Talk about a close call.
NASA is just now talking about how it saved the Fermi Gamma-ray Space Telescope from destruction by a collision with a piece of space junk last month. And, no, this particular piece of space junk isn't a lost bolt or a paint chip.
This space junk was a 3,100-pound defunct Soviet spy satellite. Yep, a spy satellite, dubbed Cosmos 1805, that dates back to the Cold War.
At the end of March, NASA realized that the defunct satellite was heading toward the famed Fermi space telescope, which for nearly five years has been mapping the highest-energy light in the universe. The two objects, which are both speeding around the Earth at thousands of miles an hour in perpendicular orbits, were expected to come within 700 feet of each other, NASA reported.
The satellite and the space telescope would occupy the same point in space within 30 milliseconds of each other.
When you're talking about a costly telescope like Fermi, that's just too close for comfort for NASA.
As bad as it would have been to lose the Fermi, there was something else to consider.
NASA calculated that with a speed relative to Fermi's of 27,000 mph, a collision with the Russian satellite would release as much energy as two- and-a-half tons of high explosives, destroying both spacecraft, and further littering Earth's orbit with potentially dangerous debris.
While the objects were expected to miss each other, NASA scientists have learned that they can't take the risk.
In February 2009, for example, NASA scientists thought a dead Russian communications satellite would pass about 1,900 feet from a working Iridium 33 communications satellite. That wasn't the case, however. The Iridium satellite was destroyed when it was battered by clouds of debris around the Russian satellite.
After the loss of the Iridium four years ago, NASA was on high alert for this potential collision. "My immediate reaction was, 'Whoa, this is different from anything we've seen before!' " said Julie McEnery, project scientist for NASA's Fermi Gamma-ray Space Telescope.
Upon discovering that the two objects were on a near-collision course, McEnery found that being a project scientist also means being a space traffic controller.
She learned of the problem on March 29 when she was going through her email and noticed an automatically generated report from NASA's Robotic Conjunction Assessment Risk Analysis team based at the Goddard Space Flight Center.
At that point, Fermi was just one week away from trouble.
NASA reported that the only way to move Fermi a safe distance away from Cosmos 1805's path was to fire Fermi's thrusters, which were designed to be used at the end of the satellite's life to take it out of orbit.
The problem was that the thrusters had never been tested. If the thrusters had a propellant leak or an explosion, they could have taken Fermi out of commission.
On April 3, NASA instructed the telescope to stop scanning the sky and orient itself along its direction of travel. It then stowed its solar panels and tucked away its high-gain antenna to protect them from the thruster exhaust, NASA noted.
"The maneuver, which was performed by the spacecraft itself based on procedures we developed a long time ago, was very simple, just firing all thrusters for one second," said Eric Stoneking, the altitude control lead engineer for Fermi. "There was a lot of suspense and tension leading up to it, but once it was over, we just sighed with relief that it all went well."
Within an hour of the thruster push, Fermi was back to work.
"A huge weight was lifted," McEnery said. "I felt like I'd lost 20 pounds."