Comet Hale-Bopp 'Frozen to Death'

Hale-Bopp, the most dazzling comet to grace our skies in the 20th Century, has drifted beyond the orbit of Neptune, over 30 AU from the sun (1AU = one sun-Earth distance).

Curious astronomers from Hungarian, U.S. and Australian research institutions decided to track the icy vagabond as it continued its outward journey on an epic 2,500 year orbit.

In 1997, Hale-Bopp gave us Earthlings a dazzling show. Heated by the sun, the comet underwent an impressive outburst, ices subliming into space, producing a dusty coma visible to the naked eye with bright dust and gas tails (plus the discovery of a previously unknown neutral sodium tail) carried away from the comet in the direction of the solar wind for millions of miles.

But what of the comet 15 years later?

In research to be published in Astronomy & Astrophysics the team, headed by Gyula Szabó of Konkoly Observatory in Hungary, used the European Southern Observatory's 2.2 meter telescope at La Silla Paranal Observatory in Chile to check up on Hale-Bopp. At 30.7 AU, this most recent observation makes the comet the most distant ever studied.

Observing the comet in 2007, Szabó's team tracked Hale-Bopp when it was 25.7 AU distant. Back then, it still had a prominent coma suggesting that although the comet was so far away from the sun, its icy nucleus was still producing gas and dust.

"This surprising observation raised the question where this activity will cease," Szabó remarked.

It appears that Szabó now has an answer: "Compared to our previous observations, the general appearance of the comet has drastically changed during the past 3 years," he said.

The comet appears to have dimmed dramatically, potentially indicating that cometary activity has ceased. Although recent observations suggest there is some coma-like material surrounding the nucleus, it might be a cloud of left-over dust from previous outbursts being dragged with the comet's passage through the solar system.

Edit: As pointed out by science writer Paul Sutherland in the comments below, the detection of a faint coma surrounding Hale-Bopp may also indicate some low levels of cometary activity, as noted by the authors of this new research.

Interestingly, the researchers calculated the comet's surface temperature during the 2007 observation campaign, at 53.1 K (-220 Celsius or -364 Fahrenheit). So, if cometary activity has completely ceased, they are able to put constraints on the temperature at which comets, like Hale-Bopp, become frozen to death.

"Since Hale–Bopp seems to turn into inactive state, we infer that the temperature of activity cessation is somewhere between 50–53 K for a Hale–Bopp type comet."

Understanding how comets act as they pass through the solar system is of great importance, especially if one should be passing near-Earth orbit. Outgassing of ices can cause deviations in the comet's path potentially making them unpredictable chunks of ice should they steer in our direction. Knowing at what distance (indeed, what temperature) cometary activity stops will help us characterize future comet sightings.

Assuming Hale-Bopp is a frozen corpse, it won't spring back to life for over two millennia when it sweeps back into the inner solar system to be heated by the sun once again.

Publication: "Frozen to death? - Detection of comet Hale-Bopp at 30.7 AU," Gy. M. Szabó, K. Sárneczky, L. L. Kiss, 2011. arXiv:1104.4351v1 [astro-ph.EP]

Image: Comet Hale-Bopp in 1997 (NASA)