Two astronauts who took the first commercial trip to orbit have left the International Space Station. They are scheduled to return home on Sunday.

The astronauts, Robert L. Behnken and Douglas G. Hurley, traveled to the space station in May aboard a Crew Dragon capsule built and run by SpaceX, the private rocket company started by Elon Musk.

The Crew Dragon undocked from the space station at 7:35 p.m. Eastern time on Saturday, with brief thruster firings pushing the spacecraft back.

As the capsule backed away from the station, Mr. Hurley thanked the current crew of the space station and the teams on the ground that helped manage their mission.

We look forward to splashdown tomorrow, he said.

If the weather conditions remain favorable, it will splash down in the Gulf of Mexico off Pensacola, Fla., at 2:48 p.m. on Sunday, NASA announced.

A safe return would open up more trips to and from orbit for future astronaut crews, and possibly space tourists, aboard the spacecraft.

Isaias is forecast to sweep up along the Atlantic coast of Florida over the weekend. NASA and SpaceX have seven splashdown sites in the Gulf of Mexico and the Atlantic, but the track of the storm ruled out the three in the Atlantic.

We have confidence that the teams on the ground are, of course, watching that much more closely than we are, Mr. Behnken said during a news conference on Friday, and we wont leave the space station without some good landing opportunities in front of us, good splashdown weather in front of us.

NASA Televisions coverage of the undocking will continue through splashdown. You can watch it in the video player below.

The capsule is now performing a series of burns to move away from the station and then line up with the splashdown site.

For much of the trip, Mr. Behnken and Mr. Hurley will be sleeping. Their schedule sets aside a full night of rest.

Any return journey that exceeds six hours has to be long enough for the crew to get some sleep between undocking and splashdown, Daniel Huot, a NASA spokesman, said in an email.

Otherwise, because of the extended process that leads up to undocking, the crew would end up working more than 20 hours straight, which is not safe for dynamic operations like water splashdown and recovery, Mr. Huot said.

Just before a final burn that will drop the Crew Dragon out of orbit on Sunday afternoon, it will jettison the bottom part of the spacecraft, known as the trunk, which will then burn up in the atmosphere.

At re-entry, the Crew Dragon will be traveling at about 17,500 miles per hour. Two small parachutes will deploy at an altitude of 18,000 feet when the spacecraft has already been slowed by Earths atmosphere to about 350 miles per hour. The four main parachutes deploy at an altitude of about 6,000 feet.

Once the capsule splashes in the water, it is expected to take 45 to 60 minutes to pluck them out.

The storm complicated where splashdown could take place. At the splashdown site, winds must be less than 10 miles per hour for the capsule to land safely. There are additional constraints on waves, rain and lightning. In addition, helicopters that take part in the recovery of the capsule must be able to fly and land safely.

The first landing opportunity will aim for only the primary site, Pensacola. If weather there is inconsistent with the rules, the capsule and the astronauts will remain in orbit for another day or two, and managers will consider the backup site, which is Panama City.

Spacecraft can safely return to Earth in either environment.

During the 1960s and 1970s, NASAs Mercury, Gemini and Apollo capsules all splashed down in the ocean while Soviet capsules all ended their trips on land. Russias current Soyuz capsules continue to make ground landings, as do Chinas astronaut-carrying Shenzhou capsules.

When Boeings Starliner capsule begins carrying crews to the space station, it will return on land, in New Mexico. SpaceX had originally planned for the Crew Dragon to do ground landings, but decided that water landings, employed for the earlier version of Dragon for taking cargo, simplified the development of the capsule.

After launch, re-entry through Earths atmosphere is the second most dangerous phase of spaceflight. Friction of air rushing past will heat the bottom of the capsule to about 3,500 degrees Fahrenheit. A test flight of the Crew Dragon last year successfully splashed down, so engineers know the system works.

A successful conclusion to the trip opens the door to more people flying to space. Some companies have already announced plans to use Crew Dragons to lift wealthy tourists to orbit.

In the past, NASA astronauts launched on spacecraft like the Saturn 5 moon rocket and the space shuttles that NASA itself operated. After the retirement of the space shuttles in 2011, NASA had to rely on Russia, buying seats on the Soyuz capsules for trips to and from orbit.

Under the Obama administration, NASA hired two companies, SpaceX and Boeing, to build spacecraft to take astronauts to the space station. NASA financed much of the work to develop the spacecraft but will now buy rides at fixed prices. For SpaceX, the trip by Mr. Behnken and Mr. Hurley the first launch of astronauts from American soil since the last space shuttle flight was the last major demonstration needed before NASA officially certifies that the Crew Dragon is ready to begin regular flights.

The astronauts are Robert L. Behnken and Douglas G. Hurley, who have been friends and colleagues since both were selected by NASA to be astronauts in 2000.

Both men have backgrounds as military test pilots and each has flown twice before on space shuttle missions, although this is the first time they have worked together on a mission. Mr. Hurley flew on the space shuttles final mission in 2011.

In 2015, they were among the astronauts chosen to work with Boeing and SpaceX on the commercial space vehicles that the companies were developing. In 2018, they were assigned to the first SpaceX flight.

Originally, the mission was to last only up to two weeks, but Mr. Behnken and Mr. Hurley ended up with a longer and busier stay at the space station. Because of repeated delays by SpaceX and Boeing, NASA ended up short-handed, with only one astronaut, Christopher J. Cassidy, aboard the space station when the Crew Dragon and its two passengers docked.

They stayed two months, helping Mr. Cassidy with space station chores. Mr. Behnken and Mr. Cassidy performed four spacewalks to complete the installation of new batteries on the space station. Mr. Hurley helped by operating the stations robotic arm.

The men have also been contributing to science experiments in low earth orbit. They assisted in a study of water droplet formation in the low gravity environment of the space station using a shower head, and another that used fruit punch and foam to look at how to manage fluids in space. They also helped install new equipment inside the station that will be used in future scientific research.

Mr. Cassidy will remain aboard the station with two Russian astronauts, Anatoly Ivanishin and Ivan Vagner. All three are to stay on board through October when another crew of one American and two Russian astronauts will replace them.

The first operational flight of the Crew Dragon will launch no earlier than late September. It will take three NASA astronauts Michael S. Hopkins, Victor J. Glover and Shannon Walker and one Japanese astronaut, Soichi Noguchi, to the space station.

The second operational flight, tentatively scheduled for February 2021, will carry two NASA astronauts, Robert S. Kimbrough and K. Megan McArthur; Akihiko Hoshide of Japan; and Thomas Pesquet of the European Space Agency.

Ms. McArthur is married to Mr. Behnken.

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SpaceX Crew Dragon Departs, Carrying NASA Astronauts Toward Home - The New York Times