control issue On June 13, 2021, Hubble's payload computer halted due to a suspected issue with a memory module. An attempt to restart the computer on June 14 failed. Further attempts to switch to one of three other backup memory modules on board the spacecraft failed on June 18. On June 23 and 24, NASA engineers switched Hubble to a backup payload computer, but these operations have failed as well with the same error. On June 28, 2021, NASA announced that it was extending the investigation to other components. Scientific operations were suspended while NASA worked to diagnose and resolve the issue. After identifying a malfunctioning power control unit (PCU) supplying power to one of Hubble's computers, NASA was able to switch to a backup PCU and bring Hubble back to operational mode on July 16. On October 23, 2021, HST instruments reported missing synchronization messages and went into safe mode. By December 8, 2021, NASA had restored full science operations and was developing updates to make instruments more resilient to missing synchronization messages. Future Orbital decay and controlled reentry Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag. If not reboosted, it will re-enter the Earth's atmosphere within some decades, with the exact date depending on how active the Sun is and its impact on the upper atmosphere. If Hubble were to descend in a completely uncontrolled re-entry, parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalities. In 2013, deputy project manager James Jeletic projected that Hubble could survive into the 2020s. Based on solar activity and atmospheric drag, or lack thereof, a natural atmospheric reentry for Hubble will occur between 2028 and 2040. In June 2016, NASA extended the service contract for Hubble until June 2021. In November 2021, NASA extended the service contract for Hubble until June 2026. NASA's original plan for safely de-orbiting Hubble was to retrieve it using a Space Shuttle. Hubble would then have most likely been displayed in the Smithsonian Institution. This is no longer possible since the Space Shuttle fleet has been retired, and would have been unlikely in any case due to the cost of the mission and risk to the crew. Instead, NASA considered adding an external propulsion module to allow controlled re-entry. Ultimately, in 2009, as part