By Irene Klotz

CAPE CANAVERAL, Florida (Reuters) - Three crewmembers left the International Space Station for Earth on Thursday while their replacements remain delayed by an investigation into a Russian launch accident.

Former station commander Andrey Borisenko, cosmonaut Alexander Samokutyaev and U.S. astronaut Ron Garan sealed themselves inside one of two Russian Soyuz capsules parked at the outpost at 5:30 p.m. EDT/(2130 GMT), closing out a five-month mission.

Three hours later, the Soyuz slipped out its berthing port at the station, a recently completed $100 billion project of 16 nations. Touchdown in Kazakhstan is set for midnight EDT/(0400 GMT) on Friday.

"It's hard to believe it's time," NASA astronaut Mike Fossum, the new station commander, said during a farewell ceremony broadcast on NASA Television.

"We don't want to see you go. We like the company," he said. "We love the friendship. We've lived together closer than families in many ways. We wish you all the best."

The returning crew's replacements -- NASA flight engineer Dan Burbank and cosmonauts Anton Shkaplerov and Anatoly Ivanishin -- were scheduled to arrive September 24 but the flight was delayed following an August 24 launch accident involving an unmanned Russian cargo ship bound for the station.

The upper-stage motor that failed on the Russian Progress rocket, causing it to burn up in the atmosphere and shower debris across part of Siberia, is virtually identical to one used to fly crew to the station on Soyuz rockets.

The new crew is now scheduled to launch on November 14 and arrive at the station two days later.

The departure of Borisenko, Samokutyaev and Garan will leave Fossum, Japanese astronaut Satoshi Furukawa and Russia's Sergei Volkov, on their own for an extended two-month period.

They will have little time to prepare the new crew to take over the station before heading home themselves around November 22.

The Progress launch failure is believed to have been caused by blockage in a kerosene fuel line. Russia plans to fly another cargo ship on October 30 before launching the next station crew.

With the retirement of the U.S. space shuttles this summer, crews can reach the station only aboard Russian rockets. China, the only other country able to fly people into orbit, is not a partner in the space station.

The current U.S. plan is to pay Russia to fly NASA astronauts, at a cost of more than $50 million per person, until U.S. companies are able to do so.

(Editing by Bill Trott)