An ancient myth tells the story of Daedelus and Icarus, a father and sun who used wings made of wax and bird feathers to fly out of captivity. Daedelus flew cautiously, staying close to the ground, and made it to safety. Icarus, his reckless son, couldn't resist the temptation to fly higher and higher – until the sun's heat melted the wax holding his wings together, and he plunged to his death.
Comet ISON has some lessons to learn from Icarus. This is ISON's first trip around the sun, and like Icarus's first – and only – flight, its projected course comes dangerously close to the sun. According to current predictions, ISON is a "sungrazer," meaning it will pass within 750,000 miles of the sun on November 28, 2013, Thanksgiving Day. If it survives, it will have a gorgeously long tail streaming across the heavens throughout December, visible to the naked eye even in daylight – or it could burn up entirely and never be seen again.
This comet has been dubbed the "Comet of the Century," but some astronomers think it won't survive its trip past the sun.
When a comet gets too close to the sun, heat isn't the only problem. The sun's radiation not only boils off the ice and other volatiles, but it also can physically push on the comet, as can solar wind. Throw in gravitational stress, and it's no wonder many of these "sungrazing" comets don't survive their brush with the sun.
"As a very small comet gets really close to the gigantic sun, the sun is exerting gigantic tidal forces – exerting different forces on different sides of the comet – possibly ripping it apart," explains Max Mutchler, with the Space Telescope Science Institute (STSci) in Baltimore, Md., in a press conference. .
So if ISON is doomed like Icarus, why all the fuss?
"There's two basic reasons for the hype," said Dr. Mutchler. "It's a fresh comet, coming in from the Oort cloud, full of fresh volatiles that have never seen sunlight up close and personal like this. Also, it's a sungrazing comet ... not all comets get that close to the sun."
A comet's path is easy to map out, but other than that, comets are notoriously unpredictable.
Like all comets, ISON is made of bits of rock and volatile compounds like ice and dry ice. Those volatiles are even more vulnerable to the sun's heat than wax wings. "As the comet gets closer and closer to the sun," explains Bonnie Meinke, an STSci planetary scientist, "volatiles come off the nucleus itself, making the real structure of what we think of as a comet." As the frozen material sublimates into water vapor, carbon dioxide, and carbon monoxide, it frees tiny fragments of rock and dust to stream out behind the comet.
The released dust and rocks linger like breadcrumbs behind the comet, making a long track across the sky. "When Earth passes a comet's orbit, a lot of particles fall into our atmosphere, making a meteor shower," said Jian-Yang Li, a research scientist with the Planetary Science Institute.
Earth will cross through ISON's inbound path on January 17, 2014. "Maybe, just maybe, there will be a nice little meteor shower somewhere in the middle of January 2014, from the particles left behind when ISON flew through before," said Tony Darnell, creator and host of Deep Astronomy, during the press conference.
"ISON is a long-period comet," says Dr. Meinke. Long-period comets don't orbit on a regular loop, like Halley's Comet, but instead make a single curved trip to our solar system, swinging in from the Oort Cloud, through the solar system, and off into space. "The Oort Cloud is a reservoir," explains Alberto Conti, an STSci astrophysicist, containing "trillions of these objects just sitting out there, waiting to be disturbed gravitationally and then fall ... which I find mind-boggling."
Comet ISON is right now crossing the "frost line," says Dr. Li, the imaginary line between Earth and Mars where volatiles stop being stable in frozen form and start to sublimate away. "That's why we care about the frost line, as it relates to comets," says Meinke. "We get this beautiful tail on a comet, and the coma, and all that stuff starts to appear."
Now that ISON is crossing the frost line, its tail should start to get dramatically bigger and brighter. Some scientists have estimated that ISON could be as bright as the full moon, but that's "a very risky prediction to make," says Mutchler, an expert on the cameras of the Hubble Space Telescope.
The question on everyone's mind: Will ISON, unlike Icarus, emerge unscathed from its brush with the sun? That's a function of four things: the distance of perihelion (the moment when ISON is closest to the sun), composition, structure, and size. "How tightly or loosely assembled is it? We know a lot of comets are not tightly assembled. It may be barely hanging together," says Mutchler.
"Everybody cares about whether the comet's going to survive perihelion," says Li. He examined Hubble's first images of ISON, back in April, to try to measure the size of the nucleus. His team wasn't able to get a precise measurement, but they can say that it's no more than 4 km (2.5 miles) across. "That's the upper limit," he emphasizes. "It could also be much smaller."
But it's not (just) size that matters, it's how close it gets to the sun.
A comet that comes too close to the sun must break apart, like Icarus's wings. But if it's far enough away, it can survive to continue on its journey. Comet ISON will be traveling precisely at the boundary between too-close and just-far-enough-away, so even the experts are left waiting and watching to see what will happen on November 28. If it survives to put on a show for December and January, we'll all have something to be thankful for.