Julie McEnery is NASA's Project Scientist for the Fermi Gamma-ray Space Telescope. When she checked her email on March 29, 2012, she was startled to find an automatically generated message stating that in six days, her half-billion-plus dollar satellite was going to cross paths with Cosmos 1805, a Soviet-era spy satellite. The predicted encounter had the two satellites occupying the same coordinates only 30 milliseconds apart. Not only that, but Cosmos was in an orbit moving nearly perpendicular to Fermi such that their collision would be equivalent to tons of high explosives. Essentially total destruction.
Launched into low Earth orbit in 2008, Fermi is a space observatory that performs gamma-ray astronomy observations of the heavens. It measures about 1.8 m (4.6 ft) square by 2.9 m (9.2 ft) high, with solar wings that extend to give Fermi a total width of about 15 m (50 ft). Fermi is not a light satellite, having a mass of 4,300 kg (9,500 lb).
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Fermi's instrumentation was developed to study astrophysical and cosmological phenomena, such as active galactic nuclei, pulsars, gamma-ray bursts, other high-energy sources and dark matter. While its formal mission was to last five years, the spacecraft was designed to continue operations for a full ten years.
In the other corner we have Cosmos 1805 – a Russian Tselina D military communications satellite intended to monitor military naval signals. Launched from the Plesetsk cosmodrome in 1986, it was intended for an active lifetime of six months, but since no one was worrying about limiting orbital debris in those days, it is still in orbit nearly 30 years later. The satellite weighs 1,400 kg (3,080 lb) and is in a nearly polar orbit, so that its orbit crosses that of Fermi nearly perpendicularly.
The two objects, speeding toward each other at a net velocity of 11 km/s (7 miles/s) were expected to miss each other by a mere 210 m (700 ft). While a miss is as good as a mile, satellite operators have learned that they can't be too careful. In 2009, the Iridium 33 communications satellite and Cosmos 2251 collided about 800 km (500 miles) over Siberia, leaving some 1,500 kg (3,300 lb) of small metal pieces strewn over an ever widening volume of space. Before the collision, the satellite tracking reports predicted a miss of 584 m (1,900 ft).
So the decision was made that Fermi would dodge the oncoming Cosmos 1805. Ironically, this would be accomplished with a one second pulse on the de-orbit thrusters that were designed to push Fermi out of orbit at the end of its life so it would burn up in the atmosphere and not pose a threat to other satellites.
Although intended to get Fermi out of harm's way, this maneuver is not without its own dangers. The procedure involves halting Fermi's scanning of the skies, orienting the satellite along its direction of motion, folding and parking the solar panels, and tucking away its high gain communication antenna. Then, once the (untested) thrusters had been fired, the process of making Fermi safe for the maneuver had to be reversed. A single glitch had the potential to render Fermi unusable, explaining why the decision to maneuver was not taken lightly.
As it happened, all went swimmingly with the burn on April 3 and Fermi was back doing scientific observations within an hour of the maneuver. The following day, Fermi and Cosmos 1805 missed colliding by 10 km (6 miles), a much more comfortable distance than the length of two football fields.
Today, maneuvers to avoid satellite collisions have become a routine requirement. Despite this, the amount of space debris continues to increase. Not all of it can be detected, and very few of the pieces in orbit still have maneuvering capability. This was underlined on the ISS this past Monday, when Commander Chris Hatfield noticed that a small bit of debris had punched a hole in one of the space station's solar panels.
Space debris has the potential, through a chain reaction known as a Kessler syndrome, to render near-Earth space impassible, perhaps for centuries. Such a fate may still be avoided, although some aerospace experts believe that a Kessler syndrome is nearly unavoidable at this point. It seems that the space nations of the world must get serious about the space debris problem, while we can still get into space.
The following video shows how Fermi avoided possible destruction.