WASHINGTON (AFP) – The space shuttle Atlantis successfully docked Wednesday with the International Space Station and the hatch separating the crews was opened for the traditional welcoming ceremony, NASA said.

The shuttle and its six astronauts left Florida's Kennedy Space Center without a hitch on Monday on a 11-day voyage to deliver a 20,000-pound (9,071-kilogram) haul of spare parts to the ISS.

The shuttle's final approach towards the orbiting station was manually completed by the commander Charlie Hobaugh as the two spacecraft hurtled towards each other at 28,000 kilometers (17,000 miles) an hour.

Just before docking two minutes later than planned at 1651 GMT, soaring 220 miles (354 kilometers) above Australia at the time, Atlantis approached the space station at a quarter of a centimeter per second, NASA said.

Atlantis is carrying assorted gyroscopes, ammonia tanks and other equipment too large to be launched into space aboard any other vehicle, NASA said.

With only five launches left before the 2010 retirement of the shuttle fleet, NASA officials said the parts were essential for extending the life of the space station.

"This flight is all about spares, basically, we're getting them up there while we still can," said mission director Brian Smith ahead of the mission. "You'll see this theme in some of the flights that are going to come after ours as well."

This fifth and final shuttle mission for 2009 is scheduled to include three space walks to store hardware on the exterior of the space station and bring US astronaut Nicole Stott, who has been on the ISS since August, back to Earth.

The crew will also be conducting science experiments with the help of some ground-breaking worms that could explain muscle loss in space.

Thousands of the microscopic creatures have been sent from Britain's University of Nottingham to study the effect of zero gravity on the human body's muscle development and physiology.

The worms will be stashed inside the Japanese Kibo laboratory on the ISS where they will be tested with several potential treatments for muscle loss.

Ahead of the September 2010 retirement, the White House could still decide to extend the shuttle program through 2011 to reduce US reliance on Russia's Soyuz craft for astronaut transport to the ISS as the future Orion capsules are being built.

NASA's human space flight program, however, is at great risk of being grounded as it remains too underfunded to keep aloft.

The National Aeronautics and Space Administration's yearly budget is about 18 billion dollars, 10 billion dollars of which are plowed into the human space flight program, chiefly in developing the successor of the space shuttle: the Ares 1 rocket and the Orion Crew Exploration Vehicle.

The Augustine Committee, a panel tasked with assessing the future of US human space flight, has said an additional three billion dollars a year is needed for NASA to meet Constellation program goals or take human space flight the next step beyond the existing ISS.

Late last month NASA successfully launched the prototype Ares I-X rocket as part of its effort to build a new generation of space rocket to transport the Orion capsule.

Orion, which won't be ready at least until 2015, is initially being designed to take a crew of up to six astronauts on flights to the International Space Station, or a crew of four on lunar missions lasting up to 210 days.