The idea for the handheld device was brought about by a new collaboration between NASA and the US National Institute of Justice. The plan is to adapt taxpayer-funded space research to fight terrestrial crime.

Jacob Trombka, a physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, set the ball rolling. He believes X-ray fluorescence (XRF) could be a key crime-fighting technology. It was used by NASA's Near Earth Asteroid Rendezvous (NEAR) probe, which touched down on the asteroid Eros in February 2001.

X-ray fluorescence spectrometry is used to identify the chemical elements in a sample by measuring the wavelengths it emits upon exposure to X-rays.

NEAR's sensors simply recorded cosmic X-rays bouncing off the asteroid and beamed the details of the emissions back to Earth.

Trombka believes a handheld forensic tool could work along similar lines, taking X-ray fluorescence readings at the scene of a crime and beaming them to a computer for instant analysis.

This way, forensics experts could quickly detect traces of blood, semen or gunshot primer on suspects' hands. Gunshot primer is a chemical that converts kinetic energy from the gun's hammer into heat to ignite the gunpowder.

One benefit of this approach is that measuring X-ray emissions wouldn't destroy the physical evidence, as analysing a swab can often do. "Right now we have no method of doing this," says Carl Selavka, from Massachusetts State Police Crime Laboratory, who has been working with Trombka on this research.

"It could also be quite helpful in investigating suicides," he says, because roughly half of all murder investigations turn out to have been suicides. If there's gun residue on the victim's hand, it's likely they fired the fatal shot. Unlike NEAR's XRF system, the portable unit has to have its own diminutive X-ray source, wrote New Scientist.

The device will compare its spectral readings with an onboard database, or failing that beam the information back to a forensics computer for more detailed analysis. Either way it should only take a few minutes and give crime teams reliable enough feedback to arrest a suspect - or not.

Trombka found XRF particularly useful for identifying residue from gunshot primer, which can be difficult to detect, even in the lab. Traces of antimony and barium can come from gunshot primer, but may also be found on the hands of people working in jobs where they come into contact with brake fluid or solder.

However, Trombka found that these elements bind together with the rapid temperature changes they undergo when a gun is fired. So he can identify the residues as gunshot primer by checking if the barium and antimony are bound together. "It's a fingerprint of the high-temperature process," he says.

But the real triumph could be XRF's ability to detect substances without destroying the samples. It might even spot blood or semen on walls that have since been painted over. However, the device still needs to be made smaller, he says. "But by 2003 we should be testing it in real situations."

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