NASA delays shuttle launch

Plagued by troublesome fuel sensors in the space shuttle’s main propellant tanks, NASA officials on Sunday postponed the launch of Atlantis until Jan. 2 at the earliest.

For the second time in two launch attempts this week, at least one of the four engine cutoff sensors failed while technicians were fueling the craft on its launch pad. Two sensors failed during fueling Thursday, and a third failed while the tank was being emptied.

The sensors shut off the shuttle’s main engines when fuel levels in the tank drop below a specified level, either because all fuel has been used up or the tank has been emptied by a leak.

Shutting off the engines in such a situation is critical, because continuing to run them with no fuel could cause the fuel pumps to break, potentially leading to a catastrophic explosion.

The sensor problem has periodically bedeviled NASA since flights resumed after the 2003 Columbia tragedy. A September 2006 launch of Atlantis was delayed because of sensor problems, as was a July 2005 launch of Discovery.

Working Friday and Saturday, engineers thought the problems might be traced to a subtle manufacturing problem in the sensors and had hoped that replacing all four sensors might overcome the difficulties.

But that was not to be. Fueling was halted Sunday morning.

Because of sun angles and other orbital considerations, Atlantis would have to be launched by Thursday to meet up with the orbiting International Space Station. Rather than try to meet that deadline, officials decided to delay the launch and investigate the sensors more thoroughly.

“We’re determined to get to the bottom of this,” said LeRoy Cain, chairman of the mission management team.