How NASA's Demo-2 astronauts will make a historic splashdown on Earth

Editor's update: SpaceX is on track to land NASA's Demo-2 astronauts at 2:48 p.m. EDT (1848 GMT) on Aug. 2. Watch the landing live here.

Original story:

Just over 45 years since the last American astronaut splashed down in the ocean, NASA and its partner SpaceX are set to land astronauts in the sea again on Aug. 2.

The agency announced last week that SpaceX's Demo-2 commercial crew mission, carrying NASA astronauts Doug Hurley and Bob Behnken, is set to return to Earth, splashing down in the ocean near Florida in one of seven designated landing sites. The mission is expected to land at 2:42 p.m. EDT (1842 GMT), as long as the weather and technical systems co-operate, NASA said in a statement. 

Coverage will be streaming on NASA Television, with the astronauts landing in one of these locations: off the coasts of Pensacola, Tampa, Tallahassee, Panama City, Cape Canaveral, Daytona  or Jacksonville, NASA said in the same statement. The return time for the astronauts will take between six and 30 hours, depending on the exact undocking and splashdown zones chosen. For now, NASA and SpaceX expect to undock Crew Dragon from the International Space Station at 7:34 p.m. EDT (2334 GMT) on Aug. 1.

Related: SpaceX's historic Demo-2 Crew Dragon astronaut test flight

The Demo-2splashdown will be a historic moment. For one thing, it will be a "back to the future" moment: the last time astronauts made an ocean landing was on July 24, 1975 to complete the Apollo-Soyuz Test Project between the United States and the Soviet Union. The Demo-2 landing will also complete the first-ever commercial crew test mission and prepare NASA for operational flights with SpaceX; the next launch is set for September with a full crew of four astronauts.

The splashdown will usher in a new era for NASA, which finally (after more than a decade of work) has one version of replacement commercial crew spacecraft ready to launch American astronauts to the International Space Station. SpaceX and another partner, Boeing, are expected to largely replace the Soyuz spacecraft NASA astronauts have been using in the years since the agency's space shuttle program stopped flights in 2011 to access the space station.

We won't know the exact location for splashdown until roughly two days before landing, NASA added in a fact sheet, and the agreed-upon area will be confirmed at 6 hours and again at 2.5 hours before undocking. If conditions are "no go" or marginal at the landing site, undocking will likely go ahead anyway. NASA and SpaceX will continue to monitor as the astronauts get closer to Earth, since weather conditions can change quickly and they would rather preserve the option of splashing down if possible. 

If conditions ultimately aren't conducive for a landing, the crew has the option to "wave off" the landing for 24 to 48 hours and remain in space aboard Crew Ddragon Endeavour before making a second attempt.

"Splashdown locations are selected using defined priorities, starting with selecting a station departure date and time with the maximum number of return opportunities in geographically diverse locations to protect for weather changes," the agency said in the fact sheet. "Teams also prioritize locations which require the shortest amount of time between undocking and splashdown based on orbital mechanics, and splashdown opportunities that occur in daylight hours."

Recovery parameters will depend on factors such as wind speed (no greater than 15 feet, or 4.5 meters, per second), an ocean wave slope of no more than seven degrees, rain, lightning (which cannot happen within 10 miles, or 16 km), the availability of the recovery helicopters that may pick up the astronauts (the helicopters will be tested prior to deployment), the vessel's pitch and roll (which should be no greater than four degrees), the visible ceiling (no less than 500 feet, or 152 meters) and overall visibility (no less than half a mile, or 0.8 km, assuming daylight operations.)

Mission milestones

Here are the main milestones to expect for the splashdown, according to NASA:

Once the Crew Dragon Endeavour separates from the space station, it will complete two tiny engine burns to safely move away from the station. Next, Endeavour will perform four longer departure burns that will set it on a path for Earth. A few hours later, assuming splashdown conditions look good, Endeavour will complete a six-minute "departure phasing burn" to put it in the correct orbit for its splashdown area.

Next, Endeavour will let go of its trunk, which holds the spacecraft's solar arrays and other equipment, to clear its heat shield for re-entry. Endeavour then will perform a "deorbit burn" and begin its plunge into Earth's atmosphere, hitting the air at a speed of 17,500 mph (28,000 km/h). A communications blackout will last for six minutes as the heat shield reaches temperatures of 3,500 degrees Fahrenheit (1,900 degrees Celsius).

Once out of the worst of re-entry, Endeavour will pop two drogue parachutes at 18,000 feet (about 5.5 km) in altitude, slowing its drop from 350 mph to 119 mph (560 km/h to 191 km/h). Three main parachutes will deploy when Endeavour is roughly 6,000 feet (1.8 km) above the ground.

Next will be splashdown. The crew will be picked up by one of two recovery ships, either the Go Searcher or the Go Navigator. Each ship will have more than 40 personnel from NASA and SpaceX on board, including personnel such as water recovery experts, medical professionals, spacecraft engineers and other folks specialized in recovery operations.

The main recovery ship will send out two smaller ships with SpaceX personnel on board. One boat will make sure the capsule isn't leaking (the NASA Mercury Liberty Bell 7 spacecraft did accidentally sink in 1961 and was only recovered in 1999.) The boat will also make sure there are no propellant vapors that can put the crew in danger. The second boat will recover the parachutes that should, at this point, be floating freely from Endeavour in the water. 

Once that's all done, the main recovery vessel will move towards Endeavour and hoist the spacecraft (with the crew still inside) onto the main deck. Behnken and Hurley will be helped by medical professionals immediately after arrival, both for routine medical checks and to exit their spacecraft. Should all go to plan, the crew should be back on the recovery ship within 45 to 60 minutes from splashdown.

After finishing their medical checks, Hurley and Behnken will be brought to shore — either by helicopter (for six of the seven recovery areas) or by recovery ship (if the crew lands near Cape Canaveral.) It will take between 10 minutes and 80 minutes to get back on dry land, after which the crew will board a NASA plane for their return to their home base in Houston. It will take them several weeks at the least to recover from being in space, which is typical of returning crews who stay in microgravity for months at a time. 

Endeavour will take a separate path, ending up in Florida for SpaceX inspection and processing. Certifying the spacecraft for operational missions will take about six weeks, after which the next mission (Crew 1)  will be authorized for launch in September.

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook.