Space shuttle Endeavour astronauts Michael Fincke (L) and Andrew Feustel work outside the International Space Station during their spacewalk in this image provided by NASA and taken May 25, 2011.[Photo/Agencies]

CAPE CANAVERAL, Fla -- One of NASA's most experienced spacewalkers paired with its newest on Friday for the space shuttle program's last servicing call outside the International Space Station.

Astronauts Mike Fincke and Greg Chamitoff floated outside the space station's airlock at around 12:30 am EDT (0430 GMT) to begin the fourth and final spacewalk planned during shuttle Endeavour's 16-day mission, the next to last in the 30-year-old US space shuttle program.

The 6 1/2-hour spacewalk was the 159th in support of assembly and maintenance of the station, which began with the robotic attachment of the US Unity node with the Russian Zarya base block in 1998. Since then, the $100 billion outpost, a project of 16 nations, has grown to more than 1 million pounds (455,000 kg) of hardware orbiting 220 miles (355 km) above Earth.

During the spacewalk, the ninth for Fincke and the second for Chamitoff, NASA will hit the 1,000-hour mark of spacewalk time by astronauts working outside the station.

"In the early shuttle program, a flight with one or two EVAs (extravehicular activities, or spacewalks) was considered a pretty challenging mission. With the station missions that we fly today, with three and four EVAs the norm, the difference and the upgrade in the capability is just tremendous," said   NASA station flight director Derek Hassman.

NASA plans one last shuttle mission in July to deliver a year's worth of supplies to the station before turning over Endeavour and sister ships Discovery and Atlantis to museums.

"I am sad to see the three space shuttles be rolled into a museum here shortly," Endeavour commander Mark Kelly said during an in-flight news conference. "I think it's a necessary step so we can go on and do some more exciting things."

NASA plans to save the shuttles' $4 billion annual operating budget and develop new vehicles that can travel beyond the station's orbit where the shuttles cannot go.

During Friday's spacewalk, Fincke and Chamitoff will transfer the shuttle's 50-foot (15-metre) inspection boom to the station so it can be used to extend the reach of the station's robotic crane. With the boom's installation, 11 years of station assembly by NASA comes to an end.

Endeavour and its six-man crew are due to leave the station late on Sunday.

The shuttle delivered the station's premiere science experiment, the $2 billion Alpha Magnetic Spectrometer particle detector, and spare parts.

Landing is scheduled for 2:32 a.m. EDT (0632 GMT) on Wednesday at the Kennedy Space Center in Florida. Shuttle Atlantis is targeted to launch on July 8 for NASA's 135th and final flight.