1I/’Oumuamua updates!

[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:

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).

The Discovery One is spinning when he Alexei Leonov comes to find it because it had angular momentum but no attitude control, so eventually found the lowest energy state, which was a spin about 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:



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:

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…

10 thoughts on “1I/’Oumuamua updates!

  1. jtw13 Post author

    Well, Fraser et al. work out the details and they find it will still tumble after billions of years. My understanding is that this is not all that surprising, given that there are lots of tumbling asteroids about this size or smaller in the Solar System that must have been doing that for billions of years, too.

    But you’re right that it must eventually damp out, the question is just how dissipative the system is.

  2. Larry

    Heating/cooling cycles are about it for energy, and what ever it’s picked up in various flybys.
    No outgassing to speak of? It seems fairly inert.
    My engineer bump is itchy. Even as slowly as this thing is tumbling, I can’t figure out why. It should have damped out, entropy and all that. (???????)

  3. Larry

    Should this thing even be Tumbling? Correct me if I’m wrong, but this sucker is likely billions of years old.
    Even in a vacuum, shouldn’t it have run down by now?

  4. jtw13 Post author

    Those are best estimates. For the tumbling model, the shape is uncertain but best estimate at the longest-to-shortest axis is 5:1. That said, this model assumes an ellipsoid, so it could have a more complex shape. This model also assumes uniform surface brightness, so if it has surface features that will complicate things.

    Tumbling is complex, and the data aren’t good enough yet to get the actual orientation of the angular momentum vector, but since it’s tumbling we should eventually see all of its surface.

  5. Josh P

    One thing I have been trying to understand is if the 6:1 or 10:1 ratio that people are reporting is a minimum ratio or our actual best guess at the ratio? Are people assuming that the minimum brightness is its smallest possible face and at its brightest we are seeing the largest face? If it was actually rotating in such a way that we we never saw it head on its smallest cross section, couldn’t the ratio be much larger? Do we actually know anything about its orientation of its rotation towards earth?

  6. James *Jason* Wentworth

    This idea of “getting a free ride around the solar system by hitching a ride on an asteroid,” while it doesn’t make sense (with one, admittedly highly-speculative possible exception–“harpooning” an asteroid with a highly-elastic line), has been a popular “folk space travel” notion for many years, but:

    One possible legitimate reason to have a probe “hitch a ride” on an interstellar asteroid (or comet) would be to obtain free radiation and dust impact shielding, by having such a probe “bury itself” on the surface of the asteroid or comet. Such a buried probe would also, even at a quite shallow depth, find it easier to maintain an even temperature (the subsurface of the Moon, even just a few feet below the surface, has an unvarying, quite cold temperature, despite the very high and very low temperatures that the surface experiences throughout each lunar day).

  7. Anthony Verow

    That’s an excellent point JTW 13. I naively thought of the concept like “tagging ” an animal with a radio probe but I think you are correct. It would take an enormous amount of energy to match speeds with such a visitor and that of and by itself would get any probe to another system or interstellar space just as quickly as hitching a ride.

  8. dsjinn

    With all do respect, Dr. Wright. But why on earth would an techno civilization choose to send a probe with a straight line and go past us as another civilization? I does not make sense..

  9. jtw13 Post author

    Yes, I’ve thought about how it might be interesting to hitch a ride on one of these things, but the fact is that it takes just as much energy as sending out your own probe in the direction you’re actually interested in, so it’s not clear what the advantage would be.

  10. Tony Verow MD

    This is great stuff! I learned a lot about tumbling vs rotation. It occurred to me (and probably many others) that such sporadic visitors to our Solar System might make excellent opportunities to somehow plant a probe on them and learn more about interstellar space. Obviously would not be feasible anytime soon but perhaps in the future. We already have the Pioneer and Voyager probes heading out of our Solar System but only with a finite amount of RTG power left. Just speculating!

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