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(super?) Comet ISON

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  • (super?) Comet ISON

    Comet expected to survive close Sun encounter

    ISON is the first non-repeating comet to swing past the Sun in centuries.

    Comet ISON is winging through the inner Solar System. Astronomers have one big question: will it survive flying past the Sun on 28 November?

    If it does, ISON will likely become visible to the naked eye, blazing a path among the constellations in December. But even if the comet disintegrates, there should be plenty of science to do. ISON represents the first time researchers have watched a 'dynamically new' comet — a pristine, first-time visitor from deep space that will not return — on course to pass very close to the Sun.

    “That’s what makes it so unusual,” says Matthew Knight, an astronomer at Lowell Observatory in Flagstaff, Arizona. “There may be totally unexpected things that come up.” Knight, who has spent perhaps more time observing the comet than any professional astronomer, says that ISON may be just big and dense enough to survive its close call with the Sun. He laid out his reasoning on 9 October at a meeting of the Division for Planetary Sciences of the American Astronomical Society in Denver, Colorado.

    On 21 September 2012, Russian astronomers discovered ISON using a telescope with the International Scientific Optical Network, which became the comet’s namesake. Astronomers traced its origin to the edge of the Oort cloud, the reservoir of icy rocks lying far beyond Pluto. Predictions for the forward path suggested it would fly past the Sun at a distance of just 2.7 solar radii.

    Astronomers, both professional and amateur, have watched as more than 2,000 Sun-grazing comets were ripped apart by tidal forces or vaporized by the star’s heat. Even in such cometary deaths there can be science: in 2011, astronomers watched how the tail of Comet Lovejoy wavered as it passed through the solar atmosphere, a test that allowed them to verify models of where the Sun's magnetic field lines lay.

    A tough test

    Knight and Kevin Walsh, of the Southwest Research Institute in Boulder, Colorado, used numerical simulations to study whether ISON would be ripped apart2. Observations from the Hubble Space Telescope, as well as smaller telescopes, suggest that ISON is between 1 and 4 kilometres across. Based on the experience of other Sun-grazing comets that survived, like 1965’s Ikeya-Seki, the team suggests ISON is big enough not to be vaporized.

    But Knight says the comet could still be tidally disrupted, depending on how dense it is. If it has a typical density, ISON should make it mostly intact. If it is less dense, it may disintegrate as Comet Shoemaker-Levy 9 did before colliding with Jupiter in 1994.

    Astronomers will be looking for changes as ISON approaches the Sun, such as brightening in particular regions, or the formation of jets as volatile compounds start to boil off. They will also be looking for changes in speed or spin3. With so much advance notice that ISON is coming — Lovejoy, in 2011, gave just three weeks’ notice — astronomers have had time to marshal their resources. Along with many ground-based telescopes NASA’s Swift mission, built to study γ-ray bursts, has been imaging ISON; the US government shutdown that began on 1 October exempts active missions like Swift.

    Because it is above the Earth's atmosphere, Swift can study comets in ultraviolet wavelengths, which reveal hydroxyl (OH) molecules spraying off the comet as a measure of how much water it produces as it warms up. Observations show a huge cloud of OH particles surrounding ISON. "It expands to a huge cloud, much prettier than I expected," says Dennis Bodewits, an astronomer at the University of Maryland in College Park.

    ISON also flew past Mars on 1 October, and the orbiting Mars Express and Mars Reconnaissance Orbiter missions turned to look at it briefly. Images from both spacecraft are still being processed, although MRO did release some early pictures.

    The next few weeks will see the peak of ISON observing. On 28 November, which is the US Thanksgiving holiday, Knight and others won’t be eating turkey with their families. Instead, they will be huddled at a National Solar Observatory telescope in Arizona, watching ISON’s every move before it begins its journey out of the Solar System, never to return. “We’ve only got the one shot at it,” says Knight.

    Nature doi:10.1038/nature.2013.13924
    Dr. Mordrid
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