Ulysses is 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). By February of 2008, the power output from the RTG, which is generated by heat from the radioactive decay of plutonium-238, dwindled to the point where it was insufficient to power some critical internal heaters to keep the spacecraft's attitude control hydrazine fuel from freezing.
The nominal end of mission date was supposed to have been July 1, 2008. Nevertheless mission operations are continuing past this date, the cessation of mission operations and deactivation or hibernation of the spacecraft will probably be determined by when the attitude control fuel freezes.
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.
On its way to Jupiter the spacecraft was in an elliptical Hohmann transfer orbit with perihelion near 1 AU and aphelion near 5 AU, Jupiter's distance from the sun. At this time Ulysses had a low orbital inclination to the ecliptic.
On February 22, 2008, 17 years 4 months after the launch of the spacecraft, ESA and NASA announced that mission operations for Ulysses would be likely to cease within a few months. On April 12, 2008 NASA announced that the end date will be July 1, 2008. The spacecraft operated successfully for over four times its design life. A component within the last remaining working chain of X-band downlink sub-system failed on January 15, 2008. The other chain in the X-band sub-system had previously failed in 2003.
Downlink to Earth resumed on S-band, but the beamwidth of the high gain antenna on S-band is not as narrow as on X – so the downlink signal is much weaker, thereby reducing the achievable data rate. As the spacecraft travels on its outbound trajectory to the orbit of Jupiter, the downlink signal will eventually fall below the receiving capability of even the largest antennas (70m in diameter) of the Deep Space Network. Even before the downlink signal is lost, the hydrazine attitude control fuel on-board the spacecraft will likely freeze, as the radioisotope thermal generators fail to generate enough power for the heaters to combat the cold of space. Once the hydrazine freezes, the spacecraft will no longer be able to maneuver to keep its high gain antenna pointing towards Earth, and the downlink signal will then be lost in a matter of days. The failure of the X-band communications sub-system hastens this, because the coldest part of the fuel pipework was routed over the X-band TWTAs which, when one of them was operating, kept this part of the pipework sufficiently warm.
The previously announced mission end date of July 1, 2008 came and went but mission operations is continuing albeit in a reduced capacity. The availability of science data gathering is limited to only when the Ulysses is in contact with a ground station due to the deteriorating S-band downlink margin no longer being able to support simultaneous real-time data and tape recorder playback . When the spacecraft is out of contact with a ground station, the S-band transmitter is switched off and the power is diverted to keep the internal heaters to add to the warming of the hydrazine. The eventual date of mission cessation will probably be determined by when the hydrazine freezes or when it is depleted.
The spacecraft body was roughly a box, approximately 10 × 11 × 7 feet in size (3 × 3.3 × 2 m). The box mounted the 1.65 meter dish antenna and the RTG power source. The box was divided into noisy and quiet sections. The noisy section abutted the RTG; the quiet section housed the instrument electronics. Particularly "loud" components, such as the preamps for the radio dipole, were mounted outside the structure entirely, and the box acted as a Faraday cage.
Ulysses was spin-stabilized about the axis of the dish. The RTG, whip antennas, and instrument boom were placed to stabilize this axis. Spin was nominally 5 rpm. Inside the body was a hydrazine fuel tank. Hydrazine monopropellant was used for course corrections, and to repoint the spin axis (and thus, the antenna) at Earth. The spacecraft was controlled by eight thrusters, in two blocks. Thrusters were pulsed in the time domain to perform rotation or translation. Four Sun sensors detected orientation. For fine attitude control, the S-band antenna feed could be tipped slightly off-axis. A signal from Earth then pulsed with the 5 rpm spin. The pulsing was deconvolved into orientation, a method called CONSCAN.
The spacecraft used S-band for uplinked commands and downlinked telemetry, through dual redundant 5-watt transceivers. The spacecraft used X-band for science return (downlink only), using dual 20W TWTAs. Both bands used the dish antenna; both were prime-focus feeds, unlike the Cassegrain feeds of most other spacecraft dishes.
Dual tape recorders, each of approximately 45 megabit capacity, stored science data between the nominal 8-hour communications sessions. During peak DSN periods, the instruments recorded at lower resolution to reduce the load on the DSN.
The spacecraft was designed to withstand both the heat of the inner solar system and the cold at Jupiter distance. Extensive blanketing and electric heaters protected against cold. Heating was minimized by the 1.3 AU perihelion, meaning that Ulysses always studied the sun from a greater distance than the Earth.
Overall weight at launch was 390 kg (814 pounds).
Radio/Plasma antennas. Two beryllium-copper antennas unreeled outwards from the body, perpendicular to the RTG and spin axis. Together this dipole spanned 72 meters. A third antenna, of hollow beryllium-copper, deployed from the body, along the spin axis opposite the dish. It was a monopole antenna, 7.5 meters long. These measured radio waves generated by plasma releases, or the plasma itself as it passed over the spacecraft.
Experiment Boom. A third type of boom, shorter and much more rigid, extended from the last side of the spacecraft, opposite the RTG. This was a hollow carbon-fiber tube, of 50 mm diameter. It can be seen in the photo as the silver rod stowed alongside the body. It carried four types of instruments. A solid-state X-ray instrument, which was composed of two silicon detectors to study X-rays from solar flares and Jupiter's aurorae. The GRB experiment consisted of two CsI scintillator crystals with photomultipliers. Two different magnetometers were mounted: a vector helium magnetometer and a fluxgate magnetometer. A two axis magnetic search coil antenna measured AC magnetic fields.
Total instrument mass was 55 kg.