Gravity Tractor Could Deflect Asteroids

Hugh Pickens writes "A new study at the Jet Propulsion Labs shows that weak gravitational pull of a "gravity tractor" could deflect an Earth-threatening asteroid if it was deployed when the asteroid was at least one orbit away from potential impact with Earth. First a spacecraft would be crashed directly into the asteroid, similar to the Deep Impact mission that impacted a comet in 2005. This would provide a big change of direction, but in a less controllable fashion that could push the path of the asteroid into a dangerous keyhole. But then a second spacecraft, the gravity tractor, would come into play, hovering about 150 meters away from the asteroid, to exert a gentle gravitational force, changing the asteroid's velocity by only 0.22 microns per second each day. Over a long enough time, that could steer it away from the keyhole. In the simulation, a simple control system kept the spacecraft in position, and a transponder on the asteroid helped monitor its position and thus determine its trajectory more precisely than would be possible otherwise. 'The gravity tractor is a wimp, but it's a precise wimp,' said astronaut Jack Schweickart. 'It can make very small, precise changes in orbit, and that's what you need to avoid a keyhole.'"

Comments

OT: Orbit@Home is now NASA-funded

[Burz]

Its probably a good time to remind people that the distributed computing project to search for dangerous NEOs is soon to get under way. Test workunits have already been sent out and the news is that they ran very well.

Re:Coaxing vs Pushing

[Zocalo]

The tractor possesses mass, and therefore has a constant gravitational effect upon the asteroid (and visa versa) and the only fuel it needs to expend are the minuscule amounts necessary to keep it tracking the asteroid. The simulation shows that, given enough time, the cumulative effect of the gravitational tug can exceed that of expending all of the energy carried by the probe, whether kinetic (impact) or potential (engine burn). It's kind of like an ion engine; it's slower at first but over time it's going to leave a traditional chemical based rocket eating its space dust.

Re:Coaxing vs Pushing

[MyLongNickName]

expending all of the energy carried by the probe

You have to obey the laws of physics. There is no way you will expend less energy holding your position than using all of your fuel to build up speed and crash into the sucker.

In fact, if there is any elasticity in the collision, it it far more efficient to crash. And this does not take into account the fuel you will waste simply by having to angle your exhaust to not hit the asteroid.

Now, the down sides to crashing are that you cannot accurately know just how much you will move the meteorite. You cannot make midcourse adjustments as you learn more about its trajectory, and as you mention, not all asteroids will be landable. Soft surface or rocky surace, and you will have wasted the lander.

Re:If they ever do this...

[JesseMcDonald]

Yes, but if you add, say, 1.0e0 and 1.0e-18, the result may get rounded down to 1.0e0 (no change) as the real result, 1.000000000000000001, is not distinctly representable in the number of bits available. To avoid losing precision all the bits after the leading '1' (which is assumed) have to fit within the mantissa, which is of finite size. This applies regardless of the exponent, except for the special cases of zero, NaN, and infinity.

For a problem like this one tends to be better off using fixed-point notation, with a word size large enough to represent both the smallest increment and the largest magnitude you expect to work with. That way you don't run into the case where a small increment may or may not get rounded off depending on the magnitude of the other operand.