Last-Ditch Launch Attempt

After nearly two weeks of delays caused by bad weather and faulty electrical equipment, NASA on Friday scheduled a last-ditch attempt to launch the space shuttle Atlantis today at 11:15 a.m.

If the space agency can’t get the shuttle off the pad, the launch will have to be put off for weeks, increasing the scheduling pressure on NASA as it works to finish construction of the International Space Station before the shuttle is retired in 2010.

The launch window closes after today because a later launch would conflict with the arrival at the space station of a Russian Soyuz craft, which is carrying a new crew.

Even if Atlantis launches on schedule, it will be undocking from the space station only 12 hours or so before the arrival of the Soyuz flight.

Atlantis’ Friday liftoff was called off because of a balky fuel sensor.

The fuel sensor is one of four engine-cutoff sensors designed to shut down the shuttle’s main engines when the fuel level gets low. One sensor gave a bad reading during a test early Friday, showing the tank was full when it was supposed to show empty.

A bad sensor could shut an engine down early, preventing the shuttle from reaching orbit, a potential disaster for the crew. It could also fail to shut down the engines when the fuel ran low, which would destroy the engine.

Backup sensors are designed to prevent those things from happening. The launch of Discovery in July -- the first shuttle launch after Columbia was destroyed on reentry in 2003 -- was delayed by a similar problem. Discovery finally took off when the sensor corrected itself.

After that incident, NASA adopted a requirement that in case of a similar problem, a launch be delayed at least a day so mission managers would not be making decisions in the heat of a countdown.

On Friday, a number of mission managers were ready to waive the rule and launch anyway.

The final decision to scrub came after a representative of the astronaut office objected during a closed-door meeting of the Mission Management Team, which has final launch authority.

Atlantis was originally scheduled to launch Aug. 27, but a lightning strike to the pad and the approach of Tropical Storm Ernesto forced delays. Next came an electrical short in a pump motor connected to one of three fuel cells supplying electricity for the craft.

NASA officials decided this week that the craft could still launch because the short did not pose a safety threat.

Atlantis is carrying 17 tons of construction equipment in its payload bay for the space station. Its 11-day mission includes three spacewalks to install a new truss on the station, as well as solar power arrays that will double the amount of power produced by the station.

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john.johnson@latimes.com