EAST HARTFORD, Conn. — A Pratt & Whitney subsidiary has won a $1.2 billion NASA contract to design and develop rocket engines for the next generation spacecraft that would someday send astronauts back to the moon, the company announced yesterday.

The J-2X engine will power the upper stages of the Ares I and Ares V launch vehicles. The Ares rockets are part of the Orion program that NASA hopes will deliver astronauts and cargo to the moon no later than 2020.

The contract calls for the engines to be developed and evaluated through December 31, 2012. Engines for operational missions will be purchased through a separate contract.

The J-2X has evolved from the J-2 engine that propelled the Apollo-era Saturn IB and Saturn V rockets, and the J-2S, a simplified version of the J-2 that was developed and tested in the early 1970s. Pratt & Whitney Rocketdyne designed and developed the J-2 and J-2S and has produced, refurbished and improved the engines. The J-2X engine will incorporate significant upgrades to meet higher performance and reliability requirements for the Ares vehicles.

“We are very proud to have been selected by NASA to power the return of U.S. astronauts to the moon and beyond,” the president of Pratt and Whitney, Stephen Finger, said.

Ares I is a two-stage rocket that will transport the Orion crew exploration vehicle to low Earth orbit. It will accommodate as many as six astronauts and will link with Ares V, launching science and exploration payloads and key components, and head to the moon, a spokesman for Pratt & Whitney Rocketdyne said.

Orion is scheduled to fly astronauts in 2015, with a moon landing scheduled for no later than 2020.