How Would We Know if an Alien Asteroid Impacted the Moon?

This article was originally published in the Bad Astronomy Newsletter (November 16, 2023 Issue #645) by Phil Plait, an astronomer and science communicator, as well as a friend of Asteroid Day.
He writes his Bad Astronomy Newsletter three times a week at badastronomy.substack.com.

A few years ago a revolution in astronomy occurred: For the first time, an asteroid-like interstellar object was unambiguously seen moving through the solar system. Called 1I/‘Oumuamua, it was moving far too rapidly to be bound to the Sun, meaning it must have come from outside the solar system. That was a pretty big deal.

Then, not long after, a comet called 21/Borisov was discovered that was also interstellar in origin. It became clear that these things are zipping past us all the time, and we’ve only recently become good enough at scanning the skies to find them.

It’s hard to extrapolate from two examples, but using their sizes, motion, and dates of passage astronomers estimate that at any given moment there are likely thousands of them passing through the solar system.

What happens when one hits a planet or moon?

That’s an interesting question. The whole point here is that these things are typically moving faster than objects like comets and asteroids from our local neighborhood, sometimes by a couple of dozen kilometers per second. More speed equals more energy, so kilo for kilo an impact by one of these alien beasties is likely to carve out a bigger hole.

But how can you tell? Some astronomers tackled that question in a paper published in October 2023. The Moon, for example, has likely been hit by such interlopers over its 4.5-billion-year history, though only a handful of times (they estimate about 5 times). If this occurs, what would distinguish such a crater from the zillions and zillions of ones from impacts by locally sourced objects?

They have some ideas. For one, most (roughly 95%) craters on the Moon formed during a heavy bombardment that occurred in the solar system’s early days, so right away looking at younger craters narrows the search by a factor of about 20.

When an impact occurs, the rock under ground zero melts, and how much melts depends on a lot of different circumstances. But for craters less than a kilometer or so across you’d expect only 1% of the rock to melt by volume, so any smallish craters like that exhibiting high melt volumes would be suspicious. Quite a few like that have been identified previously, interestingly.

An unnamed 11.5 km wide crater on the Moon’s far side, seen at an oblique angle, with a clear if somewhat oddly shaped concentric ring. This is unlikely to be from an interstellar impact, but it does highlight that a lot of craters on the Moon are weird. Credit: NASA/GSFC/Arizona State University

However young craters with anomalously high melt can form for impactors that hit almost vertically, confusing the issue. The good news is that due to geometric reasons those tend to happen mostly near lunar poles, so only looking between latitudes of -70° to 70° narrows things down even more.

Taking all this into account, they estimate that a search can boost detectability by a factor of about 100 when keeping these ideas in mind. That’s still a lot of craters, though! They don’t actually do a search in this work, since their research was designed to be more along the lines of a thought experiment, just to see what can be done to help find these remnants of interstellar impacts. That’s perfectly fine; it’s like a call to others that this might be possible.

I doubt we’ll find a (not literal but aptly metaphorical) smoking gun, but at the very least this will get people thinking about the idea, and maybe figure out new and better ways to look. And who knows, perhaps a grad student will pick this up as a project and pore over lunar maps to find suspicious craters. I hope so. This sounds like fun, and if they find anything, well, that would be interesting, wouldn’t it?

News Roundup
Who can keep up with everything these days?

• I’m not a huge fan of the colonial-inspired idea of introducing new species to regions, because it generally leads to havoc. But under some circumstances… virus-caused dengue fever kills tens of thousands of people a year. The biggest vector of this is infected mosquitoes biting people. Scientists infected mosquitoes with a variant of the bacterium Wolbachia, which shortens the lives of the mosquitos and in principle kills older bugs (which have more viral load). They released mosquitoes in Colombia, and found a huge drop in dengue infections there. That’s amazing. Also, offspring of female mosquitoes so infected can also be infected, so the effect is self-perpetuating. Hopefully this will remain a good thing, and not cause unexpected consequences. I really, really hope.
• I’m happy to see that the aerospace company Sierra Space (located near my old stomping grounds near Denver, CO) has completed the construction of their Dream Chaser spacecraft, which is like a ten-meter-long mini-Space Shuttle. It launches atop a rocket, can deliver a payload to the space station, and then land like an airplane. This has been a long time coming — I wrote about tests on Dream Chaser almost exactly ten years ago! They hope to launch it on a ULA Vulcan rocket early next year.
• Astronomers using JWST have found the second and fourth most distant galaxy ever seen! The two galaxies have redshifts of z=12.393 and 13.079, which means the light we see left them 13.365 and 13.391 billion years ago. That’s less than 500 million years after the Big Bang (depending on what values you use for the expansion of the Universe, but these numbers should be pretty close). The redshifts were confirmed spectroscopically, so they’re pretty firm (link to research paper). Wow. Seeing galaxies this far away really helps astronomers understand what was going on with galaxy formation in the very early cosmos, which up until now has been essentially impossible to observe.