While there has been some slightly more positive discussion about the NASA Asteroid Redirect Mission since my previous blog post and SpaceNews Op-Ed, there have still been a steady stream of criticisms and suggestions of alternatives to the ARM mission being discussed lately. I’ve always been a bit of a knee-jerk defender of the underdog, and I still don’t think ARM is being given a fair shake by many in the space policy community, so I want to take the opportunity to respond to some of these criticisms and alternatives. And by respond, I don’t mean just dismiss–some of the suggestions have real merit, and could lead to ways to improve the existing ARM mission.
- We should be looking for asteroids not trying to redirect them — This criticism is that before sending a mission to an asteroid, we should be making more of an effort to find the vast majority of NEOs that we still haven’t identified. I actually agree that a strong effort at identifying more of the NEO population would be money well spent. As I pointed out in a previous post based on a talk by Josh Hopkins about LM’s Plymouth Rock mission concept, more knowledge of the NEO population and orbits gives us more options and can only make missions like ARM and others better over time. I don’t think this should preclude doing ARM as well, but finding ways to invest more effectively in this area would be useful. Some suggestions for how we could do better here range from traditional approaches such as funding a NASA asteroid hunting mission or doing a competition for industry provided asteroid finders, to providing matching funds or a free rides for more commercial missions such as what Sentinel and/or the prospecting spacecraft Planetary Resources and Deep Space Industries are developing, to potentially offering a bounty for detection and verification of new asteroids. It doesn’t have to cost billions, but spending a bit more money in this area is likely to make ARM better, and increase our odds of finding dangerous asteroids with enough time to do something about them.
- OSIRIS-REx is already doing this — I’ve heard people criticize ARM because the OSIRIS-REx mission is already going to be returning samples from an asteroid before the ARM mission is flown, and thus we don’t need another mission. I find this argument as silly as Obama dismissing the Moon with his “Buzz has already been there, done that” argument. Just as the Moon is a world the size of Africa, and you can only learn so much from a half-dozen landing missions, how much are we really going to learn about the millions of asteroids in the NEO population from a few hundred grams of materials returned by a handful of sample return missions? How much do we really know about the consistency of material composition even within one single decent-sized asteroid? What are the odds that even two C-type asteroids are going to be identical enough that additional samples wouldn’t be worth it?
- We need a large number of samples, not a large sample — I partially agree with this argument–diversity of samples is important, but so is having enough quantity to actually be able to do useful ISRU experimentation. The multi-teaspoon sized samples provided by current missions might provide you with some idea of the chemical composition of at least that part of a given asteroid, but learning how to mine asteroids (and if we can do so in a way that makes economic sense) is going to take larger samples. One possible compromise that could give you the best of both worlds might be a multi-lander/grasper ARM concept. Instead of having one big 4m diameter boulder grasping system, ARM could potentially do 6-7 smaller (~2m) separable grasper/landers, attached to an ESPA-ring like structure, and the spacecraft could also possibly visit more than one asteroid during the mission. As commenters have pointed out previously, there are actually relatively low-delta-V multi-asteroid tours that can be done that go from Earth to several interesting locations along the way before returning to earth. That way you can get sizable samples and variety, maybe 1-2 carbonaceous chondrite boulders, throw in a nickel-iron sample or two, and then one or two samples from another asteroid types. While this may sound a lot more complicated, NASA has already demonstrated multi-asteroid rendezvous with missions like Dawn, and building 6-7 copies of a lander grasper system would actually mean that a lot of the complexity is offset by larger efficiencies of scale–with a Prospector-like Option B grasper mechanism, you’d be making dozens to hundreds of most individual piece parts instead of the one or two that you often see for more traditional science missions. If I get more time, I’d like to flesh this idea out in its own blog post, but I wanted to get it out there publicly in case I can’t find that time.
- You shouldn’t pick the boulder just on its pluckability — One concern was that the boulder would be picked entirely from a standpoint of ease of extraction. I agree with this concern wholeheartedly, but NASA has already indicated that they were are planning to include at least a basic sensor suite to help with picking an interesting boulder, not just an easy one.
- You should bring the boulder back to Earth Orbit instead of DRO, because that’s just make-work for SLS/Orion — This one also comes up a lot. The fact that ARM is bringing the boulder back to DRO instead of LEO is seen as somehow indicating that this is all just a make-work stunt. But the more you study the problem, the more DRO seems like a reasonable choice. Spiraling in to earth orbit from escape velocity takes >5km/s of delta-V with a low-thrust system, on top of all of the other . This would require either a refueling or two, or a much bigger spacecraft (about 2-3x the size), and would take a really long time. High Thrust-to-Weight SEP stages can take 6 months to 1 yr to spiral out from LEO to escape. But with a 40-80 tonne boulder attached, the T/W ratio for the return spiral would be 5-10x worse, which would mean 3-10 years spiraling through the van Allen Belts. If you use aerocapture/aerobraking instead, with such a large mass, you would need either a large aerobrake, a lot of time, or something like the magnetoshell aerocapture technology we’ve been supporting MSNW on. I’m obviously not opposed to that last option, but this would be a non-trivial additional system development. Plus, even if you could magically snap your fingers and get an asteroid into LEO, there would still be challenges. An asteroid in LEO would be easier to visit but would also be a debris hazard (especially as you try to mine it and accidentally knock dust or rock chunks off or it), would have to deal with a much worse micrometeorite/orbital debris environment than it would in DRO, would be unlikely to have enough T/W to dodge a detected conjunction with other dead space objects in LEO, and would require constant propellant for reboost. It’s not an entirely impossible, but it’s not as much of a slam-dunk as some seem to think.
- Grabbing a boulder has nothing to do with planetary defense — This is one of the more ridiculous statements I’ve heard repeated by otherwise very intelligent people. The reality is that unless you’re going to use nukes, the gravity tractor is probably one of your better bets for asteroid deflection. And because the mutual gravitational attraction is proportional to the masses of the two objects multiplied together, there’s a big benefit for being able to increase the mass of the spacecraft using local mass. What better way is there to rapidly increase the mass of your spacecraft via in situ materials than to grab one or more big boulders off the asteroid?
- We should do ARM just minus the whole going to an asteroid and bringing a sample back thing — That’s like saying we should go to Mars but without that whole going to Mars thing. I think people are laboring under a false belief that the boulder grasping mechanism is most of the cost of ARM–it probably isn’t. The spacecraft bus and human spaceflight follow-on mission are likely a much bigger chunk, and NASA has already indicated they’d like to do those even if ARM was canceled. Canceling the grasping mechanism is unlikely to save you much at all–maybe the equivalence of a CRS mission or two, or a few months of SLS or Orion development. Spending the vast majority of the cost of the mission but without actually achieving useful exploration or ISRU development would be a waste. Why do people think that play-acting at being astronauts out at DRO without an actual useful mission for them to be performing is somehow more grown-up than doing actual exploration and potential ISRU research?
- We should skip the asteroid and go to Phobos instead — This is one of the best alternatives (not surprising considering the source–I have a ton of respect for Wayne Hale), and while I think it’s not the best option, I wouldn’t be heartbroken if ARM was refocused in this way. One of the selling points of ARM was that it is relevant to future Phobos/Deimos missions–the ARM spacecraft can and should be designed so that it can be refueled and “re-clawed” and used for another destination. The marginal cost of a Xenon tank and another copy of the claw is going to be trivial compared to the overall mission development costs, and there are tons of good reasons for an ARM-like mission to go to one or both of those moons. We didn’t explicitly analyze the case of grabbing a boulder from Phobos/Deimos, but a NASA Langley team did, and found that you could get a 1-2m diameter boulder off of them using the existing Option B hardware–notice this is the same size as the multi-lander/grasper concept mentioned above. But by skipping out on the asteroid first, you would lose the ability to test gravity tractor techniques, which could be important, and asteroids are also interesting in their own right. So I’m torn. I’d rather do both.
- We shouldn’t do anything that isn’t directly on the quickest path to Mars — I probably won’t convince Zubrinites, but it turns out we have this whole Solar System that doesn’t just consist of Earth and Mars. If manned Mars exploration was something we could do quickly, within NASA’s existing budget, or if there were no other interesting or useful destinations along the way, it might be one thing. But even the committee members who are advocating for this have admitted we don’t have the money to do a manned Mars mission in the next 25 years without significant increases in NASA’s funding. While it has been poorly marketed, Flexible Path wasn’t just about “doing asteroids first” or doing them instead of the Moon or Mars. To me the underlying point was that even if Mars is the long-term goal, we should find ways to do interesting exploration along the way to Mars, even if some of those destinations involve slight detours along the way. When you’re talking about a destination over 25 years out, acting like a 3 month delay is somehow insufferable is flat out ridiculous.
- We should just fly an SEP module to Mars and back instead of ARM — While the concept of skipping the asteroid and going straight for a Phobos or Deimos boulder return option actually made some sense–I think the concept of building a big SEP just to fly out to Mars and back is plain ridiculous. We’ve already demonstrated the ability to use SEP systems to do multiple rendezvous with celestial bodies, as mentioned earlier. SEP technology is likely going to shift so much over the next 25 years that the only good reason to spend a lot of money building and flying a demo SEP system now is if we’re using it for something useful like ARM. Building an ARM-class SEP system and just flying it around with no greater purpose seems like a waste to me. And as mentioned previously, you’re not actually saving that much money by ditching the whole grasper thing.
I could go on, and there are other positive suggestions I could provide, like using a COTS model on the SEP module to make something that gets us the experience we want while still being commercially relevant. But I wanted to provide some more thoughts for the ongoing conversation. ARM may not have very good odds of being funded to completion, but it’s not because the arguments against it are actually all that sound.
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