The U.S. Senate passed a compromise NASA bill Thursday night that would order the agency to fly an extra space shuttle mission next June and immediately start work on a new heavy-lift rocket for human voyages beyond Earth.

The vote on the Senate floor occurred late Thursday night, just before lawmakers leave Washington for the traditional August recess.

The Senate's authorization of NASA was unveiled in July as it was approved by the body's commerce committee.

The legislation calls for the launch of another shuttle flight in June 2011, at the earliest. NASA's official shuttle manifest now includes two more missions launching in November and February.

The extra flight, designated STS-135, would keep much of the shuttle workforce in place for another year and resupply the International Space Station.

Workers are already preparing equipment for a "launch-on-need" mission that would fly as a rescue if the two scheduled shuttle flights ran into serious trouble. The STS-135 flight would use the existing hardware if a rescue mission is not required.

Under the Senate authorization bill, NASA would also begin developing a new heavy-duty booster this year for human missions to asteroids and Mars. The legislation directs NASA to use existing contracts, workers and capabilities from the space shuttle and Constellation programs, including the Orion and Ares 1 vehicles.

The heavy-lift rocket should be ready for orbital missions by the end of 2016, according to the Senate. The authorization act budgets more than $11 billion through 2013 for the government-owned launch vehicle and capsule.

About $1.6 billion would be set aside in the next three years by the Senate authorization act for up-and-coming commercial space transportation systems, including capsules to take over the job of sending astronauts to and from the International Space Station by 2015.

The bill was presented as a compromise last month, but a White House spokesperson was not immediately available for comment on Thursday night's passage.

In a statement July 15, the White House told Spaceflight Now the Senate bill "contains critical elements necessary for achieving the president's vision for NASA." The compromise "represents an important first step" in fulfilling President Obama's goals for NASA, the July statement said.

The White House's original NASA policy proposed terminating the Constellation program, providing more than $3 billion to private space companies through 2013, and planning for piloted deep space expeditions at an indefinite time in the future. The proposals did not include adding another shuttle flight.

In a visit to Florida in April, President Obama set a timetable to begin development of a heavy-lift rocket by 2015. Senate legislation would move up the development to fiscal year 2011, which begins in October.

The Senate's NASA appropriations bill is still awaiting consideration on the floor. The spending legislation passed the Senate Appropriations Committee in a July 22 hearing.

The House is working on its own NASA authorization bill, but its language is far different from the Senate legislation.

NASA would still fly the STS-135 shuttle mission and retain key parts of the Constellation program if the House legislation was adopted. But the House provides much less funding for the commercial spaceflight initiative and does not call for building a heavy-lift rocket as soon as the Senate bill.

Instead, the House would direct the agency to continue developing a government-owned rocket and capsule like the Ares 1 and Orion architecture that was to be scrapped by the White House.

The legislation was approved by the House science committee July 22, but plans to bring the bill to a vote on the House floor last week faltered after intense opposition. The House probably won't take up the NASA bill until it reconvenes in September.

The differing versions must be reconciled through a conference committee between the Senate and House.