By Deborah Netburn
Los Angeles Times
Comet ISON's day of reckoning has arrived at last.
On Thanksgiving Day the icy comet is due to whip around the sun at more than 200 miles per second, the harrowing culmination of a journey that began about 5 1/2 million years ago in the farthest reaches of the solar system. ISON will experience temperatures of 5,000 degrees Fahrenheit as the sun's powerful gravity threatens to tear it apart.
Many thousands of Earthlings have been tracking the comet's journey for more than a year since it was spotted by Russian astronomers in the neighborhood of Saturn's orbit. So have nearly two dozen spacecraft, including NASA's Hubble and Spitzer telescopes.
For ardent sky watchers, it has been a reality show unlike any other. And now comes the cliffhanger: Will Comet ISON survive?
"We are all standing side-by-side as we witness a complete scientific mystery unfold before us," astrophysicist Karl Battams, NASA's primary Comet ISON blogger, wrote in a post this week. "We have absolutely no idea if ISON will survive past the Sun or not, and how it might look in our December night skies, if it ever gets that far."
Whatever happens, the space rock has been a gift to science. It is already the most-observed comet in NASA's history, and the data scientists have collected will help them study the sun and how our solar system came to be.
Carey Lisse, an astronomer at the Johns Hopkins University Applied Physics Laboratory, put it this way: "It has given us quite a ride."
PHOTOS: Comet ISON journeys through the solar system
Comet ISON made a grand first impression.
When it was discovered in September 2012, it was unusually bright for an object so far from the sun. That led some observers to declare it the "comet of the century."
It might have lived up to that billing if it had continued to brighten as it made its way into the inner solar system. But that didn't happen. Scientists believe its initial brightness was probably due to a burst of gas that converted directly from a frozen-solid state after the comet encountered the sun's warmth for the first time.
When ISON passed by Mars, instruments aboard the Mars Reconnaissance Orbiter measured its nucleus as about two-thirds of a mile across, relatively small by comet standards. The nucleus is made of ice and space dust as fine as the dust you might find at the bottom of a vacuum cleaner, and it's about as dense as a well-packed snowbank.
"Picture it like the dirtiest snowball you've ever seen," Lisse said.
The snowball is surrounded by a gas bubble about 250,000 miles wide, roughly the distance from Earth to the moon. In recent images that bubble is glowing an emerald green, a result of the sun's radiation hitting the carbon gas released from the comet. The comet's tail is white and stretches a few million miles.
But there's more to Comet ISON — named for the International Scientific Optical Network, a night-sky survey program managed by the Russian Academy of Sciences — than good looks and pop culture appeal. It's a time capsule from the days before the planets.
Its orbit indicates it came from the Oort cloud, a giant sphere of icy objects that lies at the very edge of the solar system. Out there, the sun's gravity barely holds sway. When a star passes by the cloud, which happens every few million or billion years, the comets are perturbed and some get bumped into different orbits.
Comets in the Oort cloud formed billions of years ago at the dawn of the solar system. Some of their cousins joined forces to become the gaseous planets Jupiter, Saturn and Neptune; others were expelled into interstellar space. The rest, including ISON, have remained frozen and unchanged for milleniums.
"Think of this comet as a relic, a dinosaur bone of the solar system," Lisse said. "It has been in deep freeze for 4.5 billion years, and now we are watching it bake and boil as it moves closer to the sun."
Over the last year, scientists have kept a close eye on ISON and its jets of gas. That its jets have released lots of carbon dioxide, diatomic carbon, tricarbon, ammonia, methane and nitrogen was not a surprise; these molecules are necessary for life as we know it, and scientists have long thought that they were brought to Earth by comets. But the fact that ISON has very little carbon monoxide was unexpected.
"We're learning that comets do vary," said Lisse. "They are not made in the same shop."