May 1, 2013
Fermi Has Close Call With Space Collision
[ Watch the Video: Fermi's Close Call with a Soviet Satellite ]
Lee Rannals for redOrbit.com — Your Universe Online
Julie McEnery, a project scientist for Fermi, opened her email at the end of March and found an automatically generated report arrived from NASA's Robotic Conjunction Assessment Risk Analysis (CARA) team. The document said Fermi was just one week away from an unusually close encounter with Cosmos 1805, which is a defunct Russian spy satellite from the Cold War.
Fermi and Cosmos 1805 have been speeding around Earth at thousands of miles an hour in nearly perpendicular orbits. The two objects were predicted to miss each other by just 700 feet.
"My immediate reaction was, 'Whoa, this is different from anything we've seen before!'" McEnery said.
Although predictions showed a 700-feet gap between Fermi and Cosmos, satellite operators have learned they can't be too careful. In February 2009, a dead Russian communications satellite called Cosmos 2251 collided with US commercially-owned satellite Iridium 33 in low-earth orbit, becoming the first known satellite-to-satellite collision.
The 2009 crash generated thousands of fragments large enough to be tracked, and they still cause hazards for astronauts aboard the International Space Station (ISS). Fermi has a relative speed of 27,000 miles-per-hour, so a direct hit by the 3,100-pound Cosmos 1805 would release as much energy as two and a half tons of high explosives, destroying both spacecraft and creating a flood of space debris.
Eric Stoneking, NASA's Goddard Space Flight Center attitude control lead engineer for Fermi, said predicting close approaches in space is like forecasting rain at a specific time and place a week in advance.
"As the date approaches, uncertainties in the prediction decrease and the initial picture may change dramatically," Stoneking said.
This is not the first time the Fermi team had been alerted to potential collisions in space, but those other threats faded away. However, an update before the predicted dates showed the satellites would occupy the same point in space within 30 milliseconds of each other.
"It was clear we had to be ready to move Fermi out of the way, and that's when I alerted our Flight Dynamics Team that we were planning a maneuver," McEnery said.
Fermi's team determined how much they needed to move the spacecraft in order to mitigate the threat. They selected possible times for the primary maneuver and up to three additional ones just in case.
"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," Stoneking explained. "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."
After performing the maneuver, Fermi and the 3,100-pound defunct satellite were able to miss each other by a comfortable margin of six miles.
"A huge weight was lifted," McEnery said. "I felt like I'd lost 20 pounds."