The US space agency had planned to launch Atlantis on Wednesday but the discovery of a glitch in a fuel cell that makes electricity forced managers to delay the launch.
The mission will be the first to carry new construction modules to the International Space Station since the 2003 Columbia disaster,
“There’s more data that has to come in but right now the plan is to make a launch attempt on Friday,” Dean Acosta, the Nasa press secretary, said on Thursday.
The power problem, a voltage spike in one of the three fuel cells on the shuttle, was found shortly before the shuttle was to be loaded with half a million gallons of propellants for launch on Wednesday.
The launch time from Cape Canaveral is expected to be at 11.41 AM (1541 GMT).
The 250-pound (113-kg) fuel-cell units combine oxygen and hydrogen to produce electricity for the shuttle’s systems and water that is used for cooling and for the crew to drink.
Atlantis’ crew is to deliver and install a $372 million solar power module to the space station. Nasa needs to launch by Friday to assure there is enough time for the complex assembly to be conducted and to have an extra day in case any problems arise.
If Nasa misses this week’s launch window, the next opportunity for Atlantis to fly would be October 26.
Construction of the $100 billion space station has been on hold since shuttle Columbia disintegrated over Texas more than 3 years ago, killing all seven astronauts on board.
Nasa has just four years to finish building the orbital complex before the space shuttles are retired.
The station’s structural trusses, modules and other major components were designed to be launched only on the shuttles.
Managers had planned to launch Atlantis and its six-member crew last week, but a lightning strike and a storm triggered a series of postponements.
The mission already had been delayed more than three years while Nasa recovered from the Columbia disaster.