Close Call: Europe’s Just-launched Sentinel-1A Dodges Dead Satellite
PARIS — Europe’s just-launched Sentinel-1A satellite was obliged to perform a collision-avoidance maneuver some 34 hours after separation from its carrier rocket April 3 to prevent a collision with a dead U.S. satellite, according to the European Space Agency.
The decision to maneuver Sentinel-1A was made following the usual procedure — the U.S. Strategic Command, which manages the U.S. Space Surveillance Network of ground- and space-based sensors, notified the European Space Agency’s European Space Operations Centre (ESOC) in Darmstadt, Germany, of the approach of NASA’s Active Cavity Radiometer Irradiance Monitor Satellite, or ACRIMSat.
The environment-monitoring ACRIMSat was launched in December 1999 on a five-year mission and is now one of many defunct satellites traversing low Earth orbit.
Because of the uncertainty about exactly where a rocket will leave a satellite, Sentinel-1A managers had no idea at launch of what was in store for them as they prepared already complex LEOP, or launch and early orbit phase, maneuvers to deploy Sentinel 1A’s solar arrays and radar antenna.
ESA officials had said before the launch that the LEOP maneuvers were forecast to take 38-40 hours following the satellite’s separation from the Europeanized Soyuz rocket’s Fregat upper stage. That was the time needed to perform 10 separate sequences to avoid having the solar arrays banging into the radar antenna as both were deployed.
ESA had planned two teams, each on 12-hour shifts, to follow Sentinel-1A during LEOP and assure that the antenna deployments occurred as planned.
Once ESOC — where ESA’s orbital debris office is also located — received the exact coordinates of Sentinel-1A in orbit, it informed Sentinel-1A managers that the chance of a collision with ACRIMSat was about one in 1,000.
The 693-kilometer-altitude polar low Earth orbit is a region stretching up to about 2,000 kilometers in which there is a high concentration of orbital debris. The 2009 collision of an active mobile communications satellite with a dead Soviet/Russian spacecraft occurred at 789 kilometers.
ESA Flight Director Pier Emanuelli said ESA, like other agencies including NASA, uses a one-in-10,000 probability of collision as the trigger point for when to perform a collision-avoidance maneuver.
In an April 10 interview, Emanuelli said that to his knowledge, no satellite has ever been put through a collision-avoidance maneuver during its LEOP operations. And ESA had not simulated such a scenario either, he said — a situation that will now change as the agency prepares the launch of a constellation of Sentinel Earth observation satellites as part of the European Commission’s Copernicus program.
The Soyuz launch and orbital insertion on April 3 went smoothly, but Emanuelli said the slight difference between the target and actual drop-off point made the collision-avoidance maneuver all the more necessary.
That meant the ESOC team needed to speed up the 10-point deployment procedure for the solar arrays and the radar antenna. Instead of 38-40 hours, they were able to get the deployments completed in 28 hours.
“We were very lucky to have two good teams on this,” Emanuelli said. “The hand-off from one to the other was made easier because we had experienced people involved. It also helped that we had a very good network of ground stations at Svalbard, in the Arctic Circle; and in Kiruna, Sweden, and Alaska.”
The avoidance maneuver to change Sentinel-1A’s altitude, once uploaded to the satellite, occurred while the spacecraft was outside the radio line of sight of ESOC. The latest projections given by ESOC showed that ACRIMSat could pass to within 50 meters or less of Sentinel-1A.
Some 2.5 hours before ACRIMSat and Sentinel-1A would have had their close encounter, the satellite’s orbit was changed, a sequence that lasted just 39 seconds.
Emanuelli acknowledged the irony of the fact that ESA was obliged to avoid a dead U.S. satellite just days after the international space station, managed by NASA, performed a similar maneuver to avoid a large piece of debris, later identified as a satellite support structure used for dual satellite launches of Europe’s Ariane 5 heavy-lift rocket.
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