Ulysses has added comet spotter to its list of talents. Two papers published in Nature today report that on 1 May 1996, the spacecraft flew through the tail of comet Hyakutake whose nucleus was more than 3.5AU (one AU equals the Sun-Earth distance) away at the time. "This makes it the longest comet tail ever recorded", says Geraint Jones from Imperial College, London who is a member of one of the two instrument teams that made the discovery. "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. George Gloeckler from the University of Maryland who is a member of the other instrument team, says: "The discovery was made quite by accident. It was a bit like finding a needle in a haystack when you weren't even looking for a needle in the first place". Jones, Gloeckler and their colleagues stumbled across the telltale signature of a comet quite independently when poring over old Ulysses data.
"I was looking for changes in the ionisation levels of the solar wind that would tell me about unusual solar activity," says Gloeckler who is Principal Investigator of the SWICS (Solar Wind Ion Composition Spectrometer) instrument. "The solar wind normally consists of multiply charged ions. The signature stood out because the number of singly charged ions jumped to several thousand times the background level. I thought this might be due to strange solar eruptions. But when I looked at the composition of the ions, I knew immediately that they were cometary in origin." Cometary tails are rich in oxygen and carbon ions compared with the solar wind, but depleted in nitrogen and neon. Jones and colleagues found their evidence in data from the Ulysses magnetometer.
"The magnetic field lines were draped in a way that you'd expect in a comet's tail. The solar wind is slowed down at the centre of the tail compared with the edge, which gives the magnetic field associated with the travelling ions a characteristic hairpin shape," says Jones. The findings from both teams corroborate an earlier Ulysses discovery, reported in 1998 by Pete Riley and colleagues at the Los Alamos National Laboratory, of a drop in the proton density in the solar wind on 1 May 1996. "They wondered whether the drop could have been due to a comet, but went no further," says Jones. With the evidence now mounting, the Imperial College team decided to look for a comet that would have been in the right place at the right time to account for the Ulysses data. "Hyakutake was the first comet we looked at. When I compared the orbit of Ulysses with the orbit of the comet, I found that Ulysses was extremely close to Hyakutake's orbital plane at the time," says Jones. On 1 May 1996, Ulysses was aligned with the Sun and the position Hyakutake had occupied eight days earlier. Jones calculated that eight days was the time needed for material leaving the comet's nucleus to travel the 3.5AU distance to Ulysses. One of the most surprising aspects of the discovery is the length of Hyakutake's tail, which must have been at least 3.8AU as the nucleus had moved further away from Ulysses during the eight-day travel time. Cometary experts had thought that the molecules and ions that make up a comet's tail would mingle with the solar wind and eventually become indistinguishable.
"We found that the whole thing is preserved as an entity and doesn't spread out very much," says Gloeckler. "The comet is like a point source. It emits neutral atoms and molecules which become ionised by the solar wind as they move away from the nucleus. After several million kilometres, they are all ionised. But instead of then mingling with the surroundings, this ionised sample gets picked up by the solar wind which shoots it out," he explains. By comparing the Ulysses findings with those of the Giotto spacecraft for comets Halley and Grigg-Skellerup, Gloeckler and his team have even been able to determine where the material that they detected originated - in Hyakutake's coma, the diffuse shell of gas surrounding the comet's nucleus. One reason for the tail's survival is probably that it was travelling in the fast solar wind, a steady stream of charged particles flowing out from near the Sun's poles. Comet tails flowing through the more variable slow solar wind, which emanates from near the Sun's equator, are more likely to be disrupted.
"The fast solar wind helped to maintain the magnetic field signature over such a large distance. 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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Comets are not rocks