Santa Clarita / Antelope Valley : Shuttle’s Landing a Booming Success : Aviation: Atlantis sets down at Edwards Air Force Base after sending shock waves through Los Angeles during its descent.

A returning space shuttle set off twin sonic booms over Los Angeles early Monday, jolting residents whose nerves were already frayed by the Northridge earthquake and countless aftershocks this year.

The safe touchdown by Atlantis and its six-member crew was the third Southern California shuttle landing in three months, but the last two descents approached Edwards Air Force Base from the west and the north, so that the booms were barely heard in the city.

On Monday, Atlantis hurtled back to Earth from the south, passing directly over the Los Angeles Basin on its way to Edwards. It set off the twin booms about 7:30 a.m., while many residents were still swallowing their coffee and corn flakes.

“Was that a sonic boom?” asked a puzzled Lori Farmer, 26, of Reseda.

“I woke up and I was like, was that an earthquake? I am so paranoid of earthquakes now, and I was thinking of my escape route.”

Many were caught off guard by the thunder-like blasts because NASA officials, watching storm conditions in Florida, did not officially move the landing site to California until about 2 a.m. Monday. Some residents contacted Edwards to inquire about the noise or express their irritation.

“I have an idea that people are more sensitive to something like this because of the earthquake,” said NASA spokesman Don Haley.

During a press conference Monday afternoon, the six astronauts chuckled when informed that some residents mistook their man-made booms for fresh Earth tremors.

Air Force Lt. Col. Curtis Brown, the mission’s pilot, said the shuttle was traveling three or four times the speed of sound--or up to 2,800 m.p.h.--when it passed over Los Angeles.

Sonic booms are created when a speeding aircraft pushes aside air molecules in its path, forming shock waves. The sharp release of pressure from these shock waves creates the boom.

All supersonic aircraft set off twin sonic booms, one at the nose and one at the tail. But when small military jets break the sound barrier, the booms follow one another so quickly that the people usually hear it as one sound.

Space shuttles, at 122 feet long, produce a gap of about one-half a second between the nose and tail sounds, resulting in the distinctive double booms familiar to residents of the high desert.

NASA officials said sonic booms sound louder in canyon areas, where the noise reverberates, and are more intense on days like Monday, when few clouds are present to muffle the sound.

Atlantis pilot Brown said he was paying more attention to the sights outside his window than the sounds that his descent was creating.

“We had a very great view of Los Angeles--the downtown and the city itself, the marina and the harbors,” he said. “I had a perfect view of the L.A. area.”

Last year, NASA designated the Kennedy Space Center in Florida as the primary landing site for its shuttles, with Edwards as the backup. But bad weather in Florida forced the agency to divert Discovery to Edwards on Sept. 20 and Endeavour to Edwards on Oct. 11.

Landings at Edwards add about $1 million to the cost of each mission, NASA officials say, largely because the vessel must be ferried back to Florida atop a modified Boeing 747 jumbo jet.

Monday’s safe return by Atlantis marked the end of an 11-day scientific research mission in which the astronauts studied the Earth’s atmosphere and the effects of low-gravity on developing rats.

The last-minute switch in landing sites provided little time for spectators to gather at the base, but more than 100 arrived at a viewing area.

Among the first to sit in the bleachers were Fresno residents Kathleen Stecko and Alan Smith, who arrived at 5 a.m. Smith said he and Stecko had wanted to view a shuttle landing for years.

“Work always interfered,” he said. “But this time we decided work wouldn’t interfere. We played hooky.”