epicycles are still a useful expansion for perturbations around a circular orbit. I work with a bunch of planetary physicists, and some of them still use epicycles to this day to calculate weird effects like wave phenomena and density perturbations in protoplanetary nebulae, the rings of Saturn, and the like.
is the book you want. It's by Bate, Mueller, and White, and it works from first principles up to "how we designed the Apollo lunar trajectories".
The easiest way to conceive of interplanetary orbits is to first pretend that they lie in a single plane (the plane of the ecliptic) and then pretend that the planets themselves are insignificant for most of the trip -- so you consider only the gravitational field of the Sun. Then your orbit is an ellipse. It's pretty easy to show that, if you're going at Earth's orbital velocity, the ellipse that gets you from Earth's orbit to any other nearly circular orbit with the least change in velocity (ie rocket fuel) is an ellipse that is tangent to both orbits.
Once you've figured that out, you have to figure out when to launch to get to Mars's orbit in the same place that Mars happens to be. Those times happen at a particular phase of Mars's and Earth's orbit.
You can do pretty well by pretending that you can neglect the Sun entirely until you get far enough from the Earth, then you can neglect Earth and Mars entirely until you get close enough to Mars. That is the technique that was used for Apollo trajectories -- the "method of spliced conics". You can hear some evidence of it in the Apollo 13 movie, when they talk about "entering the Moon's gravitational field" or something like that -- the Moon's gravitational field extends throughout the Universe, of course, but to simplify the calculations they neglected everything but the mass with the strongest gravitational force on the capsule.
Nowadays you can get really, really good orbital elements for each of the planets online, which lets you calculate exactly where each planet is at any given time. You can just code up an insanely cheesy inverse-square-law integrator in PDL or one of the other free languages -- or even a spreadsheet -- and find a good orbit by trial and error using the gravitational fields of all the large bodies in the solar system.
I never spellcheck and freely admit it. Save your karma for more worthwhile "lol erorrs" replies.
Wye knot? Eye dew. Butt four sirious - dough ay spill checquer cant ripless gooed riting, gooed spilling his quay two gitting yore massage too duh raider. Bat spilling jest machs ewe luck stupid.
lbf is "pounds-force", a slightly more specific unit than "lb", which could refer to a mass (0.454 kg) or a force (4.54 N).
As a scientist I think in SI these days though it took years to unlearn the training of my youth, and I still vascillate between F and C for my preferred temperature unit.
Nobody uses perfect units. Why aren't you measuring your car's efficiency in inverse square millimeters?
1GB USB-2 flash drives cost $80 (see Froogle). A better solution is probably to software-RAID four USB-2 flash drives plugged into a USB hub. Total cost is about the same as a maxed-out iRAM, with comparable speed and latency -- but no battery issues.
We're using flash RAM for an embedded controller in extreme conditions (sounding rocket payload): we're running Linux booting off of a PCMCIA 6GB flash-RAM device. That's a little more expensive (a couple of $k for the device) but then it's a little more rugged and a lot smaller, too.
Currently NASA has two spacecraft "at" the L1 point -- SOHO and ACE. They share a volume several hundred times bigger than the volume of Earth itself. There's plenty of room there -- every nation on Earth could build a colony the size of Manhattan out there, and they'd never collide. "Defending" the Lagrange points is pretty ludicrous.
TFA just says that you have to consider breakeven when planning your municipal system. That's not surprising -- it's just like any other business venture.
It shouldn't be hard to make the numbers work, provided that one can divide:
$150,000
Amortized cost over 5 years (whole system)
$2500
Cost of whole system per month, amortized for 5 years
$25
Benefit to each regular user per month (from TFA)
100
Number of regular users to break even
Finding 100 users per square mile should not be hard. Medium-density suburban lots are typically 0.3 acres, for an average of about 1000 houses per square mile (including a factor of 50% for infrastructure). So if one in ten households uses the WiFi regularly, the system breaks even at the stated price.
City centers might have a factor of 10 more people in them; so if 1% of city core dwellers use the WiFi regularly, the system is working.
On the other hand, low-density suburban areas might have only 100-500 households per square mile; those areas might not get enough users to make sense.
Re:Headlines running together in my head
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This is/., so I'm going to get pedantic.
A virus does not rely on any vulnerability in the platform to propogate.
If you're going to be pedantic, learn to spell "propagate".
... and, for what it's worth, it works great. I find the patent pretty dodgy (how is this fundamentally different than a CD holder with built-in FM transmitter?) but it is a good product. Works a darn sight better than the iTrip I used to own, which worked only slightly better than a small, white turd.
Well, the analogy is a bit strained since it's between a noise floor limited discrimination and a single bit; but in both cases the problem is finding enough "signal space" to hold all the data you want to transmit.
Normal radios use a simple discriminator: the carrier frequency. UWB devices use a code-multiplex discrimator that operates on frequencies much as a hash function operates on bit values.
The rub is -- what is "properly designed" and how likely is it?
A looong time ago, I tried the "Green Laser Pointer" scheme, a protytype of this one (I think also run by Niu). That was about three years ago. I never got the green laser pointer, but my mail server still gets about 800 spams/day addressed to the pseudo I used.
The problem with UWB is that it works great for one single device, but not so great once you have 100 million of the buggers running around. There's only so much bandwidth in the whole spectrum, so the "low noise due to wide-band modulation" argument would not hold once millions of these devices got made.
In the software world we're used to super-duper-ultra-wideband spaces: MD5 hashes are a good example. You don't have to bother decolliding MD5 hashes -- there are so many that no two documents are likely to ever collide by chance. But you can't just "add more bits" to the electromagnetic spectrum: once you get down below about a centimeter, you might as well be using infrared instead of radio.
It's the same problem as those RF-excited plasma light bulbs that were all the rage a while ago: the first 10,000 or so work great -- but by the time you deployed 10 of 'em to every household in America, nobody's radio would work any more.
This highlights a common misconception. If the nozzle is firmly affixed to the rocket, it doesn't matter where it is placed: if the thrust vector fails to pass through the center of gravity, the rocket will thrust in circles.
We're used to thinking about stability in terms of things that we are pushing or pulling -- carts pulled from in front are dynamically stable, and carts pushed from behind are generally dynamically unstable. The problem is a false analogy between cart motion and rocket travel. When you're pulling or pushing a cart, you are referring the force to an external reference frame (the world at large): if the cart turns a little bit, you will continue to thrust in the same direction relative to your surroundings. But when a rocket under power turns a little bit, the line of thrust turns with it.
Small rockets intended for use in an atmosphere generally have fins near the back. The fins are a simple way to refer the rocket's direction to the fixed medium around it. Fins become impractical for larger rockets, or for rockets flown outside an atmosphere. For those applications you have to have another direction reference (like, say, a gyroscope) and gimballed engines or some other way of vectoring the thrust. If Goddard had flown larger rockets he would have used gyro control systems too.
IEEE is a trade organization. While maintaining the archive would be expensive for individuals, it is rather cheap when spread across the whole organization.
Author-pays works OK for the American Astronomical Society, which offers inexpensive access to journals (e.g. Astrophysical Journal) but charges authors for submission.
Like just about everyone else who posted to the same effect, you're failing to realize that Windows Update is the official pathway for updating (e.g.) Internet Explorer, Microsoft Outlook, and Microsoft Office. Those applications run just fine under Wine but are blocked from being updated.
I do run Orifice and Exploder from time to time (I purchased Orifice2000 for just that reason) under the Codeweaver wine, and it would be nice if M$ would supply me the effing updates they told me I'd get.
Many anti-nuclear protests were valid, back in the day -- and many are now, I suppose. But much current anti-nuclear protesting appears to be pretty thoughtless.
For example: Yucca mountain isn't perfect, but it's a LOT better than the waste storage we have now, and it took 30 years of trying just to get a good start at it. Thought experiment: would you rather put existing high-level waste in a secure facility at Yucca Mountain, or leave it in open holding pools in populated areas? Many holding pools (Trojan in Oregon and San Onofre in California, to name two) are within 50 miles of over a million people, in seismically active areas.
Another example: the anti-Cassini protests, which ignored the huge amount of radioactive materials that are already naturally dispersed through the Earth's biosphere. One argument against Cassini was that the plutonium it contained, properly distributed among the population, could in principle kill nearly half of humanity. That sounds pretty scary, till you realize that the semen contained in a typical teenage boy, properly distributed among the population, could in principle impregnate nearly half of humanity.
My point isn't that all protests are necessarily stupid -- just that there have been a lot of pretty stupid, counterproductive protests over the years. That makes it hard to do even the things that one should do, when they involve "hot-button" topics.
Many of those people live near me, here in Boulder, Colorado.
The waste isn't nearly the issue that it's made out to be. The problem is that the risks are overhyped. Tens to hundreds of thousands of deaths a year are attributable to cancers and respiratory disease caused by fossil fuel burning to produce electricity. For example, most of the southwest corner of Colorado and the northwest corner of New Mexico have high levels of sulfur, cadmium, mercury, and even radioactives in the air because of coal-burning plants that sell electricity over a wide territory. Respiratory disease in those areas is climbing rapidly to be on a par with the rates in the LA basin in the late 1970s, when children were being found with lung-tissue scarring normally seen in long-time smokers.
We should not think of nuclear waste in terms of the total population risk it generates, but rather in terms of the change in population risk it entails. A few hundred deaths a year attributable to the nuclear waste chain would be miniscule compared to the tens of thousands of deaths a year they would eliminate (that are attributable to the fossil fuel waste chain).
I recently had occasion to examine every email that I received over the last 10 days, trying to find a particular email. 10,228 messages were received in just under 10 days. More than one every 90 seconds, around the clock. My mail server is a 300MHz Pentium running BSD, and its load average hovers around 25%-30% just processing my spam.
And I *don't* run a million-hit-a-day website.
I *do* run my own domain, and about half of the spam is bounces from third-party forged spam. About 10% of it, or about 100 messages a day, is to a pseudo I made up for the purpose of clicking -once- on a Green Laser Pointer web ad. I didn't get the laser pointer.
Slashdot Mirror
... together with back-issues of National Geographic. That should avoid the problem in the future by raising the grade level by 5-7 meters.
I'll grant you it's big -- but automotive assembly lines (which could be considered "machines") are larger, and CERN is certainly larger.
when will people learn to spell "propagate" correctly?
Parent and grandparent are quite good -- but "propogate" sounds like a Washington neologism for a scandal involving bee pollen.
It's hard to sound authoritative if you can't even spell.
Jeez, the lengths some people will go to, to avoid the google cookie...
... though a full upgrade to "X-windows" seems to avoid most viruses.
epicycles are still a useful expansion for perturbations around a circular orbit. I work with a bunch of planetary physicists, and some of them still use epicycles to this day to calculate weird effects like wave phenomena and density perturbations in protoplanetary nebulae, the rings of Saturn, and the like.
The easiest way to conceive of interplanetary orbits is to first pretend that they lie in a single plane (the plane of the ecliptic) and then pretend that the planets themselves are insignificant for most of the trip -- so you consider only the gravitational field of the Sun. Then your orbit is an ellipse. It's pretty easy to show that, if you're going at Earth's orbital velocity, the ellipse that gets you from Earth's orbit to any other nearly circular orbit with the least change in velocity (ie rocket fuel) is an ellipse that is tangent to both orbits.
Once you've figured that out, you have to figure out when to launch to get to Mars's orbit in the same place that Mars happens to be. Those times happen at a particular phase of Mars's and Earth's orbit.
You can do pretty well by pretending that you can neglect the Sun entirely until you get far enough from the Earth, then you can neglect Earth and Mars entirely until you get close enough to Mars. That is the technique that was used for Apollo trajectories -- the "method of spliced conics". You can hear some evidence of it in the Apollo 13 movie, when they talk about "entering the Moon's gravitational field" or something like that -- the Moon's gravitational field extends throughout the Universe, of course, but to simplify the calculations they neglected everything but the mass with the strongest gravitational force on the capsule.
Nowadays you can get really, really good orbital elements for each of the planets online, which lets you calculate exactly where each planet is at any given time. You can just code up an insanely cheesy inverse-square-law integrator in PDL or one of the other free languages -- or even a spreadsheet -- and find a good orbit by trial and error using the gravitational fields of all the large bodies in the solar system.
Wye knot? Eye dew. Butt four sirious - dough ay spill checquer cant ripless gooed riting, gooed spilling his quay two gitting yore massage too duh raider. Bat spilling jest machs ewe luck stupid.
lbf is "pounds-force", a slightly more specific unit than "lb", which could refer to a mass (0.454 kg) or a force (4.54 N).
As a scientist I think in SI these days though it took years to unlearn the training of my youth, and I still vascillate between F and C for my preferred temperature unit.
Nobody uses perfect units. Why aren't you measuring your car's efficiency in inverse square millimeters?
We're using flash RAM for an embedded controller in extreme conditions (sounding rocket payload): we're running Linux booting off of a PCMCIA 6GB flash-RAM device. That's a little more expensive (a couple of $k for the device) but then it's a little more rugged and a lot smaller, too.
Currently NASA has two spacecraft "at" the L1 point -- SOHO and ACE. They share a volume several hundred times bigger than the volume of Earth itself. There's plenty of room there -- every nation on Earth could build a colony the size of Manhattan out there, and they'd never collide. "Defending" the Lagrange points is pretty ludicrous.
It shouldn't be hard to make the numbers work, provided that one can divide:
$150,000 Amortized cost over 5 years (whole system) $2500 Cost of whole system per month, amortized for 5 years $25 Benefit to each regular user per month (from TFA) 100 Number of regular users to break evenFinding 100 users per square mile should not be hard. Medium-density suburban lots are typically 0.3 acres, for an average of about 1000 houses per square mile (including a factor of 50% for infrastructure). So if one in ten households uses the WiFi regularly, the system breaks even at the stated price.
City centers might have a factor of 10 more people in them; so if 1% of city core dwellers use the WiFi regularly, the system is working.
On the other hand, low-density suburban areas might have only 100-500 households per square mile; those areas might not get enough users to make sense.
If you're going to be pedantic, learn to spell "propagate".
... and, for what it's worth, it works great. I find the patent pretty dodgy (how is this fundamentally different than a CD holder with built-in FM transmitter?) but it is a good product. Works a darn sight better than the iTrip I used to own, which worked only slightly better than a small, white turd.
RTFA. The reviewer complains that lots of inferior material is inserted instead of the good jokes that are pulled out.
Well, the analogy is a bit strained since it's between a noise floor limited discrimination and a single bit; but in both cases the problem is finding enough "signal space" to hold all the data you want to transmit.
Normal radios use a simple discriminator: the carrier frequency. UWB devices use a code-multiplex discrimator that operates on frequencies much as a hash function operates on bit values.
The rub is -- what is "properly designed" and how likely is it?
A looong time ago, I tried the "Green Laser Pointer" scheme, a protytype of this one (I think also run by Niu). That was about three years ago. I never got the green laser pointer, but my mail server still gets about 800 spams/day addressed to the pseudo I used.
In the software world we're used to super-duper-ultra-wideband spaces: MD5 hashes are a good example. You don't have to bother decolliding MD5 hashes -- there are so many that no two documents are likely to ever collide by chance. But you can't just "add more bits" to the electromagnetic spectrum: once you get down below about a centimeter, you might as well be using infrared instead of radio.
It's the same problem as those RF-excited plasma light bulbs that were all the rage a while ago: the first 10,000 or so work great -- but by the time you deployed 10 of 'em to every household in America, nobody's radio would work any more.
We're used to thinking about stability in terms of things that we are pushing or pulling -- carts pulled from in front are dynamically stable, and carts pushed from behind are generally dynamically unstable. The problem is a false analogy between cart motion and rocket travel. When you're pulling or pushing a cart, you are referring the force to an external reference frame (the world at large): if the cart turns a little bit, you will continue to thrust in the same direction relative to your surroundings. But when a rocket under power turns a little bit, the line of thrust turns with it.
Small rockets intended for use in an atmosphere generally have fins near the back. The fins are a simple way to refer the rocket's direction to the fixed medium around it. Fins become impractical for larger rockets, or for rockets flown outside an atmosphere. For those applications you have to have another direction reference (like, say, a gyroscope) and gimballed engines or some other way of vectoring the thrust. If Goddard had flown larger rockets he would have used gyro control systems too.
IEEE is a trade organization. While maintaining the archive would be expensive for individuals, it is rather cheap when spread across the whole organization.
Author-pays works OK for the American Astronomical Society, which offers inexpensive access to journals (e.g. Astrophysical Journal) but charges authors for submission.
Like just about everyone else who posted to the same effect, you're failing to realize that Windows Update is the official pathway for updating (e.g.) Internet Explorer, Microsoft Outlook, and Microsoft Office. Those applications run just fine under Wine but are blocked from being updated.
I do run Orifice and Exploder from time to time (I purchased Orifice2000 for just that reason) under the Codeweaver wine, and it would be nice if M$ would supply me the effing updates they told me I'd get.
For example: Yucca mountain isn't perfect, but it's a LOT better than the waste storage we have now, and it took 30 years of trying just to get a good start at it. Thought experiment: would you rather put existing high-level waste in a secure facility at Yucca Mountain, or leave it in open holding pools in populated areas? Many holding pools (Trojan in Oregon and San Onofre in California, to name two) are within 50 miles of over a million people, in seismically active areas.
Another example: the anti-Cassini protests, which ignored the huge amount of radioactive materials that are already naturally dispersed through the Earth's biosphere. One argument against Cassini was that the plutonium it contained, properly distributed among the population, could in principle kill nearly half of humanity. That sounds pretty scary, till you realize that the semen contained in a typical teenage boy, properly distributed among the population, could in principle impregnate nearly half of humanity.
My point isn't that all protests are necessarily stupid -- just that there have been a lot of pretty stupid, counterproductive protests over the years. That makes it hard to do even the things that one should do, when they involve "hot-button" topics.
Many of those people live near me, here in Boulder, Colorado.
The waste isn't nearly the issue that it's made out to be. The problem is that the risks are overhyped. Tens to hundreds of thousands of deaths a year are attributable to cancers and respiratory disease caused by fossil fuel burning to produce electricity. For example, most of the southwest corner of Colorado and the northwest corner of New Mexico have high levels of sulfur, cadmium, mercury, and even radioactives in the air because of coal-burning plants that sell electricity over a wide territory. Respiratory disease in those areas is climbing rapidly to be on a par with the rates in the LA basin in the late 1970s, when children were being found with lung-tissue scarring normally seen in long-time smokers.
We should not think of nuclear waste in terms of the total population risk it generates, but rather in terms of the change in population risk it entails. A few hundred deaths a year attributable to the nuclear waste chain would be miniscule compared to the tens of thousands of deaths a year they would eliminate (that are attributable to the fossil fuel waste chain).
Lyx rocks, and it just keeps getting better. I use it for most of my word processing (emacs gets some, kontact gets my email stuff).
And I *don't* run a million-hit-a-day website.
I *do* run my own domain, and about half of the spam is bounces from third-party forged spam.
About 10% of it, or about 100 messages a day, is to a pseudo I made up for the purpose of clicking -once- on a Green Laser Pointer web ad. I didn't get the laser pointer.