"Mission engineers confirmed at 2:30 p.m. PST [22:30 GMT] Thursday that the spacecraft was placed in safe mode, ceasing all science operations. After the decommissioning was confirmed, Spitzer Project Manager Joseph Hunt declared the mission had officially ended," NASA said in a statement.
The telescope was launched in 2003. It studied comets, asteroids, star and planet formation. NASA decided to close the Spitzer mission in 2016 in anticipation of the launch of the James Webb Space Telescope in 2018.
However, with the postponement of the Weeb telescope until 2021, Spitzer was given additional time in space.