Six American astronauts and a Russian cosmonaut are scheduled to lift off at 6:38 a.m. Eastern Daylight Time (10:38 GMT) Thursday on a delay-plagued International Space Station (ISS) maintenance mission.

Click to see an interactive graphic of the Atlas 3 in Flash format

Their flight scrubbed three times in late April due to windy weather, the Atlantis crew aims to repair station electrical systems, mount construction cranes outside the outpost and nudge the unfinished lab into a higher orbit.

A Thursday launch, however, will not be an option if the Atlas 3 flight -- now scheduled between 5:37 p.m. and 7:57 p.m. EDT (21:37 and 23:57 GMT) -- is postponed until Wednesday.

"If they require a launch opportunity Wednesday," said NASA Test Director Jeff Spaulding, "then well toggle our launch to Friday morning."

The reason: It would take all day Thursday to reset equipment on the Air Forces Eastern Range, which provides tracking, range safety, scheduling and weather forecasting services for all launches from Floridas Space Coast.

Despite the potential one-day shuttle slip, routine launch preparations are continuing at KSCs launch pad 39-A, where Atlantis is being readied for flight.

Engineers early Tuesday began loading liquid oxygen and liquid hydrogen into storage tanks aboard the winged spaceship. The chemical reactants will be used to power the shuttles three fuel cells, which generate electricity to run spaceship systems during flight.

Also today:

• Switches that control small explosive charges used to separate the shuttle from its launch platform, as well as its twin solid-fuel rocket boosters and external tank in flight, will be put through a routine test.
• A towering water tank at complex 39-A will be filled up. More than a half a million gallons from the tank will be dumped on the launch pad in the seconds leading up to liftoff to deaden the thundering roar of launch and reduce acoustical vibration that could damage the shuttle.
• The shuttles three liquid-fueled main engines will be prepared for final fuel-loading operations and flight. The engines are powered by more than 500,000 gallons (2.3 million liters) of liquid oxygen and liquid hydrogen that will be stored in the shuttles 15-story external tank.

If the Atlas 3 flies as scheduled Tuesday night, Spaulding said routine launch preparations would continue Wednesday and then culminate in an Atlantis launch attempt early Thursday.

On the other hand, a one-day slip in the Lockheed Martin mission would prompt NASA to put the shuttle countdown and launch preparations on hold for 24 hours.

In any case, NASA will be ready to launch Atlantis Thursday if the opportunity presents itself.

"At this time, were working no significant issues at the pad," Spaulding said. "All of our flight and ground systems look real good, and everybody is looking forward to a launch on Thursday morning."