WASHINGTON — A Senate panel issued a subpoena ordering NASA to produce internal documents related to the agency’s plans for the Space Launch System (SLS), the heavy-lift rocket Congress ordered NASA to make ready for flight by Dec. 31, 2016.

The Senate Commerce, Science and Transportation Committee, whose members were the architects of the NASA Authorization Act of 2010 that created SLS, took the unusual step of exercising its subpoena authority July 27.

The subpoena came one day before internal NASA briefings containing the agency’s proposed flight schedule and manifest for SLS surfaced online.

“I can confirm that the committee sent a subpoena yesterday,” Vincent Morris, a spokesman for the committee, wrote in a July 28 email to Space News. Morris works for Sen. John Rockefeller (D-W.Va.), the chairman of the Senate Commerce Committee.

In a June 22 letter to NASA Administrator Charles Bolden, the committee’s leadership demanded that the space agency turn over documents detailing “the data and analysis NASA has relied on to comply with the 2010 Act’s space launch system and crew vehicle requirements.” The panel said in the letter that if it did not see the documents it wanted by June 27, it would subpoena them.

“While we share the Senators’ commitment to human space exploration and implementation of the Authorization Act, we also have a commitment to be responsible stewards of taxpayer dollars,” NASA spokesman Michael Cabbage said July 28. “The Space Launch System is the most important — and expensive — decision NASA will make for the next decade, and we want to get it right so we don’t repeat the mistakes of the past or get pushed into making a premature decision about our nation’s deep space exploration plans.”

The website nasaspaceflight.com said July 27 that it had unearthed more details about those deep space exploration plans.

According to nasaspaceflight.com, the fully evolved SLS — the version capable of lifting 130 metric tons to orbit — would not be ready until 2032. The first crewed mission to be launched by SLS would not fly until 2021, and no incarnation of SLS would fly more than once a year. The website said it got its information from a “budget restricted manifest” created by NASA.

The first SLS launch, an unmanned test flight, would take place in 2017, as Bolden told members of the House Science, Space and Technology Committee July 12. At that hearing, Bolden acknowledged for the first time that NASA had selected — and sent to the White House Office of Management and Budget for review — an SLS design that was almost identical to Ares 5, the heavy-lift rocket planned as part of the now-defunct Constellation Moon-return program.

In the 2017 test flight, an SLS core stage comprising three space shuttle main engines and two five-segment, shuttle-derived solid-rocket boosters would send an empty multipurpose crew capsule into orbit. The capsule would perform a Moon loop, return to Earth and splash down.

Meanwhile, the Senate’s inquiry into NASA’s progress on SLS is effectively on hold until the agency either turns over the documents sought by the committee, or refuses to do so. The committee would not divulge the deadline it had set for NASA to comply with the subpoena.

If NASA refuses to comply, the full Senate would have to sign off on a resolution of contempt before agency officials could be held accountable.