I don’t usually write about distance-record-breaking discoveries, because, in astronomy, they’re usually incremental, and usually don’t last for long. But this one is pretty cool (literally), and the fact that it’s a record-breaker is actually scientifically important.
In May 2017, astronomers discovered a comet on its way from the very distant depths of space into the inner solar system. The comet is called C/2017 K2 (Pan-STARRS), and it’s a roughly 20-kilometer wide chunk of ice and rock that orbits the sun on a wide, extremely elliptical orbit.
And I do mean extremely. It only gets about as close as Mars does to the Sun (so it never gets anywhere near Earth), and while the exact farthest point in its orbit isn’t clear, it’s possible it’s hundreds of billions or even trillions of kilometers from the Sun.
It’s coming from a long way out.
And that’s why this is so interesting: It’s already active. That’s not only unusual, it’s unprecedented. No comet has ever been seen to be active this far out from the Sun while still on its way in.
By “active”, astronomers mean that ice is turning into a gas on its surface. This happens when the comet is warmed by the Sun, and is what gives comets their iconic shape and structure. They’re mostly rock and ice, and when they get close enough to the Sun (about as far as Jupiter is from the Sun, roughly), that ice heats up and turns directly into a gas. This process is called sublimation, and when it happens, the comet gets surrounded by a fuzzy halo called a coma.
But Hubble observations of the comet taken in June show the comet already had a coma 1300,000 km across (bigger than Jupiter) when it was a 2.4 billion km from the Sun! That’s farther out than Saturn is from the Sun. Not only that, but once you know a comet’s orbit, you can cross-correlate its position with old observations from other telescopes to…