A new NASA satellite with powerful gamma-ray vision is set to launch on 3 June. It will observe the deaths of massive stars, probe the gamma-ray sky for unknown objects, and might even pin down the nature of the mysterious dark matter that pervades the universe.

The $700 million Gamma-ray Large Area Space Telescope (GLAST) is set to launch into low-Earth orbit at 1145 EDT (1645 GMT) from Cape Canaveral, Florida, US. It carries a gamma-ray telescope of unprecedented sensitivity and a monitor that can detect radiation from violent cosmic events called gamma-ray bursts.

The mission will provide the first detailed survey of the sky in a largely unexplored part of the energy spectrum of gamma rays, the highest-energy form of radiation.

The observatory's Large Area Telescope (LAT) is sensitive to gamma rays with energies between 20 mega-electronvolts (MeV) and 300 giga-electrovolts (GeV). The 10 to 100 GeV range is mostly invisible to ground-based telescopes and was poorly sampled by GLAST's predecessor, the Compton Gamma Ray Observatory.

"History tells us when you open up a new band for exploration of the electromagnetic spectrum, there are new discoveries," said GLAST chief scientist Steven Ritz of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, at a GLAST press briefing at Goddard in 2007.

Annihilating WIMPs

The spectrum around 100 GeV offers the possibility of an especially big breakthrough – the chance to identify the nature of dark matter. Dark matter is an invisible substance that outweighs ordinary matter in the universe and has so far only been detected by its gravitational influence on ordinary matter.

The most popular explanation says that dark matter is made of exotic elementary particles that rarely interact with ordinary matter. A zoo of such weakly interacting massive particles (WIMPs) have been proposed, such as neutralinos and axions, but Earth-based experiments have so far failed to confirm their existence.

WIMPs are expected to annihilate and release gamma rays when they hit one another. When GLAST opens its eyes to the sky, it might see bright spots due to clumps of dark matter sprinkled throughout the galaxy. Such clumps are predicted by theory, but whether GLAST will be able to detect them is highly uncertain.

It depends on exactly what the dark matter is made of – some kinds of WIMPs would produce a stronger signal than others – and on how well GLAST can distinguish a dark matter signal from other sources of gamma-rays, such as the debris from a supernova.

Black hole evaporation

"The burden of proof is very high on a big discovery like this – you have to show that your signal is not due to some other astrophysical background," Ritz said. For about half of the kinds of dark matter particles that theorists envision, the signal may be too weak to distinguish from more mundane sources, he added.

While the potential for a dark matter detection is uncertain, GLAST is bound to provide a wealth of information on some of the universe's most violent events. These gamma-ray bursts can put out more energy in a matter of seconds than the Sun will over its entire lifetime. Most of the bursts are thought to result from the collapse of massive stars and from collisions between neutron stars, while the origins of others are still mysterious.

The GLAST Burst Monitor (GBM) is sensitive enough to detect around 200 gamma-ray bursts each year, about twice the rate seen by NASA's Swift satellite, a gamma-ray burst observatory that launched in 2004. GLAST can see gamma rays across a much wider spectrum of energies than Swift, which will give scientists a more complete view of these events.

There is also an outside chance that GLAST could observe gamma rays from the explosion of microscopic black holes. According to some theories, these tiny primordial black holes would have formed in the violence of the big bang itself. Depending on what mass they were born with, they could be evaporating today in bursts of gamma rays through a process called Hawking radiation.

Cosmology - Keep up with the latest ideas in our special report.

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