Feb23
Diverting Asteroids
on February 23, 2013 at 10:53 amNASA’s astronomy pic of the day a couple days ago featured a gravity tractor. Which works fine, if you have enough warning (of course, a nuclear-electric vehicle is politically, if not technically problematic).
The image, though, is copyright by Dan Durda. NASA didn’t fund the picture, let alone the tractor. Ares V is more importsnt.
That’s what happens when you make Jeff Bingham chief rocket designer.
Anyone with high school physics and 15 minutes to spare can figure out that a “gravity tractor” doesn’t compare favorably to any method of direct coupling, of which there are plenty to choose from. Even running the probe into the asteroid will do more than flying nearby it for long durations. At least when the B612 Foundation folks talk about using a gravity tractor they’re careful to make clear that it’s for fine tuning of a prior diverting maneuver.
Did you bother clicking on the link? This picture is from the B612 Foundation.
.. and?
John has some thoughts (with some admirable humility.) Yes, I admire that greatly about him. He’s a true frontiersman.
I really don’t understand the need for a gravity tractor. To me, it seems like going about it the hardest way. For just one thing, you’re losing a lot of thrust due to the need to have the thrusters firing at angles. Secondly, you’d need one hell of a lot of ISP, such as nuclear-electric, and that’d probably mean a very massive spacecraft.
What I don’t understand is why nuclear warheads are not the easiest, simplest approach? No need to spend a lot of delta-v matching course, head on or or an angled high velocity intercept will work just fine.
There’s no need to vaporize it, all it needs is a nudge, and that means you don’t need or want to hit it. Detonate close but not on it – perhaps one asteroid diameter away. In space there’s no shockwave to worry about, and the energy transfer is radiative. It would heat and ablate the surface, vaporizing a few inches, causing some of the material to depart the asteroid at fairly high velocity. That’d impart delta-v to the asteroid, and if you’re a few years out, you only need a very small nudge in any direction. If it’s closer to impact (say a few months) do it a few times. I don’t see this imparting enough force to break up an asteroid, especially a large one, but it should give it a slight shove. I’m thinking of an existing, well-tested warhead that’s designed for the kind of stresses spaceflight implies, such as the warheads used on Minuteman or the Trident D-5. It wouldn’t need to be ejected from the carrier craft, just detonated.
What are the technical problems with doing this? It’s all very well to talk about gravity tractors and massive laser arrays, but with the tech we have today, those seem both highly impractical and would perforce need to be deployed in space well in advance, whereas something like this could be cobbled together within a few months, maybe. As a guess, modify a geosynchronous transfer bus as the carrier (for one thing, you’d need imaging capability, and for another, longer-range comms), use the lighter payload and the largest available launcher to give it escape velocity capability, and go for a high relative velocity intercept. I’m guessing that something like this could be cobbled together within a year.
Obviously, I’m either very wrong somewhere in my above technical assumptions, or this would be being looked at far more closely than it is.
So, any ideas why using a mid-range three-stage nuke, such as the W88 (weighs around 800 pounds and yields around 500kt) in a standoff method (detonating near, but not on, the targeted asteroid or comet) wouldn’t work?
So, any ideas why using a mid-range three-stage nuke, such as the W88 (weighs around 800 pounds and yields around 500kt) in a standoff method (detonating near, but not on, the targeted asteroid or comet) wouldn’t work?
I keep picturing sticking the nuke into something like a mortar packed with a whole lot of sand. Think high-power shotgun. A very large number of high velocity but individually miniscule impacts. Adding even slight directionality with regular explosives is quite useful, but the needed weight of the barrel andor tamping is unknown to me.
It’s impossible to accurately model the effect of a nuclear weapon on an asteroid without making assumptions about its composition — which may or may not be correct. A rubble pile will react differently than a solid body.
To develop plan that’s sure to work will require more data. A sensible program would include close reconnaissance of a large number of asteroids, detailed studies of a smaller number (probably including manned missions), and some experiments in actually moving asteroids. The last might overlap with resource recovery.
It’s insane that we spend less of asteroid research than we do on Mars. The god of war hasn’t hit us recently.
Would a ruble pile be disrupted by a gentle shove, which is what a standoff nuclear device would give?
I’d very much like to see a test program that would determine composition of several comets and asteroids, and also try changing the orbits of a few. We’d get a lot of science out of it, along with the practical knowledge. The Deep Impact mission was IMHO a good start.
Computer simulations have shown the pile would be disrupted but reform in a short period of time. Merely disrupting the cohesiveness of the mass without altering its trajectory does little good, though. And we don’t actually know that much about the internal structure — we infere it based on the density. Right now, trying to move an asteroid would be like civil engineering without good geological data.
Doesn’t conservation of momentum suggest that blasting part of the asteroid in a generally eastward direction is going to move the rest of the asteroid in a westward direction? If the asteroid were a solid rock instead of a rubble pile you could use successive nukes to form a nozzle and get higher and higher efficiencies as time goes on, but so long as you don’t use a big enough nuke to disrupt the asteroid it seems to me that you ought to be able to nudge it into a different orbit.
Trent is right about the gravity tractor being a poor coupling idea. I have never understood why anybody would consider it a useful idea.
Because then you don’t get evil people contaminating noble asteroids. Hands off, just like the enviro-weenies like it.
Those same envoronazis also objected to the intentional impact of a Centaur upper stage on the moon, for reasons ranging from predictions that it would change the moon’s orbit and cause tidal waves, to declaring it an “act of war against the moon”.
http://rense.com/general86/moonbomb.htm
You simply can’t parody these people, it’s not possible.
A gravity tractor is a very weak coupling, and thus very slow. But unlike impulsive methods is unlikely to disassemble a rubble pile. And unlike a tow line doesn’t require de-spinning or finding a solid anchor point.
An electrostatic tractor or firing an ion thruster at the asteroid are other methods that could be considered.
Some of us would consider a gravity tractor to be a useful idea because we are old enough to remember the old “Asteroids” arcade game of the 1980s. Giant asteroid coming right at you, big enough to cause total obliteration of it hits. Nuke the sucker. Now four large asteroids coming right at you, each large enough to cause total obliteration if one of them hits. Next step, sixteen asteroids…
Asteroids may be fragile enough that any impulsive momentum transfer (even distributed over the entire surface) or any point momentum transfer (even distributed over many months) sufficient to divert them from impact, may also fragment them – and the momentum will not be evenly distributed among the fragments. Good chance some of them will still be on a collision course, and the people of Earth will not thank you for converting the cosmic 12-gauge slug headed their way for an equal charge of interplanetary buckshot.
The gravity tractor delivers its propulsive momentum uniformly throughout the entire mass of the asteroid and over several months, without requiring any complex coupling or detailed knowledge of the asteroid’s construction. It is the one thing we know how to do, that we know won’t fragment the asteroid and possibly make things worse.
It is also, as noted, somewhat inefficient. For large asteroids and/or those located late in the game, more violent and dangerous approaches will be the only possible solution. Ideally multiple low-yield nuclear proximity detonations; for the worst cases, large H-bombs impacting the surface and about fourteen billion fervent prayers. But the gravity tractor is certainly a useful thing to have in our arsenal for the cases where it can be made to work.
Gravitational coupling is weak, but can be effective in a reasonable period of time. For a 60 m diameter rock, massing .34 million tons, a 200 ton ion drive vehicle hovering 20 m above the surface (r=50 m) is at about a microgravity acceleration, and needs about 2N of thrust. That 2N will move the rock 2600 km in the first year, 10,000 km by the end of year 2, 24000 km in three years.
The ion acceleration of the 200 T vehicle would be about half that of the Dawn probe currently on its way from Vesta to Ceres, but would require close to a megawatt of power rather than a kilowatt. Not an insurmountable engineering problem, but not a trivial matter either.
Slow and steady can eliminate all the problems of actually *grabbing* the object. There’s always lots of strong opinions in the absence of analysis….
Rock to be moved:
Radius 30 m
Density 3000 kg/m3
Volume 113097 m3
Mass 339292007 kg (339 kTon)
G 6.67E-011 m3/kg.s2
Rtug 50 m (Radius from center of rock)
Mtug 200000 kg
A_tug 9.06E-006 m/s2 (0.78 m/s.day)
F_tug 1.81 N (Needed to hover above rock)
Vexh 30000 m/s
Mdot 0.001667 kg/s (144 kg/day)
Power 750000 W (750 kW)
A_rock 5.34E-009 m/s2 (0.0005 m/s.day)
Dist/yr 2658485 m/yr (2658 km/yr)
Dawn Probe
Thrust 0.09 N (90 mN)
Mass 4000 kg
Accel 2.25E-005 (1.94 m/s.day)
Vexh 30000 m/s
Mdot 3.00E-006 kg/s (0.26 kg/day)
Power 1350 W
Hmm, since tethering was brought up in the other post, could tethering be used to adjust the spin of an asteroid?
Probably not of a rubble pile.
I kind of like using graffiti to alter the orbit of an asteroid–change its albedo and let the sun do the rest. Not so easy if the asteroid is spinning.