The End of Ulysses

It’s been three years this weekend since one of the longest missions in space exploration ended. Known as Ulysses, this joint ESA/NASA spacecraft was finally turned off on June 30 2009. Planned back in the 1970s as a dual mission to study the Sun, only the European spacecraft was eventually built, although it carried instruments from both Europe and the US. The Challenger disaster delayed the launch until October 1990, when it was carried into space by the Space Shuttle Discovery. Ulysses headed first to Jupiter for a gravitational kick that flipped it out of the plane of the solar system. The Jupiter approach shifted Ulysses into a southward approach to the Sun, allowing the space craft to pass over the south pole of the Sun in 1994 and the north pole in 1995. It didn’t get particularly close to the Sun – Ulysses was at a distance of 300 million km when it passed over the poles of the Sun and at its closest approach to the Sun it was still beyond the Earth’s orbit, but it was able to survey the Sun from a previously unexplored direction. The spacecraft operated for three orbits, each one a little over 6 years in length. It passed over the poles of the sun three times in total, discovering how the space environment varied in the three spatial dimensions and with time. The passes were at times of varying solar activity and showed that on a large scale, the complex magnetic field near the solar surface simplifies into a field like a bar magnet inside the Sun. When solar activity is at a minimum, this bar magnet is aligned with the poles. Six years later, at maximum, the bar magnet has moved to lie at right angles to the poles. It then continues moving so that by the time of the next minimum, it is aligned with the poles again, but in the opposite orientation.

Ulysses, like its namesake, the hero of Greek legend, travelled into unexplored territory, including passes through the tails of three comets – Comet Hyakutake in 1996, Comet McNaught-Hartley in 2004 and Comet McNaught in 2007. It also became part of an international team of gamma ray burst detectors. Gamma ray bursts are believed to occur somewhere in the sky at a rate of about one per minute, coming from sources billions of light years away. There are different types of bursts; the fireball model attributes them to matter moving at near the speed of light that collides with other material, but what gets the matter moving so fast is still being studied. Bursts release as much energy in a few seconds as the Sun will release in its entire lifetime. Determining where the burst come from has been problematic, but widely separated detectors provide an excellent way of triangulating their position. Ulysses, with an 18 year flight time and a position well out of the plane of the solar system was a key part of this astrophysical data collecting.