Scientists plan space mission that will be out of this solar system

Scientists at the Jet Propulsion Laboratory (JPL) are laying the groundwork for a mission to reach beyond the solar system. Such a trip, backers say, could return a wealth of information on planets such as Saturn and Pluto, the solar system as a whole, and conditions in space beyond the sun's influence. The spacecraft would also measure distances to stars with increasing precision.

``I feel a bit like the old cathedral builders, who started a project knowing that it would be up to future generations to finish it,'' says Aden Meinel, who along with his wife, Marjorie, heads a team of JPL scientists working on the 50-year-long mission.

The spacecraft would reach a distance of 93 billion miles from the sun - or 1,000 astronomical units - some 50 years after launch, which could occur as early as 2010.

Of primary interest is using the craft to measure distances to stars. More accurate measurements to greater distances would help fine-tune calculations about the age of the universe, which in turn bears on the question of whether the universe will expand forever or ultimately collapse.

The most accurate method uses the diameter of Earth's orbit as the base of an imaginary triangle, with the target star as the third point. But because the base line is limited, this method is good only for measuring distances of less than 400 light years. By comparison, Earth is some 27,710 light years from the center of this galaxy.

The ability to extend that base line 500 times by using a spacecraft means that the technique can be used not only to measure stellar distances in the Milky Way but to stars in nearby galaxies as well.

Work on the concept began three years ago, when JPL director Lew Allen threw down the gauntlet to his staff.

``Originally, I tried to stir up people thinking of interstellar missions by trying to encourage them to challenge each of the various precepts that say, `You can't do that,''' he says. ``I tried to extend the time scales that would be acceptable out to several hundred years, which already caused some people to choke a little bit.''

The Meinels, along with six Cal Tech undergraduates, took up the gauntlet and found that a visit to the nearest stars didn't look feasible with technologies that will be available in the 1990s. But further studies indicated that a mission to reach out to 1,000 astronomical units (AUs), giving the project its name TAU, was achievable and had substantial scientific merit.

``Everything is essentially ready except one item,'' Dr. Meinel says of the enabling technologies. ``We're lacking the nuclear power plant needed to run the ion propulsion system.''

Space-based nuclear reactors are under intensive study as part of the Strategic Defense Initiative, popularly known as the ``star wars'' program. In a report released earlier this month, the National Research Council recommended that NASA play a much larger role in the SDI's SP-100 reactor program to help ensure that its own future need for large amounts of energy in space will be met.

Once TAU cleared Earth, its reactor would begin suppling energy to a particle accelerator, which would emit streams of ions to provide the thrust. Acceleration would build gradually; by the time the spacecraft was about 6 billion miles from Earth, it would be traveling at 225,000 miles an hour.

Dr. Meinel says that the project would cost roughly the same amount as the Hubble Space Telescope, which is awaiting a June 1, 1989, launch. He adds that although it would take half a century for the craft to clear the 1,000 AU mark, it would begin relaying data three years after launch, giving scientists plenty to chew on during transit time.

The TAU craft would carry a 1.5-meter telescope for astronomy and a package of detectors to measure such phenomena as cosmic radiation and magnetic fields. High-powered lasers would be used for communications to and from the craft.