Comet ISON is now inside the orbit of Earth as it plunges headlong toward the sun for a fiery close encounter on Nov. 28th. Although the comet is not yet as bright as many forecasters predicted, the comet is putting on a good show for observatories around the solar system. NASA spacecraft and amateur astronomers alike are snapping crisp pictures of the comet's gossamer green atmosphere and filamentary double-tail.
Lowell Observatory astronomer Matthew Knight, a member of NASA's Comet ISON Observing Campaign, lays out some of the possibilities.
"I've grouped the possible outcomes into three scenarios, discussed in chronological order," says Knight. "It is important to note that no matter what happens, now that ISON has made it inside Earth's orbit, any or all of these scenarios are scientifically exciting. We're going to learn a lot no matter what."
Comet ISON is being observed by a tremendous variety of telescopes on Earth and beyond. If ISON does disintegrate, it would be the best-observed case of cometary disruption in history and would likely contribute vast new information about how comets die.
Assuming ISON survives the next few weeks intact, it faces an even more daunting challenge: making it around the Sun. At closest approach to the sun, the comet's equilibrium temperature will approach 5000 degrees Fahrenheit, hot enough to cause much of the dust and rock on ISON's surface to vaporize.
Destroyed comets can still be spectacular, though. Sungrazing Comet Lovejoy, for instance, passed within 100,000 miles of the sun's surface in December 2011. It disintegrated, forming a long tail of dust that wowed observers on Earth.
The best of all possible worlds would be if ISON broke up just a bit, say, into a few large pieces. This would throw out enough extra material to make the comet really bright from the ground, while giving astronomers pieces of a comet to study for months to come.
"I'm clearly rooting for #3," says Knight.
"Regardless of what happens, we're going to be thrilled," he predicts. "Astronomers are getting the chance to study a unique comet traveling straight from 4.5 billion years of deep freeze into a near miss with the solar furnace using the largest array of telescopes in history."