After more than a million years of travel to reach the inner solar system, the comet ISON is set to brush past the sun on Thanksgiving, generating what astronomers and ordinary earthlings alike hope will be a visually stunning and scientifically revealing show.
Unlike other comets humans have observed, ISON (named for the International Scientific Optical Network project that discovered it) has never traveled around the sun before. The ice and dust that make up ISON are pristine, never having been affected by solar heat. Now, as the comet heats up near the sun, some never-before-seen chemistry is expected to shed light on the role of comets in the early solar system. It’s thought that comets crashing into Earth may have seeded the planet with water and prebiotic organic molecules, from which life began.
Comets are the most ancient objects in the universe, created some 4.5 billion years ago from rocky detritus left over from the formation of the planets. Billions of comets now reside in the Oort cloud—a sphere of material just outside our solar system—and on occasion gravitational forces pull comets into trajectories that bring them into the solar system.
“With ISON, we can watch how the comet falls apart—the reverse of how it got put together,” said Carey Lisse, an astronomer at Johns Hopkins Applied Physics Laboratory, at a press conference on Nov. 26.
The sun’s heat will vaporize rocky pieces embedded in the comet, unleashing swaths of magnesium, iron, calcium, aluminum, and potassium ions. “We will see spectral emission lines that we normally don’t see at all from other comets,” Lisse said.
ISON has been drawing closer to the sun, and astronomers have already noted its unusual, striking green tail, which is produced by unstable two- and three-carbon molecular species formed by light-driven reactions of water, methane, formaldehyde, and other simple organics. Most comets are richer in carbon monoxide than ISON is, and as a result they have a bluish cast produced when CO is ionized to CO+.
Astronomers spotted ISON a year ago, giving them an unprecedented amount of time to prepare for the solar flyby. Telescopes are trained on the object. Plus, scientists had time to launch a spacecraft, the Far-Ultraviolet Off Rowland-Circle Telescope for Imaging & Spectroscopy (FORTIS), for the sole purpose of studying ISON’s chemistry up close as it nears the sun—for a mere six minutes.
“This is perhaps the most observed comet ever,” noted Jim Green, director of NASA’s Planetary Science Division.