HOUSTON, April 10 (Reuters) - The four Artemis II astronauts, returning from the world's first crewed moon voyage in over half a century, hurtled back toward Earth on Friday as they prepared their Orion spacecraft for the final phase of their descent and splashdown in the Pacific Ocean off Southern California.
On Friday afternoon, the autonomously piloted Orion crew capsule executed one last eight-second firing of its jet thrusters to fine-tune the flight course, a critical maneuver to ensure a safe return after a nearly 10-day mission.
Roughly 20 minutes before entering Earth's atmosphere, the gumdrop-shaped Orion vehicle jettisoned its service module housing its main rocket engines, exposing the capsule's heat shield that will face temperatures of up to 5,000 degrees Fahrenheit (2,760 Celsius) through a fiery atmospheric re-entry.
Blazing-hot plasma enveloping the capsule will trigger an expected six-minute communications blackout. The atmospheric friction and a set of parachutes are expected to slow the capsule from 32 times the speed of sound to a soft 17 miles per hour (27 kph) at splashdown a few hundred miles off the San Diego coast.
If all goes well, U.S. astronauts Reid Wiseman, Victor Glover and Christina Koch, along with Canadian astronaut Jeremy Hansen, will end up bobbing safely in the ocean aboard their Orion capsule, dubbed Integrity, shortly after 8 p.m. ET (0000 GMT).
The quartet blasted off from Cape Canaveral, Florida, on April 1, lofted into an initial Earth orbit by NASA's giant Space Launch System rocket before sailing on around the far side of the moon, venturing deeper into space than any humans before them.
STEPPING STONE TO MARS
They became the first astronauts to fly in the vicinity of the moon since the Apollo program of the 1960s and '70s. Glover, Koch and Hansen also made history as the first Black astronaut, the first woman and first non-U.S. citizen, respectively, to take part in a lunar mission.
The voyage, following the uncrewed Artemis I test flight around the moon by the Orion spacecraft in 2022, marked a critical dress rehearsal for a planned attempt later this decade to land astronauts on the lunar surface for the first time since Apollo 17 in late 1972.
The ultimate goal of the Artemis program is to establish a long-term presence on the moon as a stepping stone to eventual human exploration of Mars.
In a historical parallel to the Cold War era of Apollo, the Artemis II mission has played out against a backdrop of political and social turmoil, including a U.S. military conflict that has proven unpopular at home.
CRITICAL TEST OF HEAT SHIELD
The four Artemis astronauts spent much of the final 24 hours of the mission stowing equipment and configuring the crew cabin for the re-entry and splashdown to come.
The return to Earth will put the Lockheed Martin-built Orion spacecraft through a critical test of its heat shield, which sustained an unexpected level of scorching and stress on re-entry during the 2022 test flight. As a result, NASA engineers altered the descent trajectory for Artemis II in order to reduce heat buildup and lower the risk of the capsule burning up.
As is typical in such return descents, the intensity of heat and air compression will form a red-hot sheath of ionized gas, or plasma, that engulfs the capsule, cutting off radio contact with the crew for several minutes at the start of re-entry.
Moments later, two sets of parachutes will be deployed from the nose of the free-falling capsule, slowing its descent before Orion gently hits the water.
Just as critical as the performance of the heat shield and parachutes are several other factors, including achieving the spacecraft's precise descent path and re-entry angle through a series of course-correction blasts of its jet guidance thrusters.
The last of three such jet thruster "burns" was conducted on Friday afternoon, roughly five hours before splashdown. A final angle adjustment of the spacecraft was to take place as the vehicle nears the top of the atmosphere.
Once the capsule hits the top of the atmosphere, it takes less than 15 minutes before two sets of parachutes are deployed and the capsule floats into the sea.
NASA says it will take about another hour for recovery teams to secure Orion, assist the astronauts out of the capsule one by one and hoist them into helicopters hovering above.
At the flight's peak, the crew reached a point 252,756 miles from Earth, exceeding the previous record of roughly 248,000 miles set in 1970 by the crew of Apollo 13.
Last week's launch was a major milestone for the SLS rocket, handing its principal contractors, Boeing and Northrop Grumman, long-sought validation that the launch system more than a decade in development was ready to safely fly humans to space.
(Reporting by Steve Gorman in Los Angeles and Joey Roulette in Houston; Editing by Bill Berkrot)
