An End Foretold
Like every great comet, C/2012 S1 — better known to the world as Comet ISON — is dying. You could say the story of its demise began 14 months ago, when two observers near Kislovodsk, Russia, stumbled across a dim, fuzzy object while they scanned the sky near the constellations Cancer and Gemini. That fuzz was the outer layers of Comet ISON disintegrating and dispersing as it accelerated toward the warmth of the sun.
You could push further back and trace the comet’s downfall to a fateful event a few million years earlier. At the time, it was an inert chunk of ice, dust and frozen gases, floating nearly motionless in the outermost region of the solar system, a thousand times more distant than Pluto. Then some unknown disturbance, perhaps the nudge of a passing star, dislodged the comet from its stasis and sent it on a self-destructive sunward plunge.
Or you could say that Comet ISON’s end was foretold 4.56 billion years ago, when it and trillions of others like it formed during the birth of the solar system. Some of those comets collided with the infant planets; on Earth, they helped build the atmosphere and fill the oceans. But some of the comets were flung outward into distant cold storage. In that sense, the sunward journey of Comet ISON is a homecoming after a long exile.
Regardless of when the story begins, we know exactly when it will reach its climax: at 1:41 p.m. EST on Nov. 28, 2013, when the ice-packed Comet ISON reaches perihelion — the point closest to the sun — passing less than 750,000 miles (1.2 million kilometers) above the solar surface. There, ISON will roast at more than 2,000 degrees Celsius (hotter than 3,600 degrees Fahrenheit), boiling off layer after layer of its frozen surface. In the process, it will offer a firsthand look at the raw material that Earth and the other planets were built from when the solar system was formed.
Maybe some diminished portion of the comet will remain intact; maybe it will break apart and disperse entirely. Either way, the public unraveling of Comet ISON will be cause for celebration, not mourning. “Comet ISON is an extra-ordinarily rare object,” says Carey Lisse of Johns Hopkins University, who is coordinating an international observing campaign. “It isn’t just hyperbole. We are going to go to town on it. And we are going to learn a lot.”
Day of Discovery
Almost as soon as they sighted the comet on a cloud-streaked autumn night in Russia’s North Caucasus, Vitali Nevski and Artyom Novichonok realized it was something special. They were performing a routine sky survey on
Sept. 21, 2012, as part of the International Scientific Optical Network, or ISON, a project that normally tracks rocky asteroids and space debris. This one smudge in their images was neither. “My heart missed a beat. Is it really a comet?” Nevski jotted down in his broken English.
When the two observers calculated the object’s orbit, their excitement mounted. The newly named Comet ISON was headed almost directly toward the sun, falling into the group of daredevil objects that astronomers picturesquely call sungrazers. Cold comets are small and unspectacular, but when they heat up, the ice and dust that blow off catch sunlight, forming a round cloud (the head, or coma, of the comet) and long streamers (the tail). That is what makes comets so beautiful and spectacular.
Because of its Icarus-like trajectory, Nevski and Novichonok realized, Comet ISON would put on an extreme performance, boiling away madly under intense illumination. “The brightness of the comet in the maximum could reach the full moon,” Nevski commented in his notes on Sept. 25, after the comet was formally announced. If so, Comet ISON would rank as the most brilliant comet since the Great January Comet of 1910 became easily visible in daytime skies. Even the newer, less upbeat forecasts suggest ISON will be lovely in binoculars.
For the casual sky gazer, a bright sungrazing comet means a delightful spectacle. For researchers like Lisse, it presents a rare opportunity to see exactly how a comet is constructed: By passing its light through a spectrograph that detects wavelengths, they can identify the visual fingerprints of the various chemical components that are present. From estimating how much of each component is present, they can learn a lot about where and how the comet formed.
“I look at comets as the dinosaur bones of solar system formation. If we can crack open a comet, we’re going to be able to learn about the initial conditions that helped lead to the planets, and to us,” Lisse says.
There’s no need to figure out how to crack a comet; solar radiation is already doing the job. “Comet ISON is going to get close enough that the sun will not only boil off all the icy parts of the comet; it’s going to boil the rock as well. It’s going to give us a chance to literally count atoms in the gas and the dust.”
The Plot Thickens
Soon after the discovery in Russia, other researchers traced Comet ISON’s orbit backward to identify its point of origin. They found a path to the sun that looked nothing like the circle or oval traversed by most comets. Instead, Comet ISON’s trajectory was more like a straight line. Its journey started in the Oort Cloud, a vast swarm of inert comets that stretches from the edge of our solar system nearly halfway to the next star.
Suddenly the story got even more interesting.
Most of the comets that astronomers study have made previous journeys into the torrid inner reaches of the solar system, often multiple times, as they orbit the sun. As a result, they have been cooked, evaporated and eroded in ways that erase much of their original nature. Comet ISON, on the other hand, appears to be making its first trip in from the Oort Cloud. Extending Lisse’s metaphor, Comet ISON is not just a bunch of dinosaur bones. It is more like the dinosaur itself.
Comet ISON’s long trajectory has given astronomers yet another precious gift: a chance to prepare. Most sungrazing comets are scrappy objects spotted just hours before they race past the sun. That isn’t nearly enough notice to set up observing campaigns at big observatories, which must be reserved months in advance. “But Comet ISON was found when it was far from the sun, six times Earth’s distance, so we’ve had a lot of time to do observational planning,” says Jian-Yang Li of the Planetary Science Institute in Tucson, Ariz.
The Witnesses Assemble
The first phase of that plan kicked off around the beginning of 2013, as observers around the world began monitoring Comet ISON and sending reports to the Minor Planet Center in Cambridge, Mass., the main clearinghouse for information about comets and asteroids. By spring, Karen Meech of the University of Hawaii had scheduled a session on the giant Gemini North telescope atop Mauna Kea. From those observations, she documented a vigorous outpouring of gas and dust from the comet.
Around the same time, Li led a team working with the famed Hubble Space Telescope, which clarified the origin of the outpouring. The Hubble images captured an energetic geyser, almost 2,500 miles high (4,000 kilometers), shooting out from the sunward side of the comet. “I did not expect such an obvious jet, nothing as spectacular as that,” Li says.
That geyser partly explains why Nevski and Novichonok spotted the comet so far ahead of perihelion, but it opened up a deeper mystery. The most common ingredient in a comet is ordinary ice, so comets typically remain fairly sedate until they receive enough sunlight to vaporize water. Comets do not generally get that hot until they approach Mars. But there was Comet ISON, puffing away in the frigid realm beyond Jupiter.
Meech thinks such flamboyant behavior reflects the unusual makeup of comets that have never experienced any warmth from the sun. She notes that as a new arrival from the Oort Cloud, Comet ISON might be covered with a layer of frozen carbon monoxide and carbon dioxide, gases that vaporize rapidly when exposed to even a slight trickle of heat. “Carbon dioxide would be strongly outgassing as far out as the orbit of Saturn,” twice the distance of Jupiter, Meech says.
Veteran comet researcher Michael A’Hearn of the University of Maryland points to another possible reason why Comet ISON was unraveling so rapidly as it plummeted toward the sun. Solar wind creates a huge magnetic bubble, known as the heliosphere, that protects Earth and the other planets from energetic subatomic particles that constantly zip around in deep space. But in the faraway Oort Cloud, Comet ISON received no such protection.
“After four and a half billion years outside the heliosphere, the outer 10 or 20 meters of the comet has been completely irradiated,” A’Hearn says. “Essentially all the chemical bonds are broken down. You get lots of unstable molecular fragments. When they warm up, even a little, they recombine and tend to do so explosively.” The outer layer of Comet ISON was like a thick blanket of TNT waiting for the slightest spark of heat to set it off.
Around April 2013, Meech and others noted that Comet ISON was no longer brightening so quickly. “I would interpret that as the end of the bomb,” A’Hearn says. The super-reactive molecules had mostly been consumed, and ISON was now behaving more like an ordinary comet.
At that point, astronomers realized they might have misjudged Comet ISON based on its front-loaded performance. Drawing on his data from the Hubble observations, Li estimates that the nucleus — the solid body of the comet itself — is no more than about 2.5 miles (4 kilometers) across, smaller than what many astronomers initially expected for such an energetic comet. Although the comet’s tail may ultimately stretch millions of kilometers through the solar system, its nucleus is likely smaller than New York’s Central Park.
Likewise, Comet ISON probably will not become as bright as the early extrapolations had suggested; contrary to Nevski’s early hope, the comet is unlikely to rival the full moon. On the other hand, the dramatically changing nature of Comet ISON as it boils away is exactly what scientists were hoping for. With each new part of the comet exposed, a little more of its past — and our past — is revealed.