SpaceX Crew Dragon astronauts return to Earth

Hurley and Behnken's Crew Dragon spacecraft just lit up its engines, orienting the vehicle as it beelines for Earth.

There's no going back now.

The spacecraft is traveling about 17,000 miles per hour, and Crew Dragon's exterior will heat up to about 3,500 degrees Fahrenheit as it cuts through the atmosphere.

Inside the spacecraft cabin, the temperature should stay below 85 degrees Fahrenheit.

A thick heat shield will protect Behnken and Hurley, but the extreme heat and plasma buildup will cause a six-minute communications blackout, according to NASA.

A large circular trunk attached to the bottom of Hurley and Behnken's spacecraft just popped off the capsule. This is intentional — the trunk contains the solar panels and power systems that Crew Dragon needs as its flying through space. But it's not needed for the splashdown.

Letting go of the trunk also exposes the Crew Dragon's heat shield on the bottom of the spacecraft, which will protect Hurley and Behnken from the intense the spacecraft is about to face as it reenters the atmosphere.

It's a fairly quick trip from deorbit to the ground. Splashdown is still slated for 2:48 pm ET.

The Crew Dragon will ignite its thruster engines as it slices back into the thickest part of Earth's atmosphere while traveling at 17,500 miles per hour. Rapid air compression and air friction will heat the outside of the spacecraft to about 3,500 degrees Fahrenheit — a dangerous part of the journey. But a heat shield on Crew Dragon's exterior will keep Behnken and Hurley safe inside.

Behnken previously described his experience reentering the atmosphere aboard the Space Shuttle:

In some ways, however, the ride home on Crew Dragon will be much different from what the astronauts experienced during their prior Space Shuttle missions. The G-forces will be much more intense but won't last quite as long.

As the Crew Dragon approaches Earth, it'll deploy a set of parachutes, called "drogue parachutes," to begin slowing its descent before a large plume of four parachutes fans out to slow the vehicle down even further. If all goes well, Crew Dragon will be traveling less than 20 miles per hour when it hits the water.

Reisman said that after getting used to the microgravity environment in space then experiencing the intense G-forces of the return flight, it takes awhile to readjust, he told CNN Business.

Weather officials are still keeping an eye on the winds and waves near the Crew Dragon's splashdown zone.

But so far, it's looking good.

SpaceX took a moment to show off the heat shield, attached to the bottom of the spacecraft, that will keep Behnken and Hurley safe while the vehicle plunges back into the atmosphere.

The heat shield is made out of a high-tech material called PICA, which was developed by NASA in the 1990s. And SpaceX worked with the space agency to develop its own variant.

CNN Business is now streaming the official NASA webcast at the top of this page. Just refresh this page to watch it live.

Rough winds and high waves are exactly what SpaceX and NASA are hoping to avoid.

But Tropical Storm Isaias is barreling toward Florida's east coast on Sunday, threatening intense winds capable of damaging roofs and buildings.

NASA and SpaceX predetermined seven potential splashdown sites for this mission, four of which are to the west of Florida in the Gulf of Mexico: Pensacola, Tampa, Tallahassee, and Panama City. And in that area, the wind and wave heights appear calm enough for the Crew Dragon to land safely.

Still, weather officials are keeping a close eye on the forecast, and they could call off the splashdown any time before the spacecraft begins reentering the Earth's atmosphere.

But calling off the splashdown is a situation NASA and SpaceX really hope to avoid.

The Crew Dragon will have to remain in orbit and circle back for another splashdown attempt in the next 24 to 48 hours, but if conditions still remain unfavorable, the spacecraft will likely have to make an emergency splashdown in some other area.

Behnken and Hurley have only about three days worth of the life-saving chemical (Lithium Hydroxide) needed to scrub the carbon dioxide out of the air on their spacecraft.

It's been a long time since astronauts splash landed in the ocean.

The last time NASA executed a splashdown was in 1975, when it completed an experimental mission that docked a NASA Apollo spacecraft with a Soviet Soyuz capsule in orbit.

Water landings were a common practice in the earliest days of the US space program: Every vehicle made during NASA’s Mercury, Gemini and Apollo programs were built for ocean splashdowns. But the practice was retired when NASA moved on to the Space Shuttle program. The winged white orbiters landed on runways, much like commercial aircraft.

The other modern crewed vehicles — including Russia’s Soyuz, China’s crew spacecraft and Boeing’s forthcoming Starliner spacecraft — are all designed to land on the ground. 

SpaceX, however, does have plenty of experience recovering spacecraft from the ocean. The cargo-only version of its Dragon spacecraft, which has flown resupply missions to the ISS for years, routinely lands in the ocean. And Crew Dragon was flown on an uncrewed test mission last year.

NASA's forthcoming Orion crew capsule, which the space agency hopes will take humans back to the Moon, will also conduct splashdown landings.

GO Navigator is a rescue ship that's ready to speed out to the Crew Dragon's splashdown site in the Gulf of Mexico, and haul the spacecraft out of the ocean. It's equipped with a medical facility and helicopter pad, ready to give the astronauts an initial checkup before they head back to shore.

The ship will also have a crew of about 40 SpaceX and NASA personnel — including engineers, water recovery experts, medical professionals, and NASA cargo experts.

Hurly and Behnken will be whisked by helicopter to the Pensacola Naval Air Station. From there, the they'll board a NASA airplane that will shuttle them to Johnson Space Center in Houston, NASA’s official home base for astronauts.

That's where Behnken and Hurley will be reunited with their families.

Astronauts Robert Behnken and Douglas Hurley — both married to fellow NASA astronauts — have young families waiting at home for them.

After getting some sleep during their slow descent back toward Earth last night, their sons gave them their wake-up call Sunday morning.