Solar Probe Awaits Day in the Sun

WHILE technicians ready Columbia for its long-delayed Astro astronomy mission, European Space Agency (ESA) officials have their eyes on its sister shuttle Discovery; it sits on a nearby pad with the Ulysses solar probe in its cargo bay. Ulysses is the gem of the space shuttle autumn manifest - the mission whose time-critical launch the National Aeronautics and Space Administration (NASA) is struggling to protect.

Should Columbia fail its fourth launch attempt in three months tomorrow or Wednesday, it will have to take a back seat to the joint NASA-ESA mission.

Ulysses will get to its solar rendezvous by way of Jupiter. It needs the giant planet's gravity to flip out of the ecliptic plane - the plane in which the planets orbit - so it can pass over the north and south poles of the Sun.

For this to work, Discovery must lift off its pad between Oct. 5 and 23. Otherwise, the mission - already seven years late - would have to wait another 13 months for Earth and Jupiter to come into favorable alignment again.

While the many delays of the shuttle program have embarrassed NASA, ESA officials have often pointed out that they, too, have found them burdensome.

The Ulysses project began as part of the old International Solar Polar Mission for which both ESA and NASA promised to supply shuttle-launched spacecraft for a mission in 1983.

Yielding to White House budget pressure, NASA abruptly canceled its craft in 1981. This forced planners to redesign the mission so that the European craft could do the work originally assigned to two probes.

For ESA, this raised the issue of American reliability as a space partner. ESA scientific program director Roger Bonnet points out that delays to Ulysses, beginning with the cancellation and including the Challenger disaster, and delays in orbiting the Hubble Space Telescope, which carries a major ESA instrument, caused the bulk of a $107 million ESA budget overrun.

ESA officials have repeatedly told NASA - and testified to the US Congress - that they are very concerned about the fate of NASA's Freedom space station. ESA is supplying a billion-dollar Columbus laboratory module for the station.

The several redesigns of Freedom that budget cuts have forced on NASA have raised concern within ESA that its costly commitment may risk a unilateral cancellation of this program too.

Doubt over US reliability as a space partner, as well as the mission's critical timing, has given Ulysses' October launch added importance for NASA.

The 804-pound Ulysses spacecraft, with American and European instruments on board, is to explore unknown space beyond the north and south polar regions above the Sun.

It will be looking at solar and extrasolar cosmic rays, at the ``wind'' of particles that flow out of the Sun, at magnetic fields, and at other aspects of this uncharted part of the solar system. Ulysses also will monitor space conditions on route to its target.

Timing is critical not only for the Jovian gravity assist but also for cooperation with four other spacecraft.

NASA's two Pioneer and two Voyager spacecraft, now heading out of the solar system, are to make simultaneous observations with those of Ulysses. Scientists hope to map out a three dimensional chart of the Sun's sphere of influence from these data. If this October's launch opportunity is lost, the backup November 1991 window would be Ulysses' last chance for that kind of research.

If launched next month, Ulysses will reach Jupiter in February 1992. It should begin its transit over the south solar pole in May 1994, cross the solar equator in February 1995, and pass through the north polar region in May 1995. Ulysses' flight path will keep it well out of solar heat: 1.3 astronomical units at its closest solar passing; 5.4 astronomical units at its most distant passing. One astronomical unit is 93 million miles, Earth's average distance from the Sun.

Anticipating a successful launch, ESA's Ulysses project manager Derek Eaton has said: ``Despite all the disappointments and frustrations from delays and cancellations over the past 12 years, I remain extremely enthusiastic about Ulysses. This is because we are doing something completely new and something on which future generations of spacecraft will build.''