Ulysses space probe makes surprise trip into comet's tail
ROYAL ASTRONOMICAL SOCIETY NEWS RELEASE
Posted: April 6, 2000
Comet Hyakutake, a bright comet seen by many people in 1996, developed the longest comet tail ever recorded. At 570 million km (360 million miles) it beat the previous claimed record of 330 million km (206 million miles) held by the Great March Comet of 1843. The discovery was made recently, when Dr Geraint Jones and Professor Andre Balogh of Imperial College, London, together with Dr Tim Horbury of Queen Mary and Westfield College, London analysed 1996 data from the Ulysses spacecraft.
The joint European Space Agency-NASA spacecraft Ulysses was launched in 1990, and is in an orbit taking it over the poles of the Sun. It makes continuous measurements of the stream of charged particles called the solar wind which flows outwards from the Sun past the spacecraft. On May 1, 1996, Ulysses was 560 million km (347 million miles) from the Sun, when decidedly unusual things happened to the solar wind. The first odd feature to be noticed was a dramatic drop in the number of protons at Ulysses, which was reported in 1998 by another team of scientists led by Dr Pete Riley, then of the Los Alamos National Laboratory. They mentioned that a comet could explain some aspects of the odd results.
Comet nuclei are small bodies that were formed when our solar system was young. They are typically a few kilometres across, and are composed of a mixture of ice and dust. When their orbits bring them close to the Sun, the rise in temperature makes them release gas and dust. The tiny dust particles are pushed away from the Sun by the pressure of sunlight, forming a dust tail. The gas particles eventually become electrically charged, forming ions. These ions join the solar wind flowing away from the Sun, forming an ion (or plasma) tail. When Jones and colleagues looked closely at the data returned from Ulysses's magnetometer instrument at the time, they realised that the solar wind's magnetic field lines displayed a herringbone pattern - a sign that the centre of whatever Ulysses had crossed had been moving slower than its edges. This is expected at comets, because the comet's ions slow down the solar wind when near the nucleus. This convinced them that the event was indeed due to a comet; so they began to search for the comet to which the tail belonged.
Apart from the great scientific value of an encounter with a fourth comet (comets Giacobini-Zinner, Halley and Grigg-Skjellerup have been visited by other spacecraft), several aspects of the tail crossing are particularly intriguing. The tail's length is most surprising - Hyakutake's tail was over 570 million km (350 million miles) long. This breaks the record for the longest measured tail, which is generally regarded to have been previously held by the Great March Comet of 1843, which had a visible tail around 330 million km (205 million miles) long. Had Hyakutake's tail been visible at the time from the Earth, it would have stretched over 80 degrees across the sky - a very impressive length for a comet so far away. However, at this time, it was invisible from Earth because its head was very close to the Sun in the sky.
Comets' ion tails are generally thought of as pointing almost straight away from the Sun. The magnetometer data from Ulysses reveal that at the spacecraft, the tail was definitely not doing this - it was travelling almost sideways. Jones and colleagues explain this by the comet's rapid motion around perihelion. Like the jet of water from a lawn sprinkler, Hyakutake's tail started out pointing away from the Sun. The further it got from the Sun however, the more it twisted away from the anti-sunward direction, as a lawn sprinkler spray twists. Ion tails are therefore curved, especially when comets are around perihelion. This has implications for some Earth-based comet observations.
When Ulysses crossed the tail, the comet's head was being observed by the LASCO coronagraph aboard the SOHO spacecraft, even though it could not be seen from Earth. "At this time," says Jones, "what was happening at the head of the comet didn't have any relevance to the tail at Ulysses. If you want to study the part of the tail crossed by Ulysses, you need to look at images of Hyakutake obtained around April 23. Unfortunately, few images were obtained then, as Hyakutake was sinking into twilight as seen from Earth." Nevertheless, the Ulysses results are providing unique information on the magnetic structures of ion tails.
The discovery, and identification of the parent comet by Jones and colleagues are only the beginning of the event's analysis. The study of the data returned from other Ulysses instruments will undoubtedly lead to a fuller picture of what happened when a distant spacecraft crossed an incredibly long tail. In the same issue of 'Nature', colleagues of Jones and co-workers, led by Professor George Gloeckler of the University of Maryland, report their independent discovery of cometary ions during the same event using another instrument aboard the spacecraft.
The research that led to the discovery is supported by the UK Particle Physics and Astronomy Research Council.
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