Hartley-2, a New Breed of Comet?

NASA's EPOXI mission reveals groundbreaking new information about Hartley-2.

At the heart of every comet lies a remnant of the dawn of the solar system. Or is that remnants? Astronomers don't know, but the answer would give them a clearer picture of exactly how comets were born eons ago at the birth of the Solar System. Did thin tendrils of dust and ice get drawn slowly inward and pack themselves into a single, uniform mass? Or did a hodge-podge of mini-comets come together to form the core for a comet of substance?

For Hartley-2, the answer so far is neither. "We haven't seen a comet like this before," says Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Md. "Hartley-2 could be the first of a new breed."

Both data collected by Mumma's team and detailed images of the comet taken by NASA's EPOXI mission reveal that the comet's core is not uniform. "We have evidence of two different kinds of ice in the core, possibly three," says Mumma. "But we can also see that the comet's overall composition is very consistent. So, something subtle is happening. We're not sure what that is."

The researchers observed Hartley-2 six times during the summer, fall and winter of 2010, both before and after the EPOXI mission's Deep Impact spacecraft had its November rendezvous with the comet. Using telescopes perched high in the mountains of Hawaii and Chile, Mumma's team studied the comet's coma—the aura of gas, dust and ice particles that surround the core. The findings of Mumma and his colleagues at Catholic University of America in Washington, D.C., the University of Missouri in St. Louis, the University of Hawaii in Honolulu, the California Institute of Technology in Pasadena, the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and Rowan University in Glassboro, N.J., are being reported in a special issue of Astrophysical Journal Letters on May 16, 2011