NASA boffins: Space 'scope JUST missed dead Cold War spy sat
In space, no one can hear you beep
NASA has released rare details of a near miss between a delicate space telescope and a disused Cold War spy satellite.
Julie McEnery, project scientist for NASA's Fermi Gamma-ray Space Telescope, was alerted to the threat when she received an automatic warning generated by the space agency’s Robotic Conjunction Assessment Risk Analysis (CARA) on 29 March, 2012.
It predicted that Fermi was just a week away from a perilously close encounter with Cosmos 1805, a one-and-a-half tonne Russian spy satellite launched in 1986 which was whizzing around the world at 15,000 mph (24,140km/h). A direct hit would have resulted in an explosion with the force of almost three tonnes of explosive, more than enough to obliterate both spacecraft.
Realising the two craft were travelling at almost perpendicular orbits and were predicted to come within 700 feet (213m) of each other, the equivalent of an interstellar hair’s breadth, McEnery immediately began to panic.
She said: "My immediate reaction was: 'Whoa, this is different from anything we've seen before'."
Fermi is able to pick up gamma ray radiation, allowing it to study black holes and other phenomena which would be invisible to the human eye.
The Fermi team had received two similar threat reports before, which had both proven to be false alarms. It’s a difficult business forecasting what will happen to objects in orbit, so often the predicted disasters come to nothing.
But then came an update on 30 March, which suggested the two orbiting craft would be in the same spot within 30 milliseconds of each other, inevitably resulting in a collision.
"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 was built with thrusters which were designed to guide it into the atmosphere at the end of its life, where it would burn up.
The team had never before used these boosters, for fear that a fuel leak would cause a catastrophic explosion and bring the mission to an abrupt end.
The risk of the manoeuvre was weighing heavily on McEnery. "You can't help but be nervous thinking about highly flammable fluids heading down pipes they'd never flowed down before," she added.
The Fermi team then called up CARA to work on plotting a course for the telescope, first making sure it wouldn’t simply avoid one threat and then end up smashing into another.
After drawing up several different escape routes, they used data from the US Space Surveillance Network, which keeps an eye on all orbiting objects which are more than 4 inches (10cm) wide, to assess the possible risks. The surveillance team are responsible for keeping astronauts safe from collisions, by making sure nothing nasty is heading for its spacecraft.
On noon on Tuesday 3 April, after they had decided everything was in place, the team furled the solar panels on Fermi and retracted the antenna to avoid any damage from the rocket boosters.
They then fired the thrusters for just one second, sending Fermi speeding out of danger. One hour later, it was back at work, sending back data to scientists down on earth. The two craft had missed one another by six miles (9.6km).
"A huge weight was lifted," McEnery said. "I felt like I'd lost 20 pounds."
Orbital collisions are becoming an ever-increasing problem. Last year, the CARA team helped organise seven avoidance collision missions. Just a month before the Fermi incident, they helped Landsat 7 to avoid debris from a Chinese weather satellite which had been destroyed in 2007 as part of a military test.
In May and October, NASA's Aura and CALIPSO Earth-observing satellites were forced to dodge fragments from Cosmos 2251, the craft involved in the first recorded collision with another satellite in 2009. It smashed into Iridium 33, a communications satellite, resulting in a huge cloud of debris which still remains a hazard to orbiting spacecraft. ®