Space Shuttle Endeavour’s last flight to orbit is delayed at least 72 hours because of a problem with heaters on one of its three auxiliary power units (APUs).

The APUs are fueled by highly toxic hydrazine and provide power to the hydraulic systems that help steer the shuttle on ascent and landing. On launch, they gimble the main engines to steer the shuttle to orbit. During landings, they are responsible for controlling the brakes and wing surfaces.

NASA officials say the delay will be at least 72 hours. Astronauts were on their way to the pad and President Barack Obama was en route to the Kennedy Space Center when the scrub was announced. Endeavour will now be drained of fuel and engineers will inspect the units and determine how long to delay the launch.

President Obama and his family arrived at the space center about 2 p.m. and were expected to take a tour before Obama was to speak at a community college commencement in Miami.