Hale-Bopp: The Comet That Doesn't Quit

Eight years after Comet Hale-Bopp dazzled astronomers as it passed through the inner solar system, the dirty snowball is still detectable (about 20th magnitude) despite being a whopping 21 astronomical units from the Sun. On January 8th MIT astronomers Andrew S. Rivkin and Richard P. Binzel observed the comet with Magellan Observatory's 6.5-meter Clay telescope in Chile.

Rivkin and Binzel were aiming for a "Goldilocks" observing moment — the comet would have cooled off, the coma would be gone, and yet the nucleus would still be bright enough to observe. "There's not a lot of spectra of the nuclei of comets," says Rivkin. They are hard to capture because the nucleii are obscured once comets develop comas.
They didn't find what they bargained for. "When we tried to see the nucleus," says Rivkin, "the thing had a tail!"

Although they couldn't see the nucleus, the observations raised a key question: why is Hale-Bopp still active?
"Hale-Bopp is just acting like a normal, bright, long-period comet," says Paul R. Weissman (NASA/Jet Propulsion Laboratory). "Long-period comets stay active so far out that one can never reliably say that we are looking at a bare nucleus."

Weismann says that comets like Hale-Bopp have far more volatile ices close to their surfaces than short-period comets do, which can contribute to activity at much greater distances. "It is not at all surprising, now that the comet is far from the Sun, that there is still enough energy to sublimate the CO (carbon monoxide) ices, even if they are buried somewhat below the immediate surface," he adds. Even while on perihelion approach, it takes a while for the Sun's heat to penetrate into the nucleus's middle.

"It's just like cooking a steak — the longer you cook it, the warmer it gets deep inside," says Weismann.