RTEMS is a Real-Time Operating System commonly used in ESA missions. It is the only _Space Qualified_ operating system approved by ESA.

RTEMS was initially developed by the US Depart of Defense as the Real Time Executive for Missile Systems in the late 1980s. It has since been adopted by ESA, and NASA, and is in use on (among other missions) the Curiosity Rover, the Parker Solar Probe, and the HERA mission.

It provides a POSIX API, and has been ported to the SparcV8 (the architecture used in the LEON family of chips by Frontgrade Gaisler).

Given its use in high-profile missions, ESA has invested in qualifying RTEMS for space missions.