Puzzling New Comet Next to the Sun Picked Out by SOHO

NASA's Solar and Heliospheric Observatory (SOHO) has discovered a rare periodic comet, one which follows its orbit around the sun more than twice and has an orbital period of less than 200 years.

Given the official designation P/2007 R5 (SOHO), the newly discovered periodic comet was first seen in September 1999. It made a second appearance in September 2003 when German Ph. D. student Sebastian Hoenig realized that the two sightings seemed to actually be the same object. To test his theory, he calculated a combined orbit for the comet and consequently predicted that it would return on Sept. 11, 2007. Once again in view near the Sun, P/2007 R5(SOHO) has proved Hoenig's theory to be accurate, and we have a new periodic comet.

Only approximately 190 of the thousands of comets seen are classified as periodic. Halley's Comet returns in its orbit around the Sun every 76 years and is the most famous periodic comet. Halley's most recent pass close to the Sun was in 1986.

P/2007 R5(SOHO) has an orbit around the Sun that is much smaller than Halley's Comet's orbit: P/2007 R5 (SOHO) has been seen every four years since 1999 giving it a four-year solar orbit as opposed to a 76-year solar orbit.

P/2007 R5 (SOHO) presents a puzzle, though, in that is that it does not look exactly like a comet. It has no visible tail or coma of dust and gas, as is traditionally associated with the phenomena of comets. This initially led some scientists to wonder if P/2007 R5 (SOHO) was actually an asteroid, which is a chunk of space-rock, rather than a comet, which is a chunk of space-ice.

Observations of P/2007 R5 (SOHO) do show that it exhibits some of the characteristics that are consistent with a comet. As scientists watched the object pass close to the sun, drawing to within 4.9 million miles, they saw it brighten by a factor of around a million. Solar brightening is a common feature of behavior for a comet.

But this comet faded just as quickly as it brightened. Soon it was rendered it too faint for SOHO's instruments to see. Estimates show that P/2007 R5 (SOHO) is a small comet that measures probably only 100 to 200 yards in diameter. Further observations from Earth-based telescopes are also difficult given how small and faint the object is and how close it still is to the sun. This makes a more specific identification of object P/2007 R5 (SOHO) rather difficult.

One other possibility is that P/2007 R5 (SOHO) is an extinct comet. Extinct comets are those that have expelled most of their volatile ices and retain little to form a tail or coma. They are theorized to be common objects among the celestial bodies orbiting close to the sun. Karl Battams of the Naval Research Laboratory, Washington, who runs SOHO's comet discovery program, has said, "[P/2007 R5 (SOHO)] is quite possibly an extinct comet nucleus of some kind."

Knowing for certain that P/2007 R5 (SOHO) is present in an orbit around the Sun means that astronomers will be watching closely for it during its next return, which is scheduled for September 2011.

Credit for the original discovery and recovery of the object goes to Terry Lovejoy (Australia, 1999), who also discovered another comet earlier this year that was named the Lovejoy Comet in honor of him, Kazimieras Cernis (Lithuania, 2003) and Bo Zhou (China, 2007).

SOHO is a cooperative project between NASA and the European Space Agency.

Stuart Clark/European Space Agency (ESA), "SOHO Mission Discovers Rare Comet," NASA/ESA.