Ulysses was the first space probe to explore the region of space above the Sun's poles, as a joint project of the European Space Agency (ESA) and NASA. The spacecraft was provided by ESA and the launch by NASA. The idea to send a probe out of the ecliptic (the plane in which the planets orbit the Sun) and over the Sun's poles was first put forward by a group of U.S. scientists in 1959, just two years after the launch of Sputnik. Even though the scientific importance of an out-of-ecliptic mission was recognized early in the space age, the technical means to accomplish it became available only in the early 1970s as a result of knowledge gained through the Apollo program and by sending probes to the outer planets.
Planning for Ulysses (first called the Out-of-Ecliptic mission and later International Solar Polar Mission—ISPM) began in 1974. The idea was to launch two spacecraft from the Space Shuttle toward Jupiter and to use its gravity to swing them far out of the ecliptic. The two probes, each carrying a suite of scientific instruments, were to orbit the Sun in opposite directions. Launch was foreseen for February 1983, with ESA and NASA each providing one of the probes. The ESA spacecraft was built by a European consortium led by West Germany's Dornier Systems and carried nine instruments: a pair of magnetometers to measure the strength and direction of the heliospheric magnetic field; a suite of sensors to measure the bulk properties of the solar wind, including its elemental and charge-state composition; a suite of energetic particle and cosmic ray detectors; an instrument to measure the natural radio emission from the Sun and planets; a gamma-ray burst detector; instruments to measure interplanetary and interstellar dust grains and neutral interstellar helium atoms.
In 1980 the project received its first setback when, as a result of difficulties with the development of the Space Shuttle, NASA announced a launch delay of two years. Work on the ESA spacecraft was already well advanced and so ESA decided to continue with its development and integration and then, following completion in 1983, to store the spacecraft until the new launch date. In February 1981 funding problems led NASA to cancel its spacecraft and delay the launch of the ESA spacecraft by one more year until May 1986. These decisions, which were taken without consultation with ESA, constituted a unilateral breach of the ISPM Memorandum of Understanding and soured NASA-ESA relations. An obvious consequence of the change from a dual- to a single-spacecraft mission was the loss of half of the scientific payload, including a number of European experiments. Enough of the original unique scientific objectives remained, however, that ESA decided to continue development of its spacecraft. The launch disruptions caused by the loss of Space Shuttle Challenger in January 1986 resulted in a further launch delay.
Finally on 6 October 1990 Ulysses began its journey on board Space Shuttle Discovery. Following a Jupiter flyby in 1992 Ulysses passed over the Sun's polar regions three times, first in 1994–95 and again in 2000–2001 and 2006–8. The first and third polar passes occurred close to solar activity minimum and revealed a relatively simple heliosphere dominated by fast solar wind streams originating in the polar regions. In contrast, the second polar passes occurred near solar maximum, with complex structure found at all latitudes. An important difference between the two solar minimum passes was the reversal of the Sun's magnetic field that occurred in 2001 and its effect on the charged cosmic ray particles.
From its unique orbit inclined at 80 degrees to the ecliptic, Ulysses gave scientists important new information about the nature of the Sun's magnetism; the creation and behavior of the solar wind, cosmic rays, and energetic charged particles; the dust and gas that surround Earth's solar system; and some of the most energetic events in the known universe, gamma-ray bursts. In addition to its scientific successes, Ulysses also exemplified the benefits and pitfalls of international cooperation in space exploration.
See also: European Space Agency, Space Science Policy, Space Shuttle
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