BioEd Online

NASA's Swift satellite, which will track the most powerful explosions in the Universe, launched successfully on Saturday 20 November, at 1716 GMT.

Gamma-ray (?-ray) bursts are sudden flashes of radiation in the sky, and their fleeting nature has made it tricky for astronomers to work out what causes them. Although some bursts last for minutes, some are visible for just a few milliseconds.

Astronomers are now fairly sure that longer ?-ray busts come from the collapse of massive stars. But the shorter bursts are more mysterious, and may be generated when two neutron stars collide (see " Bagging bursts, swiftly").

Swift is an apt name for the satellite, because it can detect these short bursts and quickly turn its X-ray sensors to soak up the afterglow, providing scientists with clues about their origin.

"The energy generated in a ?-ray burst is staggering," says Keith Mason, an astrophysicist at the Mullard Space Science Laboratory, University College London, who helped to build Swift. "In a few minutes, they release as much energy as our Sun releases over the whole of its 10-billion-year lifetime."

"They are the biggest bangs since the big one," adds Martin Ward, an astrophysicist at the University of Durham, UK. The scientists hope that Swift will detect about three ?-ray bursts every week.

Swift response

The bursts were first seen in 1969 by a satellite used to monitor the Nuclear Test Ban Treaty, explains Mason. But it was not until the Italian-Dutch orbiting telescope BeppoSAX analysed the X-ray afterglow of a ?-ray burst in 1997 that astronomers realized the explosions were coming from distant galaxies.

In fact, many of the bursts seem to date from a time when the Universe was less than a billion years old, allowing scientists to see further back into the early history of the cosmos than ever before.

Swift will study these ?-ray bursts in three different stages. The Burst Alert Telescope will be the first to spot the tell-tale rays, locking on to the signal and swivelling the satellite into the optimum observing position in as little as 20 seconds.

Just 90 seconds after the burst, an X-ray telescope studies its afterglow, allowing Swift to pinpoint the source with greater accuracy. A third telescope then looks at the source in the ultraviolet and optical part of the electromagnetic spectrum, pinning down the position of the burst with even more precision.

Within five minutes, the satellite transmits all this information back to Earth, where it is distributed to astronomers around the world by e-mail and text message. It will also trigger more powerful ground-based robotic telescopes to watch the optical afterglow that persists for many minutes after the initial burst.

Faulty receiver

The launch had been delayed several times because of a problem with the craft's communications equipment. Engineers had found faults in a receiver that is responsible for picking up signals from mission control to make the rocket self-destruct if it veers off course. They fixed the problem on 19 November.

Swift is now orbiting about 600 km above the Earth. After calibrating the instruments on the 5.5 metre-long craft, it should be fully operational by the end of March.