A couple of weeks back, I got an invitation from Josh Hopkins (a friend of mine from Lockheed Martin’s Advanced Programs group) to attend a colloquium he was going to be presenting at up at SwRI’s Boulder office. It turns out the colloquium ended up right in the middle of SpaceX’s COTS 1 mission, so I ended up heading down there after the launch.
Anyhow, Josh was presenting the “Plymouth Rock” mission concept LM has been pitching as a way to do relatively “affordable” missions to Near Earth Asteroids. You’ve probably seen pictures for this mission concept showing two Orion capsules docked to each other. Admittedly, I had been a bit skeptical about this particular mission approach, but Josh did a pretty good job of making his case.
I didn’t get a copy of his actual presentation, but here’s a more detailed paper he wrote on the topic.
Their approach was to focus on the portion of near earth asteroids (NEAs) that had relative inclination, circular orbits close to earth’s. There are probably dozens of objects that fit this description. He mentioned a couple of key characteristics of these accessible NEAs:
- The circular, low-inclination orbits close to earth’s orbit keeps the required delta-V for rendezvous and return down to a reasonably low level.
- However, because the relative orbital speed of earth and these NEAs is so similar, visit opportunities for any given target are very infrequent (once every 10-20 years).
- A corollary to #2 is that since it is easiest to discover such NEAs when they’re close to earth, typically when you discover an NEA, the best time to do a mission is either right away, or 15-20 years from now.
- Most of these NEAs are less than 100m in diameter, with many of them less than 30m in diameter
- Most of these have relatively high spin rates
- Due to the small diameter and high spin rates, some of these objects have negative effective surface gravity.
- That means that these objects are very unlikely to be rubble piles, and many are likely solid monoliths.
- However, one research group found that a rubble pile NEA with that spin rate would preferentially shed the larger boulders, but due to the high adhesion of the regolith, the regolith would stay behind. So there could actually be dust pile NEAs in this group.
- Smaller accessible NEAs are much more common than large ones. According to models discussed at the meeting, we probably know less than 10% of the 30m or smaller accessible NEAs, but most likely we know over half of the >100m ones.
- Even then, accessible NEA opportunities are rare enough that NEA missions might be better as targets of opportunity rather than the focus of the system design.
There were other points, but I think those were the highlights.
Josh then spent a bit of time talking about the work they did with the dual-Orion and then the Orion + Orion-derived Hab analysis. Pretty interesting. For the mission in question they were talking about only two people. So, having two vehicles allows you to “go to your room” and be alone in a way that a single vehicle couldn’t. Josh also pointed out that an Orion-derived vehicle that didn’t have to be used for reentry, and didn’t have to be designed for launch abort modes, or take the conical form factor could easily double the usable internal volume of an Orion at significantly lower weight. I asked him afterward if he had looked at other vehicles like Bigelow’s Sundancer or Dragon, and his reply actually made a lot of sense–he had, but there just isn’t anywhere near enough public data on either of those systems to really be able to analyze a mission around them. It might be worth further investigation once Bigelow or SpaceX have released more information.
While the technical details of the space hardware was interesting, the most interesting takeaway from the discussion was Josh’s point that one of the most important investments if we’re seriously going to go after NEAs is in NEA detection and tracking. If we only know about 10% of the 30m-class targets, that means that investing in better detection has a pretty high probability of giving a better target for any given launch window. Especially with the fact that NEAs detected from earth tend to be only up for visit again 15 years from now, there’s a lot to be said for putting money into better detection. It was suggested that a Venus-distance IR telescope, pointing out away from the sun could probably detect about 50% of the 30m or greater NEA population within only 2-3 years of operations. Between finding more of them, and getting better orbital data so we can more accurately predict their courses at future dates, you can greatly increase your options for targeting missions. This makes it a lot more likely that you can find a good interesting mission that’s easy to get to.
Anyhow, I could probably say more, but I wanted to get these notes up there before I forgot to. I’d suggest reading Josh’s paper and giving it some thought.