Washington

The US space agency NASA last week gave the go-ahead to two new medium-class Explorer spacecraft in the next five years. One will study gamma rays, and the second will map 40 million stars with unprecedented precision.

The $163-million Swift Gamma-ray Burst Explorer, scheduled to reach orbit in 2003, will have three telescopes — viewing in gamma-ray, X-ray and ultraviolet/visible wavelengths — poised to observe these mysterious bursts within minutes of their appearance.

During its three-year mission, Swift is expected to pinpoint some 300 gamma-ray bursts, and to discover about 400 black holes. The principal investigator is Neil Gehrels of NASA's Goddard Space Flight Center in Maryland.

The $162-million Full-sky Astrometric Mapping Explorer (FAME), due to launch in 2004, is intended to improve on the successful European Hipparcos mission of the early 1990s, which gave extremely accurate positions (to within 1 milliarcsecond) for 120,000 stars.

FAME will use advanced charge-coupled device arrays and a more efficient scheme for blocking out sunlight to pinpoint some 40 million stars down to fifteenth magnitude, with an accuracy of 500 microarcseconds. For stars brighter than ninth magnitude the accuracy will be 50 microarcseconds.

“There's all kinds of science you can do with these observations,” says Kenneth Seidelmann, leader of the project's science working team at the US Naval Observatory in Washington DC.

A key objective is the search for wobbles in a star's motion that betray the presence of a large planet or brown dwarf. Seidel-mann says FAME should be able to detect planets that are as small as twice the mass of Jupiter.

During its five-year, all-sky survey FAME will produce a catalogue of star locations that can be used by NASA's planned follow-on, the Space Interferometry Mission, which will be accurate to one microarcsecond. The European Space Agency is also considering an astrometry mission called GAIA, which could catalogue a billion objects with similar precision.