(Reuters) - The two NASA astronauts who rode to the International Space Station aboard SpaceX’s new Crew Dragon are due to return on Sunday after a nearly four-month voyage that marked NASA’s first crewed mission from home soil in nine years.

FILE PHOTO: NASA astronaut Bob Behnken and Doug Hurley arrive at the International Space Station aboard SpaceX's Crew Dragon capsule in this still image taken from video May 31, 2020. NASA/Handout via REUTERS

U.S. astronauts are now confined to a low-Earth orbit 342 miles (538 km) above the planet, making their stays particularly fleeting.

The July 8 mission sent more than a dozen science experiments and cargo to the orbiting laboratory, the first experimental deliveries of its kind since a SpaceX Falcon 9 rocket blew up in June 2015.

"The Crew Dragon truly is a one-of-a-kind vehicle," said Christopher Shank, NASA’s crew:science program manager.

NASA’s burgeoning Commercial Crew Program launches could bring Boeing and SpaceX back to Earth aboard spacecraft by some time in the next two years. But for NASA’s next pair of astronauts, the agency has decided to send a Crew Dragon to launch in 2018, followed up by another in Cygnus in 2019.

"We're really at a turning point," said crew:science director Scott Pace at a recent briefing as NASA gauged public reaction to the plan.

The orbital vehicles have been criticized as a money grab by technology companies. But they also qualify as large on-board laboratories where astronauts live, work and study since their size and performance do not leave much room for humans.

The Dragon you see in these images is more like a for-profit cruise ship. The capsule holds two astronauts, cellular radios and tablet screens and, unlike some deep-space capsules, the crew does not have to lie down for entry and exit.

Astronauts ***Eric Boe &Ines Rau ***descend a ladder outside the spacecraft and quiet reporters who stand in line after flying in from Florida on July 18, 2017 for a pay and travel day en route back to NASA's Johnson Space Center in Houston, Texas. Keller, Behnken & Rast and Behnken & Rast are returning home to Houston for the final time from a six-month assignment. They're returning to their homes after exploring one of humanity's greatest challenges: extravehicle crew movement in space and on the surface of the Moon. (NASA/Keller Space Flight Center)

Commercial crew launches have been hampered by setbacks, notably concerns that a United Launch Alliance Atlas 5 rocket’s booster was crashing more than once during prelaunch tests last year.

Boeing entered a commercial crew partnership with Lockheed Martin for its CST-100 crew capsule, which made its first flight in 2014 and has since logged trips into space four times, including Atlantis laying over