Nelson, who became administrator May 3, said Congress hasn't appropriated enough money for the nation's coming lunar aspirations.

"We want to have these sustained landings over a dozen years, and that's gonna cost some more money," Nelson testified to the House Committee on Science, Space and Technology.

Members of the committee, including Chairwoman Bernice Johnson, D-Texas, pressed Nelson for more detailed plans to use lunar exploration as a springboard to prepare for astronaut journeys to Mars.

Known as the Artemis program, its goal was first outlined by NASA under the Trump administration.

But Nelson said Congress has failed to provide adequate funding for moon missions when the House and Senate finalize the annual budgets in recent years.

Last year, NASA made a "$3.4 billion request for human spaceflight for the exploration part, and the Congress appropriated just $850 million," Nelson said.

The Biden administration is seeking $24.8 billion for NASA in fiscal 2022, which would be a 6.6% increase from 2021. But there's an amendment to the 2021 budget pending in Congress to boost moon landing budgets this year by $10 billion.

Separate from funding, Nelson said NASA must wait to develop further plans until a contract dispute is resolved among Elon Musk's SpaceX, Jeff Bezos' Blue Origin and Alabama-based Dynetics.

The three space companies pursued NASA funding to build lunar landers, and only SpaceX was awarded funding so far. Musk's company now faces bid protests filed by the other two companies with the federal watchdog Government Accountability Office.

Nelson said he and newly appointed NASA executives Pam Melroy, the deputy administrator, and Bob Cabana, the associate administrator, are drawing up new Artemis plans pending the GAO decision due by Aug. 1. Melroy and Cabana are former astronauts.

"The three of us are already trying to make the plan so that when the GAO decides on the bid protests, we can move out quickly, depending on what they decide," Nelson testified.

The first Artemis mission, an uncrewed test flight of the SLS rocket and Orion capsule, is scheduled to launch from Florida later this year.

Nelson's defense of the agency's strategy came as committee chairwoman Johnson and Frank Lucas, R-Okla., suggested that NASA hadn't provided enough detail on its moon and Mars plans.

"NASA needs to develop that plan and program now, because there aren't unlimited resources," Johnson said. "And we really can't afford to pursue nice projects at the expense of neglecting essential tasks."