WEEKLY WHINE

Comet tails must be pretty long

This week, various members of the science teams for the Ulysses mission published papers showing that the spacecraft sailed through the tail of Comet C/1996 B2 Hyakutake on WED 01 MAY 1996. What did they find out? The comet's tail was really long.

Ulysses is a probe that orbits nearly perpendicular to the ecliptic, the plane in which the planets orbit. This allows it to study the poles of the Sun - its next pass over the Sun's south pole is between SEP 2000 and JAN 2001. A joint project between the European Space Agency and the USA's National Aeronautics and Space Administration, the spacecraft is exactly ten years younger than I am.

Says Dr George Gloeckler, a University of Maryland physicist who serves as the principal investigator for Ulysses's Solar Wind Ion Composition Spectrometer [SWICS] 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."

The ESA's report on the encounter notes that it took nearly four years to determine the cause of the encounter, identify the comet responsible, and compile results. In 1998 Dr Pete Riley and friends published their description of one of Ulysses's observations, a precipitous drop in the solar wind's proton density.

Dr Geraint Jones, a Ulysses magnetometer team member from Imperial College in London, says that the comet that was lighting up Earth's skies was a good first guess. "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." C/1996 B2 Hyakutake was almost directly between Ulysses and the Sun. That would place Ulysses in the right position to observe the comet's tail, which is blown almost directly outward from the Sun by the solar wind.

Ulysses is actually in a better position to observe long cometary tails. The output from the Sun's polar regions is termed the fast solar wind, which is more consistent than the slow solar wind emitted from its lower latitudes. Thus there is less room to disrupt the tail. "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," continues Dr Jones. At the time of the encounter, Ulysses was about 500,000,000 km [300,000,000 mi] from C/1996 B2 Hyakutake. The comet's tail stretched about 3.8 AU, and Dr Jones suggests that it would still be observable at the heliopause, the boundary at which the solar wind gives way to the interstellar medium. It resides about 100 AU from the Sun.

Full results have been published in this week's issue of the journal Nature.

The researchers say that it is possible that Ulysses or other spacecraft have also passed through long comet tails but admit that the possibility is low. Dr Gloeckler says, "It was a bit like finding a needle in a haystack when you weren't even looking for a needle in the first place." Another of the magnetometer investigators, JPL's Dr Edward Smith, adds, "The odds that Ulysses' flight path would intersect the comet tail were probably less likely than someone breaking the bank at Monte Carlo."