European and American scientists will bid a fond farewell on July 1 to the space probe Ulysses, which has circled the sun gathering data for 17 years, almost four times its expected lifetime.
The first major collaboration between NASA and the European Space Agency (ESA), launched in 1990, "changed forever the way we view the sun and its effect on the surrounding space," David Southwood, ESA's director of science, said in announcing the end of mission.
Stuffed with 10 observational instruments, the 814-pound probe is the only satellite to have circled the sun's poles.
Its principle objective was to explore the boundaries and impact of the sun's sphere of influence, called the heliosphere.
One of its many findings was that the sun's magnetic fields, thrust outward by solar wind, extends into the solar system in ways that were previously not suspected.
"This is very important because regions of the sun not previously considered as potential sources of hazardous particles for astronauts and satellites must now be taken into account," noted the Paris-based ESA's Richard Marsden.
Scientists originally thought that the speed of solar wind -- a constant stream of particles emitted by the sun -- was about 250 miles per second.
But Ulysses proved that during much of the Sun's 11-year solar cycle, wind travels at nearly double that speed.
The probe also detected and analyzed cosmic dust flowing into our solar system from deep space, showing that it was at least 30 times more abundant than astronomers had thought.
Unexpectedly, new measurements of helium isotopes created in epochs billions of years apart also confirmed cosmological theories about the Big Bang -- and the likely fate of the universe.
The probe's "measurements support the theory by which the initial density of matter corresponds to a universe that will not collapse on itself at the end of time," explained Edward Smith, the Ulysses Project Scientist for NASA.
The Big Bang is thought to have occurred some 16 billion years ago.
Hurtling through space at an average speed of 35,000 miles per hour, Ulysses has logged over 5.3 billion miles.
The mission was originally designed to last five years, but engineers were able extend the life of the on-board generators powering the equipment by more than 12 years.
Power has now dwindled, however, to the point where fuel will soon freeze in the spacecraft's pipelines.
One of the challenges confronting space scientists at the outset was placing Ulysses into an orbit that passed over the sun's poles.
In October 1990 the Discovery space shuttle lifted the probe into space and away from the sun towards Jupiter.
Sixteen months later, the giant planet's gravity bent the spacecraft's flight path downward and away from the ecliptic plane along which all planets circle the sun, catapulting Ulysses into an orbit that went over the top and under the bottom of the sun. The closest it ever came to the sun was when it was on Earth.
In 17 years, the probe has circled the sun two and a half times.
Space Probe Ulysses Retires: Discovery News : Discovery Channel
