SpaceX cargo capsule reaches International Space Station

CAPE CANAVERAL, Florida (Reuters) - A SpaceX Dragon cargo capsule overcame a potentially mission-ending technical problem to make a belated but welcome arrival at the International Space Station on Sunday.

Astronauts aboard the outpost used the station's robotic arm to pluck the capsule from orbit at 5:31 a.m. EST as the ships sailed 250 miles over northern Ukraine.

Flight controllers at NASA's Mission Control in Houston then stepped in to drive the capsule to its berthing port on the station's Harmony connecting node. Docking occurred at 8:44 a.m. EST.

The Dragon capsule, loaded with more than 2,300 pounds (1,043 kg) of science equipment, spare parts, food and supplies, blasted off from Cape Canaveral Air Force Station in Florida on Friday aboard a SpaceX Falcon 9 rocket for the second of 12 planned supply runs for NASA.

SpaceX is the first private company to fly to the station, a $100 billion project of 15 nations.

Dragon was to have arrived at the station on Saturday but a problem with its thruster rocket pods developed soon after reaching orbit. Engineers sent commands for Dragon to flip valves and clear any blockage in a pressurization line in an attempt to salvage the mission.

By Friday evening, Dragon had fired its thruster rockets to raise its altitude and begin steering itself to rendezvous with the station.

The orbital ballet ended when station commander Kevin Ford, working from a robotics station inside the outpost, grabbed the capsule with the station's robot arm.

"As they say, it's not where you start but where you finish that counts. You guys really finished this one on the mark," Ford radioed to Dragon's flight control team in Hawthorne, California, and NASA's Mission Control in Houston.

"What a fantastic day," Ford said.

Once Dragon's hatch is open, the station crew will spend the next several days unpacking the food, clothing, supplies and science experiments from the capsule. The research includes studies on plant seedlings, mouse stem cells and combustion in microgravity.

SpaceX also sent the crew a gift of fresh fruit from an employee's father's orchard, company president Gwynne Shotwell said.

Ground controllers will use the station's robot arm again on Wednesday to unpack equipment for a future spacewalk that is stowed in Dragon's unpressurized trunk.

Once the capsule is unloaded, the crew will begin refilling it with 3,000 pounds (1,361 kg) of unneeded and broken equipment and science samples for analysis on Earth.

Dragon is the only station freighter that makes return trips, a critical service that was lost after the U.S. shuttle program ended in 2011. Cargo ships flown by Russia, Europe and Japan incinerate in the atmosphere after leaving the station

Dragon's departure and parachute splashdown in the Pacific Ocean is scheduled for March 25.

Dragon's flight is the second of 12 missions for privately owned Space Exploration Technologies, known as SpaceX, under a $1.6 billion NASA contract. Following a successful test flight to the space station in May 2012, SpaceX conducted its first supply run to the orbital outpost in October.

A second firm, Orbital Sciences Corp is due to debut its freighter this year.

NASA turned to private companies for delivering supplies to the station following the retirement of its shuttle fleet. The agency hopes to buy rides commercially for its astronauts as well beginning in 2017.

(Editing by Bill Trott)