Space Shuttle Atlantis has been cleared for launch as early as Sunday with a unanimous “go” recommendation that stands in contrast to the divided views that clouded the recent Space Shuttle Discovery launch.

Atlantis is poised to lift off at 4:30 p.m. ET Sunday, or if delays occur, sometime in a window running into September.

While safety concerns prompted two senior NASA officials to recommend “no go” for the Discovery mission, a view that NASA Administrator Michael Griffin considered but didn’t accept, the graceful Discovery performance in a brilliant July 4 launch (and later return to Earth with little damage) prompted those officials to green-light the impending Atlantis launch. The shuttle will rise from Launch Pad 39B at Kennedy Space Center, Fla.

To be sure, those officials still voice concerns that they wish to see further safety improvements in the shuttle system.

The Sunday Atlantis launch date was announced after the Flight Readiness Review, a traditional meeting in which top NASA managers and engineers determine whether the shuttle’s complex array of equipment, support systems and procedures are ready for flight and assess any risks associated with the mission.

“It was an honor to work with this team, a thrill to see another” flight review completed, Griffin said. “It was a great review, and I look forward to a great launch.”

“The teams have done a great job of getting us here. We still have some minor open work in front of us. We look forward to the return to assembly” of the International Space Station (ISS), the purpose of the Atlantis voyage, said Bill Gerstenmaier, associate administrator for space operations. He chaired the review.

That unanimous backing for the Atlantis launch means NASA leaders see new safety fixes on the shuttle fleet as being effective.

The Sunday launch could be delayed by various factors. For example, the Discovery launch was delayed several days by thunderstorms, a common occurrence in the warm, humid August atmosphere of Florida.

Also, some bolts holding the Ku-band antenna in place may not be adequate, NASA has just discovered, although shuttles have flown with the bolts for years. The decision to replace the bolts won’t delay the launch, even though Atlantis already is vertical on the launch pad, NASA officials said. Earlier, some observers estimated bolt replacements might delay a launch for two days.

Other safety improvements won’t be ready until later shuttle flights. For example, NASA may shroud the ice frost ramps in titanium.

That would aim to curb foam insulation from ripping free from the space shuttle external fuel tank, a problem that had a disastrous result three years ago.

When Space Shuttle Columbia blasted off, foam ripped loose from the tank, and a large chunk hit the leading edge of an orbiter wing, punching a hole in it.

Later, as Columbia began returning to Earth, the fiery hot gases of reentry rushed into the wing, causing structural failure and loss of the shuttle and crew on Feb. 1, 2003.

In contrast, during the recent Discovery launch, only small particles of foam broke loose, and even then they tumbled away so late in the ascent that Discovery was beyond the thick atmosphere that can accelerate a chunk of foam to damaging speed.

When Discovery later landed, it showed no major damage, though minor, unremarkable damage was detected in minute post-mission inspections.

If Atlantis returns in similar shape, that could buttress judgments of some NASA officials that new safety moves are working well. Those moves include reductions in the amount of foam insulation applied to the external tank. As well, NASA decided that protuberance air load ramps pose a risk of shedding foam, so a decision was made to remove the ramps from external tanks.

Adjustments were made to the ice frost ramps on the fuel tank exterior.

Aside from the moves to curb foam loss, NASA also moved to ensure that if damage nonetheless occurred during launch, it would be detected in orbit, averting a disastrous decision to attempt reentry.

To that end, Discovery was run through meticulous, repeated inspections in orbit.

It was viewed by the ISS crew, and then sensors and cameras peered repeatedly at the orbiter vehicle, especially at its critical heat-shielding tiles and nose area, seeking to discern even the most minute damage.

Atlantis will have all of these safety protections on its STS-115 mission. It also will undergo further inspections after it undocks from the ISS, to see whether micro- meteorites streaking in from space harmed the orbiter even slightly.

Crewing on the Atlantis mission will be Commander Brent Jett, Pilot Chris Ferguson, and mission specialists Joe Tanner, Dan Burbank, Heide Stefanyshyn-Piper and Canadian Space Agency astronaut Steve MacLean.

The crew will install the P3/P4 truss, a girder-like structure, aboard the station. The new piece will include a set of giant solar arrays, batteries and associated electronics. There will be three spacewalks to hook up the truss and prepare the arrays for operation.