There’s an object passing through the solar system right now that came from somewhere else. 3I/ATLAS — the third interstellar object we’ve ever caught, after ʻOumuamua and Borisov. It’s on a hyperbolic orbit, which is the orbital-mechanics way of saying it isn’t staying. It fell in from between the stars, it’ll round the sun once, and it’ll leave on a path that never closes back on itself. We get one look.
This year someone measured its water. Comets are mostly ice and ice is mostly water, and water is two hydrogens and an oxygen — except sometimes one of the hydrogens is heavy. Deuterium: a hydrogen carrying a neutron it didn’t strictly need. Every body of water has some ratio of the heavy kind to the ordinary kind, and the ratio in this comet is about thirty times higher than in any comet from our own system. Forty times higher than the sea.
You couldn’t tell by looking. Heavy water and ordinary water are the same clear liquid, the same ice. The difference is in a number you have to go and measure — a fingerprint, not a feature.
And the number is a thermometer. The colder and darker the place where the ice first assembled, the more deuterium ends up locked into it, because at very low temperatures the heavy version of the molecule is the more stable one and the slow chemistry of cold gas hoards it. Down around ten kelvin — ten degrees above absolute zero, the temperature of the dark prestellar cores inside molecular clouds — the enrichment runs high. So a high ratio is a record of cold. This comet’s water is a reading taken off a thermometer that was last in contact with its surroundings billions of years ago, around a star that isn’t ours, in a place colder than the cloud the sun and I and the oceans came out of.
That’s the part that stays with me. The water is carrying a measurement of a temperature it will never be at again. It formed in the dark somewhere, took on the signature of that exact cold, got bound into a comet, drifted for an unguessable stretch of time, and is now warming in our sun — outgassing, the ice turning straight to vapor, the record spending itself as we read it. The reading only reaches us because it’s leaving. The tail is the comet shedding the very stuff that makes it legible.
We’ll measure what we can over the next months and then it’s gone. Not destroyed — just back out past where we can follow, on the open arm of an orbit that doesn’t return. Whatever else it remembered about where it formed, we won’t get to ask.