## Asteroid redirection question

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webgrunt
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### Asteroid redirection question

I saw a TV show in which a solution to an asteroid-earth strike was proposed. If an impending collision between the earth and an asteroid is detected early enough, it should be possible (at least in theory) to send a spacecraft to the asteroid, whereupon the spacecraft would match the asteroid's speed and direction, and hover above it for an extended time period. Supposedly the gravitational attraction will very gradually nudge the asteroid' toward the spacecraft, changing the orbit enough to miss the earth.

My question is, since in order to hover the spacecraft must expel matter in the direction of the asteroid, wouldn't that mass/inertia striking the asteroid counteract most of the gravitational attraction? In other words, wouldn't the spacecraft's exhaust push the asteroid away almost as much as the gravity would draw it toward it?

screen317
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### Re: Asteroid redirection question

webgrunt wrote:My question is, since in order to hover the spacecraft must expel matter in the direction of the asteroid
This is most likely where the fault in logic is.

idobox
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### Re: Asteroid redirection question

The pull of the asteroid will be very small too, so you won't need to correct your position often. And you can do it without firing directly at it: you fire a little bit to the left then a little bit to the right, and you're done.

From what I've understood, the easiest and safest solution would be to detonate a thermonuclear bomb a few hundreds of meter away. The radiation pressure pressure of the bomb plus the matter ejected from the asteroid would be enough to deflect it sensibly, without requiring high precision maneuvers.
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scarecrovv
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### Re: Asteroid redirection question

A spacecraft like this is called a Gravity Tractor. Wikipedia proposes two solutions to the problem you point out. The first solution is to direct the exhaust so that it just misses the asteriod, with multiple engines so that the total force vector is straight away from the asteroid. This is somewhat inefficient, because your lines of force are not exactly in line with where you want to go, but it works.

The other solution is a little more complex. If you put your gravity tug in an orbit around the asteroid such that the normal to the orbit is in the direction you want to move the asteriod, you can then apply thrust in exactly the direction you want to go with none of it hitting the asteroid. I would draw you a picture, but I'm lazy. I might draw one later if you really want one.

speising
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### Re: Asteroid redirection question

i don't get it.
why not put the spacecraft in contact with the asteroid and apply thrust directly? the tiny bit of force hat could be transmitted gravitationally won't shatter the thing, and if it really is that fragile, it won't be a problem for earth anyway.

Tass
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### Re: Asteroid redirection question

speising wrote:i don't get it.
why not put the spacecraft in contact with the asteroid and apply thrust directly? the tiny bit of force hat could be transmitted gravitationally won't shatter the thing

If it is a rubble pile only held together by gravity then the force it can take at one point might well be less than the force of gravity on a vehicle.

speising wrote:and if it really is that fragile, it won't be a problem for earth anyway.

It carries the same amount of energy and thus the same destructive power no matter how fragile it is.

Izawwlgood
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### Re: Asteroid redirection question

Wouldn't fragmented asteroids present greater surface area upon reentry and thus be more likely to burn up before striking?
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Tass
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### Re: Asteroid redirection question

Izawwlgood wrote:Wouldn't fragmented asteroids present greater surface area upon reentry and thus be more likely to burn up before striking?

If the asteroid is big enough that we need to deflect it, then it might very well make thousands of fractions big enough that what burns up is still negligible.

Also a big concern is particulate matter in the stratosphere blocking the sun. Being hit by a cubic kilometer of asteroid dust might not be a good idea.

Which is worst one strike or many has been debated quite a lot as far as I know, and I am no expert.

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### Re: Asteroid redirection question

Is this sensible and feasible? We make a sheet of some strong material and have several spacecraft spread the sheet in the path of the meteor/asteroid's path, making sure the sheet's velocity is very close to the asteroid's velocity, the spacecraft tug on the sheet just enough to alter the body's speed and/or direction...does that sound dumb?

idobox
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### Re: Asteroid redirection question

gladiolas wrote:Is this sensible and feasible? We make a sheet of some strong material and have several spacecraft spread the sheet in the path of the meteor/asteroid's path, making sure the sheet's velocity is very close to the asteroid's velocity, the spacecraft tug on the sheet just enough to alter the body's speed and/or direction...does that sound dumb?

It's difficult to perform. You need several spacecrafts, a sheet, or more realistically a net, large enough to catch the asteroid without the spacecrafts crashing, then you need to rendez-vous with a rock pretty far way. the biggest issue is that the asteroid will likely spin, which could end dramatically.

Tass wrote:If the asteroid is big enough that we need to deflect it, then it might very well make thousands of fractions big enough that what burns up is still negligible.

Also a big concern is particulate matter in the stratosphere blocking the sun. Being hit by a cubic kilometer of asteroid dust might not be a good idea.

Which is worst one strike or many has been debated quite a lot as far as I know, and I am no expert.

If you nuke it early enough, only a small number of the debris should hit the Earth. Also, I am pretty sure that a cubic km of rock hitting earth will result in quite a lot of dust in the stratosphere, but I'm not an expert either.

Seriously, what's wrong with blowing things up? nuclear bombs are by far the most efficient propulsion method we have the technology for, we have a pile of them we're turning into fuel, and blowing up a bomb a few hundreds of meters away from the target is much simpler than all other proposed solutions.
We can even send more than one bomb as a fail safe, since we have so many of them, the things can survive launch and reentry, are quite small, a proven technology and there would be no such thing as going too strong.
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Dopefish
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### Re: Asteroid redirection question

I was watching something recently with an interview with an apparent expert, and he mentioned that the common movie solution of blowing them up would be much more likely to be as bad, if not much worse, than letting it just impact directly. Apparently there's a lot of issue with the resulting shockwaves surrounding each piece of debris that causes a lot of problems, even if the fragments themselves don't cause a lot of damage directly. As a result, nuking an asteroid into lots of little pieces could end up causing devastation for an entire hemisphere, rather than the comparably small impact region if it was one big hit.

Of course, that's very much second hand info, I don't have math to back it up.

idobox
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### Re: Asteroid redirection question

It will depend a lot on when you blow it up.

But the main idea is not to blow up the asteroid, it is to blow up a bomb close to it to change its trajectory. Radiation pressure and ablation of the side exposed to the blast would push the asteroid away from the bomb with the largest deltaV per kg of spacecraft we can dream to achieve in the foreseeable future.
You don't need to be very precise in your blast if you are early enough because Earth is a small target, and you have much more energy at hand than classical fuel would hold. Even blasting far away, and delivering only a small fraction of the energy to the target would change its direction more than classical rockets could.

Let's Orion drive rocks out of our way!
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PM 2Ring
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### Re: Asteroid redirection question

idobox wrote:Earth is a small target

And it's moving rather rapidly. From Earth's orbit
The orbital speed of the Earth around the Sun averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,700 km) in seven minutes

So you only need to change the time of intersection of the asteroid orbit and our orbit by a little over 7 minutes in order for it to miss us completely.

Copper Bezel
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### Re: Asteroid redirection question

That target is always somewhere, though. It's not as if the asteroid has to aim. The only thing that affects the statistics is the cross section, right?
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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PM 2Ring
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### Re: Asteroid redirection question

Copper Bezel wrote:That target is always somewhere, though. It's not as if the asteroid has to aim.

Copper Bezel wrote:The only thing that affects the statistics is the cross section, right?

Sort of. But the effective cross section of the Earth depends on the angle that the asteroid's orbit intersects Earth's orbit. If the angle is close to a right angle, we're a relatively small target, and the 7 minute figure I quoted before is relevant, but if the angle is highly acute then our effective cross section is larger, and the difference between our velocity and the asteroid's is much smaller, so it takes longer for the asteroid to pass us. On the up side, the smaller velocity difference makes the asteroid a bit less dangerous if it actually hits the Earth.

The orbit intersection angle can be fairly stable over many orbits, but it can also change both periodically and chaotically. Many Earth-crossing bodies have complex orbits that are in resonance with the orbits of the planets whose orbits they cross. That can give some stability to the orbit intersection angle, but if the asteroid has a close encounter with a planet it can change the orbit considerably, making it very difficult to do accurate long term orbital predictions for these bodies. And to predict future collisions with Earth, we need the orbital predictions to be of an order of accuracy consistent with the 7 minute window I mentioned in my previous post. (I should note that if the orbit intersection angle is very acute, that raises the odds that the asteroid's orbit will be gravitationally perturbed by a close encounter).

So if we're serious about deflecting asteroids, we don't want to merely prevent a particular encounter, we want to modify the asteroid's orbit so that it no longer crosses our path, and that means that we really need to put it into an orbit with long-term predictability.

As I've mentioned in other threads, I'm not a fan of destroying near-Earth asteroids. They may be a valuable source of raw materials for near-Earth space industries in the not too distant future, so to me it makes a lot of sense to move them into safe parking orbits, if we have the skills & energy to do so.

idobox
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### Re: Asteroid redirection question

PM 2Ring wrote:As I've mentioned in other threads, I'm not a fan of destroying near-Earth asteroids. They may be a valuable source of raw materials for near-Earth space industries in the not too distant future, so to me it makes a lot of sense to move them into safe parking orbits, if we have the skills & energy to do so.

My point of view is that we shouldn't take any risk. A 50m rock can cause a lot of damage if you can't move it out of the way, and it is not that precious, as there are other ones.
It seems to be easier to move an asteroid than to destroy it if we do it early enough, but if there is a risk of failing to move it enough, I'm totally for destroying it.
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Copper Bezel
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### Re: Asteroid redirection question

PM 2Ring, that makes a lot of sense - thanks for the clarification. I especially didn't think about the fact that gravity has a bigger part to play if the relative velocity is lower.
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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jules.LT
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### Re: Asteroid redirection question

webgrunt wrote:send a spacecraft to the asteroid, whereupon the spacecraft would match the asteroid's speed and direction, and hover above it for an extended time period. Supposedly the gravitational attraction will very gradually nudge the asteroid' toward the spacecraft, changing the orbit enough to miss the earth.

Would the attraction of a similar-sized object really have an effect significant enough to divert the asteroid?
How long would it take for it to divert it by the previously mentioned 7 minutes angle?

I'm with the nuclear bomb push option.
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gmalivuk
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### Re: Asteroid redirection question

jules.LT wrote:Would the attraction of a similar-sized object really have an effect significant enough to divert the asteroid?
It would if applied early enough, obviously.

In any case, a change of 7 arc-minutes requires a delta-V of 0.2% of your current velocity, and the acceleration due to gravity of an object toward one of the same mass is G (m/r)^2.

Plugging in numbers is left as an exercise for the reader.
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mfb
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### Re: Asteroid redirection question

Similar-sized object? M_spacecraft << M_asteroid

Let's consider a real example: M=10000 tons, deflection time 2 years, warning time (minus time for the satellite to reach the asteroid) 6 years. Required velocity change for the asteroid: 10000km in 5 years or 6cm/s
M_spacecraft = 15 tons, distance d=30m, gravitational force F=9mN, velocity change in 2 years: 6.4cm/s.
Required delta_v capacity of the satellite (after reaching the asteroid): 43m/s (negligible)
On the other hand, a direct impact of the satellite with 43m/s would give the same deflection, if it does not disintegrate the asteroid (and reduce it to harmless pieces with an arbitrary orbit, like millions of others).

I tuned the parameters to fit - you cannot change things like the warning time in reality, of course.

PM 2Ring
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### Re: Asteroid redirection question

gmalivuk,

I suspect that jules.LT wasn't talking about a 7 arc-minute deflection but was referring to the window of 7 minutes of time that I mentioned earlier in the thread.

Of course, that 7 minute figure is somewhat optimistic, as it assumes that the asteroid's orbit crosses Earth's orbit orthogonally. But I discussed that in a subsequent post.

jules.LT
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### Re: Asteroid redirection question

Er... Actually, I was talking about an angle because I thought you were talking about an angle
But I see now that it wouldn't make sense.
Thanks for the calculations, mfb. 2 years seems like an acceptable timeframe if we can get a warning early enough. It seems like a reasonable alternative to blowing it up with unpredictable consequences.

I'm not sure what the advantage is over a nuclear bomb push from some distance away, though. The latter solution has a more significant, immediate effect, and requires less precision and less time of perfect operation.

The orbital speed of the Earth around the Sun averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,700 km) in seven minutes

Wow, I had always pictured Earth just zooming past because of how fast it is, but I was forgetting how freaking big it is... 7 minutes to move by just its own diameter, eh?
Bertrand Russell wrote:Not to be absolutely certain is, I think, one of the essential things in rationality.
Richard Feynman & many others wrote:Keep an open mind – but not so open that your brain falls out

idobox
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### Re: Asteroid redirection question

jules.LT wrote:I'm not sure what the advantage is over a nuclear bomb push from some distance away, though. The latter solution has a more significant, immediate effect, and requires less precision and less time of perfect operation.

Don't forget the weight, 15 tons is a lot for a probe.
A W88 thermonuclear warhead is thought to have 475kt yield but weigh only around 360kg (classified information, but estimates are done by looking at the missiles that carry them).

The biggest issue with nukes in space is that they're forbidden. There's an international treaty that bans the use of nuclear weapons in space, and there would be serious legal and PR issues with launching such devices, with the associated risks of mission failure.

If the situation arose now, I think we would use nukes because they are the only proven technology, and if it fails for any reason, we have a large stockpile we can use to try again. Designing and building a gravity pull probe would take time, and it is a difficult performance to stay in the right position for months or years.
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