CAPE CANAVERAL, Fla. - NASA engineers are battling thunderstorms and rain as they struggle to understand the impact of a launch pad lightning strike that scrubbed the planned Sunday liftoff of the space shuttle Atlantis.

Shuttle officials have found at least two anomalies - one on Atlantis' Pad 39B launch pad and another on the orbiter itself - associated with a powerful lightning strike Friday that led them to postpone the Aug. 27 space shot. But a comprehensive survey of those areas must wait until heavy thunderstorms and a lightning threat pass over NASA's Kennedy Space Center (KSC) launch site.

"We think it may be the largest lightning strike in terms of the current," Cain said in a press briefing here after the scrub, adding that the bolt's strength measured around 100,000 amps. "We know just enough to know that we don't know enough to be able to press on into a launch countdown tomorrow."

Atlantis was set to launch six astronauts toward the International Space Station (ISS) at 4:29:57 p.m. EDT (2029:57 GMT) Sunday on NASA's STS-115 mission to resume construction of the orbital laboratory. The shuttle's 60-foot cargo bay is filled with a 17.5-ton pair of portside trusses and solar arrays to be installed aboard the ISS.

Severe storms have been a daily phenomenon here at NASA's launch site, highlighted by the 1:49 p.m. EDT (1749 GMT) lightning strike at Atlantis' Pad 39B.

"It was certainly not a hit to the vehicle, I want to make that perfectly clear," said NASA launch director Michael Leinbach of the strike. "But you can get an induced voltage field around the lightning strike, and that's what we're looking at now."

The lightning struck one of a network of cables designed to protect shuttle launch pad structures and orbiters from being hit. The one-inch steel runs over the top of an 80-foot fiberglass mast and stretches about 1,000 feet on either side to the ground.

After reviewing data from the lighting strike, engineers detected a small spike in the voltage readings from one of the three electrical buses that supply power to certain systems aboard Atlantis, Cain said. The spike - in a unit known as Essential Bus 1 BC - spanned just 80 milliseconds, but was enough to begin checks to ensure none of the shuttle's systems were compromised during the lightning strike.

A second area of interest is a vent arm that siphons off gaseous liquid hydrogen, which is used with liquid oxygen to fuel shuttle launches, from Atlantis' 15-story external tank. The vent arm attaches to a region near the mid-body of Atlantis' external tank and separates from the vessel via an explosive pyrotechnic device less than one minute before launch during a typical shuttle liftoff, NASA officials said.

"We are 99.9 percent sure the pyro did not fire," Leinbach said. "We don't suspect it fired, but again that's why we have to go out and look at it."

Checking the vent arm, its associated cables and electronics will take several hours, he added.

Leinbach said it typically takes about 96 hours to check all of the necessary systems at a launch pad, any orbiter that's present, and associated external tanks and boosters after a lightning strike. Engineers don't plan to check all of those systems, only those required to make sure Atlantis is safe to fly Monday.

"We need to let the folks go off and look at their data," Cain said. "And that's what we're going to do."

NASA's window to launch Atlantis runs through Sept. 7. The shuttle's chances of actually rocketing spaceward Sunday were a bit low due to a 60 percent chance of rain, thunderstorms and clouds near its launch site.

Lt. Kaleb Nordgren, of the 45^th Weather Squadron at Cape Canaveral Air Force Station, said Monday's forecast calls for only a 20 percent chance of bad weather preventing a launch. Tuesday's launch forecast is also favorable, though weather conditions will again begin to deteriorate on Wednesday, he added.

Lightning typically strikes NASA's two shuttle launch pads about five times each year, though no reports of serious damage have been recorded to date, according to NASA records. In 1983, bolts of lightning struck the launch pad while an orbiter was present three times, records show.

Lightning also struck the launch pad perimeter - not the pad itself - six days before NASA's July 4^th launch of the space shuttle Discovery's STS-121 mission last month.

"Florida is the lightning capitol of the U.S.," Nordgren said.

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