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|7 November 2008|
Ulysses, a joint ESA/NASA mission, was launched aboard the Space Shuttle on 6 October 1990 from Kennedy Space Center in Florida, heading for an unprecedented journey of discovery. It was the first mission to study the unknown environment of space above and below the poles of the Sun. With Ulysses, scientists obtained the first-ever four-dimensional map of the heliosphere – the bubble generated by the solar wind that defines the sphere of influence of the Sun.
Originally designed for a lifetime of five years, the mission has surpassed all expectations. The reams of data returned by Ulysses have forever changed the way scientists view the Sun and its effect on the space surrounding it.
After more than 17 years of observations, almost four times its expected lifetime, and having surmounted several operational challenges, Ulysses is now approaching its natural end. Its power supply is unable to keep the spacecraft warm enough, leaving it to succumb to the cold conditions in space.
ESA and NASA first studied the possibility of a joint out-of-ecliptic mission in 1974. This mission, in which two spacecraft were to be sent over the poles of the Sun, was approved in 1977. By 1981, the International Solar Polar Mission, or ISPM as it was then called, had turned into a single-craft mission, comprising a European-built probe, with half the instruments from Europe and the rest from the US.
Originally approved for launch in 1983, a number of delays meant it was eventually launched by Space Shuttle Discovery in 1990.
During the course of its mission, Ulysses has been joined in space by other missions (SOHO, Wind, ACE, Hinode, STEREO). Together, they form a network that is studying the Sun and its environment as never before.
After just over 17.5 years, the mission is approaching its end. The declining output from the Radioisotope Thermoelectric Generator (RTG), which provides power for the craft and its payload, is unable to provide enough heat. This means that the fuel for the thrusters will freeze . In mid-January 2008, the situation worsened when the main radio transmitter failed – its warmth kept the fuel from freezing.
To overcome these difficulties, ground controllers have been using a smaller transmitter to ensure that as much science as possible is returned from Ulysses in the last few weeks of its life.
The mission is expected to end by 1 July. Once it is clear that the fuel needed to keep the main antenna pointing towards Earth has started to freeze, ground controllers will put Ulysses into a stable configuration. It will continue to orbit the Sun indefinitely.
The orbit is an ellipse with the Sun at one focus (heliocentric), and is inclined 80° with respect to the Sun’s equator (polar). The orbital period is 6.2 years. Maximum distance from the Sun (aphelion) is reached at about 810 million km (or 5.4 AU; one AU or Astronomical Unit equals the average distance between Earth and the Sun, or about 150 million km) and minimum distance (perihelion) is at about 200 million km (or 1.3 AU).
The gravity-assist at Jupiter meant that Ulysses performed a polar pass first over the Sun’s south pole and then over its north pole roughly a year later. The polar passes are periods during which the spacecraft is above 70° heliospheric latitude in either hemisphere. The mission was designed to maximise the total duration of each polar pass.
Over more than 17 years of operation, Ulysses has orbited the Sun three times and performed six polar passes:
During its mission, Ulysses has travelled 8.6 thousand million km (or 57.65 AU) at an average speed of 56 000 km/hr. This corresponds to the distance a spacecraft must travel to reach more than half-way to the edge of the heliosphere, well beyond the orbit of Pluto (39.5 AU).
The science objectives of the Ulysses mission were:
Thanks to its suite of instruments and its special orbit around the Sun, in addition to the long duration of its mission – a major achievement on its own – the key discoveries of Ulysses range from the best-ever understanding of the heliosphere and the role played by its magnetic field in all the phenomena occurring inside it, to important astrophysical questions such as the evolution of the Universe. Key discoveries:
Design: the overall design was dictated by the large distances from Earth and the Sun (up to 950 million km from Earth, 810 million km from the Sun). At such distances, solar power could not provide enough electricity; instead, a Radioisotope Thermoelectric Generator was provided. A large antenna (1.65 m diameter) was necessary to communicate with Earth from large distances. The thermal design had to accommodate the widely-varying temperatures during the mission. The electronics were hardened to withstand the strong radiation near to Jupiter during the close flyby in 1992.
Mass: 366.7 kg at launch
Dimensions: 3.2 × 3.3 × 2.1 m LxWxH
Magnetometer (MAG): MAG has been performing measurements of the magnetic field in the heliosphere and studying how it varies at different heliospheric latitudes. Measurements of Jupiter’s magnetic field were also performed. Principal Investigator: A. Balogh, Imperial College of London (UK).
Solar Wind Plasma Experiment (SWOOPS): The SWOOPS experiment has been studying the solar wind at all solar distances and latitudes and in three dimensions. Principal Investigator: D.J. McComas, Southwest Research Institute (USA)
Solar Wind Ion Composition Instrument (SWICS): SWICS has been determining the composition, temperature and speed of the atoms and ions that comprise the solar wind. Principal Investigators: J. Geiss, ISSI (CH) and G. Gloekler, University of Michigan (USA).
Unified Radio and Plasma Wave Instrument (URAP): URAP has been studying radio waves from the Sun and electromagnetic waves generated in the solar wind close to the spacecraft. Principal Investigator: R.J. MacDowall, NASA/GSFC (USA).
Energetic Particle Instrument (EPAC) and GAS: EPAC has been investigating the energy, fluxes and distribution of energetic particles in the heliosphere, while GAS studied the electrically-uncharged gas (helium) of interstellar origin. Principal Investigator: N. Krupp, Max Plank Institute for Solar System Research, Lindau (D).
Low-Energy Ion and Electron Experiment (HI-SCALE): HI-SCALE has been investigating the energy, fluxes and distribution of energetic particles in the heliosphere. Principal Investigator: L.J. Lanzerotti, New Jersey Inst. of Technology (USA).
Cosmic Ray and Solar Particle Instrument (COSPIN): COSPIN has been investigating the energy, fluxes and distribution of energetic particles and galactic cosmic rays in the heliosphere. Principal Investigator: R.B. McKibben, University of New Hampshire (USA).
Solar X-ray and Cosmic Gamma-Ray Burst Instrument (GRB): GRB has been studying cosmic gamma-ray bursts and X-rays from solar flares. Principal Investigator: K. Hurley, UC Berkeley (USA).
Dust Experiment (DUST): DUST has carried out direct measurements of interplanetary and interstellar dust grains to investigate their properties as functions of the distance from the Sun and solar latitude. Principal Investigator: H. Krüger, Max Plank Institute for Solar System Research, Lindau (D).
The overall cost of Ulysses is about €1000 million at today’s prices. ESA’s share has been about €450 million Euros at current prices, including the spacecraft and about €2.5 million annually for spacecraft operations. NASA’s share has been about $520 million (about €338 million) at current prices, including development, launch and 17 years of mission operations and data analysis. The European instruments, hardware and operations have been funded nationally.
ESA Mission Manager & Project Scientist: Richard Marsden
ESA Mission Operations Manager: Nigel Angold
NASA Project Manager: Ed B. Massey
NASA Project Scientist: Edward J. Smith
For further information:
ESA Media Relations Office
DC Agle, NASA/JPL Media Relations
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