Soyuz carrying US, Russian astronauts and 32 fish docks with International Space Station

ALMATY, Kazakhstan – A Russian Soyuz spacecraft carrying three astronauts and a consignment of fish successfully docked Thursday with the International Space Station after a two-day voyage.

The arrival of NASA astronaut Kevin Ford and Russians Oleg Novitsky and Yevgeny Tarelkin on Thursday brings the crew at the orbiting outpost to six.

Novitsky gently slotted the Soyuz craft into the Russian Poisk research module around 410 kilometers (255 miles) above southern Ukraine around six minutes ahead of the scheduled 1235 GMT (8:35 a.m. EDT) arrival.

The trio blasted off Tuesday from a Russian-leased facility in the southern Kazakhstan town of Baikonur.

Incoming cargo includes 32 guppy-like fish that will be used to test how conditions in space impact on living organisms.

Akihiko Hoshide, an astronaut with Japan's JAXA space agency, spent early Thursday morning preparing an aquarium on the Japanese experiment module called "Kibo," or Hope.

"The importance of these very small fishes is that they have bones and muscles just like human beings," Hoshide told NASA TV earlier this year before the start of his mission. "What we're trying to do is have them stay in space for a longer duration and then bring them down to look at their bone structure and muscles."

The hardy Medaka fish, which can grow up to four centimeters (1.6 inches), are considered particularly suitable for the study as they have transparent bodies that enable scrutiny of their internal organs.

A spacewalk by Hoshide and U.S. astronaut Suni Williams to repair an ammonia leak from the station's temperature control system is planned for next Thursday.

NASA says ammonia is pumped through the station's system to keep electronics and other equipment cool.

Another task in the frenetic workload over the incoming crew's first week in orbit will be the unberthing Sunday of a commercial Dragon cargo vehicle.

The capsule loaded with completed scientific experiments will splash down in the afternoon in the Pacific Ocean off the west coast of Baja California.

The growing capabilities of private space vehicle companies have boosted hopes that NASA will be able to focus increasingly on more ambitious exploration projects.

Earlier this month, California-based SpaceX successfully delivered supplies to the space station on Dragon, the first official shipment under a $1.6 billion contract with NASA. It calls for 12 such shipments.

Cygnus, the first cargo vehicle to the station from Orbital Sciences Corp., of Dulles, Virginia, is scheduled for December.