Wednesday, August 25, 2010

Rock Envy

With the exception of Hayabusa, all asteroid missions to-date have been to targets bigger than 1 km in "spherical radius".

DateEncounterAsteroidRadius (km)Spacecraft
1991Flyby951 Gaspra6.1Galileo
1993Flyby243 Ida15.7Galileo
1999Flyby9969 Braille~1Deep Space 1
2000Flyby2685 Masursky~8Cassini
2001Landing433 Eros8.42NEAR Shoemaker
2002Flyby5535 Annefrank2.4Stardust
25143 Itokawa0.165Hayabusa
2006Flyby132524 APL~1.1New Horizons
2008Flyby2867 Šteins~2.8Rosetta
2010Flyby21 Lutetia95.8Rosetta

This has led a number of people to express dismay that all the asteroids which have been identified for human exploration missions have significantly smaller estimated sizes.

DateAsteroidRadius (m)
20162008 HU4~5
20171991 VG~45
20192008 EA9~6
20202007 UN12~4
20251999 AO10~35
20262008 JL24~2.5
20282006 RH120~2.5
20292000 SG344~22.5

Notice that the scale has changed from km to m. Of course, exactly why these rocks are small is most probably what makes them optimal for a human mission. Their close approaches to Earth most likely would have ended long ago if they were any bigger. Collecting samples from these asteroids will help us to understand why they are not in the main asteroid belt, which is very important to know as their larger cousins threaten the Earth.

Some mission planners at NASA and elsewhere have expressed a desire to exclude any target with a radius smaller than 25m, being referred to as "mere building sized" asteroids. This reduces the targets in the table above to just two, although the last typically scrapes by due to a lack of options. Exactly why this policy is being suggested is unknown. Some speculation includes:

  • the belief that more targets will become available with greater funding directed to finding them, so it's best to downplay the available targets.
  • the difficulty of approaching a small rapidly spinning body.
  • lack of mass for sample collection.
  • lack of surface area for exploration given extended mission duration.
  • overall "spectacle".
These are all terrible reasons, and I'd hate to be accused of putting up a "strawman" to knock down, but they are the only reasons that I have read about. If you've heard more, let me know.

The first reason is just bad politics, and it's completely unnecessary. No-one can reasonably say the asteroid surveys are getting "enough" money, but they're certainly getting some, and they're getting valuable priority time on telescopes, both optical and radio. Hopefully this is just an ugly rumor.

Approaching a spinning body is something astronauts have done before. Dale A. Gardner and others used the Manned Maneuvering Unit (aka, the astronaut jetpack) to match rotation with satellites in LEO to return them to Earth on the Space Shuttle. With this in mind, it would seem engineering and astronaut experience is more applicable to small spinning bodies than it is to large ones.

The lack of mass argument should be immediately recognized as a failure of imagination. Referring to these asteroids as "small" in the first place is suggestive of this. The smallest asteroid on the list above has an spherical radius of ~2.5m, about the size of this:

The biggest asteroid on the list above, 1991 VG, is about half the size of this:

And unlike these reference objects, asteroids are solid with few to no internal voids. So there's plenty of mass, and when it comes to surface area and mission duration, I actually think making the case for short mission duration makes a lot of sense, and besides, suggesting that just a few days is too much time to spend exploring one of those London bus sized objects is just silly. A meteorite of such a size would be investigated for years.

In regards to spectacle, I recommend watching the video I linked to above of Dale A. Gardner catching that satellite. Even with a London bus sized object you're in for one hell of a show.


  1. Anonymous1:54 AM

    Similarity of the job that Gardner did on Westar 6 to a ten-meter size asteroid should not be assumed. Gardner linked to Westar 6, and used thrusters to stop the spinning. An asteroid, even as small as Westar 6 would be HUGELY more massive, and probably very asymmetrical. So it would take a lot of propellant to stop the spin, and would be tricky to do it without inducing nutation.

    The best argument for humans not to go to a small asteroid is that the entire surface could be mapped out robotically at the resolution of the human eye, such that another vehicle could be sent to pick up things that would look interesting to an astronaut, if there is anything that looks interesting enough to pick up.

    We'd do the former anyway, I guess. We're not going to send astronauts to a place that hasn't been scouted robotically.

  2. Umm.. that's because they wanted to take the whole satellite.. unless you had a satellites sized asteroid you were visiting you wouldn't want to do that.

    As has been said quite a number of times now, robotic probes to asteroids have been incredibly lucky to return any data at all. Every programmatic lead engineer for every robotic asteroid mission that has ever flown was recently sitting at a table together at a NASA exploration workshop and they all said that a human mission to an asteroid is of incredible scientific value, and absolutely necessary from a planetary protection perspective.

  3. Anonymous5:41 AM

    I'd come at it for the opposite direction. What need is there for human beings to rendezvous with asteroids as small as 25m in radius? I usually get three answers to this question:

    1. You can build a fare habitat out of a small asteroid.
    2. Practice.
    3. Big Science.

    I'm pretty dismissive of Big Science--at least in terms of how it's managed and funded--in general, so let's take 1) and 2). First, whether or not a small asteroid makes for a suitable base is irrelevant; the question is why send out men to encounter it. To be of any use, you'll need to capture the rock and return it to the Earth sphere anyway, so why not wait until then to worry about putting people on it. Second, practice for what? Encountering asteroids (and comets) large enough to require some manned intervention to capture? That's a whole other ball game in terms of endurance.

  4. Last one was from me, Trent.

  5. cool.. my answer is that, really, the goal is to journey not the destination. NASA could just fly astronauts out to double digit lunar distances and then right back to Earth.. but the astronauts might as well be asleep.

    Asteroid targets are motivation for flying longer duration missions. The term NASA uses is "stepping stones", but I think it's a terrible analogy. The gift of close approaching near-Earth asteroids is that they are reachable with current technology, or at least we think so now. Going to them will be a learning experience that will tell us how wrong we are, and tell us what to improve and what new technology we need to develop, to get to the next distant target. Eventually, that capability will extend all the way to Mars.

    Those who say we already know how to go to Mars are fooling themselves and those who say "let's just go to the Moon, we already know how to do that" are missing the point. The goal is to do what we don't know how to do!

  6. Still Prez here, and I'm done fighting Blogger when it comes to identity. ;)

    That's one way to look at it, but consider this. We don't know how to keep a hamster alive on the surface of the Sun. Is it our goal to figure out how to do so? If you could fund anything and everything to your heart's content, then certainly.

    Destinations *do* matter, and hopefully we abide by criteria more prudent than "we know (or don't know) how to do something." After all, the goal is settling space so we can all get rich.

  7. the goal is to populate everywhere we can populate. That's the biological imperative and there's very good reasons why we're not "above" it. Asteroids are particularly interesting because they both tell us what we can do next to forward that goal and they remind us that we could be wiped out tomorrow.

    I've mentioned the other thing NASA, ESA, or someone should be doing next: send a mouse box to the Moon and demonstrate that mammals can reproduce in reduced gravity! Without that answer our direction into space will always be malguided.

  8. There's no doubt that asteroids are interesting, but that's separate from the question of whether there's a need to plant feet on asteroids of a given size on a given trajectory. As far as I'm concerned, the criteria to spend hundreds of millions to billions to set foot on anything should be economic necessity.

    We've the means to map key boundaries on reproduction in space (mammalian or otherwise) right here on Earth. Gravitotropism presents a number of challenges, though apparently not insurmountable even in the extreme case of free fall. As best as we know, we should be able to have kids, yield harvest and raise livestock at some additional cost.

    There's tons of questions about living in space like this one that have been answered to some degree or another, but will require further investigation to determine whether or not they present deal breakers substantial enough to revise our understanding of settlement risk.

  9. "the goal is settling space so we can all get rich"

    "the goal is to populate everywhere we can populate."

    These are good goals, but are these goals shared by those who control NASA's spending?

    Space enthusiasts might see the potential available to solve so many problems, with near-limitless resources and energy and room for humanity.

    However, consider that same potential through the eyes of Machiavelli and Sun Tzu - and the sorts of people in Congress and the Senate and in governing bodies all over the world.

    I'm working on a longish piece about different philosophies about space, based in part on the speech Rick Tumlinson gave at the Orphans of Apollo premiere and further expanded upon here. It ties ones' views on space into their political and economic philosophies and hopefully explains much.

  10. Anonymous4:48 PM

    "Going to them will be a learning experience that will tell us how wrong we are, and tell us what to improve and what new technology we need to develop, to get to the next distant target."

    I'm not sure we need to go to an asteroid to do that. Would you choose an asteroid over a Lagrange point or some sort of lunar mission?

    I'm all for doing any of them as long as the vehicle is reusable.

    But if you are looking for a reason, we have never needed more than "Because it's there."

    I just hope that whatever we do, it is part of a larger strategy and that there is a strategy before we decide to go anywhere.

  11. The other concern with small asteroids is that they may not be solid. It's supposed that some may just be a collection of rocks and dust, but without sufficient gravity to form a solid object.