Inside the towering assembly building at Cape Canaveral's Complex 41, United Launch Alliance technicians have begun stacking another Atlas 5 rocket in preparation for the year's third launch.

File photo shows an Atlas first stage being erected at the Vertical Integration Facility. Credit: NASA TV

Following a successful flight on April 3 that deployed a sophisticated Air Force communications satellite and the June 18 launch to the Moon with NASA's Lunar Reconnaissance Orbiter and LCROSS impactor experiment, attention has turned toward a planned mid-August liftoff of the secretive PAN mission.

The process of putting together the 19-story rocket aboard a mobile launching platform started Wednesday when the first stage was hoisted upright by cranes at the Vertical Integration Facility.

The bronze stage, which stands 106.5 feet in length and 12.5 feet in diameter, is equipped with the powerful RD-180 main engine that burns a highly refined kerosene fuel and supercold liquid oxygen during the initial four minutes of flight.

The assembly sequence continues with installation of a barrel-like interstage adapter and attachment of the Centaur upper stage. The payload, which is being readied elsewhere, will be encapsulated with the rocket's two-piece nose cone and then delivered to the rocket hangar for mounting atop the Centaur.

A previous Atlas 5 is pictured here is the 401 configuration without any solid rocket boosters. Credit: Justin Ray/Spaceflight Now

The cryogenic Centaur supplies its RL10 engine with liquid hydrogen and liquid oxygen propellants to boost the PAN spacecraft into the intended orbit. The stage stretches 41.5 feet in length and 10 feet in diameter.

No strap-on solid rocket boosters are required for this launch. It will be the eighth flight for the Atlas 5 rocket's 401 configuration.

Little is known about the PAN spacecraft or the mission it will perform in orbit.

"Lockheed Martin is the prime contractor for the PAN mission, which includes a commercial-based satellite and launch system solution for the U.S. Government," the company said in a statement.

"Additional details of the mission are of a classified nature."

The launch is targeted for August 12 during a window opening at 4:55 p.m. EDT (2055 GMT).

After PAN is delivered into space, the Atlas team will continue a busy pace through the rest of the year.

Atop the Space Launch Complex 3 East pad at Vandenberg Air Force Base in California, another Atlas 5 rocket in the 401 configuration is awaiting liftoff on September 3 to put a weather satellite into polar orbit for the Air Force.

An Atlas 5 rocket outfitted with three solid-fuel boosters will loft the Intelsat 14 commercial communications satellite to cover the Americas, Europe and Africa. That launch is targeted for October 14 from Cape Canaveral.

The year's sixth and final flight is slated for December 4 to dispatch the Solar Dynamics Observatory, a new Sun-watching spacecraft as part of NASA's Living with a Star project. Another 401 rocket of the Atlas 5 family will be employed for the SDO launch from the Cape.

SDO will mark the 20th flight for the Atlas 5 program since debuting in 2002.