The X-ray Multi-Mirror Mission (XMM), scheduled for launch Friday, marks the latest step in the burgeoning field of X-ray astronomy. But it is by no means the last step.

In early 2000, a Japanese-U.S. observatory called Astro-E is scheduled for launch. Joining the XMM and the Chandra satellite, it will complete the current generation of X-ray observatories. And beyond that, the next generation of X-ray observatories is planned.

The X-ray Multi-Mirror Mission (XMM), scheduled for launch Friday, marks the latest step in the burgeoning field of X-ray astronomy. But further stepsare planned.

In early 2000, a Japanese-U.S. observatory called Astro-E is scheduledfor launch, completing the current generation of X-ray observatories.

The next generation of X-ray observatories consists of XEUS, a projectof the European Space Agency, and a NASA project called Constellation X.

Each of the three current-generation observatories has somewhat different capabilities, notes Steve Holt, Astro-E project scientist at NASA's Goddard Space Flight Center. Astro-E (pictured) is a joint project of NASA and Japan's Institute of Space and Astronautical Science (ISAS), and is slated for launch February 8 on a Japanese rocket.

Astro-E, says Holt, will "concentrate on spectroscopic information" -- determining, for example, objects' composition and movement from the breakdown of their X-rays into spectral lines. By contrast, Chandra's strength is in producing the "sharpest images," while XMM's forte is its sensitivity and ability to capture events that are "not repeatable."

"Astro-E will concentrate on spectroscopic information."

The planned next generation of X-ray observatories consists of XEUS (X-ray Evolving Universe Spectroscopy Mission), a project of the European Space Agency, and a NASA project called Constellation X.

XEUS, a large, highly sensitive telescope, will be in effect a successor to XMM, whereas Constellation X, focusing on spectroscopic data, will be the next-generation version of Astro-E. These next-generation observatories are expected to be deployed no earlier than 2010.