It looked like a missile launch, he said, because of an optical illusion that made the contrail appear as though it started on the ground and zoomed straight up. In reality, he said, the contrail began on the horizon and ran parallel to the ground.
"It was an unusually clear day," he said. So what looked like a missile launch 35 miles off the coast of Los Angeles was actually the contrail of a jet that stretched 300 miles into the distance, he said. "At the end of the day, you really have to go with the simplest explanation," he added.
Harvard astronomer Jonathan McDowell told New Scientist that because a helicopter in flight recorded the video of the incident, it employed a visual angle that distorted the arc of the contrail. That effect was compounded, McDowell maintained, by the distinctive lighting of the sky at twilight--and the net result was a bit of an optical illusion.
"What isn't clear to me is whether anyone but this helicopter saw it," McDowell said. "If it's coming over the horizon, straight at you, then it rises quickly above the horizon. You can't tell because it's so far away that it's getting closer to you -- you'd think it was just going vertically up."
There you have it!