WASHINGTON — A United Launch Alliance Atlas 5 rocket carrying the second Advanced Extremely High Frequency (AEHF) secure communications satellite for the U.S. Air Force lifted off at 2:42 p.m. EDT May 4 from Cape Canaveral Air Force Station, Fla.

The AEHF-2 satellite successfully separated from its launcher about 51 minutes into the flight. The 6,170-kilogram spacecraft will undergo 110 days of orbit-raising operations followed by 120 days of on-orbit testing, the Air Force said after the launch.

Lockheed Martin Space Systems of Sunnyvale, Calif., is under contract to build up to six AEHF satellites to replace the five-satellite Milstar constellation the United States uses for handling critical military communications. The first AEHF satellite was launched in August 2010 but took more than a year longer than expected to reach its operating orbit due to an on-board propulsion system failure that occurred shortly after liftoff.

AEHF-2’s May 3 launch attempt was scrubbed due to a lack of helium flow from the ground support equipment to the launcher’s interstage adapter compartment.