Comet ISON isn't brightening up as fast as expected, but it isn't breaking apart, either. A new study compares Comet ISON to two historical comets: one that survived its brush past the sun, and one that disintegrated during approach.
Comet ISON is proving to be a head-scratcher of a comet. When first discovered, it was unusually bright for a comet so far from the sun, prompting "Comet of the Century" rumors. When it failed to live up to its early promise, some wrote it off as a dud.
But ISON is still chugging along toward its rendezvous with the sun, scheduled for Thanksgiving Day (Nov. 28), and nobody knows what will happen.
A new study is looking to the past to predict the future. To evaluate Comet ISON's prospects, one expert with five decades of comet-watching experience is comparing ISON to two historical comets: Seki-Lines and Hoenig. Like ISON, these were "new" comets, freshly perturbed from the Oort Cloud and heading in for their first (and only) trip around the sun.
Based on his comparison, ISON probably won't give the dazzling show that was hyped a few months ago, says Zdenek Sekanina, the principal scientist with NASA's Earth and Space Sciences Division.
"The trends, at the moment, are certainly not in favor of a spectacular comet," he says, though he acknowledges that comet predictions are a tricky business.
ISON will not be the closest comet ever to graze the sun – in fact, thousands of comets have come closer – but it is the closest "new" comet ever measured. At its closest approach, ISON will be barely a million miles from the surface of the sun.
In 1962 Comet Seki-Lines, the current record holder for the closest "new" comet, got within 3 million miles of the sun, and it survived. In 2002, Comet Hoenig was over 70 million miles away when it disintegrated.
"These are similar comets, as far as their origin is concerned, yet they behaved very differently," says Sekanina.
Seki-Lines and Hoenig present "extreme cases," he explains, mapping out best-case and worst-case scenarios for ISON. Sekanina is comparing the comets by measuring how brightly they shine while approaching the sun. In 1962, Seki-Lines got slowly but steadily brighter, while in 2002 Hoenig brightened quickly, leveled off, and then disintegrated.