2010

Comet Hartley-2

After completing the successful 2005 mission to crash a projectile into comet Tempel-1, engineers responsible for NASA’s Deep Impact spacecraft realized that there was enough fuel left onboard to operate the probe as a remote observatory for characterizing extrasolar planets using the transit method (which was also being used by the Kepler mission), and to potentially encounter a second comet nucleus as well. Deep Impact was thus recast as a new mission called EPOXI, for Extrasolar Planet Observation and Deep Impact Extended Investigation.

After three Earth flybys to provide gravity-assist trajectory tweaks, EPOXI was targeted to fly closely past the nucleus of the Earth-approaching comet Hartley-2 in November 2010. Discovered in 1986 by Australian astronomer Malcolm Hartley, Hartley-2 is a short-period comet that travels in an orbit between about 1.1 astronomical units (AU) and 5.9 AU every six and a half years. Short-period comets are further divided into Jupiter family comets, such as Hartley-2, with periods less than about 20 years, and Halley family comets (named after their most famous member), with periods from about 20 to 200 years. Many short-period comets are suspected to have once been long-period comets that had their orbits dramatically changed by a close encounter with one of the giant planets. Hartley-2, for example, may be a relatively primitive object from the Öpik-Oort Cloud that encountered Jupiter relatively recently.

The EPOXI data from the flyby support the idea of a primitive, outer-solar-system origin for Hartley-2. Powerful jets of ice, gas, and dust were seen spewing from the comet’s 1.4-mile (2.3-kilometer) peanut-shaped nucleus in the dramatic EPOXI images, and Spectroscopic measurements showed that the ice is dominated by carbon dioxide (dry ice) rather than water. Initial studies also point to the possible presence of some organic molecules, such as methanol, in Hartley-2’s jets and extended atmosphere.

At the rate that the comet is currently losing mass from the jets as well as from sublimation (evaporation of ice) of the surface in general, scientists predict that it might only survive for about another 100 orbits (700 years) or so before breaking up into smaller pieces. Thus it is likely that this little lump of primitive ices from the original solar nebula is indeed a recent interloper in the inner solar system.

The nucleus of comet Hartley-2 photographed by the NASA EPOXI spacecraft during its November 4, 2010, flyby. Powerful jets of water vapor, other cometary gases, and dust are escaping from the comet’s interior.