Comet ISON, named for the International Scientific Optical Network that discovered it, is now inside Earth’s orbit around the Sun. The comet is approaching the sun for what promises to be a fiery encounter on Thanksgiving Day, November 28.
ISON has never passed into the inner solar system before, being a first-time traveler from the Oort Cloud, a domain of comets that could extend as far away as a light-year from the Sun. ISON is an unfamiliar entity, and so its behavior in the inner solar system will be unpredictable. Matthew Knight, a member of NASA’s Comet ISON Observing Campaign at Lowell Observatory, has assembled in a NASA press release the possible fates that might befall ISON as it nears the Sun.
Scenario #1 is that ISON will simply, spontaneously disintegrate. ISON is in the region of space, within approximately 0.8 AU (astronomical unit = 93 million miles = average distance between Earth and the Sun) of the Sun, in which other comets have suddenly disintegrated. Comet LINEAR (C/1999 S4) and Comet Elenin (C/2010 X1) have both disintegrated in this region, in 2000 and 2011, respectively. If ISON disintegrates, it will be the best-observed such event in history.
Scenario #2 is that ISON will survive the approach to the Sun but will not make it around. At the comet’s closest approach to the Sun, within 621,000 miles of the star’s surface, its temperature will rise to 5,000 degrees Fahrenheit; at such high temperatures, most of the dust and rock on ISON’s surface will evaporate. ISON might survive this close encounter thanks to its speed and size; the comet might pass out of the danger zone quickly, and it must be 200 meters wide to survive, a mere fraction of its current diameter of 500 meters to two kilometers. However, even if ISON survives the vaporization of its surface, the Sun’s gravity might shatter it. A similar end came for Comet Lovejoy in December 2011, when it passed within 100,000 miles of the Sun’s surface.
Scenario #3 is that ISON survives its close shave with the Sun and emerges on the other side with enough of its nucleus intact. Even if it endures, ISON will probably lose enough material to generate a spectacular tail, much as Comet McNaught (C/2006 P1) in 2007. Alternatively, ISON might break up into a few large chunks, which would render the comet extremely bright from the ground and provide astronomers a chance to study the 4.5-billion-year-old comet with the most extensive array of orbiting and Earth-based telescopes ever fielded.
“I’m clearly rooting for #3,” said Knight.