When the crew of a KCBS television news helicopter caught sight of a growing, glowing streak in the evening sky off the southern California coast November 8, experts and amateurs alike began to wonder just what this "mystery missile" was.

To the dismay of conspiracy theorists everywhere, the sky trail seems to have been caused by an ordinary passenger plane, according to the Pentagon and some independent experts. "It's clearly an airplane contrail," says John Pike, director of GlobalSecurity.org in Alexandria, Va. Aside from the visual evidence, which Pike says looks more like the trail of a jet than the trail of a rocket, "every non-airplane explanation requires heroic assumptions," such as an undetected foreign attack off the coast. "Possibly it's the blue whales signaling to the space people," Pike says, tongue in cheek. "Possibly it's the finger of God foretelling the doom of Los Angeles."

The airplane explanation was put forth rather quickly by Contrail Science, which demonstrated how an airplane flying horizontally toward a viewer can, from certain perspectives, mimic the appearance of a rocket flying vertically into the sky. The Web site and others even identified a likely suspect for the optical illusion: U.S. Airways flight 808, from Honolulu to Phoenix.

The KCBS news copter observation can be explained by a fortuitous alignment of flight path and viewer, combined with clear skies that allow the contrail to be visible all the way to the horizon. "Normally the airplane is going to be flying at some direction other than straight toward you, and normally you're not going to see it at sunset with clear skies," Pike says. "It is unusual but not unprecedented," he adds, noting that Contrail Science has a number of photographic examples.

The simple explanation also agrees with the military's inquiry, which turned up no missile launches—planned or accidental, foreign or domestic—that would fit the bill. In an e-mail, U.S. Department of Defense spokesperson Col. Dave Lapan said he informed the Pentagon press corps November 10 that "there is no evidence to suggest that the condensation trail observed off the coast of southern California on Monday evening was anything other than a contrail from an aircraft."

But Theodore Postol, a professor of science, technology and international security at the Massachusetts Institute of Technology, is not yet convinced. The Pentagon, he wrote in an e-mail, has provided no detailed information to support the aircraft claim. "One of their jobs is to provide air surveillance for the country, and they should be able to provide a convincing analysis supported by data for their conclusion," said Postol, who furnished photographs of the mystery contrail along with remarkably similar pictures of solid-propellant missile launches. "I do not know what to think at this point," he said, "but one thing is for sure, the Pentagon has not provided a plausible explanation of the observed event."