HURRY up and get it built. That’s the message to NASA from a key group of senators, urging the agency to provide a launcher capable of carrying astronauts beyond low-Earth orbit. Yet if their measure becomes law, it could come at the expense of more advanced technologies like orbiting fuel depots.

In April, President Barack Obama said he wanted NASA to send astronauts to an asteroid by 2025, but that the agency would not develop the rocket needed to do so until 2015. By this time technologies might have become available that would offer advanced features, such as the ability …