Trade.Information

Ulysses Hanging on Valiantly Waiting for a Symbol of the Fuel Freeze

The Ulysses spacecraft, whose mission was expected to end on 1 July 2008, is hanging on valiantly as spacecraft controllers wait for a sign of the fuel freeze that would end the mission. This could happen any time now.

Controllers will know that the fuel needed to keep the antenna pointing towards Earth has started to freeze when Earth-pointing manoeuvres become less efficient and the radio signals from the spacecraft grow weaker.

In the meantime, Ulysses is still providing important science data as it pursues its exploration of the heliosphere, the sphere of influence of our star.

Although the spacecraft can now transmit data only in real time, the amazing 17.5-year-old mission is using all the time it has left to add to the wealth of information collected so far.

About Ulysses spacecraft
Ulysses was a robotic space probe designed to study the Sun at all latitudes. The spacecraft, named for the Latin translation of "Odysseus", was launched October 6, 1990 from the Space Shuttle Discovery (mission STS-41) as a joint venture of NASA and the European Space Agency. It was originally scheduled for launch in 1986 aboard the Space Shuttle Challenger. The spacecraft was equipped with instruments to characterize fields, particles, and dust, and was powered by a radioisotope thermoelectric generator (RTG). However in February of 2008, what scientists had always known began to materialize. The power output from the RTG, which is generated by heat from the radioactive decay of plutonium-238, has dwindled to the point where it is insufficient to power some critical internal heaters to keep the spacecraft's attitude control fuel from freezing. The Ulysses spacecraft was deactivated on 2008-07-01.

Until Ulysses, the Sun was only observed from low solar latitudes. The Earth's orbit defines the ecliptic plane, which differs from the Sun's equatorial plane by only 7.25 degrees. Even spacecraft directly orbiting the Sun do so in planes close to the ecliptic because a direct launch into a high inclination solar orbit would require a prohibitively large launch vehicle.

Several spacecraft (Mariner 10, Pioneer 11, and Voyagers 1 and 2) had performed gravity assist maneuvers in the 1970s. Those maneuvers were to reach other planets also orbiting close to the ecliptic, so they were mostly in-plane changes. However, gravity assists are not limited to in-plane maneuvers; a suitable flyby of Jupiter could produce a significant plane change. An Out-Of-The-Ecliptic mission (OOE) was thereby proposed. See article Pioneer H.

Originally, two spacecraft were to be built by NASA and ESA, as the International Solar Polar Mission. One would be sent over Jupiter, then under the Sun. The other would fly under Jupiter, then over the Sun. This would provide simultaneous coverage. Due to cutbacks, the US spacecraft was canceled in 1981. One spacecraft was designed, and the project recast as Ulysses, due to the indirect and untried flight path. NASA would provide the Radioisotope Thermoelectric Generator (RTG) and launch services, ESA would build the spacecraft assigned to Astrium GmbH, Friedrichshafen, Germany (formerly Dornier Systems). The instruments would be split into teams from universities and research institutes in Europe and the United States. This process provided the 10 instruments on board.

In figure 1, Artist’s impression of Ulysses

In figure 2, Ulysses's orbit