On March 29, 2012, NASA scientists learned that the space agency’s Fermi Gamma-ray Space Telescope was headed for a potential conjunction (close approach) with Cosmos 1805, a defunct Russian satellite from the Cold War era.
According to a news release from NASA’s Goddard Space Flight Center, the tale of how Fermi avoided a potential space disaster reveals an interesting component of supervising a space mission: orbital traffic control.
Upon checking her inbox on March 29, Julie McEnery, the project scientist for NASA’s Fermi Gamma-ray Space Telescope, discovered a report from NASA’s Robotic Conjunction Assessment Risk Analysis (CARA) team. After reading the document, McEnery learned that Fermi was one week away from passing within 700 feet of Cosmos 1805.
Though Fermi was expected to miss Cosmos by several-hundred feet, NASA scientists knew from experience that forecasting spacecraft positions a week in advance isn’t an exact science. For example, Iridium 33 and Cosmos 2251 collided in 2009 even though they were predicted to miss each other by approximately 1,900 feet. This was the first known satellite-to-satellite collision.
That collision resulted in thousands of pieces of space junk, some big enough to be tracked and numerous smaller pieces that elude detection. Many of these pieces continue to be a hazard to operating spacecraft.
According to NASA’s news release, a collisions between Fermi and Cosmos 1805 would release as much energy as two and a half tons of high explosives, obliterating both spacecraft.
With approximately 3,000 satellites orbiting Earth, it seems that close approaches would be more common, but the space agency says that they rarely take place.
Goddard’s Eric Stoneking compares the forecasting of close approaches to predicting rain at a “specific time and place a week in advance.” As the date approaches, meteorologists are able to adjust their predictions because uncertainties in the forecast decrease.
This is the third time the NASA scientists working with Fermi have been made aware of potential conjunctions, and on both occasions that threats to the space agency’s spacecraft vanished. While entirely possible that the threat of colliding with Cosmos 1805 would disappear as well, the update on Friday, March 30, failed to put engineers at ease. In fact, the update said that the satellites would occupy the same point in space within 30 milliseconds of each other.
McEnery sprang into action and notified the Flight Dynamics Team that her team was planning a maneuver to get Fermi out of Cosmos 1805’s path. The team knew that the only way to move Fermi would be to fire thrusters designed to move the spacecraft out of orbit at the end of its operating life.
Unfortunately, these thrusters had never been tested because a failure of this system could have ruined Fermi’s mission. McEnery said that despite her nervousness during the maneuver, she is confident that the team could attempt the maneuver again if Fermi finds itself heading toward a potential conjunction with another object.
The Goddard CARA team calculated how much thrust Fermi would need to avoid a close approach with Cosmos 1805. They also made sure that the new orbit wouldn’t put the spacecraft on course for a conjunction with a different space object. Four possible maneuver times were named.
Over the weekend, the U.S. Space Surveillance Network tracked Cosmos 1805.
By Tuesday, April 3, the threat of a conjunction with Cosmos 1805 had not vanished and NASA prepared to fire Fermi’s thrusters.
Shortly after noon EDT, the space agency fired all thrusters for one second. When it was over, everyone involved “just sighed with relief that it all went well.” By 1 p.m., the spacecraft had returned to its mission.
Fermi is gathering data that will help scientists answer a number of important questions across numerous space topics, including supermassive black holes, pulsars and cosmic rays. Fermi orbits Earth every 95 minutes, capturing gamma rays from differing sources across the universe.