Ulysses is a joint venture of NASA and the European Space Agency and was launched in 1990. Its primary goal is to observe in the regions of space above the Sun's north and south poles, but that is a particularly difficult orbit to attain. The Earth is moving so swiftly in the plain of the Sun's equator that the mission must go to great lengths to cancel that velocity and create an orbit at right angles to it.
The easiest way to reach a polar orbit is to use a gravity assist from another planet, and it takes a planet with the vast gravity field of Jupiter to meet this requirement. Ulysses still had to leave Earth's gravity field at seven miles per second (11.3 km/sec), the fastest interplanetary spacecraft ever launched. The spacecraft reached Jupiter about a year and a half after launch, and it made its closest approach to Jupiter on February 8, 1992. Ulysses made detailed and valuable measurements of Jupiter's magnetic field during the 12 days it flew within the magnetosphere. After leaving Jupiter, the craft traveled back to the inner solar system to arrive at the Sun in 1994.
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