Ulysses (spacecraft)
Solar probe that studied the Sun from all latitudes.
Ulysses was a robotic probe built to orbit the Sun and observe it from every latitude. A joint project led by the European Space Agency with NASA and Canada’s National Research Council, it launched in 1990 and completed three fast scans of the Sun’s latitudes—in 1994/1995, 2000/2001, and 2007/2008—along with studies of several comets. Its final day of operations was June 30, 2009.
To reach a high-inclination orbit around the Sun, the probe needed to leave the Solar System’s plane. No launch vehicle could provide the necessary velocity change, so mission planners used a gravity assist from Jupiter. That Jupiter flyby ruled out solar power, so Ulysses relied on a General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG).
Originally named Odysseus after Homer’s hero, the spacecraft was renamed Ulysses—the Latin form—at ESA’s request, also honoring Dante’s character in the *Inferno*. It was first scheduled for launch in May 1986 aboard the Space Shuttle Challenger (mission STS-61-F), but after Challenger’s loss on January 28, 1986, the launch was delayed to October 6, 1990, aboard Discovery (mission STS-41).
ESA designed the spacecraft, and Dornier Systems built it. The main body was a box about 3.2 by 3.3 by 2.1 meters, mounting a 1.65-meter dish antenna and the RTG power source. The box was split into noisy and quiet sections: the noisy part next to the RTG, the quiet part housing instrument electronics. Loud components like radio dipole preamps were mounted outside, and the box acted as a Faraday cage.
Ulysses was spin-stabilized around its z-axis, which roughly aligned with the dish antenna. The RTG, whip antennas, and instrument boom helped stabilize this axis, with a nominal spin rate of 5 rpm. Inside was a hydrazine fuel tank. Hydrazine monopropellant was used for course corrections toward Jupiter and later solely to reorient the spin axis (and antenna) toward Earth. Eight thrusters in two blocks controlled the spacecraft, pulsed in time for rotation or translation. Four Sun sensors detected orientation. For fine attitude control, the S-band antenna feed was mounted slightly off-axis, creating an apparent oscillation in received signals from Earth. The amplitude and phase of this oscillation indicated the spin axis’s orientation relative to Earth—a method called conical scanning, once used in early radars and infrared missiles.
Quick Facts
- Names List
- Odysseus
- Mission Type
- High-inclination solar orbiter
- Operator
- NASA / ESA
- Cospar Id
- 1990-090B
- Satcat
- 20842
- Manufacturer
- Astrium GmbH, Friedrichshafen / (formerly Dornier Systems)
- Launch Mass
- 371 kg
- Payload Mass
- 55 kg
- Dimensions
- 3.2 × 3.3 × 2.1 m
- Power
- 285 watts
- Launch Date
- 6 October 1990, 11:47:16 UTC
- Launch Rocket
- Space Shuttle Discovery (STS-41) with Inertial Upper Stage and PAM-S
Facts from the source article.
Lore & Background
The spacecraft was originally named Odysseus, because of its lengthy and indirect trajectory to study the solar poles. It was renamed Ulysses, the Latin translation of 'Odysseus', at ESA's request in honor not only of Homer's mythological hero but also of Dante's character in the Inferno. Ulysses was originally scheduled for launch in May 1986 aboard the Space Shuttle Challenger on STS-61-F, but due to the 28 January 1986 loss of Challenger, the launch was delayed until 6 October 1990 aboard Discovery (mission STS-41).
To study the Sun at all latitudes, the probe needed to change its orbital inclination and leave the plane of the Solar System. To reach the desired orbit around the Sun, the mission's planners chose a gravity assist maneuver around Jupiter. This Jupiter encounter meant that Ulysses could not be powered by solar cells; it was powered instead by a General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG). The spacecraft was designed by ESA and built by Dornier Systems, a German aircraft manufacturer. It was spin-stabilised, with a hydrazine fuel tank for course corrections and attitude control.
Ulysses carried twelve different instruments from ESA and NASA, including magnetometers, solar wind plasma and ion composition experiments, radio and plasma wave instruments, energetic particle detectors, cosmic ray instruments, a gamma-ray burst experiment, and a dust experiment. The radio communications link was also used to search for gravitational waves and to probe the Sun's atmosphere through radio occultation, though no gravitational waves were detected.
Reader's Guide
Ulysses was significant as the first spacecraft to explore the Sun's polar regions, providing data from all solar latitudes that were previously inaccessible. Its three fast latitude scans in 1994/1995, 2000/2001, and 2007/2008 covered different phases of the solar cycle, allowing scientists to study the Sun's magnetic field, solar wind, and energetic particles at high latitudes. The mission also contributed to studies of Jupiter's magnetic field and several comets. By using a Jupiter gravity assist, Ulysses demonstrated a trajectory technique that enabled high-inclination solar orbits without requiring a prohibitively large launch vehicle. The spacecraft's longevity—operating from 1990 until 30 June 2009—provided nearly two decades of observations. Its legacy includes a comprehensive dataset on the heliosphere and the solar wind's three-dimensional structure, which continues to inform models of solar activity and space weather. The collaboration between ESA, NASA, and Canada's National Research Council also set a precedent for international space science missions.
Did You Know?
- Ulysses was originally named Odysseus, after the mythological hero, but was renamed to the Latin translation at ESA's request.
- The probe was powered by a radioisotope thermoelectric generator because its Jupiter gravity assist meant it could not use solar cells.
- Ulysses made three fast latitude scans of the Sun: in 1994/1995, 2000/2001, and 2007/2008.
- The spacecraft's radio link was used to search for gravitational waves, though none were detected.
More in Missions to the Sun 1-21
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced