SOHO composite showing the movement—and demise—of the comet C/2012 S1 (ISON) as it rounded the Sun in late 2013. Credit: ESA/NASA

Dec 17, 2014 Meteors, asteroids, and comets. Oh my!

Sen—My last Sen blog dealt with an asteroid that masquerades as a comet: It has a long tail, possibly from dust ejected due to is rapid rotation. Coincidentally, a few days ago the annual Geminid meteor shower reached its peak. This is the only shower known to come from an asteroid! Usually the culprits are comets, which slowly lose material as the Sun warms the ice inside them, turning it into gas, which then releases the dust and rock with it.

The parent of the Geminids is the asteroid 3200 Phaethon, however, which dives inside the orbit of Mercury every time it approaches the Sun. It gets hot enough that some of its material vaporizes, leaving the surface of the asteroid and forming a debris trail. The meteors occur as we plow through that stream.

I watched the Geminids over the weekend, standing in the cold Colorado night and gaping as a bright flaring streak zipped across the sky every minute or so. I did a fun calculation… An observer on the ground can see about 100 meteors per hour during the shower. Knowing how much of the sky we can see at one time (about 4 million km2 at the top of the atmosphere), and extrapolating over the whole planet (~500 million km2), I get a very rough estimate that over the course of a day about 250,000 meteors burn up in our atmosphere.

That’s a lot! But they’re tiny, most smaller than a grain of sand. Phaethon is about 5 km across, so it can spew out debris like this for a long, long time. The Geminids aren’t going to disappear any time soon.

But it reminded me that even things we think are forever really aren’t. Someday, Phaethon will run out of material, and both it and the Geminids will evaporate (literally). We know this to be true due to simple math, but also because we’ve seen it.

Comets tend to lose their material faster than asteroids. Ice vaporizes at much lower temperatures than rock, so they can be much farther than the Sun and still blow out debris. That’s why we associate tails with comets far more than asteroids.

Comets, composed of rock and dust embedded in ice, can be very fragile. Sometimes, even comets coming in from very deep space for the first time can lose a lot of material as they approach the Sun, enough so that it’s catastrophic.

Such was the case just over a year ago, when comet C/2012 S1 (ISON) dove deep into the Solar System. It originated so far out past Neptune that’s it’s not clear just how distant its orbit took it; it was almost certainly part of the Oort cloud, the distant repository of perhaps trillions of such icy bodies tens of billions of kilometers from the Sun. It was out there for billions of years before it finally started to fall inward, downward, toward the Sun.

It was what’s called a Sun-diver, screaming in and practically skimming the Sun’s surface. At its closest it was an incredible 1.1 million km from the Sun’s surface! The heat pouring out from our star was, apparently, too much for the friable comet: It disintegrated. It must have lost too much of its ice to hold itself together, and at some point as it rounded the Sun it simply fell apart.

As it happened we weren’t sure exactly what we were seeing, but it became clear in the hours after the event. The picture at the top of this article tells the story pretty well. It was taken by the Solar and Heliospheric Observatory in space, which uses a small disk to block the fierce light of the Sun so fainter solar material can be seen (the Sun’s size is shown using the small white circle). You can also see stars and, of course, the comet.

The picture is a composite of several images, showing ISON as it approached the Sun—the long streak at the bottom—and then rounding the star over the next few hours. As you can see, it gets increasingly distended and fuzzy (the bright spot in the comet at the top is likely a background star or subatomic particle that hit the detector, and is not part of the comet). It basically just fell apart.

It’s a not-so-gentle reminder that yes, lifetimes are long when dealing with astronomical objects, but “long” doesn’t mean “forever”.