Gravity Tractor Could Deflect Asteroids 372
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.'"
Re:If they ever do this... (Score:5, Informative)
I hope their simulations use doubles, not floats!
I know you're joking but for just the speed values if they used time increments in the order of the second then the speed differences would be in the order of e-18, which is too small for a double's mantissa. I'd rather go with long doubles, or better (I think you can achieve something like that by using a number to store the closest representable value and another one to represent the tiny difference from what it should be).
Re:Keyhole - Is that a standard term? (Score:3, Informative)
Dupe (Score:4, Informative)
Re:Sounds overly complex (Score:3, Informative)
Even if the asteroid is solid, and there is some 'miracle' way of anchoring the rocket to the asteroid: Landing and pushing requires the assumption that the center of gravity and the shape of the asteroid is such that you can position the rocket push in a productive manner and not just cause the rock to pinwheel or split in two.
Re:I hate to accuse JPL of forgetting something... (Score:5, Informative)
Who modded the parent "insightful". The answer is pretty simple, and is even illustrated in the article. The picture shows a craft with three thrusters all angled away from the asteroid. The resulting thrust is a vector normal to the target. Sure, it sacrifices efficiency, but it works.
Compensated summation (Score:5, Informative)
(I think you can achieve something like that by using a number to store the closest representable value and another one to represent the tiny difference from what it should be).
Yup. It's the Kahan summation algorithm [wikipedia.org]. It works as you describe it and it used to compensate the error that happens when doing very big sums of very small numbers (exactly the situation in the gravity tractor's problem)
Re:Nice, but lets keep it real. (Score:4, Informative)
Gravity Tractor? You know I love these sky high fantasy ideas to deflect asteroids as much as anyone else but shouldn't we be concentrating on what is real? If an asteroid does threaten Earth in the next few years we will use nuclear demolitions on it. We will not use a gravity tractor, laser beams, or giant snow balls. Nor will we attach plasma engines or mass drivers to it. We will use nuclear demolitions because that is, simply, all we have.
[...]
We wont' use one nuke. We will blowup the big one then we will blow up the smaller ones into smaller pieces. We will do this until the chunks are small enough that the atmosphere will handle. With smaller chunks there is more surface area for the atmosphere to work on. Most importantly the smaller chunks will not "crack the crust" as one fat ass one would.
Blowing up an asteroid isn't necessary, and with only a couple of years' notice, it isn't very effective, either. For details, see:
https://e-reports-ext.llnl.gov/pdf/343984.pdf [llnl.gov]
Nuclear explosives are a good tool for this job, just not in the way that you think they are.
Re:and now ... (Score:3, Informative)
Cause I speak of the properties of love.
Pompatus [wikipedia.org].
Re:If they ever do this... (Score:1, Informative)
It's the Unix desk calculator. [wikipedia.org] The grandparent means that if an ancient Unix utility can do infinite precision math then the JPL definitely can =)
Why is this useful? (Score:3, Informative)
If you place your massy spacecraft near an asteroid and let go, the two will mutually attract each other and eventually collide. The centre of gravity of the system won't change.
So, in order for this to work at all, you need a manouevering system on the spacecraft in order to maintain its separation from the asteroid.
The thing is, though, that from a pure orbital mechanics point of view, this is absolutely equivalent to simply mounting the spacecraft's thrusters on the asteroid itself. In fact, using the gravitational tractor is probably going to be rather less efficient, because the geometry of the system is such that you have to fire your thrusters towards the asteroid --- and a certain amount of your thruster exhaust is going to bounce off the asteroid's surface, imparting momentum in the wrong direction to the asteroid.
The only things I can think of that the gravity tractor does for you that direct acceleration doesn't is:
But, given the type of accelerations we're talking about --- which will be the same regardless what technique you're using --- I wouldn't imagine that either of these would be a problem in practice. So, what makes the gravity tractor so much better than just using rockets? Indeed, what makes it better than alternative approaches like spraying the asteroid with aluminium powder (which raises the albedo, causing increased photon pressure, which alters the orbit over time)?
Re:Nice, but lets keep it real. (Score:4, Informative)
Re:We think rather highly of ourselves (Score:2, Informative)