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COMETS_2

ISS/NASA

Comet Lovejoy as seen from the International Space Station


To be spectacular, or not to be spectacular

by Meghan Leach
Nov 20, 2012


COMETS_1

SOHO/ESA & NASA

The Solar and Heliospheric Observatory's camera photographs a sun-grazing comet.

Comets are fickle creatures. But plan ahead anyway. If the recently discovered Comet ISON cooperates for Thanksgiving 2013, we might see one of the most spectacular comets ever.

“They’re generally called sun-grazing comets. They can be visible even in daylight,"  said Michael A’Hearn, an astronomer at the University of Maryland. "Predicting how bright this one will get is little more than a guess right now, but simple predictions say that it should reach naked-eye visibility."

It should be visible from the Northwern Hemisphere in November 2013 and again in January 2014. You may get lucky during December as well.

“Around next Thanksgiving and December, we’ll have a comet that will be extremely close to the sun, but you can’t really predict with any confidence how bright it’ll be because comets vary greatly in composition,” said Michal Drahus, an astronomer at the California Institute of Technology. 

It could be as bright as Venus or the full moon, he said. "It could be one of the most spectacular comets ever seen. ”

If you miss this one, it won’t be back in your lifetime. Comets are actually quite common, though.

“In any given year, you can see many comets with a pair of binoculars or a small telescope,” said David Jewitt, an astronomer at UCLA. “But bright, naked-eye comets are relatively rare, so every few years we get a comet that a person on the street would say is spectacular. Sometimes we get comets that are bright enough that people who live in cities with lots of streetlights can still see them.”

Even their composition seems very common.

“Comets mostly contain water ice,” Drahus said. “They also have some more volatile ices, like carbon dioxide and carbon monoxide. And some comets have large areas of their surfaces covered by ice, but many of them have ice under a shield or crust and only a small part of their surfaces are exposed ice.”

“We do know that in previous sun-grazing comets, that you have not just ices vaporizing, but you have rocks vaporizing because they get extremely hot, and you see emission lines in these comets of metals and silicon—all the things that go into making a typical rock on Earth,” A’Hearn said.

But just because they’re common doesn’t mean we completely understand them.

"Right now, all we know for sure is that they must have formed in the early epoch of the solar system,” Drahus said. “But we're not sure where exactly they formed or what this process looked like.  One scenario says that a particular comet completely formed in one place and then migrated throughout the solar system. 

The other scenario is that comets first formed as smaller bodies called cometesimals, and these "cometesimals," while migrating through the solar system, stuck together. There's also a suggestion that some comets might have actually formed around other stars in the sun's birth cluster."

Studying comets gives us information about the early solar system.

“The real reason for studying comets is because of the unique record they retain of conditions in the outer solar system,” A’Hearn said. “Because the cometary nuclei are small, any internal heat source gets dissipated very easily, unlike the Earth where radioactive decay keeps the entire core of the Earth very hot and much of it is molten. The cometary nuclei remain cold. This means that they retain the molecules that were present in the early protoplanetary disk—the disk that all the planets formed from.”

“The really interesting thing about comets is that they haven’t changed much--at least in terms of their composition--since they formed,” Jewitt said. “In the beginning, they trapped all these different ices, and they formed at low temperatures, and they’ve been kept at low temperatures ever since. We know that because if they had been heated up substantially, all the ice would have vaporized. The significance is that we can study and then learn things about the very young solar system.”

Comets have always had special meaning to humans.

“Comets are very beautiful, and so they were attractive to ancient people centuries ago,” Drahus said. “They were often considered as a body that announced a disaster or an important message. They intrigued people a lot because they can be very bright and spectacular and because they can appear unexpectedly. You have a global panic because they think the end of the world is coming.”

Comet ISON isn’t the only bright comet coming in 2013. Look out for Comet PanSTARRS too.

"First of all, we have a starter comet, almost like an appetizer to the main one in December,” Drahus said. “Comet PanSTARRS will be closest to the sun in mid-March 2013. It should be visible to the naked eye at dusk and dawn around Easter in the Northern Hemisphere."