Probing the edge of the solar system

For Earth dwellers, the Sun dominates the sky. But from an interstellar perspective it's just another average star. It's influence peters out somewhere beyond the farthest planet. Scientists call this frontier with interstellar space the heliopause. They don't know where it is. But, as Richard P. Laeser of the NASA Jet Propulsion Laboratory (JPL) puts it, ``The United States has an armada of spacecraft out there trying to explore [it].''

It's a retirement job for the Pioneer and Voyager planet-probing ships.

Having fulfilled their missions to study Jupiter and Saturn, these craft are on course for interstellar space. As they pass through the heliopause, scientists can begin to outline the shape and extent of the Sun's domain. Once through it, the ships will be the interstellar world reporting on regions where humanity's instruments have never been before. It will literally be a new frontier for on-site scientific exploration.

Launched in March 1972, Pioneer 10 flew by Jupiter 21 months later. Pioneer 11 left Earth in April 1973, flew to Jupiter 20 months later and swung by Saturn in late summer of 1979. Voyager 1 left Earth in September 1977, reached Jupiter in March 1979 and Saturn in November 1980. It's twin, Voyager 2, which was launched in August 1977, surveyed Jupiter in July 1979 and Saturn in August 1981. It passed Uranus last January and will go by Neptune in August 1989.

Voyager 2 is by far the hardest working ship in this interstellar fleet, since NASA stretched its mission well beyond the original targets of Jupiter and Saturn. Now Laeser and his colleagues are talking about using it well into the next century.

As far as consumables are concerned, he says ``we're feeling confident about flying until 2005, if not 2010.''

One critical item is the ship's hydrazine attitude control gas. Small thrusters use this gas to maneuver and stabilize the spacecraft. Even after exploring three planetary systems, the tank on Voyager 2 is half full.

Also, Voyager's radio-isotope powered generator is slowly running down. This uses the heat released as radioactive material decays, transforming that heat into electricity. In the 12 years that Voyager 2 has been in space, the generator's output has dropped from around 470 watts to a little under 400 watts right now. It takes a little over 200 watts to power the spacecraft plus extra power when its instruments are working. Here again, there's plenty of energy left in the power pack to keep the spacecraft operating for decades.

Other items on Laeser's ``consumables'' list aren't physical supplies. They're capabilities that diminish as the spacecraft moves away from the Sun.

They include such functions as sending back data at a useful rate. The farther away the spacecraft is, the more energy it takes to send back a data bit -- the smallest unit of information. That will limit the amount of data Voyager 2 can beam home. Beyond Neptune, it would become difficult to send back lots of pictures. But for the kinds of measurements the spacecraft will be making in its retirement, there should be plenty of energy available. Laeser says JPL should be able to communicate with Voyager 2 ``out to 2050 at least.''

Perhaps the most critical item is the Sun itself. Voyager needs to see the Sun to know where to point its main antenna and beam its bits to Earth. Yet even at Neptune -- 30 times more distant from the Sun than is Earth -- the Sun will be little more than a bright star. The Voyagers weren't designed for interstellar research. So Laeser considers the sensitivity of their Sun sensors as the biggest uncertainty on his ``consumables'' list.

He feels confident they will work through the rest of this century. ``After that,'' he says, ``it gets a little mushy because we never really thought we'd get that far.''

Having already gotten so much extra service out of Voyager 2, though, the project team is unlikely to be satisfied with just studying interstellar magnetic fields and particles. Laeser explains: ``The ultraviolet instrument on the spacecraft has proved to be very valuable for ultraviolet astronomy. There is a push, in fact, to turn it into an observatory ... post-Neptune.''

Observing stars by the ultraviolet light they emit would be a totally unanticipated job for Voyager 2. That would be quite an achievement for a spacecraft whose mission was supposed officially to end when it flew by Saturn 5 years ago.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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