[note: As I wrote in November, I don’t think ‘Oumuamua is an alien spacecraft. While other astronomers have made that suggestion, and while I’m happy to engage in such speculation in a SETI context, I think ‘Oumuamua is interesting in its own right as an asteroid and because of how it is getting us thinking about how to find alien probes in the Solar System.]
Three updates to the ‘Oumuamua story!
First, it appears to be tumbling:
Why does 1I/ʻOumuamua vary in brightness & colour on uneven timescales? Our model: it is tumbling, & has a redder spot@wtfastro, P. Pravec, @FitzsimmonsAlan, @pedrolacerda, @astrokiwi, @colinsnodgrass, @pseudotrabanthttps://t.co/IH6atFWiVB pic.twitter.com/ZBTkDDPSjb
— Michele Bannister (@astrokiwi) December 1, 2017
This explains a lot about the confusion over its shape and color. The data keep giving different answers because the object is spinning in a complicated way. To understand deeply, you need a quick primer on principal axes (skip to the slo-mo parts, especially the “unstable” axis around 2:00):
The key is that in space, things generally rotate in a very simple way, about the “principal axis” with the largest moment of inertia (smallest radius). This is because this is the axis for which a given angular momentum has the least energy, and over time objects will lose energy but not angular momentum. The Earth, for instance, is oblate, and rotates along the shortest axis it has.
But if you just start something spinning arbitrarily (or, say, you knock it around) or if you start it spinning with some motion along its intermediate axis, it will execute a much more complex motion (around 2:00 in the video above) called tumbling. It will do this in space until the changing distortions of the body from the changing centrifugal forces eventually cause the rotational energy to dissipate away as waste heat and it ends up a principal axis rotator again (that’s why the Discovery One in 2010:Odyssey Two is spinning that way, along its shortest axis).
So why is ‘Oumuamua tumbling? It’s unclear, but it may be related to its elongated shape: unlike typical Solar System “rubble pile” asteroids and icy comets, it seems to have more rigidity (apparently not uncommon in smaller Solar System objects), and so it dissipates its rotational energy more slowly—so slowly that it can tumble for a long time.
Second, I wrote a AAS Research Note correcting a small point made by Jean Schneider, who showed that ‘Oumuamua could not have been sent into it’s current orbit via gravitational slingshot with any known planet, or the hypothetical Planet Nine. I pointed out that in fact there is no way any Solar System object could have done it, hypothetical or not (I supect that this point is trivial to people that think about this for a living, but it is nontheless surprising to those of us who don’t). I think Alt Mars Crater put it best:
— Alt Mars Crater☄ (@Comet2013A1) December 7, 2017
Somehow I completely missed this paper on 'Oumuamua:https://t.co/5NexVK6zkI
They show that even a complex interaction involving binaries doesn't really work to kick #Oumuamua into its current orbit (https://t.co/jSJLzhooYO)
— Jason Wright (@Astro_Wright) December 15, 2017
Third, Breakthrough Listen is taking a look (listen?) to see if it is emitting radio waves as one might expect (?) if it is an alien probe:
Yuri Milner has directed Breakthrough Listen to check 'Oumuamua, the interstellar asteroid, for radio signals https://t.co/SmXqVrO19T
— Marina Koren (@marinakoren) December 11, 2017
This is neat! We should be thinking about what we will do if something that looks (more) like an alien craft comes through the Solar System. Now the Breakthrough Listen team has a protocol for tracking Solar System objects with Green Bank and analyzing the data they collect.
Such a discovery would imply that there are lots of these things in the Solar System at any given moment (even if they are deliberately targeting the Sun, they are hard to spot and we’ll miss most of them), and so lots of opportunities to study them.
Why would there be so many of them? Part of the argument that it is possible to settle the entire Galaxy is that exponential growth is possible, because the only limiting resource is the stars (and the material around them) themselves. Exponential growth can be achieved via Von Neumann probes: self-replicating spacecraft that go to a system, make lots more of themselves, and then go to more systems.
Now even if these have purposes that don’t involve coming near the Sun, you might expect some fraction to eventually go derelict (space is a harsh environment, and an optimal design will likely have a nonzero failure rate). Such derelict craft would, if they are not traveling so fast that they escape the Galaxy, eventually “thermalize” with the stars and end up drifting around like any other interstellar comet or asteroid.
In fact, since they (presumably) no longer have attitude control, one would expect that they would eventually begin to tumble, and if they are very rigid that tumbling might distinguish them from ordinary interstellar asteroids… and in fact, just because their propulsion is broken doesn’t mean that their radio transmitters would be broken…