CAPE CANAVERAL, Fla. - The United States and Russia might be the most powerful players in the International Space Station program, but the future of the $60 billion construction project now hinges on an underrated partner: Canada. Circling some 238 miles (381 kilometers) above Earth, the 17-story station -- which eventually will span an area as large as a city block -- simply can't be finished without a Canadian robotic servicing system also needed to maintain the outpost over the next two decades. What's more, the ongoing assembly of the station will come to a quick halt if a critical Canadian construction crane isn't put in place by the visiting crew of shuttle Endeavour after its planned launch Thursday. "The robot arm is absolutely essential to the rest of the space station assembly, as well as to operations after assembly is complete," said Endeavour mission specialist John Phillips. "It's what's going to allow us to construct the greater portion of the station from now on," added U.S. astronaut Jim Voss, one of three tenants now serving a tour off duty aboard the outpost. A joint project of 16 nations on four continents, the station initially consisted of a Russian space tug and the American Unity module, which were launched and linked in orbit in late 1998. A long-delayed Russian command post that doubles as crew quarters finally was launched last July, and since then the station has undergone rapid expansion. The first piece of what eventually will be the station's 356-foot (108-meter) central truss was erected in October. A $600 million U.S. electric power tower - which features solar wings that measure 240 feet (73 meters) from tip to tip - was added in December. Then a shuttle-borne construction crew delivered the station's first science center - the $1.4 billion U.S. Destiny laboratory - in February, doubling the size of the outpost, which now stretches 171 feet (52 meters) from end to end. Coming up next: The so-called space station remote manipulator system, which is an advanced version of the shuttle's 50-foot (15-meter) Canadian robot arm. Considered the heart of Canada's $1.4 billion (US$900 million) station investment, the sophisticated construction crane will be delivered by an international crew scheduled to blast off aboard Endeavour at 2:41 p.m. EDT (18:41 GMT) Thursday. A marvel of modern technology, the 57-foot (17-meter) crane will be capable of moving from work site to work site outside the station, crawling to places that the shuttle's fixed robot arm cannot reach. Snare-like hands on either end of the arm will be able to grasp so-called power and data grapple fixtures that will electrically connect the crane to computer control stations inside the complex. The crane then will be able to move end over end, from one pin-like anchor to the next, much like a toy Slinky rappelling down a staircase. "It's a bit like an inchworm," said veteran Canadian astronaut Marc Garneau, now executive vice president of the Canadian Space Agency (CSA). "We've all seen an inchworm sort of put its nose down and then lift its whole body and then sort of flip over. And then its tail becomes like its nose and then it can flip end-over-end and move like that." The crane will become even more mobile with the addition next year of a rail cart also built by the Canadians. Perched atop the cart, the robot arm will be able to move up and down the station's central truss. Riding atop the so-called mobile transporter and its mobile base, which now are scheduled for launch in January 2002 and March 2002, respectively, the robot arm will be able to carry out assembly and servicing work anywhere along the station's metallic backbone. A prime example: The erection of three more pairs of power-producing American solar wings at either end of the lengthy station truss. "It would be used to put the solar wings out on the end of the trusses and to continue the assembling of the station as we build outward," said NASA space station program manager Tommy Holloway. "The arm would be absolutely mandatory to get that work done, so it is extremely critical." The final part of the Canadian servicing system is a two-armed robot dubbed the special purpose dexterous manipulator, or SPDM, which is to be launched in October 2003. Also known as the "Canada Hand," the robot will be able to ride the cart or attach itself to the end of the construction crane. Nimble enough to work with power tools, the robot will be able to swap out dead batteries and electronics boxes outside the station, work that otherwise would have to be done by spacewalking astronauts in a dangerous vacuum environment. "The purpose of this dexterous robot is to reduce as much as possible the number of [spacewalks], which is always kind of risky for astronauts," said Sylvie Beland, the CSA's flight and launch systems manager. As it stands, astronauts already have carried out 18 spacewalks at the station and 150 more will be required before outpost construction is completed in mid 2006. The two-armed Canadian robot, consequently, will free up time for scientific research aboard the outpost. "We also want the astronauts to be working on experiments in the space station - not just maintaining the station," said Beland. "So that's why we have this robot." That the Canadians have such a key role in the station project really shouldn't be too surprising. After all, Canada in the early 1960s became only the third nation to build its own satellite, which was launched by a U.S. rocket, and the country has a long history when it comes to space exploration. "I think that for Canadians, space is a natural fit," said Garneau, who became the first Canadian to fly in space on a 1984 shuttle mission. "We're a big country with a small population and right from the beginning of Canada's space history, we've become involved with using space to our advantage." The Alouette 1 satellite was launched in 1962 to study the northern lights, a shimmering curtain of color that emanates from the upper section of the ionosphere, disrupting radio communications. Seven years later, Canada helped the U.S. reach the moon by building the landing gear of the Apollo 11 lunar module that touched down on the Sea of Tranquility. And then in 1972, Canada became the first nation to launch its own domestic communications satellite system into an orbit 22,300 miles (35,680 kilometers) above Earth. The world's first direct broadcast television satellite was launched by Canada in 1976, and the nation began forging its reputation as a global leader in robotics with the 1981 launch of the shuttle remote manipulator system, also known as the Canadarm. A pioneering effort to make a satellite map of Antarctica - a region the size of Canada and Alaska that had never been completely charted - was carried out in 1997, and Canada next year will loft an advanced satellite designed to deliver high-speed Internet access to people living in urban, rural and remote areas throughout the nation. The country's contribution to the station project is seen by many as a natural next step for Canada's space industry, which comprises 250 firms, employs 5,500 people and generates $1.8 billion (US $1.15 billion) in annual revenue. Monetarily, it might only represent 1.5 percent of the $60 billion required to build the station, but it is a crucial contribution nonetheless. "We're seeing this as a great opportunity for our nation -- for our engineers, for the people on the ground who devised and have perfected the station robotics -- to show their expertise and to contribute," said Julie Payette, one of six active Canadian astronauts. "However small that contribution is, it is very essential, and we feel very proud."