Sorry, but interstellar visitor 3I/ATLAS really is a comet, not aliens

The interstellar object 3I/ATLAS is, once again, doing something strange. This interloper from another star is currently concealed from view behind the sun, leading some people to wonder what it has to hide. There can only be one explanation, they whisper: it’s an alien spacecraft.

Except, of course, this is absolute rubbish. It makes no sense whatsoever for a spacecraft to obscure itself for just a few days, when it was plainly visible before and will be again. If this really is an alien ship hoping to remain incognito, either the extraterrestrials piloting it are remarkably stupid or they think we are.

How can I be so sure? Well, to paraphrase something an astronomer told me recently: if this really is a spaceship, it’s done a bloody good job at camouflaging itself as a comet. That is what makes the clickbait discourse about 3I/ATLAS (named after the eagle-eyed planetary-defence apparatus that spied it in July) both frustrating and unintentionally hilarious. It’s so patently, screamingly obvious that it’s a comet.

Let’s tick off the reasons why. It has a coma, an envelope of vaporised ices, surrounding it. It has a glittering tail. It is moving on a trajectory that can be best explained if it is an icy projectile entering the solar system from elsewhere. Oh, and the reason why it is “hiding” behind the sun? That’s because it has just reached perihelion, the closest point to our home star on its tour of our planetary neighbourhood. All spacefaring voyagers – from planets and comets to asteroids – have perihelions; 3I/ATLAS just happens to be nearest to the sun while it is behind it, from Earth’s point of view.

Sure, there are some odd things about 3I/ATLAS that are getting astronomers excited. For one, it has a lot more carbon dioxide ice in it than water ice. But that still makes it a comet, not a space probe. It also has both nickel and iron in it, which – wait a minute, those are metals! Spaceships are made of metals! Does that mean 3I/ATLAS actually is a spaceship after all? No – we see metals inside the rocky-icy mudball cores of comets all the time. In fact, our own solar system’s comets come in a variety of flavors, and some of them are quite weird.

That 3I/ATLAS at one point had a dusty tail pointing towards the sun, rather than away from it, was also a little unusual. Some suggested that it wasn’t a tail at all, but rather a thruster plume on a spaceship trying to decelerate. The problem is that 3I/ATLAS’s trajectory is consistent with an interstellar comet, not someone slamming on the brakes of their interstellar spacecraft. Its peculiar tail direction was due to the type of icy-dusty particles being blasted off its surface. This ephemera is usually pushed away from the sun by solar radiation pressure, but some of this cometary dandruff was so heavy that it couldn’t be pushed backwards and instead tumbled towards the sun.

3I/ATLAS is only the third interstellar object ever observed, so you would expect to find some anomalous features when perusing such a small population. The first known interstellar object, ‘Oumuamua, was far stranger: it was probably cigar-shaped and it rapidly accelerated as it left the solar system. Although bizarre, it was nevertheless perfectly explicable as a flamboyant comet-like object. Both 2I/Borisov and 3I/ATLAS, the next two interstellar objects, also have their share of curious features. But suggesting they aren’t comets, but spacecraft, would be like referring to the ice cubes in your freezer as pineapples. You are welcome to do it, but you are going to need to provide a lot more evidence if you want to convince me.

Many of us, myself included, anxiously yearn for the day we discover that we aren’t alone in the universe, so I don’t find it surprising that people have leapt on the (unfounded) idea that 3I/ATLAS could be a sign this day has finally arrived. But in an era infested by misinformation, to suggest this comet might be aliens, in the face of all evidence, isn’t just silly – it’s deeply irresponsible.