On Monday during a conference held in Houston, several veteran astronauts endorsed NASA’s plan to return to the Moon. However, they also characterized the goal of landing humans there by 2024 as aspirational rather than realistic.

“It’s quite aggressive,” said four-time astronaut Michael López-Alegría of the Artemis Program’s five-year timeline. López-Alegría, who is president of the Association of Space Explorers, made his comments during the organization’s annual meeting.

He added that it was not a bad thing to have an aggressive plan. Rather, it was good for NASA and its international partners to have a clear goal to work toward. “I think that in any complex program like that, somebody needs to draw a line in the sand,” he said. “It may be aspirational, but without something like that, it’s really difficult to get people pulling in the same direction.”