Decommissioned Earth science satellite to remain in orbit for centuries

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LAS CRUCES, N.M. — A U.S.-European satellite that completed its mission earlier this month has been decommissioned but will remain in orbit for as long as 1,000 years, far beyond existing orbital debris mitigation guidelines.

Jason-2, a joint mission of NASA, the National Oceanic and Atmospheric Administration, the French space agency CNES and European weather agency Eumetsat, ended its mission to study sea-level height Oct. 1. The spacecraft, also known as Ocean Surface Topography Mission (OSTM), launched in June 2008 for a mission originally expected to last three years.

The decision to shut down Jason-2 came after telemetry indicated the spacecraft’s power system was deteriorating. Earlier problems with Jason-2 in 2017 forced controllers to move the spacecraft into a slightly lower orbit and delete its excess propellant reserves to avoid any interference with its successor, Jason-3, launched in 2016.

“Today we celebrate the end of this resoundingly successful international mission,” Thomas Zurbuchen, NASA associate administrator for science, said in an Oct. 4 statement about the decision to end the mission. “Jason-2/OSTM has provided unique insight into ocean currents and sea level rise with tangible benefits to marine forecasting, meteorology and our understanding of climate change.”

According to that statement, “final decommissioning activities” for Jason-2 started once the mission formally ended, and would be done by Oct. 10. NOAA said Oct. 9 that those activities had been completed. That decommissioning, the statement said, was intended in part to comply with “French space law,” although an English-language CNES statement about Jason-2 referred to work “to comply with international space law.”

That decommissioning process, NASA spokesman Steve Cole said Oct. 8, involves steps to remove stored energy on the spacecraft. Besides depleting any remaining propellant, gyros and reaction wheels used for attitude control were spun down. Instruments and other systems were powered off, the solar arrays disconnected and batteries run down until the spacecraft’s computer shut down because of a lack of power.

However, controllers will not implement the portion of orbital debris mitigation guidelines that call for deorbiting spacecraft in low Earth orbit within 25 years of the end of the mission.

“The OSTM/Jason-2 spacecraft was designed and built prior to the adoption of the 25-year reentry requirement,” Cole said. The spacecraft contained 28 kilograms of propellant at launch, which was not enough to change its orbit much from its operational orbit of 1,336 kilometers, a relatively high low Earth orbit.

The spacecraft was, prior to the end of the mission, placed in a lower “graveyard” orbit with an altitude of 1,309.5 kilometers. “This orbit was chosen to minimize risk by avoiding current and proposed constellation deployments,” Cole said, and will decay at an initial rate of only about 40 meters per year.

“The OSTM/Jason-2 spacecraft is estimated to remain in orbit for at least 500 to 1,000 years after decommissioning,” Cole said. “This non-compliance with the international orbital debris standard of a 25-year post-mission orbital lifetime was known and accepted by both CNES and NASA prior to launch.”

Few satellites operate at those high orbits, but that could change with some megaconstellations under development. Telesat plans to place some satellite sin orbits as high as 1,248 kilometers, and SpaceX has proposed satellites for its Starlink constellation in orbits of 1,275 and 1,325 kilometers, although the company is initially focusing on satellites in lower orbits.

Jason-2 is not the first such satellite left in a high orbit. The original Jason satellite, launched in 2001, ended its mission in 2013 in an orbit of about 1,325 kilometers. Jason-3, which is continuing the sea-level monitoring work of Jason and Jason-2, will go through a similar decommissioning process at the end of its mission, NOAA spokesman John Leslie said Oct. 9.

The next spacecraft in the series is a pair of satellites known as Jason-CS or Sentinel-6, with the first scheduled for launch in 2020. Leslie said that those spacecraft will deorbit as part of their decommissioning plan.