ISON appears to have broken up after brush with sun

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Comet ISON has disintegrated in the sun’s intense heat and gravity, according to a growing consensus among astronomers. But even as it broke up, ISON provided valuable information about cometary composition.

ISON’s closest approach to the sun occurred on Thanksgiving Day. Images from the Solar and Heliospheric Observatory, jointly run by NASA and the European Space Agency, showed a bright object flying away from the sun, giving astronomers hope that ISON had survived, even if diminished (SN Online: 11/29/13).

But more recent images strongly suggest that little remains of ISON except dust, says Alex Young, a solar physicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. After brightening in the hours before its closest solar approach, the comet faded and has remained dim, suggesting its ice has been exhausted and only loose rocks and dust remain. Young also notes that stars beyond the comet’s remains are visible through the thin dust, because no solid nucleus blocks starlight.

Matthew Knight, a comet researcher at the Lowell Observatory in Flagstaff, Ariz., who has studied ISON, agrees that not much of the comet seems to be left. “It’s not looking good,” he says.

On the plus side, ISON has given astronomers their first closeup view of a comet’s demise, which will help researchers better understand this process, says Knight. “We know comets break up, but we’ve never been able to study one as it broke up.”

As they mourn the comet, scientists are puzzling over why NASA’s Solar Dynamics Observatory didn’t capture ISON on its journey to and from the sun. Astronomers expected the observatory to detect ISON in very short ultraviolet wavelengths because previous sungrazing comets had shown up brightly in this range. But for comets to emit light in the extreme ultraviolet, they need to give off oxygen atoms, which most easily come from ice. So ISON’s no-show with the observatory may mean it had lost most of its ice before it even reached the sun, Young says.

If careful studies of ISON data confirm this hypothesis, astronomers may need to revise their theories about comets’ structure. For decades, many astronomers have thought that comets are made mostly of ice with some rocks and dust. But several recent comets, including perhaps ISON, have appeared more like masses of rock and dust held together by ice. “People talk about comets being dirty snowballs,” Young says. “But some recent comets have seemed more like icy dirtballs.”

HAPPY TRAILS  Comet ISON’s final hours appear from several perspectives in videos taken by the Solar Terrestrial Relations Observatory and the Solar and Heliospheric Observatory. In all observations, ISON appears to end its journey as a puff of dust.

Credit: STEREO/NASA, SDO/GSFC/NASA, SOHO/ESA and NASA