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So, 'Oumuamua is likely a comet after all
It was thought to be a comet, then an asteroid — though never seriously a spaceship — and now it's back to being a comet again.
'Oumuamua* has certainly been giving astronomers quite a ride. And that's really saying something for an object that's screaming away from our solar system at nearly 100,000 kilometers per hour, and originally came from deep interstellar space.
The object was first discovered in images taken using the Pan-STARRS observatory on October 19, 2017, and it was quickly determined to be very strange indeed: The shape of its orbit indicated it was not orbiting the Sun like a comet or asteroid, but instead came from another star! It was the first interstellar object ever seen to be passing through our solar system.
It was expected that such an object would be a comet, not an asteroid, because comets — large chunks of rock and ice — form in the outer parts of solar systems, and are easier to eject by giant planets than asteroids, which tend to orbit closer to their parent star.
But the object (named 'Oumuamua, meaning "first scout," appropriately enough for an alien object) still had more surprises for us. Normally, when comets get near the Sun, the ice on or just under the surface warms and turns into a gas (this process is called sublimation). This blows away from the comet along with fine-grained dust, forming a fuzzy head and tail.
Even though it was close enough to the Sun to do this, 'Oumuamua didn't show any activity at all, even in very deep images that would have shown it had it been there. For this reason astronomers then decided it must be an asteroid, not a comet. Asteroids generally have little or no ice, and wouldn't show activity.
But now a new paper has come out that changes things again. Extremely deep images using a variety of observatories including Hubble, the Very Large Telescope, and the Canada France Hawaii Telescopes tracked 'Oumuamua up to January 2018, when it was at a distance of 435 million kilometers from the Sun (well past the orbit of Mars). Even though it faded to a magnitude of about 27 (the faintest star you can see with your naked eye is 250 million times brighter than that), they were able to take these observations, combine them with ones taken earlier, and calculate a very precise orbital trajectory for the object.
What they found is fascinating: As its distance increases from the Sun, it's slowing down. The thing is, it's not slowing as much as expected. If you include the gravity from the Sun, the major planets, Pluto, the largest asteroids, and even very small effects due to relativity, 'Oumuamua is still moving too quickly. In other words, something other than gravity is affecting it.
That's common with comets: As they warm and emit gas, that expulsion acts like a very weak rocket motor, giving the comet thrust. We call this a non-gravitational force. And that's exactly what it looks like is happening with 'Oumuamua. The Sun is weaker out there, but it's still enough to warm the interstellar visitor, trigger some ice to turn into a gas, and give it a bit of a boost.
The force is very small, but clear in the data. Gravity alone cannot account for the motion, and the force appears to be mostly along the direction away from the Sun, as you'd expect from warming gas. They also calculated how much material is being sublimated from 'Oumuamua to create this thrust (assuming it's coming from water), and it's very low, only a couple of kilograms per second. That explains why it wasn't detected; that's far too little to be seen from Earth.
So it looks like we're back to where we started: It really does look like a comet from another star. Nifty.
I'll note that some asteroids do contain ice, but at that point you're arguing over semantics more than physics; the dividing line between an icy asteroid and an ice-deprived (or so-called dead) comet is fuzzy.
I'll also note that when the distinction for 'Oumuamua was being determined, I wrote that its lack of activity might be due to the fact that it spent a long time in deep space, where it was exposed to interstellar cosmic rays. These little subatomic particles zipping around could have hit the object over the millennia, creating a hardened shell around it. That still may be the case then. The shell might have cracks in it that allow a teeny bit of gas to escape, but not very much.
'Oumuamua was discovered when it was already on its way out of the solar system, so we didn't have very long to observe it before it grew too faint. But the fun thing about an object being the first of its kind ever seen is that there will be a second, and a third … Eventually we'll get lucky and see one early enough that we can get more observations of it, and maybe even get spectra and more so that we can determine what it's made of. Estimates are that there is always an object like this, something that came from another star, somewhere in the inner solar system! They're just too faint to find. Yet.
We'll find more. And then we'll learn even more about our Universe, which is the whole point.
*The word is Hawaiian, and is pronounced "[glottal stop, like a tiny cough] Oh-oo-moo-ah-moo-ah".