With NASA poised to continue flying the space shuttle through mid-2011, the U.S. space agency has agreed to pay ATK Launch Systems an additional $42.1 million to continue to provide pre- and post-launch support of the shuttle’s reusable solid rocket boosters through Dec. 31, 2011.

The three-month contract extension announced Nov. 30 increases the total potential value of Alliant Techsystem’s contract to $4.13 billion.

NASA said the contract modification is based on an extension of the current space shuttle launch schedule, which shifted the last two scheduled missions — STS-133 and STS-134 — into the agency’s 2011 budget year, which began Oct. 1.

The extension also covers work ATK will perform after the last scheduled shuttle flight — Endeavour’s STS-134 mission in late February — to clean and preserve the reusable solid rocket motors.

U.S. President Barack Obama signed a NASA authorization bill into law in October directing the agency to fly a third shuttle mission, STS-135, no sooner than June 2011. NASA, which is still waiting for Congress to approve funding for the mission, has not added the flight to its manifest. The STS-135 mission, if formally manifested, would use the space shuttle orbiter, external tank and reusable boosters NASA is preparing for a launch-on-need mission to rescue STS-134 astronauts should a cripple shuttle strand them at the international space station.