(Every day until December 25, I’m posting a science-related image or video and description.)

Day 12

Perhaps no transient event in astronomy is as evocative as a comet. They appear rarely enough to be significant occasions, and when they pass close to Earth can stretch across a significant amount of the sky. To me at least, it seems that something that big and dramatic should make a sound, yet comets are silent.[1] I remember fondly the big comets of 1996 and 1997: Hale-Bopp and Hyakutake, both of which were very impressive sights.

So many people were looking forward to the possible appearance of Comet ISON in the skies this winter. (As its name suggests, astronomers at the International Scientific Optical Network [ISON] discovered it, first observing the comet in the fall of 2012.) Many news sources hyped it as the “comet of the century”, which was a risky promise: based on its trajectory, astronomers knew it was going to pass very close to the Sun. Such sungrazer comets are pretty common, with some surviving the passage and others either melting or fragmenting under the Sun’s strong tidal force.

As it turned out, Comet ISON seemed to have suffered the fragmentation fate. While some pieces of the comet survived perihelion (the closest approach to the Sun), it faded quickly and is now invisible. Both the Hubble Space Telescope and the Arecibo radio observatory in Puerto Rico have planned searches for any remains over the next week, but by most people’s standards, ISON is a comet no more.

Comets are chunks of ice and rock, with a bunch of interesting chemicals in the mix. “Ice” in this case means both water ice and other “volatiles”, like methane, ammonia, and carbon dioxide, that are gasses on Earth, but frozen solid in a comet. When the comet falls toward the Sun, ice and dust particles stream off the nucleus due to warming and bombardment by solar wind particles. The chemicals that get a lot of attention are amino acids — long molecular chains made of carbon, oxygen, and other atoms — because certain of them are part of life’s chemistry; early comet impacts on Earth might have helped kickstart life.

Though the Sun ate ISON, I don’t recommend that you do the same. In photos of the comet, you can see a lovely blue-green tint, which is likely due in part to cyanogen. Tasty, but deadly.

Notes

1. Whatever science fiction movies have taught us, most space is silent, since the density of atoms is too low to carry the pressure waves that constitute sound.