spacetime to be "moving" near a black hole. The math might even work out exactly the same! the problem is that it tries to simplify the discussion too much: nothing 'magic' happens at the event horizon. Time (from an infinite observer's point of view) is dilating continuously as you approach the event horizon. Once you cross it, that's when strange things can happen - you can start receiving images/messages/information from the 'future', etc., but no matter what, you are heading to the singularity.)
When you drop that bit of line into the EH (event horizon) you're fine and dandy to that point. But every elementary particle that crosses that threshold is gone. I would suggest that pulling the line back up would be just as easy as letting it down. But it would be cleanly cut with a very exact curvature. At least that is my guess... Since any sublightspeed communication is not going to happen across the point where spacetime itself exceeds lightspeed the forces that would hold/connect particles inside the boundary to one outside the boundary would be cleanly severed.
Ah, reference frame issues. To you, you never see anything cross the event horizon. Nothing at all. All you see is things ever increasingly slowly approach the event horizon. It takes infinite time for the last photon that the object crossing the event horizon emits to reach you.
If you try to pull upwards on something, you're asking the molecules in that "something" to communicate your upward force to the molecules they're attached to, and likewise, the downstream particles need to communicate their downward force (preventing the upstream particles from moving away, producing tension) upward. This "upward" communication will take an increasingly long amount of time, which means that the "stretch" between adjacent molecules will increase as you get closer to the event horizon. At some point, this stretch will be great enough that the bond will just break. This does -not- occur at the event horizon, but before it. The deeper the particle falls in, the less likely you're going to be able to get it out, because the energy required continues to go up, and at some point, the energy required will be more than you can convey to the particle.
And plenty of otherthings to think about. Such as if spacetime is moving, in relation to what?
Well, as I've said above, spacetime itself isn't really moving. But, there are cases where it does (like, the expansion of the universe). In those cases, it's moving in relation to a flat coordinate system, at infinity.
Other questions occur, might be silly but... Where does spacetime come from? Is a coordinate system inexhaustable? Where does it go?
Spacetime is spacetime - it's a coordinate system. The coordinate system deforms, yes, and "the minimum amount it can deform" is up for grabs. Can you increase the volume of spacetime? Sure - all you need to do is stretch it, which is quite easily doable. It doesn't "come from" or "go" anywhere, because it "isn't" anything. It's just a coordinate system. It's how long it takes light to get from one place to another one.
Picture two "squares" of 2D spacetime. In one square, put a black hole, in another put nothing. The length of each size of the square is 1 light-year. Now, crossing the square diagonally for the square
My understanding (and flawed it probably is), is that the event horizon and the singularity are quite different things. I'll agree what you state is likely true in reference to the singularity but as for the event horizon I must disagree.
They are. The event horizon is not a true discontinuity in space. To an observer moving inward, nothing weird happens at the event horizon. The singularity is a discontinuity in space, and an indestructible 'observer' would see something weird happen at the singularity.
However, the event horizon does separate space into two regions that cannot communicate with each other - that is, the light cones of the two regions are physically distinct. There are certain coordinate systems where this is extremely obvious, like Kruskal-Szekeres coordinates (see this nice discussion on different coordinate systems for more info).
Please correct me if I'm wrong but the event horizon (EH from now on) is simply that boundary where the escape velocity exceeds the speed of light.
You're right, but you're not right. Gotta love that. The whole "escape velocity exceeds the speed of light" is a useful mnemonic to remember the Schwarzschild radius, but it is *not* where the event horizon comes from. This is an unfortunate coincidence of physics, that "bad" Newtonian gravity can give the "right" GR answer. Ah, well. The event horizon is where the denominator blows up in the Schwarzschild/Kerr/Neumann/Kerr-Neumann equation, in Schwarzschild coordinates. In non-pretentious speak, the equation describes the geometry of space around a black hole (normal, charged, rotating, or charged-rotating), and Schwarzschild coordinates are "normal" coordinates: 1 time coordinate, 3 spherical coordinates.
Now, there's nothing special about those coordinates, and that's why the event horizon isn't a real discontinuity in spacetime (though it *is* a causal discontinuity).
Let's restrict ourselves to Schwarzschild black holes only - the other ones have weird shaped event horizons (donut-shaped) and ring singularities, and weird things like that.
The "Minkowski metric", the metric of normal flat spacetime, looks like this: ds^2 = -dt^2 + dr^2 + r^2 do^2. You might have seen something similar in the flat space form: ds^2 = -dt^2 + dx^2 + dy^2 + dz^2 - this is the "4-dimensional Pythagorean theorem": if you ignore the "dt^2", which is relativity, the other part says that the distance between three points (squared) is the sum of the squares of the displacements along each axis.
However, note that time is negative, and space is positive: you can switch that (space negative, time positive) as all that matters is "ds^2", but there is still a sign difference between space and time.
Now, past the event horizon, in the Schwarzschild metric, radius is negative, and time is positive. That is, your radial distance from the singularity becomes timelike, and your time since crossing the event horizon becomes spacelike. In this case, that means that all worldlines lead to the singularity once you pass the event horizon. It's like saying "you can't move backwards in time" - same thing. Inside the event horizon, you can't move outwards. Period. End of discussion. Game over, player one.
(The astute reader may conclude that maybe you can, in fact, move backwards and forwards in time once you're inside the event horizon. Well, maybe. That fact is outside the realm of our discussion.:) )
Anyway, the best way to realize that nothing can "come out" of the event horizon is to view the event horizon from infinity. Imagine that you send a probe in, designed to emit one beep every picosecond (10^-12 second). As the probe gets closer to the event horizon, time dilates dramatically, so every picosecond for the probe takes incredible amounts of time for an observer at infinity - hours, days, months, years, etc. The other way to think about thi
Once you're outside the atmosphere there are no outstanding reasons for not moving faster. That is why I mentioned magnetic drive/coupling instead of any physical connection. And of course carbon nanotubes can be configured to carry power, it should also be relatively simple to configure part of the elevator as a track for a magnetic linear accelerator. No inefficient rocket power required (although to be honest I'm not sure what the efficiency of a linear accelerator is).
Damage to the ribbon is a good reason. However, avoiding the Van Allen belts is a better reason to move quicker once you're a bit farther up, so point. However, counterpoint is that you don't torque the ribbon as heavily once you're up at a higher point, as you're shifting the center of mass less.
However, the best reason for not moving faster is, of course, because you don't really have the power. It's silly to carry the power with you (reduction of launch mass), so you beam it up from the ground. I've a feeling that this will *stay* the main method for scaling a space elevator for a long time to come, simply because it's the easiest, and the most launch-mass efficient.
Also, the nanotube power carry thing is a bit of a red herring. There are two problems:
1) The elevator won't use continuous nanotubes - it'll be using composite nanotube fiber, which, of course, cannot carry power, as it's just glue with a bunch of really weird things called 'nanotubes' inside it, and the glue probably won't conduct.
2) Even if they end up using continuous nanotubes (maybe multiwall nanotubes, as it appears you may want to fuse them), nanotubes conduct power along their axis, and a continuous string of nanotubes would have no way of conducting power off-axis. Plus, when you're talking a length of 60,000 miles, the nanotubes would have to superconduct (which they don't - they do weird things with electrons, but it doesn't look like true superconductivity) - even the best conductor known to Man would appear to be a gigantic resistor when it's 60,000 miles long.
Last but not least, I do want to thank you for an interesting discussion. I find real thinking to be a rare pleasure these days and I do enjoy it.
Whew! I normally call lack of thinking death. Interesting discussion is the only reason to wake up in the morning.:)
...*GRIN* So you should be able to make the trip in 6'ish hours. That
6 hours? No way. 2 weeks, nominally.
The whole reason a space elevator wins over a rocket is because you can avoid air resistance by going slow. In 6 hours, you might as well be using a rocket. You're talking about 60,000 km, so roughly 10,000 km an *hour*.
There's another problem with a 6 hour trip, too - the speed of sound in the elevator produces about a 7 hour natural vibration. A 6 hour trip means that you're actually travelling supersonically up the elevator (supersonic with respect to the cable).
Anyway, with a 2 week trip, you can easily see that even a very gentle push (I think it's in the "few newtons" range) can counteract the slight lean of the elevator.
Remember that the Earth rotates--right now, you're moving at, what, about 2,000 mph? As you go straight up, relative to a point on the surface, your speed increases because you're still completing one rotation / day. Once you get up to geostationary orbit, you've got (at least) the necessary escape velocity. (I also suspect that e.v. goes down as altitude increases, but I'm not certain.)
No. Escape velocity is what you get when you set kinetic energy = potential energy. Kinetic energy is T = 1/2 mv^2, potential energy is U = GMm/r. Set the two equal, and poof, you get escape velocity.
If you have escape velocity, you are no longer in orbit. At all. You will always be moving away from the Earth. Technically we call this a "parabolic orbit" (the C3=0 orbit), but it's not what people normally consider an orbit - that is, you'll never come back to Earth.
If you're at geostationary orbit, you don't have escape velocity, because, well, you're still orbiting the Earth, now aren't you? GEO is still shy of escape velocity by something like a few km/s (2 or 3-ish, I think, I can't remember. Don't correct me if I'm wrong and it's 6 or 7, though it really can't be - it's too late for me to use real numbers).
Escape velocity depends on your altitude, yes. v_esc = sqrt(2GM/r), where r is your altitude above (the center of) the Earth.
There is no way you could build a space elevator out of a black hole. It's not possible - not for material science reasons, or practical reasons, or anything - it is simply not possible to build *anything* that could cross the event horizon of a black hole. The intermolecular forces required would be infinite - literally, infinite. Unbounded.
The energy that it would take to ascend the indistructable space elevator would be rather high, of course. And they'd be worse the deeper into the hole you were.
You're right it'd be high. Infinite. And the deeper you got, the worse it'd be: infinity+1, infinity+2, infinity+3..
I'm kidding, of course. Again, you can't. It's infinite. Period. The integral is unbounded.
The shuttle's orbit would change if it grabs a satellite, unless the center of mass of the extended "satellite/shuttle" system is at the same altitude as the shuttle was originally.
If the shuttle approached the satellite from along its orbital path (the path of its velocity) or the path perpendicular to it, then it wouldn't change (in front of/behind it, or left/right of it). If it approached along its radial vector, it would change (above/below it).
If it grabbed along the radial vector, the shuttle and the satellite would start rotating around each other slowly, around the center of mass. Depending on when the shuttle let go (and the original orientation of the satellite/shuttle - i.e., which was higher, which was lower), it could be boosted into a higher orbit, or lowered to a lower orbit, or returned to its original orbit.
This is actually the whole idea of orbit-changing space tethers.
Actually I would suggest some liquid movement tanks high and low on the counterweight to maintain the balance without actually having to shorten the cable/distance to the balance point. Or you could have some type of elevator/freight system moving large plugs/masses of rock up and down to do the job. Better yet, both, having a dual system (or even ternary) would make sense considering the importance of balancing the system...*GRIN* I would do a great deal of work making it as foolproof as possible cause goodness knows we keep making the fools.
Actually, no...
Think about this for a second.
Picture an object in a circular orbit. Gravity is F = GMm/r^2, and the centripetal force required to keep it in that orbit is F = mv^2/r. So, so long as v^2 = GM/r, everything's happy.
Now, let's suppose that something grabs onto the elevator, say, something that's 10K pounds (pounds, because pounds is a force, and that simplifies things). So, now, the force that's pulling on the object is an F > mv^2/r. But it's still *moving* with v^2 = GM/r, so it can't simply be "pulled downwards" - instead, it starts to *lean forward*. All you've done is increased the centripetal acceleration, which means all you're going to do is bend the velocity vector such that it *starts* to head towards you. If it were a satellite, you'd be putting it into a lower elliptical orbit that still keeps that point-of-increased-a_c as its perigee. Since its an elevator, it starts "falling down" - leaning forward.
OK, now send the object up the elevator. As it goes up, it's now *torquing* the elevator (because the radial vector - perpendicular to the velocity vector - is no longer in the same direction as the force vector, so there's a torque), trying to make it lean forward.
Once the mass gets beyond geosynchronous orbit, it's now shifted the center-of-mass of the elevator above GEO, and it's now trying to make the elevator lean *backward*.
See a pattern? Sending a load up the elevator will induce an oscillation in the ribbon. It won't make it slowly fall, unless you send loads constantly to less-than-GEO and let them go, and don't bother trying to actively damp the oscillations at the base.
Yah, you could just actively damp the oscillations. The elevator starts leaning one way, you pull the elevator the other way. The two oscillations cancel each other out. No balancing act needed.
(Yes, you can keep a satellite in the "wrong orbit" - imagine if the satellite was too close to the Earth, such that it tried to fall. Now imagine constantly blowing on the underside of the satellite, such that the sum of your force and the force of gravity equals the necessary centripetal force. Poof. Satellite in an orbit again. That's what active damping would do).
Orbital paths are dictated by velocity and altitude of the center of mass.
Move the center of mass, you change the orbit of the object. So the point that he made is valid, if not for the reason he thought (or you thought that he thought).
This isn't a problem in this case, as others have pointed out, for many reasons. First, it's a trivial mass compared to the elevator mass. So it's not going to significantly change things at all.
Second, even slightly altering the center of mass like this would do won't cause an orbital change, because the elevator is fixed at one point - the anchor. It'll cause a torque, which will make the elevator try to lean. The anchor will hold it in place (it's only a *small* torque), transferring the torque against the Earth's rotation, slowing down the Earth's rotation (trivially, of course).
So, you get an oscillation, which can be damped by the base, and has already been modeled and simulated. It's not a problem, and no, it will not wobble and collapse like the Tacoma Narrows bridge.
Great. Now explain to them that a 1 GHz machine is twice as fast as a 500 MHz machine.
Oh, wait, now you'll have to explain that 1 GHz is 1000 MHz.
So 0.5GHz is 500 MHz, but 0.5GB is 512MB.
It's stupid. GB is 1 x 10^9 bytes. Whoever originally figured "hey, 1024 is close enough to 1000, so we'll just use 'k'!" should be shot. Why didn't he use "bogoK"? Was a perfectly good solution for convincing people bogoMIPS aren't real.:)
Oh - and it's "terabyte", not "TerraByte". kilo, mega, giga, tera, peta, exa. Ah, the joys of working in high-energy physics.
You can directly and simply relate bits, frequency, and rate (like MB/second)
Except that frequency and rate are in powers of 10 prefixes, not powers of 2. GHz means 1,000,000,000 cycles per second, and data transfer rate is usually determined by the frequency of the clock used to transfer data. It'd be silly to measure it in terms of "1024*1024" clock cycles - there's really no point.
The only place where powers-of-2 prefixes make sense are memory (because of SDRAM's poor granularity - they only come in sizes increasing by powers of 2: so 256k, 512k, 1M, 2M, 4M, 8M, etc). In storage, frequency, data transfer rate, all of those things, powers of 2 don't matter, and base 10 measurement devices are already out there for frequency.
Honestly - why is it more convenient to, say, break things up into 1k blocks rather than 1000 byte blocks? Because the address space of a computer is likely to be in powers of 2, and 1k blocks will divide evenly (plus you can shift, rather than divide, which is of course the main reason). This is why powers of 2 are considered better in compsci - because you can just lshift/rshift. In the 'real world', it's easier to divide/multiply by 10, since we use base 10 numbers.
Storage manufacturers will never stop reporting disks in SI units until we convince people to start using base 2. After all, would you want to explain to someone that no, that 0.96 TB drive is, in fact, twice as large as the 500 GB drive?
We have mandates on product labeling for many other products I think its time we force the industry to be upfront.
But... but... they ARE being up front. They're the ones who are correct. Is it honestly their fault that everyone else uses bizarre (and incorrect) prefixes?
Honestly, think about this for a second. How could drive manufacturers estimate the size of a platter? They know the bit density (say, 16Mbit per sq. mm., so 2Mbyte per sq. mm.), so, just multiply by the area (say 500 sq. mm), and boom, that's an estimate (1GB platter). In numbers that have nothing to do with 1024, or base 2. In this case, it's easier to use base 10. This'd be more instructive if the numbers were irregular and smaller, but I'm lazy, and so are they, probably.
Think about a drive researcher, who's just calculated that they can increase the bit density by a factor of 100 on drives, and a PR person asks him how large that will make the drives. He knows they have 100 GB disks now, and then quickly says "20 TB" - and wow, he's correct. If he used binary prefixes, he wouldn't be. The point is that hard drive densities aren't required to constantly double, so binary prefixes are dumb. You have 20GB, 30GB, 40GB, 60GB, 80GB, not 8GB, 16GB, 32GB. With SDRAM, the poor granularity requires size increases in doubles - so 256MB, 512MB, 1GB, so binary prefixes "kindof" make sense.
Anyway, the whole case is ludicrous. Can you imagine going to court with this?
"So, Mr. Hard Drive Company, why exactly did your company use deceptive advertising?"
"Um, we didn't, your Honor. The drive is 20 GB in size. According to the SI system of units, this means 20 x 10^9 bytes."
"But the rest of the industry disagrees."
"Your honor, the rest of the industry has been confused about this issue from the beginning. We've been extremely consistent about this. 1 GB is 1 x 10^9 bytes."
If you want the planetary parameters, why not go to the obvious source:
Because Google gave it to me in a few seconds? If I really wanted to get it from a "more scientific" source, I've got plenty of other sources within arm's reach.
It is NOT possible that this little hydrogen could cause an increase of a factor of 100 in the brightness. There is always plenty of hydrogen left over in the overlying layers.
What I said was that if the planet reaches the core, it could cause something akin to a nova. I'm still REALLY skeptical of this, because the only reason a nova causes that significant a brightening is because the white dwarf is generating so little light anyway. A red giant has so many obscuring layers that the core wouldn't be visible (hence the reason that OV/IR stars exist).
(You can't invoke gravity, because the surrounding material quickly matches the planet's hydrogen density and buoyancy with do it's bit. The core will probably slowly down to the stellar core, but that's not a lot of power and there is only a limited amount of hydrogen fuel there anyway.)
OK, so you do agree the core of the planet would at least reach the burning region.
I'm not so sure this is irrelevant. This was the whole point that I was thinking of. I don't think it's likely that the star will necessarily bleed off all of the hydrogen of the planet (the outer layers, yes, but the inner layers are too dense - the envelope of a red dwarf is what, about as dense as water?), but I don't think it's that important. The point is that once it reaches the core, it's going extremely rapidly, and if nothing else, it's going to seriously dredge the star and stir up a lot of newer material.
After all, once it reaches the burning region, it's got a TON of momentum (what did I quote before? 0.05 R_sun is the burning zone in a red giant?) That's a lot of velocity to strike an essentially solid object with.
But anyway, as I said several times, I don't think this applies here. I don't think that it will cause a measurable brightness increase (especially in this case - if it was something very near a planetary nebula, yah, quite possibly). If nothing else, the best objection to all of this is that the impact will likely occur deep within the star, and unless they're talking about a couple-magnitude increase in the infrared, we'd never see it.
What I *do* think this could do is seriously mess with the composition of a star (it's another dredge-up, if nothing else) and *possibly* cause a spike in certain wavelengths.
Remember however that the line is under very high tension, many thousands of tons equivalent. If it breaks and drifts it's not so easy to catch, hold on to, and reel back in.
You reel down from orbit, not from the ground. You don't need to grab it. It'd bounce, and oscillate a bit when it broke.
Not really, though - the tension is not coming because it's attached to anything - it's coming from the cable's existence. Cables normally spring upwards and bounce because the tension on them goes from "huge" to "zero": in this case, the tension merely changes a little because the weight changes slightly. It's still under tension, because gravity's still there.
It'd be trivial to spool more outwards, considering you better be able to do that when you deploy the elevator.
Let's review out giant planets, shall we? Given that both planets are around 18 Earth-masses
First off, last time I checked, Uranus is 14.5 M_earth, not 18 (Neptune is 18 M_earth: even Google can tell you that: search for "mass of Uranus / mass of the Earth" and "mass of Neptune / mass of the Earth") , so it's core BETTER be less than 15 M_earth. Don't jump at someone about checking facts when you make a mistake like that.
Anyway looks like I got the core masses for Uranus and Neptune from dated sources, or confused radius with mass (though I had 5 M_earth for Uranus's core - I think I -really- mixed things up there).
When I said "mostly core", I meant by volume - you apparently meant by mass.
The metallic hydrogen is a layer right above the core, not the core itself.
That's what I said.
core is probably metallic hydrogen (way cool, that) covering iron/nickel
I guess "liquid metallic ocean" would probably be better than 'core', especially in Jupiter's case, where it's tremendously larger than the core. When it was taught to me, it was taught as a "metallic hydrogen core layer", because you can't call it an atmosphere. Ocean's definitely a better term, though.
In this case, it's an F-class star. And you missed my point entirely, which was that if the star can convect the planet's hydrogen into the shell-burning zone, it can damn well convect its own hydrogren reserves down there, which are vastly in excess of what the planet could provide. So the star should never notice the miniscule addition of the planet's hydrogen.
I said it wasn't applicable to that case - the star's not late-stage enough. I did however say it was possible to cause a large increase, IF the planet made it to the core and the star had nearly exhausted its hydrogen reserve. Hell, if the star was a good way to becoming a planetary nebula, this would essentially be the equivalent of a nova, as the amounts that normally cause a nova are actually pretty consistent with the mass of a giant planet (10^-4 M_sun), and the brightening they were talking about is consistent, though a little low as well.
The other possibility to think of is that the rocky core is actually going to drag hydrogen down to the stellar core as it descends into the thin photosphere by drag. I don't really see how this would be a huge explosion, though, but maybe I'm missing something (seems to me like it would cause a very gradual brightening, and also show some periodic variability with the planet core's period). I also don't see how the core wouldn't be ripped to shreds by tidal forces, but that I'd actually have to calculate. 10 M_earth or so in about an Earth radius is going to be pretty difficult to tidally distort, though it's gotta get down to about 0.005 R_Sun in order to reach the H-burning shell, which would have pretty significant tidal forces.
This COULD be the mechanism they're working with, though it seems a bit of a stretch offhand.
Re:A Couple Thoughts/questions
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Planet-Gobbling Star
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· Score: 3, Informative
No - you don't fuse iron. You neutron-stuff them. During the supernova, the outbound shock wave carries so much energy (and neutrinos) that neutrons are literally "shoved" into nuclei. You get ridiculous things like iron with hundreds of neutrons, which then decay down into normal elements by alpha emission and beta decay. This is r-process stellar nucleosynthesis. (There's also s-process stellar nucleosynthesis, which is also neutron stuffing, but on a much longer timescale. Essentially everything past iron is formed by r-process stellar nucleosynthesis. Check Carroll & Ostlie pp 527-528 for more info.
Stars die when they hit the iron stage because they can generate no outward pressure from fusing iron. They can't even fuse iron at all! It's actually a really complex procedure - basically, the iron starts to lose all of its electrons (from proton capture and other mechanisms) so the core rapidly loses electron degeneracy pressure, which is what was (briefly) supporting it. The inner core collapses very uniformly to a little neutron star, and the outer core decouples from the inner core, and the outer core rushes inwards at extreme velocities. The collision of the two is one of many explosions in a supernova. (Again, see Carroll & Ostlie's section on the Death of Massive Stars)
Anyway, the fuel isn't insignificant depending on what stage the star is in, and also depending on how fast the planet's orbit would decay once it's inside the photosphere. If it meets with the star's core without significantly losing mass, that could cause a VERY large brightening. Functionally it's equivalent to a nova, or the pulsing of Wolf-Rayet stars (without the mass shell shielding it).
Re:A Couple Thoughts/questions
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Planet-Gobbling Star
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· Score: 2, Informative
2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.
Ah, nitpicking.
When the core of the star gets to iron it CAN blow up, because, of course, iron is king when it comes to nuclear stability. Can't fuse it, can't fission it, it's just... iron. Hence the reason that cosmic rays are generally considered to be either particles, or iron.
Anyway, the star still won't blow up if it does start to burn stuff heavier than carbon - it'll only blow up if it has enough mass to overcome the electron degeneracy pressure. Most people merge these two - "duh, if the star has enough mass to burn up to iron, it'll have enough mass to overcome the electron degeneracy pressure!" This isn't *completely* clear (and as far as I know, it's still ambiguous) because you don't know how much matter the star's going to slough during its helium burning/carbon burning dying phase. The planetary nebulae around planets contain a huge fraction of the star's mass.
Anyway, blah, this is nitpicking. Any star which honestly seriously gets to silicon burning is going to supernova, except for some really really weirdo situations.
Other thing is that the materials that were likely formed completely in supernovae are actually only elements past iron. Anything below iron could have formed in a star, and been sloughed off, or shredded away somehow. Anyway, the point that you made was that everything past helium comes from a supernova - that's definitely not true. Probably a majority of the elements past about, oh, oxygen comes from a supernova (but probably not virtually all). Carbon, nitrogen, and oxygen can, and probably do, come from other sources than supernovae.
Being a TOTAL nitpick, only everything past iron is formed in a supernova. Elements higher than oxygen are probably spread via a supernova. The reason that C, N, O can be spread easily is because, as with the triple-alpha process, stars balloon when they manage to reach a temperature where they start burning new elements, and they slough off a ton of material, and the stages near C,N,O last long enough that they probably can shove a significant amount of material off (hundreds/thousands of years - still virtually no time cosmologically).
No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.
Depends on the kind of star. The core of the star - that is, the "dense" part - is not that inconsistent with the size of a gas giant. After the triple-alpha process ignites, the outer regions of a star are so incredibly not dense that they probably wouldn't produce ANY drag, and you could get significant enrichment when the planet actually encountered the star's core, if the resultant nova didn't blow the contents of the star out to kingdom come (i.e. escape velocity) because you'd probably isotropize the shell, and get a locally "metal-heavy" shell about the star which would show up in spectroscopy.
Note that this, of course, would only result in an enrichment of red giants, and again, isn't applicable for main sequence stars.:)
Um, yeah granted, but the aerobraking window at Mars is particularly narrow- the 'atmosphere' of Mars is very tenuous. Let's put it this way: Buzz Aldrin doesn't like it at all.
Hence the reason that a computer should be the one to do it. Computers don't have to like it. They just have to do it.:) Yes, it's difficult, and the margin is small, but you can do it, and I believe they have in fact done it already.
Mommy state will help you out? Maybe, if there's votes in it.
Nah, this is simple economics. If you've got 5 missions planned to Mars in the next 5 years that cost $1 billion dollars, and the elevator would save you, say, $400 million and cost you $250 million, obviously, you're going to use the elevator. Hell, the government doesn't even have to okay it, depending on how it's budgeted, since it's the same amount of money and the same amount of science gets done. The point is that it's an immediate obvious cost benefit.
Well, the tether is millimeters wide, and you can't hit it with any speed otherwise you cut the tether and possibly open your vehicle to a vacuum. You're approaching it from thousands of kilometers away with an initial approach speed of maybe 1km/s or so. It's pretty much the same as docking with the ISS, only slightly harder and the stakes are even higher.
No no no - you're missing the point. The point is that the elevator is traveling at different velocities at different points along its length (i.e. its linear velocity is omega*R, since its rotational velocity is constant), and you choose the point of approach that will have the same linear velocity as you do when you're at the same point. So you're not approaching it with an initial approach speed of 1 km/s: you've got the same speed it does. There's going to be a little differential speed difference between you since it's rotating, and you're not -quite- rotating (it's got centripetal acceleration from tension, and you've just got gravity), but over a reasonable time frame, that should be minimal, since to a reasonable degree of accuracy, you're both going around the Earth. The differential velocity between you would be VERY minimal - probably meters an hour, not meters a second. Plus if your velocity isn't quite what you thought it would be, you just aim higher or lower.
The obvious addition here is that you could easily put a capture platform at the Mars approach point to make it safe. The main reason this is easier than docking with the ISS is because you're using gravity (and very simply kinematics!) to match velocity, rather than using thrusters.
Aerobraking at Mars is a somewhat tricky proposition. If you don't brake enough you die.
Well, the "if you don't do it right, you die" comment is true for ANY space operation. If you don't calculate the release time correctly, you miss Mars completely. Zip! Off you go!
This is why computers need to handle this sort of thing, and not rely on engineers entering in data in the correct units. It's a simple calculational problem, but the penalty for making a mistake is huge.
With enough time, and enough debugging (and a few failed missions - though likely not MANNED ones), you'd get the failure rate down to zero. Of course, for manned missions, you'd very likely have backup systems. Granted, for current manned systems, those "backup systems" would be your primary systems, and you'd still need backup systems. So, this would still save you a component.
Yeah right, no stress, unfortunately, if you don't catch the tether you go off into space and die... no pressure, no pressure:-)
(Still, to be fair you can probably have a backup rocket to circularise your orbit around Mars and then they can come and fetch you.)
But the point is that there's very little relative velocity between you and the elevator - which means you'd probably have a rather large time to have multiple attempts.
Not really, you just build it on earth and bung it up the elevator and sling it over and deploy it. It's pretty trivial, in theory. Financing it might be harder though.
Mars has two moons, one that's below AMO (areosynchronous Mars orbit). You need to have active avoidance with whatever elevator you build. That's the remote engineering I was talking about. Plus the whole difficulty of tethering something to the ground without anyone there.
Financing shouldn't be hard, though again, a government agency would likely be the one to do it. The cost would likely be in the hundreds of millions, if not less. You'd likely save that on the first few missions that didn't need to aerobrake.
The Space Elevator would be a tempting target for terrorists, since it could be attacked using low tech weapons, if they could be delivered. We shouldn't underestimate their creativity in doing this.
Why? Why would they want to strike it? Would it cause a big commotion? No. Would virtually anyone even know it happened? No.
And here's the big reason why terrorists would NEVER bother going after the space elevator:
Would it even bring it down? No.
Terrorists would likely strike the elevator far below GEO - remember the elevator is almost 100,000 km long, and they'd be striking it within the bottom few km. This would do nothing. The operators would be like "Oh, jeez, those stupid terrorists tried to do something again, the elevator's drifting. OK, spool out another km of cable." The ONLY place that striking it would do ANYTHING is if you struck it near GEO, and if terrorists develop the technology to do an orbital strike at GEO, I've got a feeling they'll target other things besides the space elevator.
The second main reason that attacking the Elevator would be useless is that even if they broke the first cable, this wouldn't even be that impressive. The marginal cost for deploying a second cable is trivial (the Conference notes said $2B, but I think they'd win out far more than that due to economies of scale - plus they doubled several things like power distribution which wouldn't be necessary for a 'backup cable'. The ribbon itself was estimated at $400M).
You could imagine it on the news. "Elevator cable #21 was damaged beyond repair today by an explosive package concealed within a launch satellite. Consortium members have already stated that a replacement cable has been moved into position and unspooling has already begun. Full operation is expected to resume in a few weeks."
I mean, seriously. Saying the Space Elevator is a tempting target for terrorists is like saying the International Space Station is a tempting target for terrorists. Sure, it might be. But it's not like it would EVER happen.
Actually, the rockets needed to go to Mars become nonexistent - you can aerobrake for Mars entry as well. You'd also be crazy not to build a similar elevator at Mars as well, though this would be a major feat of remote engineering (but probably worth it, as the cable is much shorter, and all of the incidental costs would be nil - no expensive anchor platform needed, no climbers, etc). Then you'd have free transit back and forth from Mars - literally free - you climb to a certain point on the elevator, wait until the launch window approaches, then let go, and lo and behold, several months later, you aerobrake through Earth's (or Mars's) atmosphere, and dock with the Earth's (or Mars's) space elevator.
Yah, of course, you'd still have chemical rockets for course correction - it'd be silly otherwise. But those rockets would be so small that they wouldn't even be considered rockets, and the only REAL reason for them is contingency.
It may even be far more interesting than that. The whole reason you need to aerobrake at entry points is because of a velocity difference between a transfer orbit and a normal Earth orbit - that is, you're moving faster than a normal Earth orbit. However, so is the elevator. With very clever timing, you might not even need to aerobrake at all, which makes it even easier. You just time your approach so that you and the elevator are at the same point, and that you're at the height on the elevator such that your velocity is the same as the elevator's, and you just grab hold. No stress, no problems.
You are missing the point, which is that government spending is never for the purpose of accomplishing anything other than getting politicians elected.
Actually, I said that, two posts up. To quote myself:
The only thing that moves them [Congress] to action is fear of not being reelected.
This is the reason that politically it may never happen, at least in our country. Other countries have rulers which have longer-term thinking, and there, it will probably happen. China's already stated it plans on setting up a permanent presence on the moon, and I don't think they're kidding, and I don't think they're deluded.
It's also completely beside the point of what I was originally talking about, so, more correctly, you're missing the point. My original statement was something like "The US public would support a mission to Mars, but it probably would not happen because politically, all politicians do is try to get themselves reelected and there's no immediate benefit for them." Your reply argument was that this is all well and good, but we shouldn't be spending government money on it. The point here is that we should: this is, in fact, one of the ideal candidates for government spending. Something that private industry would never do, but would return far more money than it cost, like Apollo.
What you're saying is that you are eager to spend SOMEONE ELSE'S money on what YOU think is important.
All government spending is spending someone else's money on what an individual thinks is important. I don't think a standing military in this day and age is important, and neither do hundreds of thousands of other people, but military spending is less questioned than scientific spending! I also don't think bailing out tourism and insurance industries is important, but they went and did that anyway.
Hell, I don't believe that homeland security is important, and they went and spent money on that anyway.
By your argument all government spending, every cent of it, is wrong. And that may be your belief, but that's a very old political argument - that governments shouldn't exist - and that idea has never been successfully tried.
It's also not something in which I could agree on: the simple point is that 99% of humans are self-interested and lazy. They don't care HOW they get to where they are, just where they are. There are about 1% of the population that realizes "you know, damnit, the only reason this country has as significant resources as it does is because someone said it would be good to explore a long, long time ago. Maybe we should continue doing that, before we significantly deplete our own resources."
Government spending is the art of saying "I do not have time to figure out what is best for this country, so I'm paying you to figure it out." That, ideally, is what it is, and it's the truth. Most people don't - if you think space exploration isn't important, apparently you don't have time to figure it out either, because a simple practical analysis shows that it's a good investment, a good long-term strategy, and inevitable.
1) The cable, like everything else, is not built to last forever. They were shooting for a 20-year operating lifetime.
2) Once the first cable is built, all the other ones are essentially free - probably on the order of multiple MILLION, rather than multiple billion. That's the improvement we're talking about in terms of launch costs.
3) Near the end of the elevator's lifetime (if not far before!), inevitably several new cables will be lofted to replace it.
OK, so losing the cable is not that big deal, but you may be quite skeptical that you could simply "replace it" if it broke, since from meteorites or other random stuff, you wouldn't have any warning. This is true - but again, once the first elevator's up, everything else is free. Most likely what would happen is that the first thing done would be to loft up a large amount of "spare cable" that in case of a break, the elevator could simply spool more cable down to keep itself in geosynchronous orbit.
Simple answer is that they have thought about this stuff, and meteor damage can be mitigated by making the elevator MUCH wider in the area where space debris is common (so it could survive more hits). Read here on meteor mitigation strategies.
Oh, and random point: meteoroid: rocky object in space meteor: rocky object burning up in the atmosphere meteorite: rocky object from space that landed on Earth.
What you're concerned with is meteor damage, not meteorite damage.
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I have other things to spend my money on. I hear many retired folks complaining about socal security, I always respond that my parents were not old enough to vote when they sent their socal security money to the moon, so don't blame me for the mess we are in. (Yes that situation is complex than that) I'd like to keep my tax money. Selfish perhaps, but if you won't let me keep it, at least spend it on something that is of use, not waste it on space.
Here's the hint: humans are all selfish - it's in our nature. However, humanity can't afford humans being selfish - and thankfully, people in our government managed to realize that.
The problem is that humans are lazy . You all want flying cars, and cures for cancer, and a cure for aging, but you don't really have a damned clue how to get there.
The destination doesn't have to be the goal - it's the trip that matters . This is the real reason for "pure science" research. Pure science is more valuable than anything else you can possibly spend your money on. Anything.
That isn't to say we should stand still. Lets develope something of use here. We can catch up to the Chinese anytime. (if only because the spys mean they can't keep the technology secert for very long...)
If we don't keep pushing the envelope of what we can do, we might as well roll over and die as a country - at least as a technology leader. We pretty much already are as far as space goes - in five to ten years, I think people'll be talking about the "heyday of the US", and talking about the amazing new fighter and spy plane designs coming out of Europe, Russia, and China.
Very little scientific research needs to be done in space.
Ah, the voice of ignorance. You mean, very little research like, oh, astronomy (SNAP: Supernova Acceleration Probe, which requires the IR, which you can't see through the atmosphere), basic physics (Gravity Probe B), solar flare monitoring (SOHO), searches for terrestrial planets (the Terrestrial Planet Finder), etc.? Good call. No scientific research needs to be done in space. Sure. Can you do us a favor and let scientists decide what needs to be done in space? Most of us think that there's a metric crapload left to do.
And then if you've just got objections to human space exploration, the point, as always, is that the technology required to have human space exploration massively increases your ability to do things in space. It makes you far better at doing things, and more importantly, it means that you have more flexibility, more capability, and more possibilities than if it's purely robotic.
Hubble'd be half-blind without human intervention. What, you want us to stick around on the Earth until the need arises to leave? Infrastructure can't be built out of nothing. The smart man plans for contingencies.
I mean seriously - where to start. Fusion, alternate propulsion mechanisms, pure material generation, exotic material searches, and then in time, asteroid mining and resource searching.
Look at it this way. If you're one of those nutballs that thinks that Earth has enough of everything it needs from now till eternity, get a clue. Do a Web search on how much helium we have - by the end of this century, it's likely to be gone . Yah. Helium. You know. The stuff they put in balloons. No kidding. Gone.
That's not to say anything of the more exotic heavy metals. We will run out , eventually, and want more. Now, the smart man says: does he look for more materials before he runs out, or after? What you're suggesting is "let's wait a bit, we've got plenty of time!"
The problem with that argument is that it's always valid. Until, of course, we run out of time. And then, we're screwed.
overseas American military target, they'll get nuked off the moon.
How? What missiles do we have that could reach the Moon? Oh wait - we don't have any. And even if we did, they'd take more time than the "rocks" launched from the Moon, and would be relatively trivial to intercept/divert/destroy. Plus with the lack of atmosphere on the moon, you'd pretty much have to have a nearly-direct strike in order for it to physically destroy whatever's there.
domestic military target, their military will get nuked off the face of the planet, the US military will probably invade China and they'll get nuked off of the moon.
How would this fix the basic problem of "they have a base on the moon, and can drop stuff on us"? China might be gone, but, hell, that'd make whoever still controlled the Moon base even MORE powerful. "They just invaded our country! Oh well, hope they can defend every major city, all at once." I'm sure NORAD would love to see "50,000 incoming projectiles".
I'm even ignoring the basic physics of "how do you divert something that's huge and has a ton of kinetic energy".
If China mounts a full-scale attack of the US, then it will be then end of the world.
That, I agree with you on.
But, you might say: "The moon is strategically superior to the Earth, so the US military would be at a disadvantage." The US military will never be at a disadvantage when it can launch a thermonuclear warhead anywhere on the globe with mere minutes of notice. The US military has the hardware and the will to do this.
That's like saying that a dying man isn't at a disadvantage just because he can bludgeon a healthy man to death. If the Chinese would set up a base on the moon, with the capability to launch projectiles at the Earth, the US has two options: armageddon, or talk treaty. A base on the moon, with military goals, is a massive advantage, simply because all of the weaponry, upkeep, etc. are virtually free (in the long term), whereas the Earth-based defense is expensive. You'd win by attrition.
Then again, the US military tends to be nuts, so they'd probably choose armageddon.
The fact is that if China did set up a moon base, then the US, the EU, Russia and India would have telescopes in lunar orbit before China even got their Porta-Potties set up. Not to mention the other 500 tracking systems that would be deployed. So, no surprise attack, no secret construction, no military reason to build a mass driver.
Sure, set up telescopes. Set up tracking systems. What're you going to do? Oh, no, you knocked down one of their projectiles. Big deal. Now prevent the other several hundred that are coming your way as well. Go ahead. Keep preventing them. Knocking them down takes fuel, money, and accuracy. Sending down projectiles, though. That's virtually free.
The point of the mass driver in The Moon is a Harsh Mistress wasn't that they couldn't be blocked. It was that the relative cost for blocking one was much higher than the cost for sending it down.
I'm not arguing that it wouldn't be a bad thing. Not at all. It would be another arms race, and another chance for humans to kill themselves. Not good. I'm however talking from the pure militaristic viewpoint, and there is a pure advantage to a military Moon base there.
There are things more militarily powerful than a nuclear weapon . This shouldn't come as a shock.
I was having real trouble thinking about this until you mentioned that spacetime itself is moving/accelerating
I don't remember saying that... that's definitely not true. The Schwarzschild solution has an event horizon even if it's static.
The problem is that you've got to worry about reference frames quite a bit here. For the observer at infinity, space and time seem infinitely stretched out as you approach the event horizon.
For the observer falling INTO the black hole, nothing weird happens at all near the event horizon - it still takes finite (small!) time to reach the singularity.
(note: I guess in some sense you *could* consider spacetime to be "moving" near a black hole. The math might even work out exactly the same! the problem is that it tries to simplify the discussion too much: nothing 'magic' happens at the event horizon. Time (from an infinite observer's point of view) is dilating continuously as you approach the event horizon. Once you cross it, that's when strange things can happen - you can start receiving images/messages/information from the 'future', etc., but no matter what, you are heading to the singularity.)
When you drop that bit of line into the EH (event horizon) you're fine and dandy to that point. But every elementary particle that crosses that threshold is gone. I would suggest that pulling the line back up would be just as easy as letting it down. But it would be cleanly cut with a very exact curvature. At least that is my guess... Since any sublightspeed communication is not going to happen across the point where spacetime itself exceeds lightspeed the forces that would hold/connect particles inside the boundary to one outside the boundary would be cleanly severed.
Ah, reference frame issues. To you, you never see anything cross the event horizon. Nothing at all. All you see is things ever increasingly slowly approach the event horizon. It takes infinite time for the last photon that the object crossing the event horizon emits to reach you.
If you try to pull upwards on something, you're asking the molecules in that "something" to communicate your upward force to the molecules they're attached to, and likewise, the downstream particles need to communicate their downward force (preventing the upstream particles from moving away, producing tension) upward. This "upward" communication will take an increasingly long amount of time, which means that the "stretch" between adjacent molecules will increase as you get closer to the event horizon. At some point, this stretch will be great enough that the bond will just break. This does -not- occur at the event horizon, but before it. The deeper the particle falls in, the less likely you're going to be able to get it out, because the energy required continues to go up, and at some point, the energy required will be more than you can convey to the particle.
And plenty of otherthings to think about. Such as if spacetime is moving, in relation to what?
Well, as I've said above, spacetime itself isn't really moving. But, there are cases where it does (like, the expansion of the universe). In those cases, it's moving in relation to a flat coordinate system, at infinity.
Other questions occur, might be silly but... Where does spacetime come from? Is a coordinate system inexhaustable? Where does it go?
Spacetime is spacetime - it's a coordinate system. The coordinate system deforms, yes, and "the minimum amount it can deform" is up for grabs. Can you increase the volume of spacetime? Sure - all you need to do is stretch it, which is quite easily doable. It doesn't "come from" or "go" anywhere, because it "isn't" anything. It's just a coordinate system. It's how long it takes light to get from one place to another one.
Picture two "squares" of 2D spacetime. In one square, put a black hole, in another put nothing. The length of each size of the square is 1 light-year. Now, crossing the square diagonally for the square
My understanding (and flawed it probably is), is that the event horizon and the singularity are quite different things. I'll agree what you state is likely true in reference to the singularity but as for the event horizon I must disagree.
:) )
They are. The event horizon is not a true discontinuity in space. To an observer moving inward, nothing weird happens at the event horizon. The singularity is a discontinuity in space, and an indestructible 'observer' would see something weird happen at the singularity.
However, the event horizon does separate space into two regions that cannot communicate with each other - that is, the light cones of the two regions are physically distinct. There are certain coordinate systems where this is extremely obvious, like Kruskal-Szekeres coordinates (see this nice discussion on different coordinate systems for more info).
Please correct me if I'm wrong but the event horizon (EH from now on) is simply that boundary where the escape velocity exceeds the speed of light.
You're right, but you're not right. Gotta love that. The whole "escape velocity exceeds the speed of light" is a useful mnemonic to remember the Schwarzschild radius, but it is *not* where the event horizon comes from. This is an unfortunate coincidence of physics, that "bad" Newtonian gravity can give the "right" GR answer. Ah, well. The event horizon is where the denominator blows up in the Schwarzschild/Kerr/Neumann/Kerr-Neumann equation, in Schwarzschild coordinates. In non-pretentious speak, the equation describes the geometry of space around a black hole (normal, charged, rotating, or charged-rotating), and Schwarzschild coordinates are "normal" coordinates: 1 time coordinate, 3 spherical coordinates.
Now, there's nothing special about those coordinates, and that's why the event horizon isn't a real discontinuity in spacetime (though it *is* a causal discontinuity).
Let's restrict ourselves to Schwarzschild black holes only - the other ones have weird shaped event horizons (donut-shaped) and ring singularities, and weird things like that.
The "Minkowski metric", the metric of normal flat spacetime, looks like this: ds^2 = -dt^2 + dr^2 + r^2 do^2. You might have seen something similar in the flat space form: ds^2 = -dt^2 + dx^2 + dy^2 + dz^2 - this is the "4-dimensional Pythagorean theorem": if you ignore the "dt^2", which is relativity, the other part says that the distance between three points (squared) is the sum of the squares of the displacements along each axis.
However, note that time is negative, and space is positive: you can switch that (space negative, time positive) as all that matters is "ds^2", but there is still a sign difference between space and time.
Now, past the event horizon, in the Schwarzschild metric, radius is negative, and time is positive. That is, your radial distance from the singularity becomes timelike, and your time since crossing the event horizon becomes spacelike. In this case, that means that all worldlines lead to the singularity once you pass the event horizon. It's like saying "you can't move backwards in time" - same thing. Inside the event horizon, you can't move outwards. Period. End of discussion. Game over, player one.
(The astute reader may conclude that maybe you can, in fact, move backwards and forwards in time once you're inside the event horizon. Well, maybe. That fact is outside the realm of our discussion.
Anyway, the best way to realize that nothing can "come out" of the event horizon is to view the event horizon from infinity. Imagine that you send a probe in, designed to emit one beep every picosecond (10^-12 second). As the probe gets closer to the event horizon, time dilates dramatically, so every picosecond for the probe takes incredible amounts of time for an observer at infinity - hours, days, months, years, etc. The other way to think about thi
Once you're outside the atmosphere there are no outstanding reasons for not moving faster. That is why I mentioned magnetic drive/coupling instead of any physical connection. And of course carbon nanotubes can be configured to carry power, it should also be relatively simple to configure part of the elevator as a track for a magnetic linear accelerator. No inefficient rocket power required (although to be honest I'm not sure what the efficiency of a linear accelerator is).
:)
Damage to the ribbon is a good reason. However, avoiding the Van Allen belts is a better reason to move quicker once you're a bit farther up, so point. However, counterpoint is that you don't torque the ribbon as heavily once you're up at a higher point, as you're shifting the center of mass less.
However, the best reason for not moving faster is, of course, because you don't really have the power. It's silly to carry the power with you (reduction of launch mass), so you beam it up from the ground. I've a feeling that this will *stay* the main method for scaling a space elevator for a long time to come, simply because it's the easiest, and the most launch-mass efficient.
Also, the nanotube power carry thing is a bit of a red herring. There are two problems:
1) The elevator won't use continuous nanotubes - it'll be using composite nanotube fiber, which, of course, cannot carry power, as it's just glue with a bunch of really weird things called 'nanotubes' inside it, and the glue probably won't conduct.
2) Even if they end up using continuous nanotubes (maybe multiwall nanotubes, as it appears you may want to fuse them), nanotubes conduct power along their axis, and a continuous string of nanotubes would have no way of conducting power off-axis. Plus, when you're talking a length of 60,000 miles, the nanotubes would have to superconduct (which they don't - they do weird things with electrons, but it doesn't look like true superconductivity) - even the best conductor known to Man would appear to be a gigantic resistor when it's 60,000 miles long.
Last but not least, I do want to thank you for an interesting discussion. I find real thinking to be a rare pleasure these days and I do enjoy it.
Whew! I normally call lack of thinking death. Interesting discussion is the only reason to wake up in the morning.
...*GRIN* So you should be able to make the trip in 6'ish hours. That
6 hours? No way. 2 weeks, nominally.
The whole reason a space elevator wins over a rocket is because you can avoid air resistance by going slow. In 6 hours, you might as well be using a rocket. You're talking about 60,000 km, so roughly 10,000 km an *hour*.
There's another problem with a 6 hour trip, too - the speed of sound in the elevator produces about a 7 hour natural vibration. A 6 hour trip means that you're actually travelling supersonically up the elevator (supersonic with respect to the cable).
Anyway, with a 2 week trip, you can easily see that even a very gentle push (I think it's in the "few newtons" range) can counteract the slight lean of the elevator.
Remember that the Earth rotates--right now, you're moving at, what, about 2,000 mph? As you go straight up, relative to a point on the surface, your speed increases because you're still completing one rotation / day. Once you get up to geostationary orbit, you've got (at least) the necessary escape velocity. (I also suspect that e.v. goes down as altitude increases, but I'm not certain.)
No. Escape velocity is what you get when you set kinetic energy = potential energy. Kinetic energy is T = 1/2 mv^2, potential energy is U = GMm/r. Set the two equal, and poof, you get escape velocity.
If you have escape velocity, you are no longer in orbit. At all. You will always be moving away from the Earth. Technically we call this a "parabolic orbit" (the C3=0 orbit), but it's not what people normally consider an orbit - that is, you'll never come back to Earth.
If you're at geostationary orbit, you don't have escape velocity, because, well, you're still orbiting the Earth, now aren't you? GEO is still shy of escape velocity by something like a few km/s (2 or 3-ish, I think, I can't remember. Don't correct me if I'm wrong and it's 6 or 7, though it really can't be - it's too late for me to use real numbers).
Escape velocity depends on your altitude, yes. v_esc = sqrt(2GM/r), where r is your altitude above (the center of) the Earth.
There is no way you could build a space elevator out of a black hole. It's not possible - not for material science reasons, or practical reasons, or anything - it is simply not possible to build *anything* that could cross the event horizon of a black hole. The intermolecular forces required would be infinite - literally, infinite. Unbounded.
The energy that it would take to ascend the indistructable space elevator would be rather high, of course. And they'd be worse the deeper into the hole you were.
You're right it'd be high. Infinite. And the deeper you got, the worse it'd be: infinity+1, infinity+2, infinity+3..
I'm kidding, of course. Again, you can't. It's infinite. Period. The integral is unbounded.
The shuttle's orbit would change if it grabs a satellite, unless the center of mass of the extended "satellite/shuttle" system is at the same altitude as the shuttle was originally.
If the shuttle approached the satellite from along its orbital path (the path of its velocity) or the path perpendicular to it, then it wouldn't change (in front of/behind it, or left/right of it). If it approached along its radial vector, it would change (above/below it).
If it grabbed along the radial vector, the shuttle and the satellite would start rotating around each other slowly, around the center of mass. Depending on when the shuttle let go (and the original orientation of the satellite/shuttle - i.e., which was higher, which was lower), it could be boosted into a higher orbit, or lowered to a lower orbit, or returned to its original orbit.
This is actually the whole idea of orbit-changing space tethers.
Actually I would suggest some liquid movement tanks high and low on the counterweight to maintain the balance without actually having to shorten the cable/distance to the balance point. Or you could have some type of elevator/freight system moving large plugs/masses of rock up and down to do the job. Better yet, both, having a dual system (or even ternary) would make sense considering the importance of balancing the system...*GRIN* I would do a great deal of work making it as foolproof as possible cause goodness knows we keep making the fools.
Actually, no...
Think about this for a second.
Picture an object in a circular orbit. Gravity is F = GMm/r^2, and the centripetal force required to keep it in that orbit is F = mv^2/r. So, so long as v^2 = GM/r, everything's happy.
Now, let's suppose that something grabs onto the elevator, say, something that's 10K pounds (pounds, because pounds is a force, and that simplifies things). So, now, the force that's pulling on the object is an F > mv^2/r. But it's still *moving* with v^2 = GM/r, so it can't simply be "pulled downwards" - instead, it starts to *lean forward*. All you've done is increased the centripetal acceleration, which means all you're going to do is bend the velocity vector such that it *starts* to head towards you. If it were a satellite, you'd be putting it into a lower elliptical orbit that still keeps that point-of-increased-a_c as its perigee. Since its an elevator, it starts "falling down" - leaning forward.
OK, now send the object up the elevator. As it goes up, it's now *torquing* the elevator (because the radial vector - perpendicular to the velocity vector - is no longer in the same direction as the force vector, so there's a torque), trying to make it lean forward.
Once the mass gets beyond geosynchronous orbit, it's now shifted the center-of-mass of the elevator above GEO, and it's now trying to make the elevator lean *backward*.
See a pattern? Sending a load up the elevator will induce an oscillation in the ribbon. It won't make it slowly fall, unless you send loads constantly to less-than-GEO and let them go, and don't bother trying to actively damp the oscillations at the base.
Yah, you could just actively damp the oscillations. The elevator starts leaning one way, you pull the elevator the other way. The two oscillations cancel each other out. No balancing act needed.
(Yes, you can keep a satellite in the "wrong orbit" - imagine if the satellite was too close to the Earth, such that it tried to fall. Now imagine constantly blowing on the underside of the satellite, such that the sum of your force and the force of gravity equals the necessary centripetal force. Poof. Satellite in an orbit again. That's what active damping would do).
Well, close.
Orbital paths are dictated by velocity and altitude of the center of mass.
Move the center of mass, you change the orbit of the object. So the point that he made is valid, if not for the reason he thought (or you thought that he thought).
This isn't a problem in this case, as others have pointed out, for many reasons. First, it's a trivial mass compared to the elevator mass. So it's not going to significantly change things at all.
Second, even slightly altering the center of mass like this would do won't cause an orbital change, because the elevator is fixed at one point - the anchor. It'll cause a torque, which will make the elevator try to lean. The anchor will hold it in place (it's only a *small* torque), transferring the torque against the Earth's rotation, slowing down the Earth's rotation (trivially, of course).
So, you get an oscillation, which can be damped by the base, and has already been modeled and simulated. It's not a problem, and no, it will not wobble and collapse like the Tacoma Narrows bridge.
A TerraByte is 1024 Gigabytes
:)
Great. Now explain to them that a 1 GHz machine is twice as fast as a 500 MHz machine.
Oh, wait, now you'll have to explain that 1 GHz is 1000 MHz.
So 0.5GHz is 500 MHz, but 0.5GB is 512MB.
It's stupid. GB is 1 x 10^9 bytes. Whoever originally figured "hey, 1024 is close enough to 1000, so we'll just use 'k'!" should be shot. Why didn't he use "bogoK"? Was a perfectly good solution for convincing people bogoMIPS aren't real.
Oh - and it's "terabyte", not "TerraByte". kilo, mega, giga, tera, peta, exa. Ah, the joys of working in high-energy physics.
You can directly and simply relate bits, frequency, and rate (like MB/second)
Except that frequency and rate are in powers of 10 prefixes, not powers of 2. GHz means 1,000,000,000 cycles per second, and data transfer rate is usually determined by the frequency of the clock used to transfer data. It'd be silly to measure it in terms of "1024*1024" clock cycles - there's really no point.
The only place where powers-of-2 prefixes make sense are memory (because of SDRAM's poor granularity - they only come in sizes increasing by powers of 2: so 256k, 512k, 1M, 2M, 4M, 8M, etc). In storage, frequency, data transfer rate, all of those things, powers of 2 don't matter, and base 10 measurement devices are already out there for frequency.
Honestly - why is it more convenient to, say, break things up into 1k blocks rather than 1000 byte blocks? Because the address space of a computer is likely to be in powers of 2, and 1k blocks will divide evenly (plus you can shift, rather than divide, which is of course the main reason). This is why powers of 2 are considered better in compsci - because you can just lshift/rshift. In the 'real world', it's easier to divide/multiply by 10, since we use base 10 numbers.
Storage manufacturers will never stop reporting disks in SI units until we convince people to start using base 2. After all, would you want to explain to someone that no, that 0.96 TB drive is, in fact, twice as large as the 500 GB drive?
We have mandates on product labeling for many other products I think its time we force the industry to be upfront.
But... but... they ARE being up front. They're the ones who are correct. Is it honestly their fault that everyone else uses bizarre (and incorrect) prefixes?
Honestly, think about this for a second. How could drive manufacturers estimate the size of a platter? They know the bit density (say, 16Mbit per sq. mm., so 2Mbyte per sq. mm.), so, just multiply by the area (say 500 sq. mm), and boom, that's an estimate (1GB platter). In numbers that have nothing to do with 1024, or base 2. In this case, it's easier to use base 10. This'd be more instructive if the numbers were irregular and smaller, but I'm lazy, and so are they, probably.
Think about a drive researcher, who's just calculated that they can increase the bit density by a factor of 100 on drives, and a PR person asks him how large that will make the drives. He knows they have 100 GB disks now, and then quickly says "20 TB" - and wow, he's correct. If he used binary prefixes, he wouldn't be. The point is that hard drive densities aren't required to constantly double, so binary prefixes are dumb. You have 20GB, 30GB, 40GB, 60GB, 80GB, not 8GB, 16GB, 32GB. With SDRAM, the poor granularity requires size increases in doubles - so 256MB, 512MB, 1GB, so binary prefixes "kindof" make sense.
Anyway, the whole case is ludicrous. Can you imagine going to court with this?
"So, Mr. Hard Drive Company, why exactly did your company use deceptive advertising?"
"Um, we didn't, your Honor. The drive is 20 GB in size. According to the SI system of units, this means 20 x 10^9 bytes."
"But the rest of the industry disagrees."
"Your honor, the rest of the industry has been confused about this issue from the beginning. We've been extremely consistent about this. 1 GB is 1 x 10^9 bytes."
The judge would have to be an idiot to disagree.
If you want the planetary parameters, why not go to the obvious source:
Because Google gave it to me in a few seconds? If I really wanted to get it from a "more scientific" source, I've got plenty of other sources within arm's reach.
It is NOT possible that this little hydrogen could cause an increase of a factor of 100 in the brightness. There is always plenty of hydrogen left over in the overlying layers.
What I said was that if the planet reaches the core, it could cause something akin to a nova. I'm still REALLY skeptical of this, because the only reason a nova causes that significant a brightening is because the white dwarf is generating so little light anyway. A red giant has so many obscuring layers that the core wouldn't be visible (hence the reason that OV/IR stars exist).
(You can't invoke gravity, because the surrounding material quickly matches the planet's hydrogen density and buoyancy with do it's bit. The core will probably slowly down to the stellar core, but that's not a lot of power and there is only a limited amount of hydrogen fuel there anyway.)
OK, so you do agree the core of the planet would at least reach the burning region.
I'm not so sure this is irrelevant. This was the whole point that I was thinking of. I don't think it's likely that the star will necessarily bleed off all of the hydrogen of the planet (the outer layers, yes, but the inner layers are too dense - the envelope of a red dwarf is what, about as dense as water?), but I don't think it's that important. The point is that once it reaches the core, it's going extremely rapidly, and if nothing else, it's going to seriously dredge the star and stir up a lot of newer material.
After all, once it reaches the burning region, it's got a TON of momentum (what did I quote before? 0.05 R_sun is the burning zone in a red giant?) That's a lot of velocity to strike an essentially solid object with.
But anyway, as I said several times, I don't think this applies here. I don't think that it will cause a measurable brightness increase (especially in this case - if it was something very near a planetary nebula, yah, quite possibly). If nothing else, the best objection to all of this is that the impact will likely occur deep within the star, and unless they're talking about a couple-magnitude increase in the infrared, we'd never see it.
What I *do* think this could do is seriously mess with the composition of a star (it's another dredge-up, if nothing else) and *possibly* cause a spike in certain wavelengths.
Remember however that the line is under very high tension, many thousands of tons equivalent. If it breaks and drifts it's not so easy to catch, hold on to, and reel back in.
You reel down from orbit, not from the ground. You don't need to grab it. It'd bounce, and oscillate a bit when it broke.
Not really, though - the tension is not coming because it's attached to anything - it's coming from the cable's existence. Cables normally spring upwards and bounce because the tension on them goes from "huge" to "zero": in this case, the tension merely changes a little because the weight changes slightly. It's still under tension, because gravity's still there.
It'd be trivial to spool more outwards, considering you better be able to do that when you deploy the elevator.
"mass of Neptune / mass of the Earth"
Hey, that doesn't work in Google. What the heck? Neptune's not good enough for Google?
Anyway, Neptune's really 17.1 M_earth - I used 18 because it was 18 in the parent post, and didn't stop to remember - I knew it sounded "close".
Let's review out giant planets, shall we?
Given that both planets are around 18 Earth-masses
First off, last time I checked, Uranus is 14.5 M_earth, not 18 (Neptune is 18 M_earth: even Google can tell you that: search for "mass of Uranus / mass of the Earth" and "mass of Neptune / mass of the Earth") , so it's core BETTER be less than 15 M_earth. Don't jump at someone about checking facts when you make a mistake like that.
Anyway looks like I got the core masses for Uranus and Neptune from dated sources, or confused radius with mass (though I had 5 M_earth for Uranus's core - I think I -really- mixed things up there).
When I said "mostly core", I meant by volume - you apparently meant by mass.
The metallic hydrogen is a layer right above the core, not the core itself.
That's what I said.
core is probably metallic hydrogen (way cool, that) covering iron/nickel
I guess "liquid metallic ocean" would probably be better than 'core', especially in Jupiter's case, where it's tremendously larger than the core. When it was taught to me, it was taught as a "metallic hydrogen core layer", because you can't call it an atmosphere. Ocean's definitely a better term, though.
In this case, it's an F-class star. And you missed my point entirely, which was that if the star can convect the planet's hydrogen into the shell-burning zone, it can damn well convect its own hydrogren reserves down there, which are vastly in excess of what the planet could provide. So the star should never notice the miniscule addition of the planet's hydrogen.
I said it wasn't applicable to that case - the star's not late-stage enough. I did however say it was possible to cause a large increase, IF the planet made it to the core and the star had nearly exhausted its hydrogen reserve. Hell, if the star was a good way to becoming a planetary nebula, this would essentially be the equivalent of a nova, as the amounts that normally cause a nova are actually pretty consistent with the mass of a giant planet (10^-4 M_sun), and the brightening they were talking about is consistent, though a little low as well.
The other possibility to think of is that the rocky core is actually going to drag hydrogen down to the stellar core as it descends into the thin photosphere by drag. I don't really see how this would be a huge explosion, though, but maybe I'm missing something (seems to me like it would cause a very gradual brightening, and also show some periodic variability with the planet core's period). I also don't see how the core wouldn't be ripped to shreds by tidal forces, but that I'd actually have to calculate. 10 M_earth or so in about an Earth radius is going to be pretty difficult to tidally distort, though it's gotta get down to about 0.005 R_Sun in order to reach the H-burning shell, which would have pretty significant tidal forces.
This COULD be the mechanism they're working with, though it seems a bit of a stretch offhand.
No - you don't fuse iron. You neutron-stuff them. During the supernova, the outbound shock wave carries so much energy (and neutrinos) that neutrons are literally "shoved" into nuclei. You get ridiculous things like iron with hundreds of neutrons, which then decay down into normal elements by alpha emission and beta decay. This is r-process stellar nucleosynthesis. (There's also s-process stellar nucleosynthesis, which is also neutron stuffing, but on a much longer timescale. Essentially everything past iron is formed by r-process stellar nucleosynthesis. Check Carroll & Ostlie pp 527-528 for more info.
Stars die when they hit the iron stage because they can generate no outward pressure from fusing iron. They can't even fuse iron at all! It's actually a really complex procedure - basically, the iron starts to lose all of its electrons (from proton capture and other mechanisms) so the core rapidly loses electron degeneracy pressure, which is what was (briefly) supporting it. The inner core collapses very uniformly to a little neutron star, and the outer core decouples from the inner core, and the outer core rushes inwards at extreme velocities. The collision of the two is one of many explosions in a supernova. (Again, see Carroll & Ostlie's section on the Death of Massive Stars)
Anyway, the fuel isn't insignificant depending on what stage the star is in, and also depending on how fast the planet's orbit would decay once it's inside the photosphere. If it meets with the star's core without significantly losing mass, that could cause a VERY large brightening. Functionally it's equivalent to a nova, or the pulsing of Wolf-Rayet stars (without the mass shell shielding it).
2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.
:)
Ah, nitpicking.
When the core of the star gets to iron it CAN blow up, because, of course, iron is king when it comes to nuclear stability. Can't fuse it, can't fission it, it's just... iron. Hence the reason that cosmic rays are generally considered to be either particles, or iron.
Anyway, the star still won't blow up if it does start to burn stuff heavier than carbon - it'll only blow up if it has enough mass to overcome the electron degeneracy pressure. Most people merge these two - "duh, if the star has enough mass to burn up to iron, it'll have enough mass to overcome the electron degeneracy pressure!"
This isn't *completely* clear (and as far as I know, it's still ambiguous) because you don't know how much matter the star's going to slough during its helium burning/carbon burning dying phase. The planetary nebulae around planets contain a huge fraction of the star's mass.
Anyway, blah, this is nitpicking. Any star which honestly seriously gets to silicon burning is going to supernova, except for some really really weirdo situations.
Other thing is that the materials that were likely formed completely in supernovae are actually only elements past iron. Anything below iron could have formed in a star, and been sloughed off, or shredded away somehow. Anyway, the point that you made was that everything past helium comes from a supernova - that's definitely not true. Probably a majority of the elements past about, oh, oxygen comes from a supernova (but probably not virtually all). Carbon, nitrogen, and oxygen can, and probably do, come from other sources than supernovae.
Being a TOTAL nitpick, only everything past iron is formed in a supernova. Elements higher than oxygen are probably spread via a supernova. The reason that C, N, O can be spread easily is because, as with the triple-alpha process, stars balloon when they manage to reach a temperature where they start burning new elements, and they slough off a ton of material, and the stages near C,N,O last long enough that they probably can shove a significant amount of material off (hundreds/thousands of years - still virtually no time cosmologically).
No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.
Depends on the kind of star. The core of the star - that is, the "dense" part - is not that inconsistent with the size of a gas giant. After the triple-alpha process ignites, the outer regions of a star are so incredibly not dense that they probably wouldn't produce ANY drag, and you could get significant enrichment when the planet actually encountered the star's core, if the resultant nova didn't blow the contents of the star out to kingdom come (i.e. escape velocity) because you'd probably isotropize the shell, and get a locally "metal-heavy" shell about the star which would show up in spectroscopy.
Note that this, of course, would only result in an enrichment of red giants, and again, isn't applicable for main sequence stars.
Note: much of Uranus's core is hygrogr
Um, yeah granted, but the aerobraking window at Mars is particularly narrow- the 'atmosphere' of Mars is very tenuous. Let's put it this way: Buzz Aldrin doesn't like it at all.
:) Yes, it's difficult, and the margin is small, but you can do it, and I believe they have in fact done it already.
Hence the reason that a computer should be the one to do it. Computers don't have to like it. They just have to do it.
Mommy state will help you out? Maybe, if there's votes in it.
Nah, this is simple economics. If you've got 5 missions planned to Mars in the next 5 years that cost $1 billion dollars, and the elevator would save you, say, $400 million and cost you $250 million, obviously, you're going to use the elevator. Hell, the government doesn't even have to okay it, depending on how it's budgeted, since it's the same amount of money and the same amount of science gets done. The point is that it's an immediate obvious cost benefit.
Well, the tether is millimeters wide, and you can't hit it with any speed otherwise you cut the tether and possibly open your vehicle to a vacuum. You're approaching it from thousands of kilometers away with an initial approach speed of maybe 1km/s or so. It's pretty much the same as docking with the ISS, only slightly harder and the stakes are even higher.
No no no - you're missing the point. The point is that the elevator is traveling at different velocities at different points along its length (i.e. its linear velocity is omega*R, since its rotational velocity is constant), and you choose the point of approach that will have the same linear velocity as you do when you're at the same point. So you're not approaching it with an initial approach speed of 1 km/s: you've got the same speed it does. There's going to be a little differential speed difference between you since it's rotating, and you're not -quite- rotating (it's got centripetal acceleration from tension, and you've just got gravity), but over a reasonable time frame, that should be minimal, since to a reasonable degree of accuracy, you're both going around the Earth. The differential velocity between you would be VERY minimal - probably meters an hour, not meters a second. Plus if your velocity isn't quite what you thought it would be, you just aim higher or lower.
The obvious addition here is that you could easily put a capture platform at the Mars approach point to make it safe. The main reason this is easier than docking with the ISS is because you're using gravity (and very simply kinematics!) to match velocity, rather than using thrusters.
Aerobraking at Mars is a somewhat tricky proposition. If you don't brake enough you die.
:-)
Well, the "if you don't do it right, you die" comment is true for ANY space operation. If you don't calculate the release time correctly, you miss Mars completely. Zip! Off you go!
This is why computers need to handle this sort of thing, and not rely on engineers entering in data in the correct units. It's a simple calculational problem, but the penalty for making a mistake is huge.
With enough time, and enough debugging (and a few failed missions - though likely not MANNED ones), you'd get the failure rate down to zero. Of course, for manned missions, you'd very likely have backup systems. Granted, for current manned systems, those "backup systems" would be your primary systems, and you'd still need backup systems. So, this would still save you a component.
Yeah right, no stress, unfortunately, if you don't catch the tether you go off into space and die... no pressure, no pressure
(Still, to be fair you can probably have a backup rocket to circularise your orbit around Mars and then they can come and fetch you.)
But the point is that there's very little relative velocity between you and the elevator - which means you'd probably have a rather large time to have multiple attempts.
Not really, you just build it on earth and bung it up the elevator and sling it over and deploy it. It's pretty trivial, in theory. Financing it might be harder though.
Mars has two moons, one that's below AMO (areosynchronous Mars orbit). You need to have active avoidance with whatever elevator you build. That's the remote engineering I was talking about. Plus the whole difficulty of tethering something to the ground without anyone there.
Financing shouldn't be hard, though again, a government agency would likely be the one to do it. The cost would likely be in the hundreds of millions, if not less. You'd likely save that on the first few missions that didn't need to aerobrake.
The Space Elevator would be a tempting target for terrorists, since it could be attacked using low tech weapons, if they could be delivered. We shouldn't underestimate their creativity in doing this.
Why? Why would they want to strike it? Would it cause a big commotion? No. Would virtually anyone even know it happened? No.
And here's the big reason why terrorists would NEVER bother going after the space elevator:
Would it even bring it down? No.
Terrorists would likely strike the elevator far below GEO - remember the elevator is almost 100,000 km long, and they'd be striking it within the bottom few km. This would do nothing. The operators would be like "Oh, jeez, those stupid terrorists tried to do something again, the elevator's drifting. OK, spool out another km of cable." The ONLY place that striking it would do ANYTHING is if you struck it near GEO, and if terrorists develop the technology to do an orbital strike at GEO, I've got a feeling they'll target other things besides the space elevator.
The second main reason that attacking the Elevator would be useless is that even if they broke the first cable, this wouldn't even be that impressive. The marginal cost for deploying a second cable is trivial (the Conference notes said $2B, but I think they'd win out far more than that due to economies of scale - plus they doubled several things like power distribution which wouldn't be necessary for a 'backup cable'. The ribbon itself was estimated at $400M).
You could imagine it on the news. "Elevator cable #21 was damaged beyond repair today by an explosive package concealed within a launch satellite. Consortium members have already stated that a replacement cable has been moved into position and unspooling has already begun. Full operation is expected to resume in a few weeks."
I mean, seriously. Saying the Space Elevator is a tempting target for terrorists is like saying the International Space Station is a tempting target for terrorists. Sure, it might be. But it's not like it would EVER happen.
Actually, the rockets needed to go to Mars become nonexistent - you can aerobrake for Mars entry as well. You'd also be crazy not to build a similar elevator at Mars as well, though this would be a major feat of remote engineering (but probably worth it, as the cable is much shorter, and all of the incidental costs would be nil - no expensive anchor platform needed, no climbers, etc). Then you'd have free transit back and forth from Mars - literally free - you climb to a certain point on the elevator, wait until the launch window approaches, then let go, and lo and behold, several months later, you aerobrake through Earth's (or Mars's) atmosphere, and dock with the Earth's (or Mars's) space elevator.
Yah, of course, you'd still have chemical rockets for course correction - it'd be silly otherwise. But those rockets would be so small that they wouldn't even be considered rockets, and the only REAL reason for them is contingency.
It may even be far more interesting than that. The whole reason you need to aerobrake at entry points is because of a velocity difference between a transfer orbit and a normal Earth orbit - that is, you're moving faster than a normal Earth orbit. However, so is the elevator. With very clever timing, you might not even need to aerobrake at all, which makes it even easier. You just time your approach so that you and the elevator are at the same point, and that you're at the height on the elevator such that your velocity is the same as the elevator's, and you just grab hold. No stress, no problems.
You are missing the point, which is that government spending is never for the purpose of accomplishing anything other than getting politicians elected.
Actually, I said that, two posts up. To quote myself:
The only thing that moves them [Congress] to action is fear of not being reelected.
This is the reason that politically it may never happen, at least in our country. Other countries have rulers which have longer-term thinking, and there, it will probably happen. China's already stated it plans on setting up a permanent presence on the moon, and I don't think they're kidding, and I don't think they're deluded.
It's also completely beside the point of what I was originally talking about, so, more correctly, you're missing the point. My original statement was something like "The US public would support a mission to Mars, but it probably would not happen because politically, all politicians do is try to get themselves reelected and there's no immediate benefit for them." Your reply argument was that this is all well and good, but we shouldn't be spending government money on it. The point here is that we should: this is, in fact, one of the ideal candidates for government spending. Something that private industry would never do, but would return far more money than it cost, like Apollo.
What you're saying is that you are eager to spend SOMEONE ELSE'S money on what YOU think is important.
All government spending is spending someone else's money on what an individual thinks is important. I don't think a standing military in this day and age is important, and neither do hundreds of thousands of other people, but military spending is less questioned than scientific spending! I also don't think bailing out tourism and insurance industries is important, but they went and did that anyway.
Hell, I don't believe that homeland security is important, and they went and spent money on that anyway.
By your argument all government spending, every cent of it, is wrong. And that may be your belief, but that's a very old political argument - that governments shouldn't exist - and that idea has never been successfully tried.
It's also not something in which I could agree on: the simple point is that 99% of humans are self-interested and lazy. They don't care HOW they get to where they are, just where they are. There are about 1% of the population that realizes "you know, damnit, the only reason this country has as significant resources as it does is because someone said it would be good to explore a long, long time ago. Maybe we should continue doing that, before we significantly deplete our own resources."
Government spending is the art of saying "I do not have time to figure out what is best for this country, so I'm paying you to figure it out." That, ideally, is what it is, and it's the truth. Most people don't - if you think space exploration isn't important, apparently you don't have time to figure it out either, because a simple practical analysis shows that it's a good investment, a good long-term strategy, and inevitable.
There are several things to remember here:
1) The cable, like everything else, is not built to last forever. They were shooting for a 20-year operating lifetime.
2) Once the first cable is built, all the other ones are essentially free - probably on the order of multiple MILLION, rather than multiple billion. That's the improvement we're talking about in terms of launch costs.
3) Near the end of the elevator's lifetime (if not far before!), inevitably several new cables will be lofted to replace it.
OK, so losing the cable is not that big deal, but you may be quite skeptical that you could simply "replace it" if it broke, since from meteorites or other random stuff, you wouldn't have any warning. This is true - but again, once the first elevator's up, everything else is free. Most likely what would happen is that the first thing done would be to loft up a large amount of "spare cable" that in case of a break, the elevator could simply spool more cable down to keep itself in geosynchronous orbit.
Simple answer is that they have thought about this stuff, and meteor damage can be mitigated by making the elevator MUCH wider in the area where space debris is common (so it could survive more hits). Read here on meteor mitigation strategies.
Oh, and random point:
meteoroid: rocky object in space
meteor: rocky object burning up in the atmosphere
meteorite: rocky object from space that landed on Earth.
What you're concerned with is meteor damage, not meteorite damage.