Over at Space News, Jon Goff has ten reasons it’s a good idea.
I agree with all of them. I’m not opposed to ARM per se, except to the degree (and it’s unfortunately a large one) that the primary reason for it is to justify SLS/Orion.
10 thoughts on “Asteroid Retrieval”
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Not one of his ten requires we bring a rock from space to lunar orbit. As others have independently suggested, send a rock from earth and we can begin getting those ten experiences starting immediately.
Experience is what we need. We get that by putting a refuelable ship in orbit and rotate crews (of 4 to 7) every six months or so that visits that lunar orbiting rock. Have we learned enough yet to design such a ship?
Several of his points, unfortunately, don’t hold up. To start with, there’s no sound reason to move the rock to lunar orbit, when it could, for actually slightly less delta/v, be aerocaptured (not aerobraked!) into LEO, where it would be far more accessible.
Secondly, size; I’ve seen estimates of the size of the rock ranging all over the place, with many being in the 3 meter range. If that’s the case, we’re talking about something the size of a desk, and thus not of much use for a future resource. As for the grav tractor concept, it’s deeply flawed for most asteroid-impact scenarios; you’re having to match course (which takes a lot of delta/v) with the asteroid, and do so with all the fuel you need (with a big rock, that’s a lot). It also requires many years of advance warning. Far better to use your delta/v (including earth-escape delta/v) to arrange more of a head-on high-energy intercept, and deliver, say, a couple of tons of sand (dispersed pre-impact to avoid shattering the asteroid) at 30kps relative. You’d impart a lot more delta/v to the asteroid that way than any grav tractor – and be able to do it on much shorter notice.
Getting experience for future missions to large asteroids and comments? Nope – the issues don’t scale up, as they’re primarily gravity related.
Testing out an inflatable deep-space hab? You can do that quite well in LEO by attaching it to ISS – where you’d be able to keep it in place long enough to do a real test.
The asteroid-retrieval aspect of the mission is one I really do support in the main, it’s the manned aspect that seems worthless. Future ISRU? Great, but the manned “exploration” of the rock is irrelevant to it. (and if it’s the size of a desk, just how much “exploring” can one reasonably do?)
Oh, one other point on the unmanned ARM concept that seems to be being the favored one lately; grabbing a boulder off a larger asteroid. No one seems inclined to attempt to explain how we’d know whether such a suitably-sized boulder exists on the target asteroid?
Aerocapture direct to a low orbit is tricky. Come in just a little low and you get an unplanned landing instead of a circular orbit. Aerocapture to a non-specific highly eccentric elliptical orbit is much easier. From there you can aerobrake in multiple passes to a lower orbit.
I think we might be using the terms aerocapture and aerobrake differently.
The way I use “aerobrake” is to describe a high-heat pass through the upper atmosphere. It requires a heat shield. The way I use “aerocapture” is to describe a multi-pass drag maneuver, such as MRO (which didn’t have a heat shield) used to enter Mars orbit.
What I had in mind was the delta/v differences between entering a retrograde lunar orbit to entering a highly elliptical orbit that would allow for multi-pass aerocapture into Leo.
This animation shows a likely capture orbital plot.
https://www.youtube.com/watch?v=3KG3kHWLSZo#t=82
The first pass upon earth approach looks to me as if it’d take less delta/v to enter a very elliptical airocapture orbit than a retrograde lunar orbit.
The only reason for ARM (the retrieval craft) that I can see is SEP development and ISRU research (which I’m all for), but to do that research you need it at ISS, or Leo, otherwise it’s not much more use than sending a probe to an in situ asteroid to do ISRU experiments. It’d be far more cost effective to do it in Leo.
As for Orion/SLS, I think that part of the ARM mission is utterly useless.
“Aerocapture” means coming in on a hyperbolic trajectory (ie. not captured yet) and digging deep enough into the atmosphere to get captured into an elliptical orbit. You’re necessarily coming in fast (hyperbolic), and you have to come in deep enough (hot) to dissipate a lot of energy in one pass, because you only get one try. If you fail to go deep enough, you’re still going to be hyperbolic going out (bye-bye).
“Aerobraking” means you’re already in an elliptical orbit (with periapsis at the edge of the atmosphere), and you want to lower your apoapsis slightly. If the radiation environment doesn’t bother you, you can come in as shallow and as cool as you want, and make as many passes as you please, because you’re already captured.
I stand corrected.
I was using the terms wrongly. Thanks for the correction.
Thanks Peter, that was very clear and understandable. One the great things about Rand’s blog is he attracts such great commentors (excluding myself of course. My function has yet to be explained!)
The article reminds me of all those polls that ask what government should do to make things better, you know, the ones always promoting more spending in which the question is never asked about the cost, and what the money could be better spent, or not spent, on.
Which strongly suggests they simply let Planetary Resources, Deep Space Industries and others compete for funding. RFP a SEP tractor, then implements that use a PTO (did I use up my quota of TLAs?)
Some ARM supporters may be trying to justify SLS, but not all.
See Lori Garver’s comment on Goff’s article. Garver was (and is) an enthusiastic supporter of ARM.
Was her support an attempt to justify SLS? No. Garver is not an SLS fan.
Orion and SLS may need ARM but ARM doesn’t need SLS. For manned visits to this rock, I’d rather see trips via Falcons and Dragons.