Hale-Bopp's Tepid Birthplace

in "Newsnotes", Sky and Telescope, V94, No. 4 (October 1997), p. 24.


Did Comet Hale-Bopp (C/1995 O1) originate in the Kuiper Belt beyond Neptune or in the more distant Oort Cloud? Astronomers have leaned toward the latter source because the comet's orbit is so large and highly inclined.

This supposition has now been strengthened by spectra from the Extreme Ultraviolet Explorer (EUVE) satellite, which homed in on the comet last September. EUVE did not detect any neon in Hale-Bopp's outflowing gas and dust. While the gas may not have been completely absent, its abundance relative to oxygen was no more than 1/25 that observed in the Sun.

Neon is a sensitive indicator of formation temperature: at 25 degrees Kelvin it condenses to ice and incorporates readily into a cometary nucleus or other solid body. The observing team, led by Michael J. Mumma and Vladimir A. Krasnopolsky (NASA/Goddard Space Flight Center), concludes that the comet must have formed in an environment too warm for neon to freeze out. That apparently rules out an origin within the frigid Kuiper Belt, the huge band of cometary objects beyond the orbit of Neptune. But it's a good temperature match to a location somewhat nearer the Sun. So Hale-Bopp's lack of neon supports the notion that Oort Cloud comets originated in the vicinity of Uranus and Neptune, only to be flung thousands of astronomical units outward after close planetary encounters.

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