Circling the Earth every 90 minutes, the International Space Station is the most expensive project ever assembled in space. Within days, it will hang by a single, costly thread. And Russia, the U.S.'s historic rival in space, is holding it.

The last U.S. space shuttle is scheduled to blast off Friday. After that, the U.S. and other nations will rely on vintage Russian spacecraft to ferry their astronauts to the $100 billion station. Russia will hold a monopoly over manned spaceflight, and tensions already are rising. The Russians are in the process of nearly tripling the cost of using their Soyuz crew capsules for transport to the orbiting base, and other countries have little choice but to pay up.

'We are not in a very comfortable situation, and when I say uncomfortable, that is a euphemism,' said Jean-Jacques Dordain, director general of the European Space Agency, one of five international agencies that jointly manage the orbiting laboratory. 'We made a collective mistake.'

The Soyuz represents the triumph of a low-cost approach to human space exploration. The Russian capsules are launched on massive expendable rockets, carrying astronauts in a kind of guided cannonball to and from orbit. By contrast, the U.S. built its space program around the most complex flying machine ever, the reusable space shuttle. While the U.S. has spent $209.1 billion on the space shuttle since its inception, the entire Russian space program currently costs just $2 billion a year.

'Today, reusable ships are a very expensive pleasure, and economically they're not really justified,' Vladimir Popovkin, the newly appointed head of Roskosmos, the Russian space agency, told a Russian newspaper last month. Officials at Roskosmos didn't provide comment for this article.

The Russian monopoly on manned spaceflight won't last forever. If all goes as NASA plans, the Russian monopoly will end in 2016 when the agency hopes to take its pick of several new commercial crew transports currently on the drawing board. NASA is now seeking a commercial space-taxi service─designed, built and operated by the private sector─to cut costs while speeding the pace of development.

'We are working aggressively to get our own crew capability,' said William Gerstenmaier, NASA's associate administrator for space operations, the chairman of the international board that oversees the space station.

Since President George W. Bush announced the end of the space-shuttle program in 2004, the Russian space agency has increased the price of taking U.S. astronauts to the space station eight times. By terms of the latest contract, each seat on a Soyuz crew capsule will cost NASA $63 million by 2016─a 175% price increase since 2005, according to a new agency audit.

The largest single price increase takes effect later this summer, coinciding with the conclusion of the last space-shuttle mission. It will cost U.S. astronauts $43.4 million apiece to fly aboard the Soyuz later this year, a 57% increase from the first-half cost.

The Russian government is unlikely to use its current monopoly over access to the space station as a diplomatic pressure point, but it would certainly take commercial advantage, several U.S. space experts said.

The Russians haven't crowed over the demise of the space shuttle. Alexei Krasnov, head of manned programs at Roskosmos, told a Russian newspaper last month, 'Even though the U.S. will be paying us to use our Soyuzes, giving up the shuttles isn't good for Russia.' His country is a big backer of the International Space Station, and he noted that it would have been impossible to build the station without space shuttles. 'It would be better for us if the shuttles continued to fly, even just once a year,' he said.

The space station was originally conceived as a platform for sending spacecraft to other planets. But its mission has changed into an orbiting laboratory to conduct experiments on how humans and other organisms fare in a low-gravity environment. The hope is to understand more about basic life function and to discover new medical treatments and vaccines. And humans are needed to conduct, or be participants in, many of these experiments.

So far, NASA has purchased 46 seats for Soyuz flights through 2016, and it wants to buy more. NASA officials attribute the price hikes to inflation and the increased cost of making more Soyuz spacecraft, which the Russians have been manufacturing for almost 40 years. The Russians have continued to tweak the Soyuz, and have a new version coming out this year.

In April, NASA awarded a total of $269.3 million to five U.S. aerospace companies to develop systems for transporting humans to the space station.

One of them─-Space Exploration Technologies Corp., based in a Hawthorne, Calif.─-appears to be furthest along, experts said. It pledged to build a reusable system that could ferry seven astronauts into orbit for as little as $20 million each─a fraction of most forecasts of future crew-transport costs.

'It has to be done for an amount of money that taxpayers are willing to pay,' said Space Exploration Chief Executive Elon Musk, who co-founded PayPal and Tesla Motors. 'That should allow NASA to transport a much greater number of astronauts and to get much more use out of the space station.'

A NASA-sponsored analysis by Aerospace Corp., one of the agency's most influential outside advisers, is less sanguine. It forecast future transportation costs at $90 million to more than $150 million per seat.

Space Exploration already has a $1.6 billion NASA contract to ferry supplies to the space station using its experimental Dragon spacecraft and its Falcon rocket, beginning next year. In April, NASA awarded the company an additional $75 million to build a launch-escape system for the Dragon spacecraft─a key component in converting it into a crew transport.

Officials at Roscosmos, however, warned in April they wouldn't let the unmanned Dragon spacecraft fly near the space station or dock with it any time soon─not until they deem it safe. In 1997, Russia's Mir space station was badly damaged when a cargo module slammed into it.

Indeed, NASA's inspector general last week warned that private companies may take so long to develop safe commercial crew transportation that it could threaten U.S. access to the space station.

'It is still a very risky bet that one or more of these companies can come up with an affordable and sustainable way for crew transport,' said George Washington University space-policy analyst John Logsdon. 'For the time being, American astronauts will be taking Russian taxis.'

In fact, NASA is already moving to buy more seats for U.S. astronauts aboard Russian crew capsules in case commercial development continues to fall behind schedule. Currently, NASA has purchased seats on the Russian Soyuz only through 2016, and doing so required a congressional waiver of legal limits on technological trade with Russia. To purchase more Soyuz seats, the agency will need additional congressional approval, which it is seeking.

Despite its rising ticket price, the Soyuz capsule is a relative bargain compared to the cost of the NASA space shuttle, largely because the vehicles represent radically different engineering approaches to human spaceflight. In many ways, the Soyuz resembles the Apollo moon capsules and Saturn rockets used by the U.S. in the 1960s.

By contrast, the space shuttle is a reusable winged spacecraft piloted by astronauts who can land it from orbit like a glider. Each shuttle contains more than 2.5 million parts and 230 miles of wiring, operating at extremes of speed, heat, cold, gravity and vacuum.

Working in the 1970s, NASA's shuttle designers promised to make civilian manned spaceflight cheap, safe and routine─ a jumping-off point for human voyages to other planets. Shuttle missions would be launched up to 50 times a year.

In 30 years of flights, the crafts deployed more than 50 satellites into orbit. They carried more than three million pounds of cargo and 355 people from 16 countries into space. They launched interplanetary probes and major orbital observatories, including the Hubble Space Telescope.

In practice, however, the space-shuttle program was never routine, reliable or cheap. A shuttle launch cost $1.5 billion─100 times the $10.5 million dollars each that NASA officials promised at the start of the program in 1972. And the agency never came close to achieving the launch rates its designers had predicted.

Without the shuttle to rely on, NASA managers have scrambled to revamp the way they operate the space station. They revised the way they plan to repair it and conduct research there in the decade to come, using their last space-shuttle flights to build up orbiting caches of large spare parts that can't fit aboard unmanned Russian, European or Japanese supply craft that currently supply the station.

In the end, the U.S. is left to ponder an irony: It won the technological race to develop a space shuttle but lost the war. 'You can argue that the Russians were on the right trajectory all along, by flying big, dumb boosters,' said Duke University space historian Alex Roland.

Robert Lee Hotz