Looking Up – Bob Eklund

NASA’s Deep Impact spacecraft has acquired its first images of comet C/2012 S1 (ISON). The images were taken by the spacecraft’s Medium-Resolution Imager over a 36-hour period on Jan. 17 and 18, 2013, from a distance of 493 million miles. Many scientists anticipate a bright future for comet ISON; the spaceborne conglomeration of dust and ice may put on quite a show as it passes through the inner solar system this fall.

“This is the fourth comet on which we have performed science observations and the farthest point from Earth from which we’ve tried to transmit data on a comet,” said Tim Larson, project manager for the Deep Impact spacecraft at JPL in Pasadena.

Deep Impact, launched in 2005, has executed close flybys of two comets, Tempel 1 and Hartley 2, and performed scientific observations on two more—comet Garradd and now ISON. The ISON imaging campaign is expected to yield infrared data, and light curves (which are used in defining the comet’s rotation rate) in addition to visible-light images. A movie of comet ISON was generated from initial data acquired during this campaign. Preliminary results indicate that although the comet is still in the outer solar system, more than 474 million miles from the Sun, it is already active. As of Jan. 18, the tail extending from ISON’s nucleus was already more than 40,000 miles long.

Long-period comets like ISON are thought to arrive from the solar system’s Oort cloud, a giant spherical cloud of icy bodies surrounding our solar system so far away its outer edge is about a third of the way to the nearest star (other than our Sun). Every once in a while, one of these loose conglomerations of ice, rock, dust and organic compounds is disturbed out of its established orbit in the Oort cloud by a passing star or the combined gravitational effects of the stars in the Milky Way galaxy. With these gravitational nudges, so begins a comet’s eons-long, arching plunge toward the inner solar system.

ISON was discovered on Sept. 21, 2012, by two Russian astronomers using the International Scientific Optical Network’s 16-inch telescope near Kislovodsk. NASA’s Near-Earth Object Program Office, based at JPL, has plotted its orbit and determined that the comet is more than likely making it first-ever sweep through the inner solar system. Having not come this way before means the comet’s pristine surface has a high probability of being laden with volatile material—mostly water ice and dust—that will become a visible “tail” after it is heated by the Sun.

ISON will not be a threat to Earth, getting no closer than about 40 million miles—a little less than half the distance from Earth to the Sun. But stargazers will have an opportunity to view the comet’s head and tail before and after its closest approach to the Sun in December—if the comet doesn’t fade early or break up before then.

(Photo of Comet Lovejoy as seen from Adelaide, Australia)


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