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This is Part 2 of a series on interstellar comets.
Read Part1. Once you start listing the properties of 3I/ATLAS, it becomes clear pretty quickly that this thing is distinctly different from any other comet we've ever seen.
Here's just a small taste. Its gas plume contains way more nickel than we've ever detected in other comets.
Metals like nickel tend to sink toward the center of a solar system during formation — they're heavy, they like to be close to the action — so finding a comet drenched in the stuff is, well, odd.
It also doesn't have a lot of water, which is a problem if you're a comet.
Comets are supposed to be dirty snowballs.
Water is kind of their whole deal. As it got closer to the Sun, it got brighter — which makes sense, because it was warming up and reflecting more sunlight.
But it brightened much more quickly than other comets do.
The light curve was steeper than expected, like it had a hair trigger for solar heating. For a long time, it didn't have a coma or a tail, but it eventually got them both.
It just took a while longer than anyone predicted.
And then, on its way out, it started accelerating.
Instead of being drawn toward the Sun by gravity like a well-behaved object, it looked like it was being pushed away from the Sun.
(More on that later.) I'm going to be honest.
That's a lot. But here's the thing about comets.
They're a lot like cats: they have tails, and they do what they want.
And just like cats, not all of them even have tails. Take any comet you want — even something famous like Halley's Comet — and once you start writing down enough of its properties, you'll spot all the ways it's completely unlike any other comet in the entire solar system.
Halley's has a retrograde orbit, meaning it goes around the Sun backwards compared to the planets.
Its nucleus is darker than coal.