I've been catching up on the Expanse lately, and just finished season 3. One of the earlier episodes in the season had the most sci-fi scene I've watched so far. It wasn't the magic rocket engines or the arm-band cancer cures that impressed me. It was the retrieval of the Nauvoo.
The Nauvoo is an enormous ship debuted back in season 1. Intended as a generation ship, it's incomplete when the intrepid heroes try to use it to batter an asteroid out of the way. The Nauvoo misses the asteroid, and ends up in a difficult to reach orbit without power or propellant.
So Drummer goes off to retrieve it. There's a lot of in-space rendezvous in the Expanse, but this one really brings home how advanced their spaceflight hardware and software really is. To retrieve the Nauvoo, they use a hundred or so autonomous engines that fly out to it, clamp on, and then redirect the orbit.
Every part of this is impressive compared to what we can do now. Let's break it down:
- Coordination: There are dozens, maybe a hundred spacecraft all working autonomously and in concert to move the Nauvoo. We don't know how to do this now (though it is one of NASA's research focuses). Coordination is a hard problem because you have to let all the spacecraft communicate with each other, tell each other where they are, and decide which spacecraft is going to do what. Swarm robotics is hard. Imagine how hard normal robots are, and multiply that difficulty by the number of robots in the swarm to get a sense for how impressive it is.
- Localization: Each of these autonomous rocket engines navigates to the proper place on the Nauvoo before clamping on. Navigating with respect to a large craft is difficult because most human crafts are pretty featureless. If you just look at the side of a battleship, it's really hard to get a sense for where on the battleship you are. You have to do a combination of computer vision and odometry to figure it out. In this case the robots might be able to localize against each other to figure out where they are next to the Nauvoo, but that's also something we haven't quite figured out how to do yet.
- Attachment: In space operations, you'll sometimes hear the term RPOD for rendezvous, proximity operations, and docking. RPOD activities are very difficult, because you want to make sure the spacecraft don't collide instead of docking. Part of this is the localization problem above, but part of it is also just how do you attach to the spacecraft? When the two spacecraft are designed to dock, this is kind of easy. We do it all the time when a new craft docks with the ISS. When the craft you're docking with wasn't designed for that, it's much harder. We've only successfully done this once, when a Northrup Grumman craft docked with a satellite earlier this year.
- Control: Finally, once the swarm of robots is attached to the Nauvoo in all the right places, they fire up their engines to reorient the thing. The control problem here is staggering. The Nauvoo is enormous. It's probably filled with some leftover propellant, some water, and a lot of gases. That means it's going to slosh as it moves, so the dynamics of the Nauvoo will change over time. Each robot needs to fire its engines at precisely the right time, for precisely the right duration, in order to get the Nauvoo going where it needs to go. They need to coordinate that firing, while also learning in real time how the Nauvoo actually responds to the thrust.
The reason I find all of this so impressive in the show is that we can almost do all of it. We're actively getting closer to being able to do this, but every part of it is still outside our grasp.
When I see the med-tech that they have, my brain just fills it in as magic and moves on. I don't know enough about biology or medicine to know how likely it is that we can cure cancer or hypoxic brain injuries using some arm-cuff. It's cool, but it's not inspiring since it feels like I can't have it.
The space robotics in the Expanse is both cool and inspiring. We can get there, and it's going to be amazing.