I read popular physics: Interstellar Interlopers

This is an entry in my series where I read physics articles in Scientific American, and get annoyed that the only good ones are about astronomy.

I skipped a month, because the September issue was a special 175-year anniversary edition. I actually liked that one, but the physics article was a review of how cosmology had changed in the past 175 years, and I don’t have much to say about that. I learned that Scientific American basically predates the scientific establishment as we know it, and it started out as a thing for like, inventors and hobbyists.

The October issue has a presidential endorsement, the first endorsement that the magazine has ever made in its long history. No points for guessing who they endorsed.

Anyway, the physics article for this month is “Interstellar Interlopers” (no paywall this time), about the first two interstellar objects ever observed in the Solar System. I had never heard of these before, but I guess they made news a while back, as reporters breathlessly speculated about aliens.

I was a bit surprised to learn that these two objects, discovered in 2017 and 2019, were the first of their kind to be observed. I guess the thing to realize is that these objects are very small compared to the solar system, and not particularly bright, so it was only recently that we’ve had the capability to observe them. Obviously the sample size is very small at this point, but they estimate that there are about 10,000 such objects within the orbit of Neptune, and 3 new ones a day. That we’ve only observed 2 of these many objects speaks to the difficulty of observation.

The first of the two objects is called Oumuamua, and there’s a picture of it at the top of the article. It’s important to know: this is an artist’s conception. When astronomers actually observed these objects, what they see is basically a point of light. We don’t actually know what they look like. But astronomers are experts in inferring as much as they can from points of light. Oumuamua is thought to have a long needle shape, because its brightness changes by a factor of 10 as it rotates.

The oddness of Oumuamua’s shape has invited speculation that it’s an alien rocket. And, in case that wasn’t enough for the pattern-matching brain, scientists have also found that its orbit is influenced by a small “rocketlike” force.

But before we get carried away, rocketlike forces could be caused by natural outgassing, or pressure from sunlight. Scientists don’t see any outgassing, but maybe the gas is invisible. Pressure from sunlight isn’t strong enough, but maybe Oumuamua is much less dense than they expect. In short, it’s a mystery, but not so mysterious that we need to resort to aliens

Also, if it’s an alien rocket ship, it’s kind of weird that it rotates every 8 hours and yet the rocketlike force is only in one direction.  And hard to imagine that there are 10,000 alien rocket ships in the solar system at any given moment.

The second object, Borisov, is much less mysterious, basically resembling a comet. Like most comets, it also has a small “rocketlike” force caused by visible outgassing. So it turns out not every interstellar object is a bizarre space needle.

It’s an interesting article, and I look forward to hearing about more of these observations approximately every 2 years. Or maybe the observations will accelerate until they stop being newsworthy.

As I mentioned at the start, I’m starting to notice that most of the physics articles in Scientific American are astronomy articles. And most of the non-astronomy articles are bad. I think fundamentally astronomy is just easier to explain than other fields of physics–at least on a surface level. Or maybe astronomers have just invested more in learning how to explain to popular audiences.


  1. Pierce R. Butler says

    … hard to imagine that there are 10,000 alien rocket ships in the solar system at any given moment.

    It’s only hard if you haven’t had much practice: read the Mutual UFO Network for your daily workout.

  2. PaulBC says

    Oumuamua was covered pretty heavily in the regular news media, so I’m surprised you missed it (if that’s what you’re saying). It was pretty clearly the first of its kind. I think it is interesting if it was only discovered because we’re better at seeing these things. So maybe there will be a lot more on the way.

    I am also reminded a little of Arthur C. Clarke’s Rendezvous with Rama, and I forget what year that was supposed to be set, but needless to say, there was no option of sending mission to intercept and study it.

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