NASA captured new images of Comet ISON hurling toward the sun over five days (Nov. 20-25).
Comet ISON, a shopping-mall-sized chunk of dust and ice that makes its closest approach to the sun Thursday, is refusing to show its hand. Researchers know it will skim roughly 1 million miles from the fiery solar surface around 1:30 p.m. ET, but what will happen next is the subject of feverish scientific speculation.
If the comet is still intact for its close encounter, there's a good chance it will become visible to the naked eye, perhaps as a glowing arc across the pre-dawn sky. On the other hand, if ISON disintegrates before its embrace with the sun, most of us on Earth will see ... nothing.
"It could very well turn out to be really awesome," says astronomer Yan Fernandez of the University of Central Florida. "Or it could all turn out to be a big disappointment."
As of Wednesday morning, the signs from the sky were encouraging. New images from one of NASA's solar telescopes show the comet is getting brighter, exactly as it should if it's still a comet rather than a lifeless rock pile. And a telescope in Spain detected signs of life in ISON Wednesday.
But whether the show will be ho-hum or spectacular probably won't be known until three or four days after Thanksgiving, says theoretical astronomer Kevin Walsh of the Southwest Research Institute. Even then, the forecast could be only "a ballpark," he says. "If there's anything about comets that we know for sure, it's that they always surprise us."
ISON has been surprising the best comet-watchers in the world for a long time now. Discovered in 2012 by two Russian amateur astronomers, it came our way from the Oort Cloud, a distant cometary parking lot halfway to the next star. On its trip to the sun, ISON has showed signs of death, revived, showed more signs of death and generally behaved in a way to confuse and exasperate astronomers watching it. Now wary scientists refuse to predict the erratic comet's fate.
"We won't know if #ISON survives until it actually does, or vaporizes," ISON observer Karl Battams of the Naval Research Laboratory tweeted Wednesday. "There's zero way of knowing."
Even if the comet falls apart before reaching the sun, it could still be visible from the backyard if it splits into a few big pieces, Walsh says. And if it falls apart just after it starts swinging away from the sun, its demise could create "a beautiful paintbrush swath on the sky," ISON observer Carey Lisse of the Applied Physics Laboratory said at a NASA briefing Tuesday.
But the best bet for a memorable display would be for the comet to stay in one piece, in defiance of the scorching temperatures and dangerous gravitational forces exerted by the sun. In astronomers' dreams, an intact comet could form a bright, curving streak that would span a large part of the sky. Any signs of ISON would be visible in the first few weeks of December just before sunset or just before sunrise, Lisse said. But no one should be surprised if there's nothing to see at all.
"In French ... there is a saying that you should not make any plans (based) on a comet," says ISON-watcher Emmanuel Jehin of the University of Liege in Belgium. "This is the trouble with these great comets. You really never know."