Ulysses feels the brush of a comet's tail

Details Ulysses feels the brush of a comet's tail Ulysses feels the brush of a comet's tail

Apr 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000xmm..pres...24.&link_type=abstract

XMM Press Release PR 24-2000

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Scientific paper

"Ulysses's prime task is to map the solar wind above the Sun's poles: it had not been looking for Hyakutake, which happened to be at its closest approach to the Sun on 1 May 1996, or any other comet", says Richard Marsden, ESA's Ulysses Project Scientist. "Ulysses was just in the right place at the right time."
The two teams stumbled across the telltale signature of a comet quite independently when poring over old Ulysses data. Jones and colleagues found their evidence in magnetic field data: "the magnetic field lines were draped in a way that you'd expect in a comet's tail," says Jones. The other instrument team, lead by George Gloeckler from the University of Maryland, found their evidence when looking at the composition of the solar wind. Cometary tails are rich in oxygen and carbon compared with the solar wind, but depleted in nitrogen and neon.
The Imperial College team identified Hyakutake as the source of the anomalous readings. On 1 May 1996, Ulysses was aligned with the Sun and the position Hyakutake had occupied eight days earlier, which Jones calculated was the time needed for material leaving the comet's nucleus to travel the distance to Ulysses.
One of the most surprising aspects of the discovery is the length of Hyakutake's tail. Cometary experts had thought that comet tails eventually spread out and lose their integrity. "We found that the whole thing is preserved as an entity and doesn't spread out very much," says Gloeckler. "If it can persist as far as Ulysses, there's no reason to presume that it wouldn't continue to the edge of the heliosphere (the boundary about 100AU from the Sun between the solar wind and the interstellar medium)," says Jones.
"This discovery makes us wonder whether Ulysses or other spacecraft have crossed a comet tail before. So we're going back to look again for other signatures. But it's probably a rare event," says Jones. The comet nucleus has to be in exactly the right position with respect to the Sun and the spacecraft for the tail to pass over the spacecraft at the right time - and the chances of that happening very often are probably small.

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