Named after the hero of Greek legend, Ulysses is a joint mission between ESA and NASA which aims to explore the unknown space above the poles of the sun. The spacecraft was provided by ESA which also manages the mission operations. NASA provided the Space Shuttle Discovery to get the craft into orbit around the Sun and the power generator for the spacecraft.

Launch

Ulysses probe during pre-launch testing. Image credit: European Space Agency Ulysses was launched by the space shuttle Discovery in October 1990 and reached Jupiter in February 1992. The probe then used Jupiter’s gravity to slingshot to a 6.2-year orbit above the Sun’s poles. Ulysses passed over the North Pole of the Sun in 1995 and 2001 and the South Pole in 1994 and 2000.

Objectives

There are 11 scientific instruments on board Ulysses, investigating phenomena ranging from the solar wind to cosmic dust and gamma-ray bursts. Ultimately the data will enable scientists to better understand the Sun and its heliosphere and perhaps the Sun’s influence on the Earth’s climate.

Results

Artist’s impression of Ulysses' path as it encountered Jupiter to enter polar orbit around the Sun. Image credit: European Space Agency By the time Ulysses had completed its first orbit in 1998, our knowledge of the heliosphere (the huge bubble which marks the extent of the solar wind) had taken a dramatic step forward. Ulysses completed a three-dimensional map of the heliosphere and discovered:

• The composition of the local interstellar medium, a mixture of gas and dust (mostly hydrogen and helium, with about 0.06% made up of heavier elements, such as oxygen and nitrogen) which penetrates into the solar system between planets
• The wind boundary between the fast and slow solar winds that creates a shock wave and causes particles to gain electric charge, ultimately leading to magnetic storms and auroras when they reach the vicinity of the Earth
• The extent, direction and flow of interplanetary and interstellar dust within the solar system.

The second set of passes over the Sun's poles took place at solar maximum, when the Sun is covered with many sunspots and outbursts such as solar storms are common. Ulysses detected the reversal of the Sun's magnetic field that takes place at the end of each 11-year cycle of activity.

In 2005 ESA approved funding for the mission to be extended until March 2008. The spacecraft will be operational during its passes over the Sun's south and then north poles in 2007. As in the mid-1990s the Sun will once again be at a minimum level of activity, but this time the magnetic field will be in the opposite direction. Astronomers are keen to see how much difference this makes to the flow of material from the Sun.

The Future

Ulysses is a highly successful mission. By 2008 the probe will have been operating for 18 years which compares very well with most other spacecraft. For much of the time Ulysses is a long way from the Sun (as far out as Jupiter's orbit) and it depends on the warmth of a small radioactive source to keep its fuel from freezing.

When Ulysses next approaches the Sun it will warm up, but by the following year will once again be in a much colder region of the Solar system. Eventually the heat emitted by the radioactive source will be too feeble for the spacecraft to continue operating and at that point the mission will come to an end.