When NASA launches its new big rocket for the first time — more than a year and a half from now, at the earliest — there will be no astronauts along for the ride.

In February, at the request of the Trump administration, NASA began studying whether it was possible to add crew for the first flight of its Space Launch System, a heavy-lift rocket under development for deep space missions.

On Friday, the space agency announced it would not. During a conference call with reporters, Robert M. Lightfoot Jr., the acting NASA administrator, said the change was technically feasible, but that the additional cost, time and risks outweighed the benefits. “It really reaffirmed the baseline plan we have in place is the best way to go,” he said.

Putting astronauts on the first flight would have added $600 million to $900 million to the $24 billion price tag, Mr. Lightfoot said, and delayed the launch until probably the first half of 2020.