HIPPARCHUS OF NICEA must have been an interesting fellow. He was a second-century B.C. mathematician, philosopher and astronomer. Using the only astronomical instrument available to him -- his eyes -- Hipparchus took on the daunting task of measuring the positions of the stars and planets as they passed overhead each night. He came up with a catalog of 1,080 stars, each of which he described simply as "bright" or "small."

Hipparchus wasn't the first astronomer to pursue the science of astrometry, as the astronomical discipline of positional measurement is now called. However, his star catalog was the first of many compiled over the centuries by astronomers using ever-better instruments and techniques. From those measurements -- all made from the Earth's surface -- astronomers have derived everything from basic stellar properties to estimates for the age of the universe.

On August 8, 1989, the science of astrometry took a long-awaited leap to the stars. Riding aboard an Ariane rocket was the High Precision Parallax Collecting Satellite, otherwise known as Hipparcos. For the next three and a half years, Hipparchus's 20th-century namesake measured the parallaxes and brightnesses of more than a million stars -- despite a potentially crippling accident that sorely challenged the project's architects.

To measure such shifts, the satellite was built to scan the sky continuously. A beam combiner would allow two sight lines, separated by 58, to be viewed simultaneously in the orbiting telescope's focal plane. The light received from each set of stars would be modulated by a grid that sliced into the focal plane with nearly 3,000 parallel slits. As a result, the stars under examination would have their relative separations in one direction measured with very high accuracy. Other directions would be picked up as the spacecraft rotated with respect to the sky.

Hipparcos's first-priority targets, 118,000 in number, were selected by more than 200 scientists working to determine a plethora of astronomical fundamentals: distances, motions, luminosities, masses, sizes, and ages for a wide range of red and white dwarfs, giant stars, radio and X-ray stars, variables, and binary stars. In addition, studies of star-cluster dynamics, stellar physics, and the interstellar medium were planned. Finally, the project would seek to establish a fundamental reference frame for the sky's visible stars and to connect that grid with objects seen in other portions of the electromagnetic spectrum.

In parallel with these space-based observations, ground-based programs were organized in cooperation with professional and amateur observers to obtain good positional and photometric data for selected program stars. This would allow mission planners to optimize their use of the satellite's limited lifetime. Major collaborative projects also began between users of the Hubble Space Telescope, very long baseline radio astronomers, and other important ground-based programs.

Excitement in the Hipparcos community of researchers, some of whom had worked with the project for 20 years, was high as Ariane lofted its precious payload into the sky. Their enthusiasm turned to disappointment, however, when the spacecraft's apogee-boost motor failed to fire. This left Hipparcos in an elongated orbit that took it through the Van Allen belts, which posed the risk of serious radiation damage to the solar panels that powered the spacecraft (S&T May 1990, page 496).

Fortunately, those panels turned out to be more resistant to radiation than expected. Over the course of several months, the European Space Agency Project Team, the European Space Agency Operation Center (ESOC), members of industry, and the Hipparcos scientific consortia turned their collective attention to the observational and scheduling problems posed by the highly eccentric orbit. As a result of their heroic efforts, the mission ran successfully from November 1989 to March 1993, gathering data 60 percent of the time.

As it turns out, Hipparcos exceeded expectations, achieving accuracies at the one-milliarcsecond level for all its principal targets; in addition, high-precision photometric measurements of these stars' brightnesses were obtained.

The satellite's star mappers were also used to survey more than 1 million stars down to magnitude 11.5; accuracies of 0.01 arcsecond and 0.02 magnitude were acheived for stars brighter than magnitude 9.5. This part of the mission was called Tycho, after the prolific 16th-century Danish astronomer, Tycho Brahe.

The Hipparcos data are best grouped into three main areas: distances based on parallax measurements, brightnesses of variable stars, and positions within multiple-star systems. For distances, the Hipparcos data are providing a dramatic increase (both qualitatively and quantitatively) in the information for nearby stars. For example, before Hipparcos, astronomers had calculated distances to only a few dozen stars with 1 percent precision. Hipparcos has provided such measurements for more than 400 stars. At the 5 percent level, ground-based astrometry had triangulated only 100 or so stars. Hipparcos has measured more than 7,000 stars with such precision. Accurate distance determinations are now available within some 500 light-years of the Sun.