This article is more than 9 years old

This article is more than 9 years old

The Pentagon and Nasa experts have concluded that an airliner was likely to have caused a contrail off southern California, which resembled a missile plume illuminated by the sun.

The phenomenon, which was captured on Monday evening by a TV news helicopter, prompted a media sensation and an explosion of chatter across the internet about the possibility of a secret missile firing. The military insisted it knew of no rockets launched in the area.

Defence department spokesman Colonel Dave Lapan said officials were satisfied it was an airplane contrail distorted by camera angle, winds and other environmental factors.

Military experts studied the video and talked to government agencies, none of which reported having launched a missile, said Lapan.

The conclusion was independently supported by Al Bowers, of Nasa's Dryden Flight Research Center in the Mojave desert, and Patrick Minnis, of the space agency's Langley Research Center in Virginia.

"A missile would look like that," said Bowers, whose 27-year career has included stints as chief or lead engineer on such programs as the SR-71 spyplanes turned over to Nasa by the air force.

"It could potentially have a contrail that shape," he said. "The motion looks a little suspect to me, and my conclusion would be that it's most probably an aircraft."

KCBS-TV cameraman Gil Leyvas told the Los Angeles Times he spotted the phenomenon north-west of Catalina Island as he was shooting the sunset from the helicopter at about 5.15pm local time.

At Langley Research Center, Minnis and his colleague Doug Spangenberg analysed a sequence of infrared images of the area collected between 5pm and 6.45pm on Monday by a geostationary operational environmental satellite.

"At 5.30pm, suddenly there is contrail extending horizontally from the left side of the image ... that bends toward the north-east, pointing directly at Catalina Island," said Minnis.

The contrail had to have started 15 to 45 minutes earlier and become quite wide to be visible to the satellite, he said.

Minnis accounted in his analysis for earlier and later contrails, conditions that would cause a contrail to persist, movement of the contrail, and nearby clouds.

The area of ocean perceived as the lift-off point is in the vicinity of a Navy ocean range where missiles are often launched from vessels, platforms and San Nicolas Island.

Further up the coast is Vandenberg Air Force Base, where satellites are lofted into polar orbit and intercontinental ballistic missiles (ICBMS) are launched on unarmed test flights to targets in the Pacific Ocean. Three ICBMS were launched between June and September.

Many launches are invisible to the metropolitan area, shrouded by the layer of moist marine air that often rolls in from the Pacific. Others are lost in the brightness of daylight.

Launches on very clear nights or at twilight, however, sometimes trigger numerous calls to media or authorities reporting unusual sky sightings from hundreds of miles away or neighbouring states.

No public response occurred following the incident on Monday night, suggesting that the image captured by the airborne camera was not apparent to ground observers.