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 view from the Ulysses spacecraft as Comet Hyakutake's ion tail rushes towards it, at a point 560 million kilometers from the Sun and 550 million kilometers from the Earth. Travelling at up to 750 kilometers per second, the tail has taken 8 days to journey from the comet's head to Ulysses, carried by the solar wind flowing from the Sun. The tail, omposed of gases released from the comet's icy nucleus, can be seen stretching for over 570 kilometers. The planets Mercury, Venus and Earth can be seen to the right of the Sun. Photo: Particle Physics and Astronomy Research Council; impression by David A. Hardy

Their analysis of the magnetic field data returned from Ulysses on May 1, 1996 led them to conclude that Ulysses had passed through a comet's tail on that date. They then found that the tail belonged to Comet Hyakutake. The discovery is reported in the journal 'Nature' on April 6, 2000.

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.

A true-color image taken on the morning of March 27th at the University of London Observatory - the comet was at the time exactly side-on to the Earth. The view is around 35,000 km across. The blue features on either side of the tail are neutral gas arcs. Photo: University of London Observatory

Finding the comet in question was not simply a case of looking for known comets between the spacecraft and the Sun on May 1 - as Ulysses was so distant, the solar wind flowing at 750 kilometres per second could take days to reach the spacecraft. This gave time for the comet to move away from the Sun-Ulysses line, making it trickier to find. Comet Hyakutake (official designation C/1996 B2) had given Earth-bound observers a spectacular display during late March and early April, 1996, when it approached close to the Earth. Discovered by Japanese amateur astronomer Yuji Hyakutake in January 1996, the comet was at perihelion (its closest point to the Sun) on May 1 - the day of Ulysses's tail crossing. When Jones looked at where Hyakutake had been 8 days earlier, on April 23, it turned out that it had indeed been on the Sun-Ulysses line, and that from that point, it would take 8 days for the ion tail to be carried to Ulysses. Using the magnetometer data, the team found that the tail was the right size to belong to Hyakutake, and that it was parallel to the comet's orbital plane, as expected. The comet had been identified.

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.

Another artist's concept of Ulysses and the comet. Photo: NASA/JPL

"A few weeks before Ulysses' tail crossing, some observers reported tail lengths for Hyakutake that were much longer than possible if comet tails are assumed to be straight, and pointing away from the Sun", says Jones. "The Ulysses magnetic field measurements show that these assumptions aren't true.", he continued, "Although it can't quite fully account for some of the longest tail lengths reported in late March and early April 1996, Hyakutake's tail would have been curved in the correct way around the Earth for observers to see a tail longer than previously thought possible."

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.