Ulysses was launched to make continuous measurements of the solar wind, a steady stream of ionized particles that flows outwards from the star. In 1998 Pete Riley and colleagues from Los Alamos National Laboratory published a paper describing a dramatic drop in the number of protons registered by Ulysses. They suggested that a comet could have caused the signal.

Recently two groups independently discovered that Comet Hyakutake was probably the source of the signal. Geraint Jones of Imperial College in London and colleagues used the magnetometer to calculate the size of the comet's tail. "The fast, polar solar wind preserved the structure of the tail over an exceptional distance," says Jones, "otherwise it would have been extremely hard to recognize it." Jones noticed that Comet Hyakutake had crossed between the Sun and Ulysses on the 23 April 1996, which was 8 days before the instruments went haywire on 1 May. The size of the tail, together with its magnetic field, speed and the position of the spacecraft, pointed to Hyakutake as the source.

Meanwhile, a team led by George Gloeckler from the University of Maryland noticed that the SWICS detector on Ulysses had picked up the same patterns of carbon, nitrogen and oxygen ions detected in Halley's comet. After hearing about the magnetic field results from Jones, they concluded that the ions were from Hyakutake. "We were greatly surprised to find cometary material so far away from the nucleus," he says, "and the discovery of otherwise invisible comets may [now] be possible."

However, Jones does not intend to spend too much time looking for comets. "The chances are really small that a comet's tail will pass in front of Ulysses," he says, "but we may well be surprised again."