A SpaceX rocket carried four astronauts into orbit Wednesday night, including the 600th person to reach space in 60 years.

The repeatedly delayed flight occurred just two days after SpaceX brought four other astronauts home from the International Space Station. They should have been up there to welcome the newcomers, but NASA and SpaceX decided to switch the order based on Monday's ideal recovery weather in the Gulf of Mexico and pulled it off.

"It was a great ride, better than we imagined," mission commander Raja Chari said shortly after the spacecraft reached orbit.

SpaceX counted down Wednesday toward a nighttime launch of four astronauts who have been grounded for nearly two weeks by weather and medical delays.

The Falcon rocket was poised to blast off from NASA's Kennedy Space Center a few hours after sunset. That would put the one German and three U.S. astronauts at the International Space Station by Thursday night to begin a six-month stay.

On Dec. 18, NASA is set to launch its next flagship mission into space. The spacecraft, called the James Webb Space Telescope, brings a lot of risks: Its roughly 270-square-foot mirror, which will collect light streaming in from the far reaches of space, will launch folded up inside a rocket, then unfurl far from Earth.

Astronomers are betting that the challenges will be worth it. Many have dubbed James Webb a successor to the Hubble Space Telescope—an instrument that was launched in 1990 and has delivered startling images of the cosmos, including The Pillars of Creation in the Eagle Nebula. Among other goals, James Webb will search the atmospheres of alien planets for what scientists call "biosignatures," or gases that could, theoretically, indicate the presence of life on the surface.