BOSTON — KESTRELS seek their prey with the help of ultraviolet light. Bees use it to find flowers. But when astronomers want to see the universe that way, they have to take their instruments into space.
That's what they will be doing when space shuttle Endeavour takes the Astro-2 ultraviolet observatory into orbit for 15 days, one of the longest flights yet attempted.
Liftoff is planned for 1:37 a.m. on March 2, though ominous weather conditions could delay the launch.
The Astro-2 went into orbit Dec. 2, 1990 aboard the shuttle Columbia for a nine-day mission carrying four ultraviolet (UV) telescopes. It now has three improved instruments that will let astronomers bypass the Earth's UV-blocking ozone layer and discover some of the universe's most intriguing features.
For example, by looking at UV light, astronomers expect to detect the primordial intergalatic medium -- a fancy name for the hydrogen and helium gas left over when the universe formed -- which they believe is spread thinly through the space between galaxies. Years of searching for it have yielded only unconfirmed hints of its presence. But, Johns Hopkins University's telescope on Astro-2 will search for it.
The observing team's principal investigator, Arthur F. Davidsen, expects the helium to reveal itself by the way it absorbs UV radiation from certain astronomical sources.
''At last,'' he says, ''we may be on the verge of seeing the stuff from which we, and everything else in our universe, were made.''
Astro's other two instruments have equally ambitious goals. The University of Wisconsin at Madison has updated its original Astro telescope, which can observe the polarization of UV radiation.
Polarized light waves vibrate in a particular way rather than randomly. Light can become polarized when it passes through dust clouds whose dust grains are aligned by magnetic fields. This instrument, then, can study the dust around stars. It also can reveal the geometry of astronomical objects.
For its part, the Astro team at the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Md., expects to bring new clarity to the study of crowded star fields or clustered galaxies. As NASA explains in describing this project: ''The clutter of objects which produce most of their radiation in visible light disappears [in the UV view]. Hot stars leap into prominence, the spiral arms of distant galaxies snap into clearer resolution, and the material hidden between the stars comes into view.''
Astronomers had only glimpses of the ultraviolet universe before the Astro-1 mission five years ago. Now they have two UV satellites in orbit plus the Hubble Space Telescope, which also has UV capability. Astro-2's instruments are designed to make measurements for which these other permanent observatories aren't well equipped.
For example, the Johns Hopkins telescope can see over a wide range of UV wavelengths. But it is most sensitive to wavelengths that the Hubble telescope and the other two observatories can't handle.
Also, while Goddard's instrument can't resolve as much fine detail as can Hubble, its view covers an area two-thirds as large as the moon -- one far larger than even the Wide Field Planetary Camera on Hubble can take in.
While Astro's other two instruments send their data by radio link, Goddard's telescope records on high-quality film, enabling astronomers to make detailed studies of the star fields it records.
The Astro package sits in Endeavour's cargo bay on a precision instrument-pointing pallet that is part of the European-built Spacelab. To ensure that it works well, two astronomers are joining five regular NASA astronauts in Endeavour's crew.