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Possible Habitable Planet Just 12 Light Years Away
sciencehabit writes "Astronomers have discovered what may be five planets orbiting Tau Ceti, the closest single star beyond our solar system whose temperature and luminosity nearly match the sun's. If the planets are there, one of them is about the right distance from the star to sport mild temperatures, oceans of liquid water, and even life (paper)."
I've got my own helmet. Where do I sign up?
plop
If there is life, it consists of paper-based organisms?
You know what they say about opinions. They're all fabulous!
Sure its not viable for us to go there ourselves but couldnt we start sending probes in the direction of planets like this with enough ingredients on them to help kickstart life on other worlds that can support it. It wont effect us but might help ensure life continues in the universe once we inevitably destroy our own planet.
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When you're talking about interstellar distances "Just" is an appropriate term to put in front of a distance as small as 12 light years. At our current space flight capabilities it would take us ages to get there. How ever 12 light years is do able, in terms of physics and engineering. We would have to be willing to commit to a very serious project to build the ship though and I don't foresee any government willing to do that in the near future.
And yet -- and yet! -- it is only 0.17 light-years away.
Voyager 1 is only 17 light-hours away from the sun. That is only around 0.002 light-years, not 0.17.
Also note that the Voyager craft only used standard chemical propellants during launch and slingshot affects around various planets to gain the momentum they currently have - they only needed enough thrust to visit the target planets within a reasonable amount of time. In fact, they didn't want them going too fast, otherwise they would have zipped past the planets even faster, reducing the amount of time available to gather data (which of course was the primary objective of the mission in the first place).
Imagine if they had an ion drive and had been accelerating continuously for those 37 years, which is certainly what any interstellar craft would be designed to do.
Better known as 318230.
Um, not even close. Voyager is currently just under 0.002 light years away. Two one-thousandths of a light year.
Which of course makes your point even more pertinent. 12 light years, or even 1 light year, may as well be infinity.
(When you said it was only 0.17 ly away I was like "what, seriously? That's actually pretty good!". Then I realised that can't be right, as I know the DSN isn't waiting anywhere near 0.17 years for signals from Voyager.)
Also note that the Voyager craft only used standard chemical propellants during launch and slingshot affects around various planets to gain the momentum they currently have
Could you get more by slingshotting around the sun?
Sheesh, evil *and* a jerk. -- Jade
Even if the planets are inside the habitable zone, they would need to be the correct consistencies... Venus and Mars are in the zone here, but neither has life or is natively habitable. Yes, we're attempting to discover if Mars may have HAD life, but as far as we can obviously tell, it has none now...
So it's fun and interesting to search these types of star systems and planets--and I think it's absolutely worthwhile to focus a SETI program on them to try to determine if there are any stray signals we can pick up--but otherwise this really is not much more than dreaming and guessing.
Assuming SETI finds no signals, but we do believe there a couple of planets into the habitable zone, then I think it would make some sense to attempt a probe mission there... but it could be a while before we're at the technology level we'd need...
I think our current speed record in space is about 150,000mph ... which is ~1/5000th the speed of light. So while 12 years seems do-able from a speed of light point of view, there is no (present) method to send a probe there in a reasonable amount of time... I'd say reasonable would be a ~36 years to get there, plus another 12 years for the return signal... so roughly 50 years from launch to first data... meaning it would likely be a two, maybe three, generation program from a NASA engineer point of view.
We'd need something capable of:
a) Traveling at least 1/3rd the speed of light (roughly a quarter billion miles per hour)
b) A power source capable of lasting at least ~40 years or more with enough juice available near end of life to complete its mission
c) Capable of complete autonomy in 100% unknown situation
d) Possibly requiring the ability to actively correct its course en route, and maybe even detect and avoid collisions
Just send a B Ship full of your Republicans, climate denialists, gun nuts and all the other right-wingers around the world.
They can propel themselves there with bombastic hot air, and they'll fuck everything up enough that we'll never see that planet again. Of course, if the other guys have had the same idea, we'll need a strategy to deflect them. Pretending we're gay might seem like a good idea, but before you know it, they'll be "adopting a wide orbital stance" and stalk us forever.
Suggestions anyone?
By who, exactly? Hippies and Death worshippers? Fuck those guys.
Keep in mind that Voyager, apart from some gravitational assists, wasn't ever really made to "go fast". Even now, there are ways to send things moving much quicker such as the Ion Thruster which although not NEARLY as powerful as a chemical rocket, is amazingly more efficient. The Weight to Thrust ratio is fantastic and could well be utilized to provide constant thrust for a long time. Once you exit the earth's gravity well, something like an Ion Thruster could over a number of years accelerate a craft to a much higher speed.
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If that planet were inhabited by a technological civilization, we should have been detecting their twelve-year-old radio transmissions, faint as they might be.
Could you get more by slingshotting around the sun?
Yeah, but you'll end up in San Francisco in 1986 so it won't do you any good.
The math is wrong - Voyager I is 0.71 light days away, or 0.0019 light years away. It will take a lot longer than 840 years to get to another star.
In point of fact, its all relative. You are using the orbital velocity of the planet plus its gravity, to transfer momentum to your Voyager craft. Or in your case, do the same with the sun, you would have to figure out what the relative solar motion is with respect to your destination star. There would be galactic rotation and other more local motions to be considered. Finally, the closer you get to the source of the gravity the bigger the slingshot, however if the source of your gravity assist is a ball of plasma with a million degree corona... you might wanna keep a reasonable distance, which might put a damper on the amount of gravity assist you'd ultimately be able to coax from the attempt.
You could try doing several loops around the inner planets to get a solid kick (some of the more recent planetary explorers used that trick.) However, if you're gonna attempt relativistically significant velocities, you're gonna have to use a mother Ion motor, or great big laser or an Orion engine, or something you can keep thrusting with for years while at the same time providing you with a meaningful ISP. Chemical reactions are simply too weak.
Correct me if I'm wrong, but doesn't that mean that the gravitational pull on surface dwellers would be four times that of Earth?
Not necessarily. You forget that both mass and distance play a role in gravitational attraction. Therefore, given a larger radius, it could in fact have a comparable or identical gravitational attraction on the surface, though the exact numbers vary depending upon the density and distribution of material in the various layers of the planet. Assuming identical density distribution, it would require a radius twice that of Earth to have identical gravity on the surface. At the same time, the rotation speed of the planet plays a not insignificant role in the perceived gravitational attraction, as the rotation would constantly be throwing surface dwellers outward with a small force, negating gravity's inward pull.
Could you get more by slingshotting around the sun?
Yeah, but you'll end up in San Francisco in 1986 so it won't do you any good.
Just stay off the LDS.
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Gravity is GM / R**2. Mass is proportional to R**3, which means that Gravity is proportional to R, if the density is the same. Inverting that, Gravity is proportional is M**(1/3), so 4 times the mass is 1.587 times the gravity (for a constant density).
So, I wouldn't rule it out.
Subtext: we don't care if we're proven wrong, so long as we learn something.
BECAUSE SCIENCE, BITCHES.
Even if it managed to get to a blisteringly fast .1c, you're still looking at longer than a human lifespan and generation ships.
Just because you're paranoid doesn't mean there isn't an invisible demon about to eat your face
Um, that's not quite how orbital mechanics works. Helios II started off from the earth - that is, with a lot of potential energy in the sun's gravitational field. It was put into an elliptical orbit around the sun, so on its closest approach, part of that potential energy was converted to kinetic energy (hence the high velocity), but at the most distant point, it's all converted back to potential energy, and there's zero gain. It's a bit like bouncing a rubber ball off the floor: it's surely going to hit the ground at a high velocity, but it's never going to bounce up higher than it started - no free lunch. The reason why slingshotting between planets works is because they move relative to each other. To use the rubber ball analogy again, if you throw a rubber ball at the front of a truck that is rapidly approaching you, the rubber ball will come back with a higher velocity. What you did is subtracting kinetic energy from the truck, just like a slingshotting probe subtracts kinetic energy from a moving planet. The tricks we use to make space probes gain kinetic energy are not unlike bouncing a rubber ball repeatedly between moving walls. To use the sun for slingshotting, one would require a very massive object in a highly eccentric orbit around the sun as a "second wall", which our solar system unfortunately doesn't have. (Or should that be: fortunately for our existence?) One could try to use pluto, but I doubt it's massive, eccentric and fast enough to be worth it.
Disclaimer: the above explanation is obviously somewhat oversimplified.
You're forgetting that we can see our own TV signals, sent out and then reflected back at us by an unknown source from 47 years ago. If a low quality mirror is enough to span 47 light years, than a direct view of something 12 light years away should be fine.
AGGGH just noticed the date on that article. I'm such a tool.
The listing I saw said Tau Ceti is about 5.8 billion years old and about 0.78 solar masses. Lifetime of main-sequence stars goes like 1/M^3, so Tau Ceti's lifetime is about twice as long as our sun's. It will be still be looking pretty healthy when our sun has got all bloated and ugly.
If they're as lonely as we are, and regardless of whether they used radio like we have, wouldn't any sufficiently advanced civilisation be transmitting radiowaves in all directions, with the purpose of getting a response from someone else? So, if they want to get in contact with us, radio seems like a good place to start, as least from a physics perspective?
We send out these kinds of signals now, but on a small scale. We should be sending these in all directions, and all the time, and the messages we should be sending are 'is there a better way to communicate with you guys? if there is, can you send us back some instructions on building a faster than light communication device?'
Seriously, Venus is in the living zone as well.
I prefer the "u" in honour as it seems to be missing these days.
It turns out that stars are pretty powerful radio transmitters, and the edge of a stellar system has considerable additional noise. And then there's the cube-square law. Even if they were deliberately transmitting directly at us 12 years ago, it's unlikely we could make out the signal from the noise.
Suns have a lot of light noise too, so we probably wouldn't see a laser transmitter either, unless it were from the very edge of the system.
What we might see if we were looking for it would be the ion emissions of decelerating incoming craft using ion engines, or the thermal signatures of interstellar craft using nuclear thermal propulsion. But by then it might be too late. Or esoteric energy uses like fusion. Or signatures of H-bombs near the periphery of the stellar system.
At 15 degrees Right Ascension, Tau Ceti is a little far off the solar system plane for an exploratory trip just now. Maybe in 50 years.
Help stamp out iliturcy.
Well - other than speed you are going to need some kind of kick-ass obstacle avoidance system. If you hit even the tiniest object at light speed, you are pretty much toast!!
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Don't feel too badly, you were on the right track. IIRC clear back in the 1970s it was determined that at TV frequencies the Earth was the brightest (known) object in the galaxy.
It's easier to be a result of the past, but more fun to be a cause of the future! http://www.spacefinancegroup.com/
Why would you put the people in charge on the second bus?
I think the parent is off by two orders of magnitude. I looks to me like 11 billion miles is 0.00187122571 light years. It sez here that Voyager is now travelling about 13 km/sec or 8 mi/sec, which is 0.00004 times the speed of light. Nevertheless, I think it's within the realm of the possible to build a probe, today, that could be accelerated to 1% of light speed - 3,000 km per second (about 250 times the speed of Voyager). That would make it to Tau Ceti in 1200 years. That's not an excessively long time for a project embarked upon by a global civilization. In the middle ages the Europeans built dozens or hundreds of cathedrals, many of which required more than 100 years to complete. Can we build electronics and machinery that last that long in space? An interesting question.
Consider that many plants and animals (not to mention slime molds and other entities) build spores or capsules of various sorts that allow their seed to last for hundreds of generations, floating on the air or in the water or just buried underground, waiting for the right conditions.
It's easier to be a result of the past, but more fun to be a cause of the future! http://www.spacefinancegroup.com/
"On a small scale" is an understatement. We have virtually sent no signals at all. To any system past Alpha Centauri we would be dead silent on a radio scan of our system 99.9999999999999999999999999999% of the time. Are you wondering why we haven't made any effort to send signals? Fear. Even many radio astronomers themselves are frightened of attracting the attention of more advanced civilizations that may be listening. If we are too afraid to do it other civilizations may be as well. So we all listen but never speak. Everyone will stay very quiet out of ignorance and fear. Hence Fermi's Paradox and The Great Silence.
Quite an experience to live in fear, isn't it? That's what it is to be a slave.
Sure, and so what? We could do generation ships if we really wanted to, and I doubt there'd be a shortage of volunteers. Heck, they might as well start looking for those at places like Slashdot - just imagine, living in what's essentially an ultra-high-tech basement for the rest of your life, all bills paid in advance!
By "it" in my last sentence, I meant Voyager I. You can get to the stars faster if you spend more delta V doing so.
An interesting tidbit is that 1 year at 1 g thrust gets you to just about the speed of light. After 1 year at one g, you don't really go much faster (from the standpoint of someone left behind) but, boy does the relativistic time dilation kick in. Factoring in time dilation, you can get to almost anywhere in a fairly reasonably subjective time (i.e., the relativistic proper time for the traveler), assuming you can accelerate and deaccelerate continuously at 1 g. , Thrusting at 1 g also has the comfort advantage that we are totally used to it.
Of course, if you go very far, humans, or even the Earth, may not be there when you get back. And, how to achieve a constant 1 g thrust has to left as an exercise for the reader...
One has to organize the transport. And knowing how well they organize, the trip should only go to our own sun, which should save us heaps of fuel.
We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
1/2 right. IIRC the rotational ('centrifugal') effect is negligible compared to gravity unless it's really spinning fast. The force is represented as mv^2/r, where m is the mass of a person or whatnot, v is the angular velocity in meters/second, and r is the radius. If the radius is 2*Earth and the rotation speed is the same, then the surface velocity is roughly 80,000 km/day or 1 km/second. So for a 100 kg person (let's assume they're wearing a space suit), we have 100 kg * (1000 m/s) * (1000 m/s) / 8000000 m = 100000000/8000000 = 12.5 kg m/s/s = 12.5 Newtons.
The surface gravity for that person would be (roughly) 100 kg * 10 m/s/s = 1000 kg m/s/s = 1000 Newtons.
That's assuming I did the math correctly. :P
It's easier to be a result of the past, but more fun to be a cause of the future! http://www.spacefinancegroup.com/
Hush! It's hard enough to get a government grant for some real astronomy, so we had to coat it into some kinda project that appeals to the common man. Or do you think you can sell a huge radio telescope to find some faint signals about the beginning of the galaxy to get closer to the big bang to the hicks who will instantly ask you "and what is it good for?"
But if you claim you're looking for E.T., they'll be pleased.
We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
it was determined that at TV frequencies the Earth was the brightest (known) object in the galaxy.
That is until they learn to decode the signals. Then they will stop thinking we are bright.
SETI is looking for intentional signals, not the alien equivalent of Abbot and Costello.
.
If we want to send people for thousands of years, I think it should be an artificial uterus with some deep-frozen zygotes in it. When you arrive you crack open the eggs, and voila! Adam and Eve.
Don't send them Windows. They may see that as a hostile act.
The Tao of math: The numbers you can count are not the real numbers.
It turns out that stars are pretty powerful radio transmitters, and the edge of a stellar system has considerable additional noise.
So what?
And then there's the cube-square law.
Don't you mean inverse square law?
Even if they were deliberately transmitting directly at us 12 years ago, it's unlikely we could make out the signal from the noise.
While noise (snr) is certainly a problem in any long range communication, if transmitting at the proper frequencies (ie. 1-10 Ghz; 34-37 Ghz; 75-80 Ghz) it isn't a major one.
The problems are more along the lines of:
1. It seems unlikely that any system within 50 ly of us would contain a planet with not just life, but intelligent life. Biogenesis has been compared to a tornado in a junkyard constructing a car. We really have no idea how to even do it ourselves. So it really may be quite rare. And even where there is life, intelligent life of the giant parabolic dish building variety is certainly not a given. Consider how many species there are on our planet brimming with life from pole to pole and only a single species seems to be intelligent enough to build computers and spacecraft and giant radio telescopes. If intelligence is such a good survival strategy why have more species not taken advantage of it? Even with all those stars out there intelligent life of the radio telescope building kind may be far more rare than those of us who enjoy science fiction may like to believe. The majority of life on earth is still simple microbial life.
2. The transmission has to either be aimed directly at us or be of orders of magnitude more power than at least we are capable of transmitting if only for economic reason. Either possibility seems rather unlikely.
3. No one really knows about us yet. We are just another star in a sky filled with them. If we embarked upon a major coming out party and transmitted signals for long periods to every star within, say, 100 ly then things might be different, but as it is there is no reason for anyone out there to point a dish in our direction except for causal surveys of nearby stars which might listen for only a few seconds each decade.
4. The aliens could be transmitting directly at us and we probably wouldn't hear them because they might be transmitting on one of the many frequencies that we are not listening on or which are attenuated by our oxygen-nitrogen atmosphere. Anything from 20-30 Ghz or above 100 Ghz is mostly blocked by our thick atmosphere. Very high frequencies tend to be more effecient at interstellar communication. Which is why setting up radio telescopes on the moon or mars would be rather nice.
Quite an experience to live in fear, isn't it? That's what it is to be a slave.
I never found that argument very persuasive. In terms of space travel propulsion systems has our technology really changed that much since the 60s? Seems like we're building pretty much the same rocket engines now that we were nearly half a century ago. It's just that now we can put fancy computers on them. And better robots. Also that 'argument' will always work. By that argument there is never a good time to begin a long space journey.
Quite an experience to live in fear, isn't it? That's what it is to be a slave.
So basically it is like high school dating on a galactic scale?
Hello out there! Any potential overlords looking for a slave race that's advanced enough to be useful without being able to defend itself effectively? And might be tasty?
Il n'y a pas de Planet B.
The Vulcans came from the habitable planet that orbited Tau Ceti, according to my Starfleet Technical Manual. Given that they invented the Prime Directive, they probably have to maintain radio silence (frex, using very directional masers where necessary for radio-band communications) to avoid clueing in lesser civilizations.
Plus the Andorians live right next to them and we all know what they are like.
The Burvixese race evolved on the planet Arcturus 1, progressing from turtle-like swamp dewellers to a benevolent, highly technological society in just over fifteen million Earth years. Although the Burvixese had the wherewithal to build crude interplanetary vessels, they preferred to remain on the comfortable damp surface of their world and explore the galaxy through HyperWave communication. Using this method, the Burvixese made contact with several neighboring alien cultures, including the Utwig, the Gg, and unfortunately, the Druuge, whom the Burvixese would have been much better off never finding. For many decades, the Burvixese exchanged information with these races, trading technological, historical and philosophical facts and theories, until the fateful year 2142. It was then that the Gg announced that they had come under attack by a unknown alien race, who appeared to want nothing less than their complete annihilation. The Gg surmised that the hostile race, the Kohr-Ah, had located them using the Gg's HyperWave transmissions. Knowing that they had little chance of survival, the Gg warned the Burvixese that, unless they restricted their own transmissions, they too might face a gruesome fate.
Being a charitable race, before the Burvixese turned off their HyperWave transmitters, they shared the Gg's warning with the Druuge. But it was too late. The Druuge's powerful advertising beacons had already attracted the attention of the murderous Kohr-Ah, who, having finished with the Gg, began moving in the general direction of the Persei constellation, home of the Druuge. Realizing their peril, the Druuge took immediate action. They ceased all transmissions and sent a task force of their fastest ships to the moon of the Burvixese world. Once there, the task force assembled a huge HyperWave broadcaster on the moon's surface. When it was complete, the Druuge activated the unit which began emitting powerful HyperWave signals, focused directly toward the oncoming KohrAh fleet. The Druuge hoped that the hostile aliens would change course toward the Burvixese planet and fail to find their own worlds. Unfortunately, this ruse was all too effective: the Kohr-Ah changed course, attacked the poor Burvixese and, sadly, destroyed them all in three days of orbital bombardment.
--Star Control 2 manual
Shutting down free speech with violence isn't fighting fascism. It IS fascism!
Interesting, but from what I have read, without a truly giant parabolic reflector and a very sensitive receiver none of those VHF/UHF signals would be detectable above background noise at even 1 ly. Those frequencies just don't work very well for point to point communication at great distance. I think the Friis equation would require truly giant reflector dishes compared to higher frequencies. I think they may also be attenuated by the interstellar medium (adds up over truly great distances) in addition to our own atmosphere. And if the receiver were on a planet with a similar sort of atmosphere...well. You really want to get at least above 1 Ghz to have much hope at all and 7-10 Ghz would be much better.
Quite an experience to live in fear, isn't it? That's what it is to be a slave.
You make me post this in every extrasolar planetary thread and it's really annoying.
Voyager 1 is nowhere near current technology. We have ion thrusters now. We have supercomputers now. Hell, your cellphone would have been a supercomputer to the guys who designed that thing. We have water on the moon, in near-earth asteroids, and a limitless supply in Ceres, and we didn't know that then. We have new methods of separating that water into hydrogen and oxygen on orbit efficiently, so it doesn't have to be hoisted out of our gravity well. We have far more understanding about long-term space missions and habitation. Plants grow in space! We didn't know that either. We have commercial rockets that can dock with the space station: an absurd sci-fi fantasy back then. We have robots who can do the work of gathering fuel without too much supervision. We have robots that could survive the kind of acceleration provided by a 1000 km railgun that it would take to put the robots there in a reasonable time, and a place in low-g to put that rail gun and robots to build it. And software to put on the robots that apparently can withstand a 24 year ping time.
In fact, recent learnings about the Voyager Anomaly point to an obvious way to propel interstellar spacecraft: Put a couple dozen 200 MW fission reactors behind some heat/radiation shields and point them in the opposite direction from where you want to go, and let them melt down. The heat provides thrust. At 745 W per HP, that's good for a few thousand horsepower of thrust. Since a Newton is a Horsepower-second, near enough, and the reactors run for many years, that's insane number of Newtons. We actually used to have a project that worked on this theory called Project Orion.
So, for example, get the robots to gather up some water and refine it into LH2/LO2. Slide some of that fuel down to LEO and pick up a commercial hydrox booster and lift it into high orbit and fill it. Repeat until you have seven of them. Now arrange them in a filled hexagon at L2 orbit just beyond the moon, and fill with hydrox. Strap your meltdown-driven spacecraft and habitat/humans/robotic exploration package on the nose, and at the most opportune time when your cislunar orbit is headed closest to the desired direction, light that shit off. Boost for 2.5 minutes at 6 g, and discard the 7 Saturn boosters. You're already several times past solar system escape velocity, and your course is assured. Then engage the thermal drive and melt down the reactors and continue to boost at something on the order of 40 billion Newtons per year as you head to the nearest star. Somebody do the math for me. I'm thinking 50 years.
"Oh, but the cost!" you might say. Well look. We don't need the work of all the people we have. It turns out that something like 1 in 4 Americans is all that's required to maintain our standard of living. According the US Bureau of Labor Statistics we have 11 million underemployed people in the US, or $500B/year worth of people who could do be doing something interesting and useful who aren't. And that's just the US, and I think that number is understated 2x. Besides, we'd like to be rid of that nuclear fuel anyway.
I'm not even in the space field and I could figure out how to get people to Tau Ceti in under one human lifespan with resources like that, or robots sooner still. We could do it, right now, with the resources and science that we have. It would be a one-way trip, but we would not lack for volunteers or robots. From one economic point of view it wouldn't cost us one whit more than we're already paying, and instead of being unhappily idle the proletariat would be excitedly engaged in a worthy endeavour. You just have to sell it.
Just because your grandparents couldn't figure out how to do this don't assume that the current generation can't.
Help stamp out iliturcy.
Don't you mean inverse square law?
No, I mean the Square-cube law. In particular that the strength of radio emissions fall off at a ratio of the square of the power to the cube of the distance. Stars are very far and this law destroys the effectiveness of radiological communications over such distances.
Help stamp out iliturcy.
No, it's a simple inertial thing. The prime jumping-off point for interstellar travel from Earth is L2, as it utilizes the moon as a gravity slingshot. But the target has to be somewhere close to the solar system plane, or it loses the advantages of Earth's orbit being so far out of Sol's gravity well, and Earth's proper motion around Sol as well.
Help stamp out iliturcy.
He had foreseen this. The planet around Tau Ceti is called Aurora. It is the home of long-living humans and mind-reading robots.
No, you do mean the inverse square law. The inverse square law states that the intensity of radio waves (or light) decreases with the square of the distance to the source. The square-cube law is about the surface relative to the volume: The surface increases with the square of the size, while the volume increases with the cube. This means that the same structure at different sizes would not have the same properties. For example, a giant ant would not be able to get enough oxygen, as its ability to get oxygen follows its surface area, while its oxygen demand follows its volume. Likewise, it would be crushed under the weight of its own exoskeleton, as its strength is proportional to the square of the size, while its weight is proportional to the cube.
If a frozen bacterium hit your head at 0.3c, I bet it would explode.
The bacterium, maybe, but not your head. Bacterium have very, very little mass.
For instance, a single E. Coli bacterium has a mass of approximately 2.9 x 10^-13. If someone flung one at you at at .3c, it could have a total momentum of only about 2.9 x 10^-13 x 3 x 10^8 x .3 = .0000261 gm/s.
This is about the equivalent momentum of a baseball (142g) moving at .00000001838 m/s (or .018 mm/s). (This is about the velocity imparted to an average baseball by an average slashdotter. So, not very fast.)
If the masses can keep you down, you're not the Ubermensch.
I think that fear of attracting attention is very misguided. Civilisations capable of mass interstellar travel automatically have much better opportunities to pursue ... than to go torment younger single-planet civs.
Who said anything about tormenting? The Vogons did not destroy earth to torment, but for practical reasons. They were described as "Not cruel, just callous". I destroy wasp nests, but not to torment them. I recently trapped a dozen mice in my attic; I actually felt sorry for them, they look cute, but knew that if I let them be they would be taking over the house.
Such an advanced civilisation might see us as we see an ants nest. And don't depend on talking our way out of it, reasoning with them. They would be on a totally different mental wavelength. Most higher animals on Earth talk to each other (that is what "birdsong" is for example), and we have lived for thousands of years alongside them, yet most people will not even accept that they do so - let alone listen.
Isn't that exactly what the GP said?
Easy, free ammo.
Just have them practice on the outside of the ship, firing in the opposite direction of travel, so it'll speed up the ship. Until turnaround for deceleration, of course.
I can mend the break of day, heal a broken heart, and provide temporary relief to nymphomaniacs.
Using this 2007 CSX document for the "Real Railroad" information, and assuming HO Scale (1:87.1) there are a few things:
... I model the 1940s, so don't really care)
.The curves given in the linked document (which only concerns itself industrial/siding track) are considered pretty small, at ~480' radius. Scaled down properly, you're looking at a radius of 5.5 feet (66").
... and derailing). Note that this isn't necessarily an exhaustive list, and there are other factors that can wreak havoc on model railroads that aren't a problem on real ones (e.g. "housecat")
1. Real railroads -- cars start off at ~20~30 (Imperial) tons (i.e. ~40,000~60,000 pounds, or 18-22 tonnes*), and will usually have a maximum weight of ~40 tons (~36 tonnes), or a load weight of 10-20 tons (9-18 tonnes). (Note, most of this is for older cars I've seen/restored... newer things might be heavier, with different loading weights
Model railroads -- cars start off at a few ounces. Per recommended practice, a car should be 1 ounce (~28g), plus an additional 0.5 ounce (~14g) per 1 inch (2.54cm) of length. This results in a scale 40' (10 meter) long boxcar weigh about 4 ounces (114g).
2. Real railroads -- smallest track switches are size 10 (1 unut of divergence per 10 units of distance). Model Railroads -- #4,5,6 are generally considered "small, med, large" with #8+ being considered "nice to have" but generally rare on anything but "basement empires"
3. Real railroads -- curves are absolutely massive
Model Railroads -- 20"radius curves are considered a minimum for most mainline operation (barring long wheelbase steam locomotives, full length passenger cars, and some modern locomotives/cars). 24-26" is about minimum to "run anything you could want", with 30" and larger being reserved for the afore-mentioned "basement empire" sized models
These three things combined (and probably others) are what help keep full-size railroads from "stringlining" the cars (i.e. instead of following this curve ')', the cars try taking the straight path '|'