The primary mission of the Ulysses spacecraft was to characterize the sun's heliosphere as a function of solar latitude. The heliosphere is the vast region of interplanetary space occupied by the sun's atmosphere and dominated by the outflow of the solar wind.
To study the heliosphere, Ulysses was placed into a six-year orbit around the sun that carried it out to Jupiter's orbit and back. Covering such a vast expanse of space provided unique and unexpected opportunities for the spacecraft. During its more than 17-year mission, Ulysses had three unplanned encounters with comet tails.
Scientists combed the data of a chance 2007 encounter Ulysses made with the tail of comet McNaught. The nucleus of this comet was some 257 million kilometers (160 million miles) from the spacecraft during encounter. Instead of using the length of the tail to measure the scale of the comet, scientists used Ulysses data to gauge the size of the region of space disturbed by the comet's presence.
Ulysses' solar wind ion composition spectrometer instrument, developed by University of Michigan heliophysicist George Gloeckler, found that even at such a great distance, the tail had filled the solar outflow with unusual gases and molecules.
In response, the solar wind that usually measures about 700 kilometers per second (435 miles per second) at that distance from the sun, was less than 400 kilometers per second (249 miles per second) inside the comet's tail, as measured by one of Ulysses' instruments called "Solar Wind Observations Over the Poles of the Sun," whose principal investigator is Dave McComas of the Southwest Research Institute in San Antonio, Texas.
Ulysses took nearly nine times as long to traverse the tail of comet McNaught in 2007 as it did during a 1996 chance encounter with comet Hyakutake - which until now held the record for longest known tail. This led scientists to believe the comet McNaught was remarkably productive in releasing gas and material from its surface.
While measuring such comet "outgassing" can define the level of activity of a comet, it does not directly relate to its size. But if both comets were equally active, McNaught would have to be much larger in size to produce such a massive tail.
The interaction between comets' tails and the solar wind has been studied for decades. A comet's ion tail always points away from the sun, whether the body is traveling toward or away from the sun along the comet's elliptical orbit. It was this finding that eventually led in 1958 to the discovery of solar wind. The magnetism and velocity of the solar wind are so strong, the effect pushes the comet's tail outward.
When space shuttle Discovery launched Ulysses on Oct. 6, 1990, it had an expected lifetime of five years. The mission gathered unique information about the heliosphere, the bubble in space carved by the solar wind, for nearly four times longer than expected. The mission ended on June 30, 2009.
The Ulysses spacecraft was built by Dornier Systems of Germany for ESA. NASA provided the launch and the upper stage boosters. The U.S. Department of Energy, Washington, supplied the generator that powers the spacecraft; science instruments were provided by both U.S. and European investigators. The spacecraft was operated from JPL by a joint NASA/ESA team and employed NASA's Deep Space Network for communications.