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.'"

How about we move this rock instead?

[Anonymous Coward]

In a sense, you could apply the same approach, except try to modify earth's orbit, which might actually be easier...

Keyhole - Is that a standard term?

[fotoguzzi]

Could someone with the proper knowledge submit a Wikipedia article for keyhole? The word is used in the summary three times and seven times in tfa. I guess the term is here to stay.

Thank you.

How to not get splattered

[Jhan]

TFA spells it out very nicely. Get there one orbit early (a year or a little longer) then gently tug. Of course that's for a small asteroid, for a dinosaur killer maybe five years. If you wait until the object is a few weeks away you are toast. Cindered toast. The entire "nuclear might" of the planet launched at the intruder will do diddley squat, Bruce Willis or no Bruce Willis. That's why NEAR should get lots more funding.

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 [psi.edu] 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:If they ever do this...

[jandrese]

Isn't the whole point of floating point that you store the exponent separately so you don't run into that problem (at least not as fast)?

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:Sounds overly complex

[zippthorne]

You are presuming that landing is possible. The object in question might be a loosly-conglomerated gravitationally bound pile of rocks and dust.

Re:Sounds overly complex

[zippthorne]

True, however in the case of a gravity tug, your maximum useful thrust is limited by the gravitational interaction, which is limited by how close you can get. Landing takes one of the variables away, and trades it for the ones you mentioned. If it's solid, your maximum effective thrust can be very high, even if you can only use it a shorter percent of the time.

In specific cases, landers probably would be more effective. But gravity tugs are a much better general solution, and mass-production favors general solutions.

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.

Different approach

[moteyalpha]

It seems that if you wanted to change it's course by a continuous amount, that simply increasing it's mass by pushing other material into it's field would make the sun do the work. Just a random thought.

why gravity?

[ILuvRamen]

Why use one of the weakest forces in existence to move it? You'd probably get the same amount of thrust by attaching a dozen black cat bottle rockets to it. Gravity is pathetic! If I've got a paperclip sitting on a desk, the entire earth is pulling at it and yet one little 1 ounce magnet could pick it up. Refrigerator magnet 1, earth 0. They could strap a tiny, low yield rocket to the asteroid and blast it away waaaaaaay faster and more effectively. The whole gravity thing is so weak and dependent on the distance between the asteroid and the weighted spacecraft that it has a high chance of not working or at least not working enough.

Re:If they ever do this...

[JesseMcDonald]

It's not always insignificant. The problem mainly comes up when you're trying to accumulate something over a large number of iterations. For example, suppose your speed is the 1.0e0 term and the 1.0e-18 is your acceleration. At first it's insignificant, but over a few thousand (or million) cycles the error in the result becomes noticeable.

You can get around the issue with approaches like the one that this AC [slashdot.org] mentioned, but that requires additional consideration up front. You have to consider the limitations of the floating-point format when choosing and implementing your formulas rather than just writing them in the most natural style.

Another use for such a device ....

[tkjtkj]

The 'Gravity Tractor' device was discussed somewhere on or off the net as it might relate to 'saving the earth'. As i recall, the topic was about the ultimate demise of our Sun .. in 5 billion years or so .. The process of the Sun's destruction will include its diameter expanding .. the Sun's surface approaching Earth's orbit. LONG before that, of course, the heat would sterilize our world. The proposal was to navigate a large asteroid to 'lead the earth' in its orbit around the Sun.. accellerating Earth to a higher orbit, adding millions of years of biology to continue to exist. I seem to recall that such an asteroid would need 64,000 years to accomplish its goal. At the time, i concluded that this would represent an engineering project of the largest possible scale .. In the meantime, our highways and bridges would continue to fail, of course.