The NASA executive tasked with strategizing a mission to the moon by 2024 has already resigned amid growing skepticism that the five-year deadline is feasible.

Mark Sirangelo on April 9 was appointed special assistant to NASA Administrator Jim Bridenstine, with NASA saying at the time he would help develop the 2024 strategy, as is the Trump administration's goal. The administration has sped up the timeline from the previous goal of 2028, with Vice President Mike Pence saying in March, "Failure to achieve our goal to return an American astronaut to the moon in the next five years is not an option."

But six weeks later, Sirangelo has resigned, and Reuters reports that his "ouster was sealed by increasing skepticism that 2024 was a realistic deadline for moon landings." This also comes after a proposal from NASA to create a separate directorate for lunar missions was rejected by lawmakers. Bridenstine said on Thursday that "given NASA is no longer pursuing the new mission directorate, Mark has opted to pursue other opportunities."

President Trump has proposed providing an additional $1.6 billion to NASA in furtherance of this 2024 goal, but the administration has been met with resistance on Capitol Hill to taking that money from a Pell grant program surplus, The New York Times reports. Should NASA not get this additional money, Bridenstine says the timeline would probably move back to 2028. Brendan Morrow