PIONEER 10 MARKS NEW EPOCH IN SOLAR SYSTEM EXPLORATION SEPTEMBER 18, 1990 RELEASE: 90-125 NASA's Pioneer 10 spacecraft, the first to leave the solar system, will reach another milestone on Saturday, Sept. 22, 1990. At 4:19 p.m. EDT, Pioneer 10 will be 50 times farther from the sun than the sun is from planet Earth. Reaching the 50 astronomical unit (AU) distance "marks a new epoch in exploration of the outer solar system," according to Pioneer experimenter James Van Allen of the University of Iowa. Van Allen, discoverer of the radiation belts around the Earth which bear his name, said reaching the 50 AU distance "has been a goal of physicists for many decades." When Pioneer 10 reaches that mark, it will be 4,647,809,899 miles from Earth. Pioneer already has travelled farther than any human-made object. The AU -- the average distance (93 million miles) between the sun and Earth -- is the primary unit used to measure distance within the solar system. Launched on March 2, 1972, Pioneer continues to make discoveries about the limits of the sun's atmosphere, called heliosphere. The spacecraft is seeking the boundary between the solar heliosphere and the true interstellar gas. Pioneer also continues to search for the first gravity waves to be detected and for data on the location of a possible 10th solar system planet. Pioneer left the solar system planets behind on June 13, 1983. Communication with Pioneer is expected to continue until the year 2000, when Pioneer will be 6.9 billion miles from the sun. The spacecraft's tiny 8-watt radio signal now has fallen to a signal strength of four billionths of a trillionth of a watt when it finally reaches the football-field-sized antennas of NASA's Deep Space Network. Round-trip transmission time for radio signals to the spacecraft and back to Earth is 13 hours, 47 minutes, travelling at the speed of light (186,000 miles per second). Pioneer's current speed along its flight path is about 28,900 mph. Pioneer was the first spacecraft to cross the asteroid belt, fly by Jupiter and return pictures and a description of the planet's magnetic field, interior structure and atmosphere and the mass of its moons. Pioneer's most important finding about the outer solar system is the extent of the sun's heliosphere, originally thought to have ended at the orbit of Jupiter. Pioneer is now almost 10 times farther away and still within the solar heliosphere. Pioneer carries a plaque for communication with any intelligent species which may find the spacecraft. The plaque shows a man and a woman, the Earth, a map of the solar system and location of the solar system in the galaxy. In the vacuum of space, Pioneer is expected to survive for millions of years, perhaps as long as the Earth itself. Pioneer 10 is managed by NASA's Ames Research Center, Monutain View, Calif. It was built by TRW Inc., Redondo Beach, Calif. Michael Braukus NASA Headquarters, Washington, D.C. (Phone: 202/453-1549) Peter W. Waller Ames Research Center, Moffett Field, Calif. (Phone: 415/604-9000)