From NASA: “The Day NASA’s Fermi Dodged a 1.5-ton Bullet”

NASA scientists don’t often learn that their spacecraft is at risk of crashing into another satellite. But when Julie McEnery, the project scientist for NASA’s Fermi Gamma-ray Space Telescope, checked her email on March 29, 2012, she found herself facing this precise situation.

While Fermi is in fine shape today, continuing its mission to map the highest-energy light in the universe, the story of how it sidestepped a potential disaster offers a glimpse at an underappreciated aspect of managing a space mission: orbital traffic control.

As McEnery worked through her inbox, an automatically generated report arrived from NASA’s Robotic Conjunction Assessment Risk Analysis (CARA) team based at NASA’s Goddard Space Flight Center in Greenbelt, Md. On scanning the document, she discovered that Fermi was just one week away from an unusually close encounter with Cosmos 1805, a defunct spy satellite dating back to the Cold War.

The two objects, speeding around Earth at thousands of miles an hour in nearly perpendicular orbits, were expected to miss each other by a mere 700 feet.

‘My immediate reaction was, ‘Whoa, this is different from anything we’ve seen before!’ McEnery recalled.

Although the forecast indicated a close call, satellite operators have learned the hard way that they can’t be too careful. The uncertainties in predicting spacecraft positions a week into the future can be much larger than the distances forecast for their closest approach.

But the update on Friday, March 30, indicated otherwise. 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.

The only way to accomplish this was by firing thrusters designed to ensure that Fermi would never pose a threat to another satellite. Intended for use at the end of Fermi’s operating life, the thrusters were designed to take it out of orbit and allow it burn up in the atmosphere.

Because a failure of this system, such as a propellant leak or an explosion, could have ended Fermi’s mission prematurely, the thrusters had never been tested, adding a new source of anxiety for McEnery.

By Tuesday, April 3, the threat still had not receded and all plans were in place for firing Fermi’s thrusters.

Shortly after noon EDT, the spacecraft stopped scanning the sky and oriented itself along its direction of travel. It then parked its solar panels and tucked away its high-gain antenna to protect them from the thruster exhaust.

‘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 Goddard’s Eric Stoneking. ‘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.’

By 1 p.m., Fermi was back to doing science. A few hours later, the various teams met to evaluate the results of the maneuver and determine if another would be required. When the two spacecraft reached their long-awaited conjunction the following day, they would miss by a comfortable margin of 6 miles, with no further actions needed.”

See the full article, about this truly spine-tingling affair, here.

The Fermi Gamma-ray Space Telescope , formerly referred to as the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden.


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