The US Senate on Thursday slashed the budget for NASA’s ambitious mission to return soil and rock samples from Mars’ surface.

NASA had asked for $949 million to support its Mars Sample Return mission, or MSR, in fiscal year 2024. In its proposed budget for the space agency, released Thursday, the Senate offered just $300 million and threatened to take that amount away.

“The Committee has significant concerns about the technical challenges facing MSR and potential further impacts on confirmed missions, even before MSR has completed preliminary design review,” stated the Commerce, Justice, Science, and Related Agencies subcommittee in its report on the budget.