NASA's Asteroid Redirect Mission ARM Option B would have a robotic probe pick up a boulder from the surface of a much larger asteroid, and put that boulder in lunar orbit. Boulders considered could have a diameter of up to 4 meters and a mass of up to 50 tons (at maximum density).

A potentially hazardous asteroid has a diameter of at least 100 meters and a mass of at least 1,000,000 tons, according to the same source (11th SBAG July 2014).

Could a 100 meter asteroid be deflected by picking up several (many) 4 meter boulders from it? I suppose at least one boulder could be manipulated per day. Not to put the boulders in lunar orbit, but dropping them off one by one in slightly divergent trajectories. Or alternatively, putting them down on another part of the asteroid if that is more effective. Planetary defense is one of the motivations for ARM, I wonder if this motive is applicable to ARM Option B.

  • $\begingroup$ Chelyabinks was thought to be 20 meters in diameter and 12,000 to 13,000 tonnes in mass. If Chelyabinsk had hit at a steeper angle it could have been much worse. If SBAG gave figures of a least 100 meters and a million tonnes, I question their competence. $\endgroup$ – HopDavid Dec 1 '14 at 16:15
  • $\begingroup$ @HopDavid Hey, don't get "opinionated" here, Hop! I think the definition of "hazardous" size is that the consequences are likely to be comparable to the greatest natural catastrophes in recorded human history. A Chelyabinsk size actually hitting a city vertically, is very very unlikely. Breaking windows without killing anyone, yes, but that doesn't motivate a billion dollar prevention mission. (Btw, this might be a cheaper way to retrieve a fresh boulder from space, the surface of which would be contaminated by ARM anyway...) $\endgroup$ – LocalFluff Dec 1 '14 at 17:02
  • $\begingroup$ I said "steeper angle" not "vertical". Chelyabinsk was thought to have come in at a relatively shallow angle, around 16º. Average incidence angle is around 45º so if you want to talk about likelihood, the odds are for steeper than 16º $\endgroup$ – HopDavid Dec 1 '14 at 19:23
  • $\begingroup$ A 50 meter asteroid coming in at 45 degrees could easily cause much more than a billion dollars damage. $\endgroup$ – HopDavid Dec 1 '14 at 19:30

ARM Option B hardware could be useful in deflecting an asteroid, but probably not by the methods you propose or the current Option B mission profile.

First of all: Picking up 50 ton boulders is going to have a minute effect on an asteroid massing 1,000,000 tons. Placing the boulders somewhere else on the asteroid will move the centre of mass (by a tiny amount) but that probably won’t do much good. Shifting the centre of mass does mean that the asteroid has moved, but unless we can shift the asteroid hundreds or thousands of kilometres we still have an earth intercept trajectory.

The other problem is that many asteroids don’t have enough easily detachable boulders to make up a significant proportion of their mass. Even if 20% of the asteroid is boulders, that’s still 800,000 tons of asteroid we can’t move. So flinging the boulders away isn’t going to solve the problem. (The other, bigger, problem with this approach is the horrible amount of propellant we’re going to waste slowing the spacecraft down and returning to the asteroid after we release each boulder)

There was a good discussion of using the Gravity Traction deflection technique (which is similar to what we’re discussing) on the NasaSpaceFlight forums a few weeks ago. Although the thread has since been deleted, in summary it concluded that the only situation in which Gravity Traction deflection would be the best solution (and more efficient that just pushing the asteroid) is when the asteroid consist of multiple smaller bodies (boulders) that wouldn’t have the cohesion to be pushed.

So in that case, where the asteroid is nothing but boulders, it’s quite possible that picking and throwing each one individually might be a good solution.

However, the significant metric in asteroid deflection (and in fact all orbits) is velocity. So the best solution for solid asteroids would be to use the propellant we have on board to push the asteroid like the full redirect Option A.

We might not need the fuel load of Option A to deflect the asteroid, after all we aren’t looking for a stable lunar orbit like the full Option A; we just want a few millimeters/sec of Δv to push it out of the way of Earth.

I can't find any Δv numbers of the ARM Option B missions, but if you can find them it would be relatively easy to estimate whether Option B would have sufficient Δv to deflect your 1,000,000 ton asteroid enough to miss Earth.

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    $\begingroup$ Could it be useful to accelerate the boulder and smash it into the asteroid? I see now that "Opportunity for kinetic impactor" is mentioned on page 22 of the SBAG link in the question. $\endgroup$ – LocalFluff Dec 1 '14 at 14:52
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    $\begingroup$ So your answer really should start with "No", not with "Yes". Because the usefulness that you describe has to do with pushing the entire asteroid, like ARM Option A, not in any way with picking up boulders from it? ARM Option B has no relevance for planetary defence. And NASA has declared that "ARM is not a science mission". That leaves one wondering what the use is to demonstrate a technology (boulder picking) which has no potential usefulness, but that's another discussion. $\endgroup$ – LocalFluff Dec 1 '14 at 15:58
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    $\begingroup$ Yes, good point. I was interpreting the question very broadly to include the hardware for Option B, not just the mission profile. Sticking to Option B mission profile as it stands, you’re right, it is completely useless. Just out of interest, do you have a source for that NASA quote? $\endgroup$ – ForgeMonkey Dec 1 '14 at 16:07
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    $\begingroup$ +1. The Keck report says it'd take about .17 km/s to park 2008 HU4 in lunar orbit. As you say, deflecting an asteroid can take much less delta V than parking it lunar orbit. $\endgroup$ – HopDavid Dec 1 '14 at 16:21
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    $\begingroup$ @ForgeMonkey Yes, I heard Brian Muirhead at JPL mention it ("not a science mission per se") 58:55 into this Kármán Lecture oct 3, 2014. And googling the phrase I find it on page 31 point 5 in this protocol of the Planetary Sciences Subcommitte and Planetary Science Division Sep 3-4, 2014. I don't know how "official" this should be interpreted. $\endgroup$ – LocalFluff Dec 1 '14 at 16:35

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