Space Telescope Sifts Earth Storms for Radiation Flashes

A NASA space telescope hunting for the most powerful explosions in the universe is turning its eye on Earth to hunt for tiny flashes of radiation to determine if they pose a rare, but deadly, threat to high-flying commercial airliners.

NASA?s Fermi Gamma-Ray Space Telescope has joined the search for mysterious gamma-ray flashes above thunderstorms which are ultra-brief, but could be a concern for air travelers, researchers said.

Just one millisecond blast of the so-called terrestrial gamma-ray flashes (TGFs) could expose passengers and crew aboard a nearby jet airliner to the same level of radiation as 400 chest X-rays, according to a recent study.

Fermi, which NASA originally launched to seek out gamma-ray bursts — immensely powerful explosions in deep space, usually from a dying star — joined in the hunt several months ago to possibly uncover more about when and how TGFs occur around thunderstorms and lightning.

To do that, scientists are using one of the telescope?s monitoring sensors.

"Fermi-GBM [Gamma-Ray Burst Monitor] has the broadest energy coverage and highest sensitivity of any instruments that have observed, or will observe TGFs," said Jerry Fishman, an astrophysicist at NASA's Marshall Space Flight Center in Huntsville, Ala.

Scientists first discovered the existence of TGFs by accident, when the Compton Gamma-Ray Observatory detected a flash in 1991. But they still don't know if it is lightning that triggers the phenomenon or whether TGFs provide the quick burst of electrons that may spark a lightning strike.

"The current hypothesis of the origin of TGFs is that they come from an avalanche of relativistic [high-energy] electrons in the intense electric fields within or above thunderstorms," Fishman told SPACE.com. The theory fits both observations and computer simulations so far.

A football-sized satellite called Firefly is slated to take simultaneous measurements of lightning and TGFs, and may launch this year or next year. But even that mission dedicated to studying TGFs won't match the sensitivity of Fermi's gamma-ray detector.

Still, retooling Fermi?s sensitive monitor to pick up TGFs required some new programming. The instrument typically relies upon a dozen low-energy detectors to observe distant cosmic gamma-ray bursts, but it also has two high-energy detectors.

Scientists uploaded new software last November that allows the instrument to trigger in response to high-energy events such as TGFs, Fishman explained. The change has allowed research teams to find seven times as many TGF events compared to before, at a rate of about two per week.

Fermi's GBM timing accuracy can also pinpoint a flash to within two microseconds, or two one-millionths of a second. That may help the Firefly mission pinpoint whether lightning or TGFs strike first, so to speak.

For now, the TGFs appear completely unrelated to the cosmic gamma-ray cousins that the Fermi Gamma-Ray Space Telescope set out to study. But it's a mystery closer to home that has proven just as intriguing as any astrophysics puzzle.

"Their spectrum [energy spread] is unlike anything ever seen from cosmic gamma-ray objects," Fishman said.

• The Strangest Things in Space
• The Fermi Gamma-Ray Telescope Part 1, Part 2
• Mysterious Radiation May Strike Airline Passengers