Astronomers from the University of Maryland at College Park and Lowell Observatory have used NASA’s Swift satellite to check out comet C/2012 S1 (ISON), which may become one of the most dazzling in decades when it rounds the sun later this year. Using images acquired over the last two months from Swift‘s Ultraviolet/Optical Telescope (UVOT), the team has made initial estimates of the comet’s water and dust production and used them to infer the size of its icy nucleus.The UVOT imaged comet ISON (center) at the end of January, when it was located about 3.3 degrees from the bright star Castor in the constellation Gemini. When this 5.5 min time exposure was taken, the comet was about 5,000 times fainter than the limit of human vision.
Based on ISON’s orbit, astronomers think the comet is making its first-ever trip through the inner solar system. Before beginning its long fall toward the Sun, the comet resided in the Oort comet cloud, a vast shell of icy bodies that extends from the outer reaches of the planetary system to about a third of the distance to the star nearest the Sun.
On 1 October, the inbound comet passes about 10.8 million km from Mars. During this close encounter, NASA and ESA spacecraft now orbiting Mars may be able to observe the comet. It may also have its picture taken by the Curiosity rover from the surface of Mars.
On 28 November, ISON will swing around around the sun. The comet will approach within about 1.2 million km of the Sun’s visible surface, which classifies it as a sungrazing comet. In late November, icy material will furiously sublimate and torrents of dust will be released as the ISON’s surface erodes because of the Sun’s fierce heat—as sun-monitoring satellites look on. Around this time, the comet may become bright enough to be seen from Earth just by holding up a hand to block the Sun’s glare.
Following ISON’s solar encounter, the comet will move toward Earth, appearing in evening twilight through December. It will swing past Earth on the day after Christmas, coming within 64.2 million km or about 167 times farther than the moon.
Image and Video Credit: NASA