The delay to launch until just after 4pm (2004 GMT) on Monday will give managers time to analyse data from shuttle systems and ground support equipment that may have been affected by the lightning bolt that struck on Friday.
Additional delays will depend on whether repairs are required, following what LeRoy Cain, the senior shuttle programme manager at Florida’s Kennedy Space Centre, called the biggest lightning strike to hit a shuttle launch pad.
“At this point, we don’t have enough data yet to really know whether or not we have any problems,” Cain said on Saturday.
“We know just enough to know that we don’t know enough to be able to press on into a launch situation tomorrow.”
The Atlantis mission, the third since the 2003 space shuttle Columbia disaster, is a critical part of Nasa’s efforts to finish building the International Space Station before the shuttle fleet is retired in 2010.
The lightning bolt struck a thick wire at the top of the shuttle’s seaside launch pad at about 2pm on Friday.
The wire is part of the launch pad’s lightning protection system.
Engineers were looking at at least two potential problems from the strike, including a ground system used to vent hydrogen from the shuttle’s external fuel tank and an electronic component aboard the spacecraft.
“At this point we don’t have enough data yet to really know whether or not we have any problems”
If Nasa can make a launch attempt on Monday, it would be under much more favourable weather conditions than had been forecast for Sunday, with just a 20% chance of a weather-related delay.
Tropical Storm Ernesto could also complicate launch plans for Atlantis.
By Thursday, the storm, which is projected to hit the Gulf of Mexico, could develop into a Category 3 hurricane on the 5-step Saffir-Simpson scale of storm intensity, with sustained winds of at least 178 kph.
It may also affect plans to ship a shuttle external fuel tank from Nasa’s manufacturing facility near New Orleans in time to support an emergency rescue mission, should Atlantis sustain Columbia-like damage during launch.
Columbia was hit by a piece of foam insulation that fell off its fuel tank during launch.
Damage from the impact triggered the shuttle’s breakup over Texas as it flew through the atmosphere for landing on February 1, 2003.
All seven astronauts aboard died.
While the shuttle fleet was grounded for repairs and safety upgrades after Columbia, work on the space station came to a halt.
Following two flights to test safety upgrades made after the accident, Nasa is now ready to restart station construction with the launch of Atlantis.
Atlantis will carry one of the heaviest shuttle payloads, including a 16-tonne power module for the space station.
It is set for an 11-day mission.
However, if Nasa should have to evacuate its Houston centre, the six-member shuttle crew would be told to leave the space station and land at the first safe opportunity.