Hours before its planned liftoff from Kennedy Space Center in Florida, the space shuttle Endeavour had endured pouring rain and nearby lightning—but was still on target for launch, based on weather conditions.
Instead, it was a problem with a line of small heaters that ultimately forced NASA to scrub today's 3:47 p.m. launch attempt.
NASA's next try will be no earlier than Monday morning, launch officials said today.
A planned launch of an Atlas V rocket from Kennedy on May 6 means that the shuttle lift-off can push to no later than May 4.
After that date, the next possible launch attempt for Endeavour wouldn't come until May 9.
Shuttle Delayed by Heater Failure
Endeavour's crew of six veteran astronauts, led by mission commander Mark Kelly, had already boarded their bus for the launch pad when the scrub was announced at 12:19 p.m. ET.
The trouble was caused by failed heaters in one of the shuttle's auxiliary power units (APUs), which provide hydraulic power for the craft's engine nozzles, landing gear, and other moving parts used in flight.
The APU hydraulics are driven by a fuel called hydrazine, Mike Moses, NASA's launch integration manager for the shuttle program, said today during a press briefing.
Without heaters, hydrazine could freeze in the chilly environment of space, creating buildups of frozen fuel. This could cause problems during atmospheric reentry, as thawing chunks of hydrazine could trigger leaks of combustable fuel.
The shuttle does have two other APUs, and if one had failed after launch, "it would not have been a bad day," Moses said. Flight engineers have contingency plans in place to safely shut off a malfunctioning APU.
But since the issue was discovered before launch, shuttle launch director Mike Leinbach made the decision to scrub.
"We don't like lifting off without redundancy, especially in a critical system like this one," Leinbach said.
Despite Scrub, Obama Makes Space Center Visit
To troubleshoot the problem, engineers first have to empty the shuttle's giant external fuel tanks—which had been filled just this morning with liquid oxygen and liquid hydrogen in preparation for launch.
When the tanks are empty—they take 24 hours to drain fully—technicians will be able to safely access the shuttle's avionics bay at the back of the craft to determine the extent of the APU glitch.
The problem could be a faulty thermostat, which would be relatively quick and easy to fix, Leinbach said. But if the thermostat isn't to blame, the glitch may trace to a switchbox called the load control assembly (LCA).
If something is wrong with the LCA, Leinbach said, the fix would be more expensive and time-consuming, possibly pushing the launch to May 9, at the earliest.
In the meantime, the shuttle crew has gone back into quarantine—remaining in special quarters to prevent contracting any illnesses before flight.
The delay means the crew will have a few days more to relax and spend time with their families before Endeavour makes its final flight.
The astronauts also enjoyed a post-scrub meeting with U.S. President Barack Obama, who arrived at Kennedy this afternoon with his family as part of a planned visit to see the launch.
"Hopefully we can lure him back for another launch in the future," said Kennedy Space Center director Robert Cabana.