The privately bankrolled SpaceX Dragon capsule made a historic arrival at the International Space Station (ISS) on Friday, and astronauts will begin unloading some of the 544 kilograms of food, water, clothing and other supplies its carrying starting Saturday.

The unmanned capsule was securely bolted to the Harmony module on the underside of the ISS around 12 p.m. ET after it was triumphantly snagged from space by astronauts wielding the Canadian-made robotic arm Canadarm2.

The capsule's hatch will be opened on Saturday, and its cargo unloaded over a period of four days. After it is filled back up with science experiments and equipment, it will return to Earth on May 31, landing in the Pacific Ocean about 400 kilometres west of southern California four hours after it detaches from the ISS.

The California-based Space Exploration Technologies Corp., or SpaceX, is the first private company to accomplish a commercial cargo delivery into the cosmos.

"There's so much that could have gone wrong, and it went right," said an elated Elon Musk, the billionaire CEO of the company, at a press conference at the SpaceX control centre in Hawthorne, Calif., after the capsule was berthed to the station.

"This really is, I think, going to be recognized as a significantly historical step forward in space travel —and hopefully the first of many to come."

Cargo unloading starts Saturday

NASA astronaut Donald Pettit used the space station's 18-metre Canadarm2 to snare the capsule just before 10 a.m. ET as it floated about 400 kilometres over northwest Australia.

Dragon is moved into position for docking by the Canadarm2 in this closeup image captured from NASA TV. (NASA TV/Reuters)

"Looks like we've got us a dragon by the tail," Pettit radioed to NASA mission control in Houston once he locked onto Dragon's docking mechanism.

NASA controllers applauded as their counterparts at SpaceX's control centre lifted their arms in triumph and jumped out of their seats to exchange high fives.

The two control rooms worked together as equal partners during Friday's tricky operation.

Afterwards, SpaceX's youthful-looking employees — the average age is 30 — were still in a frenzy as Musk took part in the televised news conference. They screamed with excitement as if at a pep rally and chanted, "E-lon, E-lon, E-lon," as the 40-year-old Musk described the day's events.

Alcohol was banned from the premises during the crucial flight operation, Musk noted, "but now that things are good, I think we'll probably have a bit of champagne and have some fun."

The crowd roared in approval.

Successful flight opens 'new frontier': NASA

Although cargo hauls have become routine, Friday's linkup was significant in that an individual company pulled it off. That chore was previously reserved for a small, elite group of government agencies.

U.S. President Barack Obama, left, speaks to SpaceX CEO Elon Musk in Cape Canaveral, Fla., on April 15, 2010. (Jim Young/Reuters)

Not only that, the reusable SpaceX Dragon is designed to safely return items, a huge benefit that disappeared with NASA's space shuttles.

It's the first U.S. craft to visit the station since the final shuttle flight last summer, and on Friday, NASA indicated it might not be the last.

"Now that a U.S. company has proven its ability to resupply the space station, it opens a new frontier for commercial opportunities in space — and new job creation opportunities right here in the U.S.," NASA administrator Charles Bolden said in a statement.

"Nearly 43 years after we first walked on the moon, we have taken another step in demonstrating continued American leadership in space," said Buzz Aldrin, who with Neil Armstrong was part of the 1969 Apollo 11 mission to the moon.

After this test flight, SpaceX has a contract to make a dozen delivery runs. It is one of several companies vying for NASA's cargo business and a chance to launch American astronauts into space from U.S. soil.

Technical checks caused 2-hour delay

A simulation of the Dragon spacecraft separating from the Falcon 9 rocket that launched it into space. (NASA)

SpaceX launched the capsule from Cape Canaveral, Fla., on Tuesday with its Falcon 9 rocket. On Thursday, the bell-shaped capsule, which is 5.8 metres tall and 3.7 metres across, came within two kilometres of the space station in a practice fly-by.

Before it was captured by Pettit and a second ISS astronaut, Andre Kuipers of the Netherlands, it went through a series of stop-and-go demonstrations to prove it was under good operating control.

NASA ordered extra checks of the Dragon's imaging systems as the capsule drew ever closer to the space station, putting the entire operation about two hours behind schedule. At one point, SpaceX controllers ordered a retreat because reflections were being cast on the capsule's primary navigation sensors by an external pallet attached to one of the ISS's science modules, the Japanese Kibo module.

Given that the Dragon is a brand new type of vehicle and this is a test flight, the space agency insisted on proceeding cautiously. A collision by vehicles travelling at orbital speed — 28,200 km/h — could prove disastrous for the space station.

NASA's space station program manager, Mike Suffredini, said the way the SpaceX team handled the problem and the entire operation was "remarkable."

A computer-generated image of the Dragon with its solar panels deployed. (SpaceX/Associated Press)

U.S. President Barack Obama is pushing commercial ventures in orbit so NASA can concentrate on grander destinations like asteroids and Mars. Obama's chief scientific adviser, John Holdren, called Friday's linkup "an achievement of historic scientific and technological significance."

"It's essential we maintain such competition and fully support this burgeoning and capable industry to get U.S. astronauts back on American launch vehicles as soon as possible," he said in a statement.

NASA contributed $381 million US to SpaceX in seed money, and the company itself invested more than $1 billion into the Dragon project over the past 10 years.

U.S. reliant on Russian shuttle flights

The shutting down of NASA's shuttle program has left U.S. astronauts no choice but to go through Russia, an expensive and embarrassing situation for the U.S. after a half-century of orbital self-sufficiency.

Once companies master supply runs, they hope to tackle astronaut ferry runs.

Musk, who founded SpaceX a decade ago and also helped to create the successful online payment system PayPal, said he can have astronauts riding his Dragon capsules to orbit in three or four years.

The space station has been relying on Russian, Japanese and European cargo ships for supplies ever since the NASA shuttles retired.

None of those, however, can bring anything of value back; they're simply loaded with trash and burn up in the atmosphere.

By contrast, the Dragon is designed to safely re-enter the atmosphere, parachuting into the ocean like the Mercury, Gemini and Apollo capsules did back in the 1960s.