328 — Anatomy of an Asteroid Break-Up: The Case of P/2013 R3

I just finished Leviathan Wakes, the space opera that has become the TV show The Expanse (which, to be transparent, I have not seen). So as I was browsing YouTube AS ONE DOES, I came across Scott Manley’s video on the physics of synthetic gravity via fast-spinning asteroids and decided to watch it (spinning asteroids is a core component of the Expanse universe).

As it turns out, the whole spinning-asteroid idea is extremely unfeasible: Asteroids are more like clumps of dirt that stuck together than solid rocks, and once you spin an asteroid fast enough to create gravity, it basically disintegrates. One of the reasons we know this — as Manley points out in the video — is the disintegration event(s!) of an asteroid observed in 2013. I took a look at the paper he cites — and there’s a JHU APL author on it.

I thought I was just watching a video, but it turns out I was finding my next paper.

P/2013 R3 is an asteroid that disintegrated into thirteen components over the course of five months in late 2013 to early 2014. The original asteroid was probably somewhere around 400 meters in radius.

So what caused it to break apart?

The Yarkovsky–O’Keefe–Radzievskii–Paddack effect, or YORP effect, is a phenomenon in which a low-drag astronomical body — such as an asteroid — picks up angular momentum due to the very small but meaningful contribution of asymmetrically radiated photons from an irregularly shaped body. It’s a bad analog, but my brain immediately jumps to Crookes radiometers — though those are powered by heat differentials rather than photon emission.

In addition to this slow accelerative force (which can spin an asteroid to death in less than one million years!), asteroids such as P/2013 R3 probably also have some acceleration applied due to the sublimation of frozen water. The authors propose that the energy released by 1 gram of water sublimating per second would be enough to double the YORP torque (or perhaps cancel it out, of course).

Alright so… How do we know it’s actually breaking apart? The observations over the course of five months show that the dust around these bodies is literally settling: Some of it probably recombined with the thirteen resultant bodies, and some of it probably wandered off Brownianly into nowhere. And each time a new body was spotted (because they didn’t all appear at once), dust clouds reappeared, clouding the view of the asteroid babies but also reinforcing the hypothesis that this asteroid was coming undone.

Anyway, the original asteroid had nowhere near enough spin to induce meaningful artificial gravity (and Manley’s video explains further why this is an awful plan for a space station). So if that was your plan to build a station, try again.