Crew of first private flight to ISS set for splashdown
After a dizzying descent, a SpaceX Dragon capsule carrying the Axiom-1 crew should hit the water at 1:06 pm (1706 GMT), with a ship close by to pick them up.
It will mark the official end of the first fully-private mission to the orbiting outpost.
Axiom Space paid SpaceX for transport services and NASA for use of the ISS, while charging the three tycoons a reported $55 million each for the privilege.
American real estate mogul Larry Connor, Canadian financier Mark Pathy and Israeli impact investor Eytan Stibbe and veteran Spanish-American astronaut Michael Lopez-Alegria had blasted off on April 8.
They were originally scheduled to spend only eight days on the space station but bad weather forced repeated delays.
"Thanks once again for all the support through this amazing adventure that we've had even longer and more exciting than we thought," Lopez-Alegria tweeted.
"We really appreciate your professionalism and with that we'll sign off."
Axiom had been keen to stress its mission shouldn't be considered tourism, unlike the recent, attention-grabbing suborbital flights carried out by Blue Origin and Virgin Galactic.
On board the ISS, which orbits 250 miles (400 kilometers) above sea level, the quartet carried out research projects, including an MIT technology demonstration of smart tiles that form a robotic swarm and self-assemble into space architecture.
Another experiment involved using cancer stem cells to grow mini tumors, then leveraging the accelerated aging environment of microgravity to identify early changes in those tumors, to help improve screening methods.
Pathy spent considerable time in the station's famous observation cupola, photographing Earth.
The mission was dubbed Ax-1 in a nod to Axiom Space, which served as a sort of space travel agency, paying SpaceX for providing two-way transportation and NASA for the use of the orbiting accommodations.
NASA has already given the green light, in principle, to a second mission: Ax-2.
The departure of the Ax-1 crew left seven people on the ISS: three Americans, a German and three Russians.
Monday's sea landing of a manned SpaceX Dragon capsule will be the fifth to date.
SpaceX, owned by billionaire entrepreneur Elon Musk, is now regularly ferrying NASA astronauts to and from the space station.
Last year, Musk's company launched another entirely private mission, which orbited Earth for three days without linking up with the ISS.