Moment of truth on Thanksgiving: Will icy comet survive close encounter with sun?

By Marcia Dunn

Associated Press

Published: Sunday, Nov. 24 2013 12:27 p.m. MST

"Whatever happens, it's bound to be interesting. The quip from my colleagues is, 'Comets are like cats: They have tails and do whatever they want.'"

Besides ISON, NASA is spying on Comet Siding Spring, another Oort cloud comet discovered in January by the Australian observatory of the same name. Siding Spring will pass within tens of thousands of miles of Mars next October, so close that scientists believe the coma of the comet — its thin but expansive atmosphere — will envelop the red planet.

"It will be blanketed in water and dust and meteorites. It moves like 50 kilometers per second, blazing through the environment," said Jim Green, director of NASA's planetary science division. That's more than 110,000 mph, so the comet will be gone from Mars "pretty quick."

Siding Spring-type events have happened before, Green noted. "We're just lucky in our lifetime" to have the right spacecraft in the right place to observe the spectacle.

The same applies to ISON.

Add small sounding rockets to the list of paparazzi chasing the comet; NASA fired up one from New Mexico on Wednesday with an ultraviolet telescope that reached 172 miles high before descending by parachute. Consider all the ground observatories peering at the comet, as well as countless amateur astronomers and astrophotographers, and ISON has become the belle of the cosmic ball.

"Comets evolve from the time they start brightening until they go all the way around the sun, and go back out," Green said. "By having and leveraging these assets, it really gives us that view — that unique view — that we couldn't get otherwise."

Some sky gazers speculated early on that ISON might become the comet of the century because of its brightness, although expectations have dimmed over the past year.

Scientists expect to know ISON's fate fairly quickly. At least three spacecraft will be aiming that way in real time.

If ISON survives, "it's going to fly right over the Northern Hemisphere," Green said with clear excitement in his voice. It should be visible with the naked eye for 30 days.

"So it's really a holiday comet. You ought to be able to see it well past Christmas," Green said. "But it's got to survive it, that's the only thing about that."

Online: NASA: http://www.nasa.gov/ison

ISON Observing Campaign: http://www.isoncampaign.org/

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