On June 19, 2013, the U.S. Geological Survey officially decommissioned Landsat 5 after an astonishing 29 years of operation. The satellite’s longevity was recognized by the Guinness Book of World Records, which dubbed Landsat 5 as the longest-operating Earth observation satellite.
I recently listened to Dr. Steve Covington — the flight systems manager for Landsat 5 since 2001 — recount some of the lucky circumstances and creative engineering that kept the satellite operating for nearly three decades. (The talk will be posted on the Library of Congress web site in the near future.) Here are some of the highlights.
Lucky circumstance 1: Landsat 5 had a twin, Landsat 4, which showed problems with its power system once it was in orbit. Those problems let engineers adjust Landsat 5 before it launched on March 1, 1984.
Lucky circumstance 2: Landsat 5 was equipped with a large auxiliary fuel tank designed to let the satellite fly down from its orbit to a lower orbit where astronauts could retrieve and repair it. The polar-orbiting space shuttle program that would enable these on-orbit repairs never got off the ground, and this left Landsat 5 with a whole lot of extra fuel. Mission operators used the fuel to extend the mission across decades.
Creative Engineering 1: In January 2005, Landsat 5’s primary solar array drive failed, and months later, in November, the backup drive failed. This key component turned the solar array to face the Sun straight on whenever the satellite was on the sunlit side of the Earth. Without the drive, the solar array was stuck in a single position, limiting the amount of energy it generated to power the instruments and spacecraft.
The failure of the drives looked to be a mission-ending event, since the Landsat 5’s batteries couldn’t be recharged sufficiently to continue science operations. But mission operation engineers came up with a novel solution: If the solar array couldn’t move, they would move the entire spacecraft. Before the satellite came across Earth’s shadow into the sunlight, they pitched the satellite to face the Sun. The satellite faced down again to acquire data, and then, approaching the shadow again, pitched out to face the Sun. This dance gave the satellite just enough extra Sun exposure to keep the batteries charged and execute its imaging duties.
Creative Engineering 2: Landsat 5 had four pathways for sending data to the ground: two communication links with relay satellites, and two direct downlinks to ground stations. The last of these failed in 2012, preventing the satellite from sending data from its primary instrument (the Thematic Mapper) to the ground. The secondary instrument, the Multispectral Scanner (MSS) had been turned off in 1995. Mission operations engineers realized that the communication links used by MSS were still good, and the mission could continue if the MSS still worked. Seventeen years after turning the instrument off, engineers powered it back on, and amazingly, it worked. This allowed Landsat 5 to acquire one more year of data until Landsat 8 was ready to take its place in early 2013.