"The delivery of the Canadarm is really crucial to the remaining assembly of the space station," NASA lead flight director Phil Engelauf told SPACE.com during a news briefing Saturday. "We're now at a point in the assembly where components to be added to the station cannot be installed without the additional reach capability that's provided by this new arm."

An advanced version of the shuttle's Canadian-built robot arm, the new crane -- which has been in the works now for 15 years -- is considered the heart of Canada's $900 million station contribution.

Equipped with force sensors that give it an artificial sense of feel, the robot arm is four times stronger and more flexible than its shuttle predecessor. And since it will have to operate in orbit during the next two decades, the arm can be retrofitted with spare joints if necessary.

Canadian Space Agency officials, meanwhile, say the deployment of the crane marks a huge milestone in that nation's space exploration program, which dates back to the early 1960s.

"This is the biggest day, really, in the history of the Canadian Space Agency," said Ed Tabarah, chief robot arm instructor with the organization. "This is a moment that we've all anticipated."

The high-flying construction work got underway about 4:30 a.m. EDT (08:30 GMT) Sunday as astronaut Jeff Ashby snatched a crane carrier from Endeavour's cargo bay with the shuttle's 50-foot (15-meter) robot arm.

Too long to fit in the shuttle's expansive bay, the so-called Space Station Remote Manipulator System was folded up and latched to the 3,000-pound (1,350-kilogram) pallet for the ride up to the station.

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Wielding the shuttle arm, Ashby hoisted the pallet up to a point three stories above the nose of Endeavour and attached it to a cradle on the outside of the station's U.S. Destiny science laboratory.

The tricky lift called for Ashby to twist and turn the pallet, snaking it around to the far side of the $1.4 billion lab without banging it into the station or a nearby outpost communications antenna.

"Great job," astronaut Ellen Ochoa told Ashby and his crewmates from NASA's Mission Control Center in Houston, Texas.

That set the stage for the two spacewalkers. With Hadfield riding on the end of the shuttle's arm and Parazynski climbing hand over hand, the pair set out to make their way up to the bus-sized Destiny lab and the crane's carrier pallet.

Once there, the two astronauts will hook up power, data and video cables so that the new station arm can be brought to life electrically and controlled from computer workstations inside Destiny.

Eight so-called "superbolts" - each four feet (1.2 meters) long - then will be unfastened and stored in quiver-like containers so that the boom-like arm can be unfolded and extended before its hinges are firmly locked in place.

During the scheduled six-and-a-half-hour excursion, the spacewalkers also plan to deploy a new station communications antenna.

Stowed on the crane carrier pallet, the Ultra-High Frequency antenna will enable station tenants to chat with spacewalkers doing construction and maintenance work outside the station. Attached to a four-foot (1.2-meter) boom, the antenna also will be used for ship-to-ship communications during shuttle dockings and undockings at the outpost.

The spacewalk represents part two of a multi-stage operation required to install and activate the new station construction crane.

Working within Destiny, station flight engineer Jim Voss will "walk" the arm off the pallet about 6:30 a.m. EDT (10:30 GMT) Monday and then place it on an anchor on the exterior of the lab. The so-called Power and Data Grapple Fixture will become the arm's base at that point, and its other end will be set free from the pallet.

A second spacewalk then will be carried out by Hadfield and Parazynski starting at 9 a.m. EDT (13:00 GMT) Tuesday. The purpose: Disconnecting power cables between the robot arm and the pallet so that the latter can be stowed back in the shuttle's cargo bay for a return trip to Earth.

That job calls for Voss to grab the pallet with the station's new construction crane and then hand it off to Hadfield, who will be operating the shuttle's robot arm. It will be the first time two robotic arms - and two robot arm operators - have ever worked in concert in space.

Launched Thursday from Kennedy Space Center in Florida, Endeavour's astronauts docked at the station Saturday and are in the midst of a weeklong stay at the outpost.

Made up of astronauts from the U.S., Russia, Canada and Italy, the shuttle crew also plans to deliver several tons of supplies and equipment to the station's current lodgers: Voss, Russian commander Yuri Usachev and American flight engineer Susan Helms.

Ferried to the station in an Italian moving van, the supplies and equipment will be unpacked after the shuttle-borne cargo carrier is temporarily mounted to the station's Unity Module Monday.

Endeavour's seven astronauts are scheduled to depart the station next Saturday, heading off on a two-day trip back to Earth. The shuttle and its crew are due to land at NASA's coastal Florida spaceport at 10 a.m. EDT (14:00 GMT) April 30.