Astronauts Doug Hurley and Bob Behnken undocked from the International Space Station Saturday aboard their Crew Dragon capsule Endeavour, heading for a parachute-assisted splashdown Sunday in the Gulf of Mexico to wrap up a 64-day test flight of SpaceXs commercial human-rated spaceship.

With favorable wind and sea conditions expected in the Gulf of Mexico Sunday, mission control gave the go-ahead for Hurley and Behnken to board their Crew Dragon spacecraft and close hatches between the capsule and the space station.

After a series of leak checks, an undocking command at 7:30 p.m. EDT (2330 GMT) Saturday commenced a series of automated steps to depart the station. Power umbilicals detached inside the docking mechanism, then 12 hooks opened before the Dragon Endeavour spacecraft fired thrusters in a pair of short pulses to boost itself away from the research complex at 7:35 p.m. EDT (2335 GMT).

Wearing custom-made SpaceX-built pressure suits, Hurley and Behnken monitored the departure on touchscreen displays inside their Dragon Endeavour spacecraft. NASA astronaut Chris Cassidy, commander of the stations Expedition 63 crew, rang the ships bell on the research complex and ceremoniously announced the Dragons undocking.

Cassidy and his two Russian crewmates Anatoly Ivanishin and Ivan Vagner will remain aboard the space station until October, when they will return to a landing in Kazakhstan on a Russian Soyuz spacecraft. Three fresh crew members will launch to the space station Oct. 14 on a new Soyuz spaceship.

During their two-month stay, Hurley and Behnken assisted Cassidy, Ivanishin and Vagner with space station duties, performing experiments and maintenance. Behnken joined Cassidy on four spacewalks in June and July to replace batteries on the space stations solar power modules.

Chris, we just cant thank you enough, Hurley said in a radio exchange with Cassidy shortly after undocking. Its been an honor and a privilege to be part of Expedition 63 with you, Anatoly and Ivan. Its been a great two months and we appreciate all youve done as a crew to help us prove out Dragon on its maiden flight.

Hurley also thanked NASA mission controllers at the Johnson Space Center in Houston and SpaceX teams in Hawthorne, California, for their support.

We look forward to splashdown tomorrow, Hurley said. Also like to wish you great success on the rest of your expedition and a safe flight home in the fall. Take care, friend.

Bob and Doug, wholeheartedly agree with those sentiments, Cassidy replied. Its been a real pleasure. Its been an honor to serve with you. Safe travels and have a successful landing. Endeavours a great ship. Godspeed.

A series of rocket burns maneuvered the crew capsule a safe distance away from the space station, and the astronauts planned to begin an eight-hour sleep period at 11:40 p.m. EDT (0340 GMT).

During their sleep period, the Crew Dragon is programmed to complete an automated six-minute phasing burn to line up with the splashdown target in the Gulf of Mexico.

Hurley and Behnken will close out their 64-day test flight designated Demo-2, or DM-2 Sunday with a braking burn to drop out of orbit and enter the atmosphere, targeting a splashdown in the Gulf of Mexico near Pensacola, Florida.

Our mission isnt over, Hurley said Saturday before undocking. The DM-2 test flight is, in some ways, just two-thirds complete. We did the ascent, rendezvous and the docking. We completed our docked objectives, and now is the entry, descent and splashdown phase.

The hardest part was getting us launched, but the most important part is bringing us home, Behnken said.

The astronauts are scheduled to wake up at 7:40 a.m. EDT (1140 GMT) Sunday to begin preparations for their return to Earth.

Hurley and Behnken will pack bags and ready the spaceships cabin for entry. They will also drink fluids in a process known as fluid loading aimed at easing their adaptation to Earths gravity after two months in orbit.

Assuming a final assessment of weather and sea conditions look favorable in the recovery zone near Pensacola, the Dragon Endeavour spacecraft flying on autopilot will jettison its unpressurized trunk section at 1:51 p.m. EDT (1751 GMT). The trunk is attached to the rear of the Dragons crew module, and contains the ships power-generating solar panels and radiators used to shed the spacecrafts internal heat into space.

The trunk will remain in a relatively low orbit and will naturally fall back into the atmosphere and burn up.

Meanwhile, the Dragon crew module will maneuver into the proper orientation for a deorbit burn using the spacecrafts Draco thrusters. The braking maneuver will begin at 1:56 p.m. EDT (1756 GMT) and last more than 11 minutes, slowing the ships velocity by nearly 168 mph, or 75 meters per second.

That change in velocity will allow Earths gravity to pull the spacecraft back into the atmosphere, which will do most of the rest of the work to slow Dragons speed for splashdown.

The spacecraft will close its forward nose cone at 2:11 p.m. EDT (1811 GMT) before it plunges into the discernible atmosphere at 2:36 p.m. EDT (1836 GMT), moving at some 17,500 miles per hour (28,000 kilometers per hour).

Hurley and Behnken will be wearing their SpaceX-made flame-resistant pressure suits during entry, the same garments they wore during their launch May 30 from the Kennedy Space Center in Florida on top of a SpaceX Falcon 9 rocket.

Flying with its blunt end facing the brunt of the airflow, the spacecrafts heat shield will encounter temperatures up to 3,500 degrees Fahrenheit (1,900 degrees Celsius) as it dives into the atmosphere.

The build-up of super-heated around the capsule is expected to interrupt communications with the crew for about six minutes during entry. Engineers expect to restore communications with the astronauts once Dragon Endeavour comes out of the hottest part of entry at around 2:42 p.m. EDT (1842 GMT).

Drogue parachutes will release from the top of the capsule at 2:44 p.m. EDT (1844 GMT), followed by the deployment of four orange and white main parachutes about a minute later.

The drogue chutes will deploy when Dragon Endeavour is descending through about 18,000 feet, or 5,500 meters, when the capsule is moving at approximately 350 mph, or more than 550 kilometers per hour. The four main chutes come out at an altitude of about 6,000 feet, or 1,800 meters, and at a velocity of around 119 mph, or 191 kilometers per hour.

The parachutes will slow the capsules speed for a gentle splashdown at 2:48 p.m. EDT (1848 GMT) in the Gulf of Mexico, targeting a location just south of the Alabama-Florida border.

Going into Sundays entry and splashdown, mission control identified a backup recovery site in the Gulf of Mexico near Panama City, Florida. SpaceX and NASA have seven Crew Dragon splashdown sites available in total four in the Gulf and three in the Atlantic but Tropical Storm Isaias is forecast to move near the missions recovery zones off Floridas East Coast on Sunday.

If weather conditions deteriorate in the Gulf of Mexico, mission control could wave-off Sundays return opportunities. NASA officials said the astronauts have food, water and other supplies for at least three days on the Crew Dragon after the undocking Saturday night from the space station.

A SpaceX recovery vessel named Go Navigator will be on station in the Gulf of Mexico to retrieve the Crew Dragon spaceship after it splashes down.

Two fast boats will deploy from Go Navigator and approach the capsule, which measures around 13 feet (4 meters) in diameter and 16 feet (5 meters). After ensuring the spacecraft is safe, the larger recovery boat will take position near the Dragon and hoist the capsule out of the water using a lifting frame.

Once in the Dragon is on the deck of Go Navigator, Hurley and Behnken willdisembark the capsule and undergo medical checks.

Benji Reed, SpaceXs director of crew mission management, said the recovery ship will have around 44 people on-board, including SpaceX and NASA officials, doctors, nurses and other medical personnel. Spacecraft technicians will also be aboard to recover and secure the Dragon capsule.

After an initial health assessment, Hurley and Behnken will ride a helicopter to Naval Air Station Pensacola, where they will board a NASA aircraft for the flight back to their home base in Houston.

The astronauts are coming back to Earth with around 330 pounds (150 kilograms) of cargo, including frozen experiment specimens, personal gear, and a U.S. flag left on the space station by the final space shuttle crew in 2011.

Hurley was the pilot on the final space shuttle flight.

The flag also flew on STS-1, the first shuttle mission, in 1981. The final shuttle crew left it on the space station to be returned by the next astronauts to fly to the research lab on a U.S. spacecraft.

In the end, SpaceX won the capture the flag competition on the high frontier.

NASA awarded multibillion-dollar contracts to develop and fly new U.S.-built commercial crew capsules to SpaceX and Boeing in 2014, following several years of preliminary design work.

SpaceX launched a successful unpiloted Crew Dragon demonstration flight to the space station in March 2019, then overcame a setback during ground testing of the Crew Dragons launch abort system last year. After redesigning part of the abort system, and verifying new modifications to the capsules parachutes, SpaceX launched the first Crew Dragon mission with astronauts May 30.

Boeings CST-100 Starliner crew capsule launched into orbit for its first unpiloted test flight last December, but it ran into software problems that prevented the spacecraft from reaching the space station. Boeing recovered the spacecraft with a successful landing in New Mexico, but officials plan to re-fly the uncrewed demonstration mission later this year before clearing the Starliner to carry astronauts for the first time in 2021.

With the Crew Dragon on the cusp of completing its first round-trip space mission with astronauts, SpaceX and NASA will analyze data from the Demo-2 test flight before formally certifying the commercial capsule for operational crew rotation launches.

The first such regular crew rotation flight, named Crew-1, is scheduled for launch this fall on a SpaceX Falcon 9 rocket from the Kennedy Space Center. Four astronauts are assigned to the Crew-1 flight, and NASA last week announced the crew assignments for the Crew-2 mission in the spring of 2021, the second operational Crew Dragon mission to the space station.

Subsequent Crew Dragon missions to the space station will also launch with up to four passengers, and the spaceship once certified after Demo-2s return will be capable of missions lasting up to 210 days.

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Hurley, Behnken heading home on final leg of Crew Dragon test flight - Spaceflight Now