Pretty much every major telescope in the world is gearing up to witness a meeting that has been scheduled since long before humans walked the Earth. Around Thanksgiving, Comet ISON, a mountain-sized chunk of primordial solar system, will approach within 2 million kilometers of the sun and either fall apart or slingshot back into deep space. Astronomers aren’t sure yet how much of a spectacle ISON will be for earthbound observers, but from their vantage point the comet is already providing a brief, unprecedented glimpse into what the solar system was like in its infancy.
“It’s the sort of thing I’ve been waiting for my whole career,” says Matthew Knight, a comet researcher with the Lowell Observatory in Flagstaff, Ariz.
ISON, officially known as Comet C/2012 S1, was discovered by the astronomers Vitali Nevski and Artyom Novichonok. The pair spotted the comet as part of a sky survey program known as the International Science Optical Network. Using a telescope near Kislovodsk, Russia, the astronomers first glimpsed ISON just outside Jupiter's orbit on September 21, 2012. Within days, scientists had recovered months’ worth of previously unnoticed images of the comet and calculated its trajectory from the outskirts of the solar system millions of years ago to its present location, and projected its future path as well.
From those efforts, astronomers learned that the comet is most likely a refugee from the Oort cloud, a loose collection of frozen objects that scientists think cluster in the solar system’s outer reaches, about a thousand times as far from the sun as Neptune. Astronomers believe the Oort cloud objects were tossed out of the inner solar system when the planets formed and have barely changed since then (SN: 10/19/13, p. 19); Knight calls them the “freezer of the solar system.” Occasionally the gravitational tug of a passing star will nudge one of these frozen bodies out of the cloud and start it on a multimillion-year journey into the inner solar system. Unlike the more common periodic comets that make regular swings through Earth’s neighborhood, Oort cloud comets like ISON are thought to come around only once.
Looking ahead, astronomers found that ISON will pass within a cosmic whisker of the sun, close enough to potentially be ripped apart by the star’s intense gravity. The first known object from the Oort cloud to approach so close to the sun, the comet promises researchers a spectacle of dust, gas and ice brilliantly illuminated by the sun’s light.
And because ISON has probably spent nearly its whole existence in cold storage, astronomers believe it has preserved a record of the conditions that prevailed in the ancient solar system. “It gives us a chance to understand what the composition was and what the environment was when the solar system formed 4 1/2 billion years ago,” Knight says.
Reports on ISON’s makeup have already started to come out. In April, a team led by Jian-Yang Li of the Planetary Science Institute in Tucson, Ariz., directed one of the Hubble Space Telescope’s cameras toward ISON and snapped sharp photos of the comet. The researchers found that ISON is average-sized as comets go, with a nucleus no more than 4 kilometers across.
Based on images taken in different wavelengths of light, Li and his team also discovered a large amount of water ice in the region around the nucleus. That probably indicates large amounts of ice near the comet’s surface, where the sun can easily drive it off, he says. The finding confirmed that ISON is on its first pass through the inner solar system, not a periodic visitor like Halley’s comet.
The Hubble observations also suggested that ISON presents only one face to the sun. Li thinks that ISON’s dark side could bear scars from bombardment by extremely high-energy particles called cosmic rays, which scientists believe whiz around in deep space. Relatively few of these particles make it to the inner solar system because they are deflected by a steady stream of particles known as solar wind and by the sun’s powerful magnetic field; both of these shields stop short of the Oort cloud. ISON could thus provide a rare glimpse of the perilous environment beyond the sun’s protective reach.
Unlike previous sun-grazing comets, ISON was detected more than a year out, giving researchers at the world's major ground-based telescopes plenty of time to watch its approach to the sun. In the October 20 Astrophysical Journal Letters, scientists led by Karen Meech, an astronomer at the University of Hawaii’s Institute for Astronomy, report on the gases coming out of ISON, which they observed using seven research telescopes and a network of amateur telescopes. The researchers found that the comet was shedding large quantities of dust as well as water vapor, carbon dioxide and carbon monoxide — all gases that scientists believe were present when the Oort cloud formed.
The researchers also investigated why ISON initially seemed to brighten faster than most comets but did not continue to do so. Because sunlight interacts with gas and dust to make comets light up, Meech thinks the best explanation for her team’s data is that ISON’s nucleus contained a buried layer of solid carbon monoxide, a featherweight molecule that rapidly turns to gas when exposed to sunlight. When this layer vaporized, as Meech believes it did beginning in late 2011, it also kicked up a large amount of water ice, causing further brightening. After the carbon monoxide had blown off, the comet no longer brightened as quickly.
Meech hopes her team’s results can help end a debate that has exasperated scientists even as it captivated the public. Early reports suggested that ISON could become as bright as the full moon. Then, when the comet didn’t continue brightening as expected, some scientists began predicting the comet’s demise. The most prominent of the doomsayers is the astronomer Ignacio Ferrin of the University of Antioquia in Medellín, Colombia, who makes such a prediction online October 2 at arXiv.org.
Meech thinks both predictions were misinterpretations of the data; ISON’s behavior so far, she says, suggests it will neither outshine the moon nor fizzle out.
Knight and his colleague Kevin Walsh of the Southwest Research Institute's campus in Boulder, Colo., have also sought to temper speculation about ISON’s impending doom. In the Oct. 10 Astrophysical Journal Letters, they analyze how likely ISON is to survive the sun’s intense gravity. The scientists simulated the comet’s fate while giving it a variety of rotation rates; in most cases, including every scenario in which the comet rotates in the opposite direction of its orbit around the sun, it should survive. However, if ISON rotates rapidly in the same direction as its orbit, like a tennis ball with topspin, the comet could break apart.
Because scientists have no data on the comet’s rotation, Knight and Walsh conclude that ISON will probably survive. However, Knight acknowledges that other comets have broken up for no known reason, and he is prepared for surprises.
Whether or not the comet breaks up, late November is when it will put on the best show, both visually and scientifically. Li thinks the comet could release a lot of ice in the bright sunlight just before brushing past the sun; he and others hope the gases coming off the comet will reveal the chemicals present when ISON formed billions of years ago. The comet will continue shedding material in the days after it rounds the sun; even if it breaks into fragments, scientists will be able to study the individual pieces.
Earthbound stargazers won’t notice these details, but motivated observers may be able to spot a new object in the predawn sky. ISON could glow as brightly as Venus does, and those willing to wake up early and find a dark spot with an unobstructed view of the eastern sky could get a nice show. As for the more detailed views that matter to astronomers, those can be found only at a few powerful observatories. As a result, Knight says, “many of us will not be spending Thanksgiving with our families.”
Whatever ISON’s fate, the combination of its unique trajectory, its early discovery, and today’s powerful telescopes will make it one of the most studied comets to date. "This is a very special comet," says Li.
Editor's Note: This story was updated on October 29, 2013, to correct the location of Kevin Walsh's office.
K. J. Meech et al. Outgassing Behavior of C/2012 S1 (ISON) from 2011 September to 2013 June. Astrophysical Journal Letters. Vol. 776, October 3, 2013. doi: 10.1088/2041-8205/776/2/L20.
M. M. Knight and K. J. Walsh. Will Comet ISON (C/2012 S1) Survive Perihelion? Astrophysical Journal Letters. Vol. 776, September 24, 2013. doi: 10.1088/2041-8205/776/1/L5.
I. Ferrin. The Impending Demise of Comet C/2012 S1 ISON. arXiv.org. Posted October 2, 2013.
J.-Y. Li. Early Characterization of Comet C/2012 S1 (ISON). American Astronomical Society’s Division for Planetary Sciences 45th Annual Meeting in Denver, CO, October 9, 2013.
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