The Japanese-U.S. satellite, Astro E, final member of the trio, is slated for launch February 7. Its siblings, the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission (XMM), were launched respectively in July and December 1999. All three observe X-rays (radiation of short wavelengths) from celestial bodies.

Yet while Chandra's strength is in producing the highest-resolution images, XMM has the greatest sensitivity and ability to capture fleeting events. Astro E's forte will be spectroscopy, determining the composition and movement of objects from the intensity of their X-rays at different wavelengths.

"It's really going to be a bonanza to have them all up there at once," says Wallace Tucker, science spokesman for the Chandra X-ray Observatory Center in Cambridge, Massachusetts.

The satellites' capabilities complement each other but also overlap, notes Tucker. Chandra, for instance, can gather spectroscopic data -- not as thoroughly as Astro E, but better than earlier generations of X-ray telescopes.

Since space X-rays are blocked by the Earth's atmosphere, orbiting telescopes perform most X-ray astronomy. Earlier X-ray telescopes also had a division of labor. Those launched in the 1990s included the Advanced Satellite for Cosmology and Astrophysics (ASCA), which was designed largely for spectroscopy, and the Rossi X-ray Timing Explorer (RXTE), which emphasized rapid-time observations.