WASHINGTON >> The U.S. Navy said it launched a second — and final — missile in a planned exercise from a submarine off the Southern California coast.

The second test launch of the Trident II (D5) missile from a ballistic submarine in the Pacific Ocean took place this afternoon, the Navy said. The blast-off took place to far less fanfare than Saturday night’s launch, which provoked residents from San Francisco to Mexico to take to social media, posting photos of an eerie-looking bluish-green plume smeared above the Pacific.

Speculations were wide-ranging, including rumors of an otherworldly alien UFO visit. In fact, the streak was generated from the Trident missile’s rocket motor.

The Navy later confirmed a ballistic submarine launched an unarmed Trident II (D5) missile in a test flight, but would not define the window of time available for conducting additional launches, nor would it disclose where the exercise was actually taking place.

“It’s important that we test these missiles for our national security,” said John M. Daniels, spokesman for the secretive Strategic Systems Programs office, which oversees the Navy’s nuclear-tipped missile arsenal.

The Navy is considering posting additional photos — and possibly video — of the missile launch after the current exercises are completed, Daniels said, but it has yet to decide.

The Navy’s fleet of 14 ballistic submarines can each carry 24 Trident missiles, each tipped with 14 independently targetable thermonuclear warheads.

The Navy annually tests the Tridents, on the West Coast and on the East Coast, near Florida.

The $31 million missile, built by Lockheed Martin Corp. in Sunnyvale, Calif., has had more than 150 successful launches since its first test in 1989. It is capable of hitting a target 4,000 nautical miles away.

The test Saturday featured the launch of a missile outfitted with a dummy warhead toward the Kwajalein Atoll, a missile test site that’s part of the Marshall Islands in the western Pacific.

While the risk of nuclear confrontation between the United States and Russia declined after the 1991 fall of the Soviet Union, it has never gone away.

The U.S. military’s nuclear weapons strategy rests on a triad of delivery systems — bombers, submarines and land-based missiles — developed early in the Cold War to deliver warheads anywhere in the world.