Imagine being the project scientist for a NASA experiment and getting an email telling you that a 3,100 lb. defunct spy satellite dating back to the Cold War might crash into your baby?

That's what happened to Julie McEnery of NASA's Fermi Gamma-ray Space Telescope,  which maps the highest-energy light in the universe, a year ago. When she checked her email on March 29th, 2012,  she had an automatically generated report from NASA's Robotic Conjunction Assessment Risk Analysis (CARA) saying that in about a week Fermi might be hit by Cosmos 1805. 

Ideally, they were going to miss each other by 700 feet but when two objects are zipping around Earth at 27,000 miles per hour in nearly perpendicular orbits, that's too close to call. 

So NASA had to do an impact analysis - literally - and try to account for all of the uncertainties in predicting spacecraft positions a week into the future. It doesn't always work. On Feb. 10th, 2009, Cosmos 2251, another dead Russian communications satellite, was to pass about 1,900 feet from the functioning Iridium 33 communications satellite but at the predicted time of closest approach, all contact with Iridium 33 was lost. Radar revealed clouds of debris traveling along the orbits of both spacecraft, confirming the first known satellite-to-satellite collision.

That crash generated thousands of fragments large enough to be tracked and many smaller pieces that evade detection. Much of the wreckage remains a hazard to operating spacecraft because only about 20 percent of the trackable pieces have reentered the atmosphere.

With a speed relative to Fermi of 27,000 mph, 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.

Why don't close calls or crashes happen more often?

Despite the apparent crowding in Earth orbit, there's usually a vast amount of space between individual objects. Close approaches -- also known as conjunctions -- with fragments, rocket bodies and active payloads remain infrequent events. Few of the potential conjunctions identified a week into the future will actually materialize.

"It's similar to forecasting rain at a specific time and place a week in advance," said Goddard's Eric Stoneking, the attitude control lead engineer for Fermi. "As the date approaches, uncertainties in the prediction decrease and the initial picture may change dramatically."

Twice before, the Fermi team had been alerted to potential conjunctions, and on both occasions the threats evaporated. It was possible the Cosmos 1805 encounter would vanish as well, and the spacecraft's observations could continue without interruption.

But the update on Friday, March 30th, 2013 indicated otherwise. The satellites would occupy the same point in space within 30 milliseconds of each other so clearly Fermi needed to move. Fermi has firing thrusters so it could not pose a threat to another satellite at the end of its life - the thrusters were installed to take it out of orbit and so it will burn up in the atmosphere.

Engineers don't routinely test a system like that during a mission because it's only going to be used when the mission is over - and a propellant leak or an explosion in testing would have ended Fermi's mission prematurely and so it's better to lose it when time is up.  But it had to be used in 2012. 

"You can't help but be nervous thinking about highly flammable fluids heading down pipes they'd never flowed down before," said McEnery. 

Credit: NASA

The Goddard CARA team determined how big a push Fermi would need to mitigate the threat. Working with the Joint Space Operations Center (JSpOC) at Vandenberg Air Force Base in California, CARA scientists also checked that the projected new orbit wouldn't put Fermi on course for a conjunction with another object. The Flight Operations Team selected possible times for the primary maneuver and, just in case, up to three additional ones.

Over the weekend, the radar and optical sensors of the U.S. Space Surveillance Network continued keeping tabs on Cosmos 1805 and every other artificial object larger than 4 inches across in Earth orbit. Of the 17,000 objects currently tracked, only about 7 percent are active satellites.

Once each day, JSpOC analyzes the updated orbits, looks for possible conjunctions a week or more into the future, and notifies the Goddard CARA team of any events involving NASA's robotic missions. Another group at NASA's Johnson Space Flight Center in Houston performs the same function for all spacecraft carrying astronauts, including the International Space Station.

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

Credit: NASA

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," 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."

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.

"A huge weight was lifted," McEnery said. "I felt like I'd lost 20 pounds."

Last year, the Goddard CARA team participated in collision-avoidance maneuvers for seven other missions. A month before the Fermi conjunction came to light, Landsat 7 dodged pieces of Fengyun-1C, a Chinese weather satellite deliberately destroyed in 2007 as part of a military test. And in May and October, respectively, NASA's Aura and CALIPSO Earth-observing satellites took steps to avoid fragments from Cosmos 2251.