May 3, 2013
How NASA Recently Saved A Billion Dollar Satellite From Disaster
One could naively assume that this sort of thing happens a lot. With the sheer number of satellites and other space junk floating around in Low Earth Orbit (LEO) it would seem inevitable that objects would occasionally bump into each other.
But LEO is a pretty big place and despite the ever-growing number of man-made objects up there, close approaches are still rare events. The trick, however, is predicting when they are going to happen.
Every day the Joint Space Operations Center (JSpOC) analyzes the updated orbits of the 17,000 objects being tracked and projects their positions out to at least a week into the future, looking for possible collisions. When a possible conjunction, as they are called, is identified, they notify the team responsible for the object in question.
Often times though, the threat goes away. “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 Gamma-ray Space Telescope. “As the date approaches, uncertainties in the prediction decrease and the initial picture may change dramatically.”
Occasionally, though, the situation does not improve and action is needed. And when proper care is not taken, the result can be disastrous; a lesson learned the hard way.
On Feb. 10, 2009, calculations showed that Cosmos 2251, a dead Russian communications satellite, would approach within 1,900 feet of the functioning Iridium 33 communications satellite within the next 24 hours. At the predicted time of closest approach, NASA lost contact with the Iridium 33 satellite. Radar later revealed clouds of debris traveling along the orbits of both spacecraft, confirming the first known satellite-to-satellite collision.
More recently, on March 29, 2012, NASA’s Fermi team was notified that their billion-dollar gamma-ray observatory would pass within 700 feet of a defunct soviet spy satellite, the Cosmos 1805, later in the week. Should the two objects meet, the collision would completely destroy both.
On two previous occasions Fermi had been notified of possible conjunctions, but the threats were quickly dismissed. This time, however, the threat was real. The next day, it was determined that the two orbiting satellites would be at the same point in space within 30 milliseconds of each other, all but assuring disaster.
The team got to work devising a plan to rescue Fermi, but the plan hinged on a detail that had many worried. To move Fermi out of the way, the team would fire thrusters, moving the telescope to a new orbit. The problem? This system was designed to send Fermi into a de-orbit trajectory to burn it up in the atmosphere at the end of its operational lifetime. This way it would not pose a threat to other satellites in the future.
Now, they needed to use these thrusters, which had never even been tested, to save the instrument. “You can’t help but be nervous thinking about highly flammable fluids heading down pipes they’d never flowed down before,” noted Julie McEnery, the project scientist for Fermi. “But having done this, we now know the system works as designed, and it gives us confidence should we need to maneuver again in the future.”
On April 3, the team maneuvered the satellite into a new orbit, and the satellites missed each other by 6 miles. “A huge weight was lifted,” McEnery said. “I felt like I’d lost 20 pounds.”
Image Credit: Photos.com