VANDENBERG AIR FORCE BASE, Calif. (AP) – An Atlas 5 rocket carrying a classified satellite for the U.S. National Reconnaissance Office has lifted off from Vandenberg Air Force Base on the central California coast. The launch vehicle’s main engine ignited Thursday at 2:39 p.m. PDT for a mission described only as being in support of national defense.

It’s the fourth launch this year for the NRO, which operates the nation’s system of intelligence-gathering satellites. The launch was delayed six weeks due to launch range equipment problems.

The Atlas is also carrying 11 tiny satellites known as Cubesats to conduct research on subjects ranging from space weather to tracking maritime shipping containers. The Cubesats include projects of four universities.