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Paintball Pellets As a Tool To Deflect Asteroids
SternisheFan sends this quote from an article at MIT's Technology Review:
"In the event that a giant asteroid is headed toward Earth, you’d better hope that it’s blindingly white. A pale asteroid would reflect sunlight — and over time, this bouncing of photons off its surface could create enough of a force to push the asteroid off its course. How might one encourage such a deflection? The answer, according to an MIT graduate student: with a volley or two of space-launched paintballs. Sung Wook Paek, a graduate student in MIT’s Department of Aeronautics and Astronautics, says if timed just right, pellets full of paint powder, launched in two rounds from a spacecraft at relatively close distance, would cover the front and back of an asteroid, more than doubling its reflectivity, or albedo. The initial force from the pellets would bump an asteroid off course; over time, the sun’s photons would deflect the asteroid even more."
That's a long shot plan right there.
I think sending Bruce Willis with a thermonuclear device and a boatload of family drama might work even better.
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That's true, but there's something more unnerving about losing the entire human race.
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The chance of getting killed by a car when crossing the road is orders of magnitude larger than the chance of getting killed by an asteroid.
True. However one asteroid can kill all of us, unlike one car.
The probability of an event must be combined with the magnitude of an event when assessing the risk.
Not really. Its just white paint, seems kind of boring.
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It would take a huge asteroid to wipe out the entire human race. We're talking once every 100 million years, or so. Before we spend any resources on detecting and deflecting asteroids, let's wait another 1000 years. On the scale of large asteroid impacts, a 1000 year delay is insignificant, but on the scale of human civilization, 1000 years is huge. If our civilization is much more advanced in 1000 years, we don't need our dated asteroid impact plans. If civilization crashes, our plans will be useless anyway.
We've known that incoming (and outgoing - the Yarkovsky effect) radiation can alter an asteroid's trajectory for ages. But such a solution needs to be implemented far in advance of any pending impact. At present, we don't know the trajectory of potential impactors, like 9942 Apophis, to sufficient precision to make a deflection strategy like this useful. While it's true the odds are exceedingly small, accidentally putting an asteroid into a dangerous orbit would be disastrous. Apollo astronaut Rusty Schweickart proposed putting a tracking beacon on Apophis in order to further refine its orbit, which would allow us to use such gentle deflection strategies as the one outlined in the article. NASA turned him down. Fortunately, the Russians are currently planning a mission to Apophis; so maybe it will end up getting deflected via a generous application of paint.
Your point is valid, except that the problem with estimates like that is while they are useful for estimating the risk, they don't say much about whether an asteroid of the required size is actually on its way. In other words, we don't get a do-over if the rock shows up earlier than we thought it would. Not to mention that rocks of the necessary size could be generated by the effects of a collision with another body which then suddenly expels a rock on a collision course with Earth. In that situation, we may well not see it coming until just before the window in which we need to take action to deflect it.
Existential threats like asteroid impacts are situations that you start planning to deal with as soon as you have the knowledge to do so. There is really no reason not to, since given the extreme consequences, it doesn't seem particularly absurd to maintain those plans in a constant state of revision. We know that an asteroid of sufficient size is going to hit again. It's only a matter of time. Maybe that time is a million years from now, maybe it's a week from now. I grant that we shouldn't be building an expensive specialized asteroid defense grid or mineshaft shelter/habitats right now, but an actual plan that could be feasible in the event that we end up with an unforeseen visitor is the right thing to do. In this case, scientists realize that it is very easy to miss Earth if you poke at the asteroid just a little bit when it is far enough out. It's a reasonable plan that really should not require that much expenditure to make happen, if required.
Increasing the albedo makes the photons bounce back, which requires a bigger change in momentum than just stopping them.
Actually 'landing' on it would be a huge problem. An asteroid is not typically one large smooth rock, after all. And it will definitely be 'tumbling' in relation to you as well. So it would be a very difficult docking maneuver on an uncertain surface. And remember these things arent large enough to generate enough gravity to notice either. So it's basically all in zero-g.
Spraying a load of paint at it would be orders of magnitutude easier, and still wouldnt exactly be easy.
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The problem is fuel. We don't have rockets that can fire for months. We have rockets that can fire for minutes. They provide a huge amount of thrust during that time, but you would need far, far more thrust than any existing rocket can provide to move an asteroid off-course.
A vague possibility is an ion engine of some sort. These have much lower thrust, but can run much longer off the fuel they carry. The technology still isn't very proven, though - and trying to land an engine, intact, on an asteroid, it a tough proposition.
The paint idea sounds more feasible. It's basically making it into a cheap, inefficient solar sail. It doesn't require tricky landings, it doesn't require a lot of fuel, you just aim some "paintballs" on a ballistic intercept path. The only downside is that, being inefficient, you have to catch it early. But if you've got a decade's warning or so, you should be good.
Imagine your company logo emblazoned across the surface of an asteroid.
Not only will your company have done something great for all mankind, but mankind will be reminded of it in perpetuity.
First we paint the whole thing white and then get computer controlled pain ball guns to splatter, like an inkjet printer, your company's logo all over the asteroid.
Think of watching a Papa John's ad every time you look up in the sky and having to say a little prayer that you can actually enjoy a large nutritious Papa John's pizza instead of having been reduced to a smokin' crater . :-)
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Timing;
According to the article the paint would have to be applied 20 years before the asteroid approach. Add to that the time to get the craft to space, load up with paint and get out to the asteroid. That may take another 20 years. That may mean a 40 year lead time at launch to be remotely viable.
Control
Paint is not a guidance system. Sure it may be able to move the rock around but it will just be in an indefinite direction. It is just as possible to move the rock closer to earth as away. Sure it moves the rock away from earth but into a trajectory that interacts with a planet that pulls the rock back toward earth.
Other celestial bodies.
As other asteroids impact or come close to the "rock on question" they will alter the path. As the rock enters the Sol system planets will exert gravitational pull on the rock. The part or all 20 years of movement may be wiped out by interaction with another object.
To me the only viable option would be to land thrusters on the rock. Use them to stop the rotation (if any), re-position to one side of the rock and apply constant thrust to alter the course. The thrusters would have to be ion based (low fuel, long duration) and probably powered by solar satellites. A solar sail could be added for additional thrust once the rotation has stopped. The issue with icy asteroids can be dealt with by limiting the thrust of the engines so as not to break the asteroid.
If the rotation was not stopped it would require many more thrusters as they could only fire part of the time.
This "proposal" sounds like "paint and pray".
Problem is, the herd thinning would not be done by brains power but by purchasing power. Now imagine an Earth where only managers and bankers will survive. The living will envy the dead.
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The Society Of Protection of Asteroids (SOPA) will not stand for this. Anything that stands in the way of an asteroids natural path is against nature and against God.
We're going to have to move the Earth out of the way instead... how much paint is that going to take?
Not as easy as it is on the drawing board. First, that rocket would have to be aligned EXACTLY with the center of gravity or all you accomplish is giving it a nice spin. Now, we don't even know for sure just what the asteroid is made of, let alone know the exact point of its COG. Many asteroids are anything but spherical, making the matter even worse. Considering how there is very little gravity acting on it, is it solid in the first place? Or composed of many smaller rocks held together by their gravity?
But let's even assume that you manage to find out the COG. Now you have to not only land a rocket fairly softly on the rock, you also have to be able to provide thrust on the OTHER end, because you want to push TOWARDS the rock after landing on it. Now, rocket engines are not really lightweight, and putting a few pounds into space is already a feat and a half, and we're not even close to talking about having enough fuel left to push that rock. There is a reason that Saturn V was an effin' HUGE rocket that put a relatively tiny payload onto the moon. And that all only worked because the moon gravity is a tiny fraction of Earth's gravity (which is, btw, one of the big problems of a manned Mars mission, but I digress). And you now don't just want to put that payload, i.e. a few rocket engines and a fair lot of fuel, up the gravity well towards the Moon but most likely beyond that, because if the asteroid is already THAT close, I guess the amount of fuel necessary to even push it from hitting New York to hitting Los Angeles would be unfeasible. At the very least I'd estimate you have to intercept that rock halfway between Earth and Mars. But people with more background in gravity astronomy might provide more insight here.
TL;DR version: We know too little about asteroids to push at the right spot, and putting an engine with fuel on an asteroid costs a damn lot of fuel.
We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
Among the many other problems already listed is whether or not paintballs will pop at 2.7 degrees kelvin.
"With patience a ruler may be persuaded, and a soft tongue will break a bone."
In order for paint to splatter and cover some thing it must be kept liquid which in space its ABSOLUTE ZERO. The paintballs will hit like rocks and bounce off.
It doesn't matter, because the amount of CO2 necessary to launch that many paintballs that distance would contribute so much to global warming we'd be better off taking our chances with the asteroid.
What we really need is a giant tinfoil hat to enhance the asteroid's reflectivity and a North Korean missile guidance system to ensure it can't hit anything.
That's the issue. On the other hand, dropping paint on the asteroid provides thrust as if powered by a solar panel the size of the painted area, so paintballs > simple rocket.
Well, maybe so, but there's another problem that nobody seems to have faced. Most of those asteroids are rotating, and they all have surfaces that are irregular at every scale. Unless you have very precise topo maps of the asteroid, down to the sub-millimeter scale, you won't be able to precisely calculae the direction of thrust of the paint splotches. And once a splotch is there, if its position is off by even a few cm, it's likely to produce a thrust in a different direction than you wanted.
A rocket that lands and produces thrust would at least be controllable by turning it on and off at different point in the rock's rotation. That's a lot more control than you could get with paint splotches.
If you want to use light pressure, I'd think you'd be far better off going with a flock of little light sails, which would land scattered over the asteroid's surface. Then you could send commands to precisely control which of the little sails are reflecting sunlight, and you'd actually be able to (slowly) make changes in the asteroid's orbit. That would probably be a lot more effective than paintballs or rocket motors.
What am I missing about the paintballs that would make their "thrust" effect predictable and controllable? Without that, they might just steer the asteroid right into your house. Is there some paint that I don't know about, whose color we can control remotely from millions of km away?
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...change the number of photons impinging on the asteroid, or increase their effect?
A photon has energy. When a mass absorbs a photon's energy it has two effects: the mass increases in temperature equal to the energy of the photon, and the mass is accelerated in the direction of the photon's path equal to the energy of the photon. This seems like we're using the photon's energy twice, but it isn't so because thermal energy of a mass is kinetic energy shifted into the time domain. All objects in the solar system suffer this "solar wind" effect. The closer they are to the sun the more its radiated photons push them away. Obviously, the sun is emitting a LOT of photons.
When the mass radiates the photon again it cools and is thrust again in the direction opposite the direction of the escaping photon. Depending on the rotation of the mass and the average time a photon is held before being emitted again (albedo), this can impact the course of the object. By changing the time factor you can cool the object and impact its trajectory. This is called the Yarkovsky effect. Dark or fast-spinning objects hold the photon's energy for so long that they are radiated in directions that are relatively random and have zero impact on course but they are hotter. Bright objects have more measurable impacts on course because the energy is released in a predictable direction that is relative to the input a vector related to the object's direction of spin but they are cooler. Believe it or not, you can use colors of paint to impact the period between absorption and emission, and use that to align the thrust opposite to the objects orbit around the sun, or in synergy with it. Our understanding of this effect has grown so great that we can tell an asteroid's mass, density, axis and rate of spin based only on its temperature and changes in its course.
Derivatives of this feature are helpful in explaining the normal expansion of the universe (not inflation), as photons push masses on each end away. When we observe some galaxy 12 billion light years away, we're absorbing its photons and it's pushing on us ever so slightly.
The difference can be illuminating. Radio Shack and others used to sell a heliotrope device that was a fan with reflectors on one side of the fins and black on the other. The relative difference in albedo would cause the fan to spin in any normal light.
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