Returning from the free-fall environment of orbit to the normal forces of gravity on Earth is often disorienting for astronauts. A water landing adds the possibility of seasickness.

During a news conference in 2020, Doug Hurley, who flew NASA’s first journey in the Crew Dragon capsule, said he had read some of the reports by the Skylab astronauts who completed water landings in the 1970s.

“There was some challenges post splashdown,” he said. “Folks didn’t feel well, and you know, that is the way it is with a water landing, even if you’re not deconditioned like we’re going to be.”

Mr. Hurley acknowledged that vomiting would not be unexpected.

“There are bags if you need them, and we’ll have those handy,” he said. “We’ll probably have some towels handy as well. And you know, if that needs to happen, it certainly wouldn’t be the first time that that’s happened in a space vehicle.”

But not long after the splash down, he asked SpaceX’s mission controllers to tell flight surgeons monitoring their health that, “we’re doing pretty good so far.”

The crew will also be returning after dark on Monday. The last NASA astronaut crew also splashed down in May in the dark, and it was the first night water landing by astronauts since 1968.

Steve Stich, manager for NASA’s Commercial Crew program, said that consistently calm nighttime weather at the splash down site, ample moonlight and additional factors made landing in the dark advantageous.

“When we weighed all those options, it just looked like this was the best time to come home,” he said on NASA TV earlier in the year.

Another advantage of a nighttime landing could be that fewer private boats are likely to be around. That was a problem in August 2020 when the first crewed SpaceX capsule splashed down. More than a dozen boats — one of them flying a Trump campaign flag — converged on the singed capsule, and a few went in for a closer look.

The episode raised concerns among NASA and SpaceX officials about security and safety procedures. If there had been an emergency, NASA officials said, the private boats might have impeded recovery efforts. They added that there could have been poisonous fumes from the capsule that posed a risk to the boaters.

With that concern in mind in May, the Coast Guard set up an 11.5-mile safety zone around the splashdown site to chase away any interlopers.

The Crew Dragon capsule containing the Crew-2 astronauts is orbiting at more than 17,000 miles per hour. At 9:39 p.m. Eastern time, the capsule’s thrusters will begin firing for 10 minutes to drop it out of orbit.

As it falls, the capsule actually speeds up until it enters the thicker part of the atmosphere. Then the drag of air resistance acts as a brake. The compression of air against the heat shield at the bottom of the capsule generates temperatures as high as 3,500 degrees Fahrenheit.

If it comes in at too shallow an angle, it will bounce off the atmosphere back into space. If it re-enters too steeply, it could burn up. But capsules like Crew Dragon have for decades successfully navigated through re-entry. It is rocket science, but it is well-understood rocket science.

For the most part, the spacecraft’s computer handles everything. It tracks the spacecraft’s position, fires short thruster bursts to keep the capsule oriented with the heat shield to absorb the heat and deploys the parachutes, while the crew members sit back for the ride.

But what if something goes wrong?

NASA’s astronauts are trained how to handle contingencies. Jared Isaacman, the billionaire entrepreneur who financed and commanded the recent Inspiration4 mission that went to orbit, described some of the details from a 30-hour session his team did in a Crew Dragon simulator before his trip in September.

Outside the simulator, the SpaceX mission controllers communicated with the astronauts as if they were in space. A separate team at SpaceX imagined emergencies that could come up and then unleashed them during the simulation. Neither the crew members in the simulator nor the controllers outside were given advance knowledge of what was happening. They had to diagnose the problem and figure out a fix on the fly.

“It was totally like an Apollo 13 moment by the time we were done with 30 hours,” Mr. Isaacman said.

The simulation included crashes of the spacecraft computer and failures in the communication system, so that there were periods where the astronauts could not talk to mission control.

When one simulated re-entry burn started, Mr. Isaacman said it became apparent that the capsule was off target. “It was way overshooting the target landing zone,” he said.

As it was programmed to do, the capsule gave the command to fire its thrusters to try to get back on the right track. But that meant the thrusters could have run out of propellant or failed from firing so long. The simulated mishaps were potentially cascading into a simulated fatal accident. Without the thrusters during the hottest part of re-entry, “you’ll tumble and it might be unsurvivable,” Mr. Isaacman said.

Mission control was able to override the computer that was trying to push the capsule to its planned landing site, Mr. Isaacman said. That preserved propellant for passage through the atmosphere.

At the end of the simulation, splashdown was far from where it was supposed to be. “But we survived,” Mr. Isaacman said.

That scenario is not far-fetched. Something similar occurred in December 2019 during a test flight with no astronauts of Boeing’s Starliner capsule, the other spacecraft that is expected to take NASA crews to the International Space Station.

By the time that Boeing’s controllers on the ground figured out what was going on and sent corrective commands, the spacecraft had expended too much propellant and a planned docking at the space station was called off. (That and a series of other problems have prevented the Starliner from carrying astronauts to orbit, but it may get another chance in 2022.)

Boeing and NASA officials said that if astronauts had been aboard, they would have quickly realized what was wrong and shut down the thrusters, which could have allowed the mission to proceed to the space station.

Megan McArthur, Crew-2’s pilot, feasted on the “best space tacos yet,” she said, which included Hatch chiles, fajita beef, rehydrated tomatoes and artichokes. Mark Vande Hei, a NASA astronaut who launched to the station with a Russian crew in April, has been cultivating the peppers and plans to send a batch back to Earth for examination.

Plans for future missions to destinations farther from Earth, like the moon and Mars, have spurred a need to grow crops in space with artificial light. And as longer missions in space take larger tolls on astronaut health — mentally and physically — tastier and healthier food is important. While astronauts have sprouted leafy greens in studies on the space station in the past, the Plant Habitat-04 experiment that yielded the chiles on Dr. McArthur’s taco is one of NASA’s “most challenging station plant experiments to date,” the agency said.

The astronauts worked on hundreds of other scientific investigations during their six-month stay aboard the orbital laboratory. In one study, Mr. Pesquet experimented with ultrasonic tweezers, a concept that uses sound to move small objects. He tested a hand-held device that silently emits a beam of sound waves that exert a force strong enough to trap and move a small object. The capability could be crucial for applications where tiny things, like viruses, can’t be touched or contaminated.

In another experiment, Dr. McArthur worked on a study using bits of engineered muscle tissue to test drugs designed to curb the loss of muscle mass, a deficiency astronauts are especially prone to while in space and, for others on Earth, a slowly progressing syndrome called sarcopenia.

Shane Kimbrough, another member of Crew-2, examined real-time protein crystal growth under a microscope as part of a study into new drugs that can treat diseases. Akihiko Hoshide, the Crew-2 astronaut from Japan’s space agency, worked on a study that examines how cells react to gravity.

The Crew-2 astronauts witnessed the making of a feature length movie backed by Russia’s space agency, Roscosmos. A Russian actress and a director launched to the space station on Oct. 5 for a 12-day shoot aboard the station for their movie, “The Challenge.” The film is about a mission to rescue an ailing astronaut, who was played by Oleg Novitsky, an actual Russian astronaut on the station.

It’s unclear whether the Crew-2 astronauts helped out on the set, but Mr. Vande Hei, the NASA astronaut who cultivated the peppers but is not part of Crew-2, assisted with the camerawork, according to Russian media.

Days later, U.S. Space Command, which tracks objects in orbit, determined that the alert was the result of a “reporting error” and “that there was never a collision threat because there was no object at risk of colliding with the capsule.” Still, the incident renewed discussion about the growing threat of space debris and other clutter in low-Earth orbit.

In July, Russia launched a new science module to be added on to the space station’s Russian segment. Just after it docked, the module, named Nauka, erroneously fired a set of thrusters for roughly 15 minutes and spun the football field-size laboratory one-and-a-half revolutions before coming to a stop upside down.

The incident sent mission control teams in Houston and Moscow scrambling to get the station back in its normal position. The Crew-2 astronauts rushed back into their Crew Dragon capsule, in case they needed to escape.

“In case something really bad did happen, we were ready to go and undock, if that was necessary,” said Shane Kimbrough, the Crew-2 commander, during a news conference on Friday. “Of course it wasn’t, thank goodness.”

The space station’s seven-person crew was never in any danger, NASA said at the time, and the laboratory’s external instruments sustained no immediate damage from the stresses imparted by the thruster’s force. Crew Dragon isn’t just an astronaut taxi to and from the space station — it’s also “designed to be a lifeboat” that the crew could have used within minutes to zip away from the space station if needed, Steve Stich, NASA’s commercial crew program manager, told reporters in July.

It wasn’t the only surprise for the crew aboard the station. Last month, another set of errant thrusters suddenly fired out of control from a different Russian spacecraft, tilting the space station about 57 degrees. Again, astronauts were directed to initiate emergency procedures as ground control teams in Houston and Moscow regained control of the station.

The flights to orbit typically take about 24 hours, but Monday’s trip home will last just over eight.

SpaceX developed the astronaut taxi with roughly $3 billion from a NASA program called Commercial Crew. The goal of the program was that private companies would own the spacecraft they build, with NASA being just one customer among many buying seats for astronauts. The agency’s previous mode of transportation to the space station was the space shuttle. But the shuttle program was retired in 2011, requiring NASA to buy expensive seats for its astronauts on Russia’s Soyuz rocket for nearly a decade as SpaceX and Boeing, the other company working under Commercial Crew, developed their capsules.

SpaceX’s first mission sending humans to space — a revival of NASA’s ability to loft humans to space from American soil — was in May 2020, with two NASA astronauts, Bob Behnken and Doug Hurley, riding Crew Dragon to the space station for a roughly two-month stay. The company has since launched two successful crewed missions for NASA with a combination of American astronauts and crews from Japan and the European Space Agency.

Boeing, the aerospace giant and plane maker, is far behind SpaceX. Development of its Starliner capsule has been marred by a range of technical issues, and it is at least a year away from flying its first crew of astronauts.

“Of course that’s suboptimal, but we are prepared to manage that in the time that we’re onboard Dragon on the way home,” K. Megan McArthur, the Crew-2 mission pilot, said during a news conference on Friday.

The acorn-shaped capsule is somewhat larger than a minivan on Earth; it doesn’t have a proper bathroom. Instead it has a toilet device with a tube and fans built into one of the spacecraft’s compartments that create suction to ensure waste goes in the right direction in the weightlessness of space.

In September, SpaceX detected a leaky toilet on another one of its capsules during the flight of Inspiration4, a three-day orbital journey of private astronauts that didn’t dock at the space station, according to Bill Gerstenmaier, a company vice president who discussed the toilet problems during a news conference in October.

A tube from the capsule’s toilet that funnels waste into an internal tank broke loose and leaked fluids into a fan, which sent urine throughout an area beneath the capsule’s interior floor, Mr. Gerstenmaier said.

When SpaceX engineers discovered the flaw, they instructed astronauts on the space station to inspect their Crew-2 capsule. They found similar traces of urine beneath the interior floor, which worried officials that it could corrode some of the capsule’s aluminum parts and pose a safety risk for the return flight.

SpaceX engineers conducted experiments on the ground to test whether urine, which is mixed with an ammonia-removing compound called oxone, could corrode the aluminum. The parts sat in a chamber that mimicked the humidity conditions aboard the space station for “an extended period of time,” Mr. Gerstenmaier said.

From the experiment and inspections of the Inspiration4 capsule, SpaceX found that the urine-oxone mixture had little effect on the aluminum parts because of heavy coats of paint on Crew Dragon that are “a great blocking agent to the liquid,” Sarah Walker, SpaceX’s mission management director for Crew Dragon, told reporters during a news conference.

“We learned that the liquid evaporates within just a couple days,” Ms. Walker added, “and that really limits the impact that we observed when we were doing all of our post-flight inspections.”

NASA officials approved the results of the experiments and deemed Crew-2 safe to return to Earth. SpaceX proposed a permanent fix to future Crew Dragon capsules that should ensure the urine tube would not come undone.

But the toilet leak on the Crew-2 capsule remains, meaning that astronauts heeding the call of nature in orbit must use the “undergarments” in their flight suits, Steve Stich, NASA’s Commercial Crew program manager, told reporters on Oct. 31.

“Our intent is to not use the system at all for the return leg home, because of what we’ve seen with the fluid,” Mr. Stich said. “Any time the crew is suited, they use an undergarment in that suit, and it’s a short mission coming home so it’s pretty typical to have an undergarment on and they can use that on the way home.”

“Spaceflight is full of lots of little challenges,” Dr. McArthur said. “This is just one more that we’ll encounter and take care of on our mission.”

The last time the space station held just three astronauts — also an American and two Russians — was in April 2020 as NASA was weaning off its dependence on Russia’s Soyuz rocket for flights to the orbital lab. One month later, SpaceX launched its first group of NASA astronauts to space, Bob Behnken and Doug Hurley, marking the first crewed flight from American soil in nearly a decade.

This time around, NASA tried avoiding the brief three-person crew scenario by launching the Crew-3 astronauts before Crew-2 departed, ensuring a clean handover on the space station between the two crews. But bad weather delayed Crew-3’s launch, initially scheduled for Halloween. Then a “minor medical issue” with one of the Crew-3 astronauts triggered another delay. NASA ultimately decided to shift its attention to returning Crew-2 while the astronaut’s medical issue subsided.

Agency officials said the medical issue was unrelated to Covid-19 but did not elaborate further or identify the astronaut affected, only saying on Saturday that they expect it to clear up before Crew-3’s Wednesday launch.

A three-person crew with just one American isn’t ideal for space station operations. With no backup American astronaut, it jeopardizes a continuous U.S. presence on the space station, a NASA safety panel warned last year. It also makes it difficult to maintain the space station if the short-handed crew lasts for long periods of time. Spacewalks to swap out some batteries on the station’s exterior were rescheduled during the 2020 lull until more astronauts arrived.

Mr. Vande Hei and Mr. Dubrov have been on the station since April, when they launched on a Russian Soyuz rocket with a third Russian, who returned to Earth in a different spacecraft last month. In September, Mr. Vande Hei’s six-month mission on the space station was extended to 353 days with a departure date in March 2022, putting him on track to set a record for the longest spaceflight by an American astronaut (Scott Kelly, a retired NASA astronaut, spent 340 days in space during a mission that ended in 2016).

“It was definitely helpful to have Mark Vande Hei staying” on the space station, Joel Montalbano, NASA’s space station manager, told reporters on Saturday during a news conference about the updated Crew-2 schedule.

He said NASA’s efforts to secure an agreement with Russia’s space agency to trade astronaut seats between Crew Dragon and Soyuz spacecraft will eventually help avoid scenarios when the station is down to three occupants. Those talks have been stalled for years, but American and Russian officials last month indicated progress was being made toward a first joint flight on Crew Dragon sometime next year.

While there will only be three people on the International Space Station after Crew-2 leaves, there will be three other people in orbit. China’s Tiangong space station, which is under construction, is currently home to three astronauts, its second crew since it reached orbit earlier this year.

During this stage of the trip, which is called atmospheric re-entry, Crew Dragon’s outer shell will hit temperatures of up to 3,500 degrees Fahrenheit. The astronauts inside will stay cool with the help of an air conditioning system. A sheath of plasma will form around the speeding capsule and cut off communications with ground control for a few minutes before the spacecraft slows to descent speeds of about 350 miles per hour.

Then, effectively slamming on the brakes, a set of two parachutes will deploy to slow the capsule further, jolting the crew inside. Small explosive devices will detach those parachutes just before another set of four bigger chutes unfurl, reducing the capsule’s speed to roughly 15 miles per hour.

The capsule is expected to splashdown off the coast of Pensacola, Fla., in the Gulf of Mexico at around 10:33 p.m. The crew’s return had been delayed several times over bad weather, but for Monday night, “the weather right now at Pensacola is looking fantastic,” said Gary Jordan, a NASA spokesman, of the forecasts.

NASA has been streaming live coverage of the journey that will continue until shortly after the capsule’s splashdown.

Shortly before undocking, NASA and SpaceX chose an area near Pensacola, Fla., for Crew Dragon’s splashdown zone. It is one of seven different locations in the waters around the Florida peninsula where the capsule can land, and NASA picks whichever area has the most favorable weather. Clear skies, calm seas and gentle winds are prime conditions for a space taxi splashdown.

Since astronauts started flying in the SpaceX capsule in May 2020, two crews have landed in the Gulf of Mexico near the Florida Panhandle. A third returned in the Atlantic Ocean near Kennedy Space Center in Cape Canaveral, Fla.

Shane Kimbrough of NASA. Mr. Kimbrough, 53, is the commander of Crew-2. He also made two earlier trips to space, once on the space shuttle Endeavour in 2008 and then spending more than six months on the space station from October 2016 to April 2017.

K. Megan McArthur of NASA. Dr. McArthur, 49, is the mission’s pilot and previously flew on the space shuttle Atlantis in May 2009 on the last mission to refurbish and upgrade the Hubble Space Telescope. During that mission, Dr. McArthur, an oceanographer by training, operated the shuttle’s robotic arm to grab the telescope and place it in the cargo bay.

Dr. McArthur is married to Bob Behnken, one of the astronauts who traveled on the first astronaut flight of the same SpaceX capsule last year. She will sit in the seat he occupied during that flight.

Thomas Pesquet of the European Space Agency. Mr. Pesquet, 43, previously spent six months on the space station from November 2016 to June 2017, overlapping with Mr. Kimbrough for most of his stay. He is from France. Most recently, Mr. Pesquet has served as the space station’s commander.