A Star-Mapping Mission Is Saved

SCIENCE COMMENTARY

WHILE planet scientists reveled over Voyager's Neptune pictures and cheered the launch of the Galileo Jupiter probe, a quiet orbital drama has been going on in Europe. The European Space Agency (ESA) has at least partially saved the mission of a major satellite from a tragic launch failure. This is the star-mapping satellite Hipparcos. Its mission is to measure the positions and motions of 100,000 stars with a precision that promises the first really accurate map of the sky.

After a flawless launch on an Ariane 4 rocket from Kourou, French Guiana, Aug. 8, the solid-fuel-rocket kicker motor that was to put the satellite in its proper orbit failed to ignite. Repeated attempts to fire the motor over the next several weeks also failed.

Last August, it looked as though astronomers had lost a mission that was to bring major revision of distances in the universe and deepen insight into our galaxy's evolution. ESA representatives visiting Voyager headquarters at the Jet Propulsion Laboratory in Pasadena, Calif., looked glum. Britain's famed amateur astronomer Patrick Moore could only hold his head and mutter ``it's a tragedy.''

But Hipparcos team members rallied. They have gotten the satellite working. They still anticipate the most accurate star map astronomers have ever had, even though it will not attain the full design precision and may not include all the targeted stars.

Hipparcos is one of those acronyms that tortures language to produce a symbolic name. It stands for High Precision Parallax Collecting Satellite. Parallax is a fancy term for measuring stellar distances by triangulation.

However, Hipparcos is also the name of a Greek pioneer of astrometry, as star mapping is called. He measured lunar parallax - obtaining the Earth-moon distance - some 2,100 years ago.

His orbiting namesake is designed to do astrometry with a resolution over a thousand times better than that of an unaided human eye. This means being able to distinguish between two points on the sky separated by an angular distance of only about 0.002 arc seconds.

To generate an all-sky map of 100,000 stars to that precision, Hipparcos needs to be in a stable orbit high enough so that Earth doesn't eclipse its vision.

It also must orbit beyond the planet-encircling radiation belts whose bombarding particles would degrade the spacecraft's electricity-generating solar cells. That means placing Hipparcos in the geostationary orbit, where communications satellites also travel, 36,000 kilometers (22,300 miles) out from Earth.

Standard procedure is to launch a satellite into a transfer orbit with a perigee (closest point to Earth) the height reached by the launch rocket and an apogee (farthest point from Earth) the geostationary height. Then a so-called apogee kick motor is fired to raise perigee to 36,000 km. This puts the satellite on a fully circular geostationary orbit. This is the motor that failed for Hipparcos.

Since then, Hipparcos controllers have used maneuvering rockets on the satellite to raise perigee slightly. Hipparcos now is on a 500 x 36,000 km orbit. While this is hardly ideal, project scientists report that the star data coming in look good. The solar cells are exposed to radiation-belt bombardment for part of the orbit, so ESA officials don't know how long the mission will last. But they do expect to gather much good data.

I haven't seen Patrick Moore since August. But I imagine he holds up his head and smiles when you mention Hipparcos these days.

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