Like everyone else remotely related to the astronomical world, this past winter’s holiday season (basically Thanksgiving through New Years Eve celebrations) was filled with people wanting to know about our solar system’s interstellar visitor. I was invited to give an update on NuSTAR observations of the comet at a recent NASA coordination meeting. Most of that information is in the public domain, so the meeting was mostly about what we can learn from different observers at different places.

Read more: Let’s talk about 3I/ATLAS

It’s not aliens.

Sorry. Just addressing this off the top. If you clicked here hoping for a hot scoop about how all of the astronomers are getting paid off to keep silent about interstellar visitors, you’re going to be disappointed. This seems to be an ongoing trope every time something remotely interesting is happening in the sky that people wonder “could it finally be aliens?!?!” and I end up at a BBQ or out for drinks with friends. I get a side eye that says, “You know you can tell me, right?”. This has accelerated (dramatically) with AI-generated images and videos on social media claiming “NASA presents new results of alien spacecraft.” Half of these things look like renders from Star Trek (or Babylon V if that had been made today and not in the mid 90s). The pinned image above is from the Hubble Space Telescope showing just about the clearest picture that it’s possible to take from Earth of the comet. If you see anything less blurry than this, it’s fake.

One reason this reason’s furor over the comet got a little out of control was that the experts who know about this stuff had to keep silent. The government shutdown meant that all NASA communications had to completely cease, right as the comet was going through perihelion (it’s closest passage to the Sun). And, of course, right over the holiday break where people had lots of extra time scrolling their phones. In a vacuum of experts, the snake oil salesman flourishes (or, in this case, the AI-generated flyover movie generator flourishes for clicks). And, because people are used to the experts going out there and debunking said charlatan (see: whackamole), when that doesn’t happen people assume it’s because there was some thread of truth in what the charlatan was saying.

I’m not going to talk about Avi Loeb here (much). He’s actually doing a reasonable job of accumulating all of the available real scientific results from the various missions. And he’s figured out that by being provocative you can drive eyeballs to your site. I don’t necessarily agree with the way that he does things, but it doesn’t seem to be doing too much harm other than as a source for “Here’s this once scientist who says it’s aliens” headlines (when in reality his papers usually say “One plausible explanation is that it could be aliens”).

Why is 3I/ATLAS exciting (other than the interstellar part)

Up front, I’ll say that I’m not really a comet guy. So I’m learning as much about comets as most of you. Before we get into the the what of the comet itself is exciting, lets talk about it’s somewhat unique track through the solar system.

The orbital path of 3I/ATLAS (white) compared with the planets. Image credit NASA/JPL-Caltech from the NASA 3I/ATLAS FAQ Page.

In the solar system, all of the planets (except Neptune and Pluto) are almost exactly aligned in a single plane referred to as “the ecliptic”. Astronomers and people who study planet formation use this fact (and the wealth of information from exoplanets) to figure out that planets and the Sun all formed together out of a collapsing dust cloud. As that cloud congealed into the Sun and its planets, the fact that the cloud was rotating means that there’s a “preferred” plane for things to drift to. My mental analogy (which isn’t quite right, but it gets the point across) for this is like spinning a pizza dough; it starts off as a ball, but when you spin it it flattens out into a the flat crust.

Okay, so all of the planets are in a plane. So what?

It turns out that 3I/ATLAS is passing through our solar system almost exactly in the plane. The image above showing the orbital path is basically a top-down, 2d view because that’s all you need to know; the comet’s orbital path is only inclined to the ecliptic by 5-degrees. This means that it provided a unique opportunity for instruments on missions headed out to the outer planets and Mars to study the comet as it passed by.

The NASA web page on 3I/ATLAS has a fantastic summary of all of the observations. Often these are scientists being clever with figuring out how to use their instruments in new configurations to study the comet. You’ve got images from Europa Clipper (heading out to the Jupiter to study its icy moon) turning its instruments on early during its “cruise” phase and taking images of the comet to a fixed camera on the Perseverance Mars Rover catching a smudge that’s the first image of a comet from the surface of another planet.

What’d we learn?

In short: lots. A simple ADS query with 3I/ATLAS in the abstract results in over 30 published papers with dozens more already in the publishing pipleine. By studying the cometary tail, we can look at the composition of the comet. Given that this comet is interstellar, it gives us an unique view of the composition of other planetary systems (presumably this comet was formed around some other star then ejected from its own solar system until it found its way to ours). We use comets from our own solar system to study how ices (both water ice and “dry ice” that we know and love on the Earth) were formed in the early solar system. Using the comet to look at how much of the different kinds of ice are in the comet (as well as other elements) can give us a unique view into the formation of solar systems beyond our own. Unfortunately, just sending a space probe to look at a neighboring star is going to be the realm of Star Trek for the foreseeable future, so using interstellar travelers like 3I/ATLAS to study our neighboring stellar systems is the best that we can do.