Cambridge, MA- Forget "Independence Day" or "War of the Worlds." A monstrous cosmic explosion last December showed that the earth is in more danger from real-life space threats than from hypothetical alien invasions.

The gamma-ray flare, which briefly outshone the full moon, occurred within the Milky Way galaxy. Even at a distance of 50,000 light-years, the flare disrupted the earth's ionosphere. If such a blast happened within 10 light-years of the earth, it would destroy the much of the ozone layer, causing extinctions due to increased radiation.

"Astronomically speaking, this explosion happened in our backyard. If it were in our living room, we'd be in big trouble!" said Bryan Gaensler (Harvard-Smithsonian Center for Astrophysics), lead author on a paper describing radio observations of the event.

Gaensler headed one of two teams reporting on this eruption at a special press event today at NASA headquarters. A multitude of papers are planned for publication.

The giant flare detected on December 27, 2004, came from an isolated, exotic neutron star within the Milky Way. The flare was more powerful than any blast previously seen in our galaxy.

"This might be a once-in-a-lifetime event for astronomers, as well as for a neutron star," said David Palmer of Los Alamos National Laboratory, lead author on a paper describing space-based observations of the burst. "We know of only two other giant flares in the past 35 years, and this December event was one hundred times more powerful."

NASA's newly launched Swift satellite and the NSF-funded Very Large Array (VLA) were two of many observatories that observed the event, arising from neutron star SGR 1806-20, about 50,000 light years from Earth in the constellation Sagittarius.

Neutron stars form from collapsed stars. They are dense, fast-spinning, highly magnetic, and only about 15 miles in diameter. SGR 1806-20 is a unique neutron star called a magnetar, with an ultra-strong magnetic field capable of stripping information from a credit card at a distance halfway to the Moon. Only about 10 magnetars are known among the many neutrons stars in the Milky Way.

"Fortunately, there are no magnetars anywhere near the earth. An explosion like this within a few trillion miles could really ruin our day," said graduate student Yosi Gelfand (CfA), a co-author on one of the papers.

The magnetar's powerful magnetic field generated the gamma-ray flare in a violent process known as magnetic reconnection, which releases huge amounts of energy. The same process on a much smaller scale creates solar flares.

"This eruption was a super-super-super solar flare in terms of energy released," said Gaensler.

Using the VLA and three other radio telescopes, Gaensler and his team detected material ejected by the blast at a velocity three-tenths the speed of light. The extreme speed, combined with the close-up view, yielded changes in a matter of days.

Spotting such a nearby gamma-ray flare offered scientists an incredible advantage, allowing them to study it in more detail than ever before. "We can see the structure of the flare's aftermath, and we can watch it change from day to day. That combination is completely unprecedented," said Gaensler.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.