The space shuttle Discovery is rushing to catch up with the international space station for a Thursday rendezvous. The chase mirrors what has been going on down on Earth for the past few years.

Just about every decision about the space shuttle — including the controversial call to launch now instead of waiting until the chronic foam problem is fixed — is tied to the space station's construction deadline.

The sole reason for the aged shuttle fleet's continued existence before the three remaining spacecraft are retired in 2010 is to finish up the construction of the half-built space station, NASA Administrator Michael Griffin told The Associated Press in a June interview.

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The shuttle "will be employed, if employed at all, for purposes of completing the international space station project," Griffin said. "So all of our decisions on shuttle are really being made with an eye to how to how does this help us achieve our larger goal, which is not one shuttle flight or two shuttle flights, but 16 flights to finish the assembly of the space station.

"We're looking at the package of flights, we're looking at the big picture," Griffin said. "Because if we don't think we can get to the end of the project, then frankly, there's no need to go halfway."

The United States has treaty obligations that keep it from abandoning the massive outpost 212 miles above Earth.

The 2010 deadline is there because after the Columbia accident 3½ years ago, President Bush came up with a plan to send astronauts to the moon and then Mars. NASA said it would retire the shuttle and spend its money on a new ship, aiming for the moon by 2020.

The space station will give NASA an off-Earth training station to learn how to live in space for months on end, because a trip to Mars would take six months each way.

Not only is the space shuttle's limited future tied to the space station, so is its distant past.

"They've been joined at the hip for 36 years now," said John Logsdon, director of the Space Policy Institute at George Washington University.

The shuttle was designed in mind with a space station — which wasn't put in orbit until 17½ years after the first shuttle launch in 1981. The space station had a series of abortive fits and starts, including the spending of $11.2 billion on its first version, Space Station Freedom, without any parts of the station being built.

Recast in 1993, the new international space station finally got hardware in orbit in November 1998 as an international cooperative for science instead of an American outpost.

Some critics, like space analyst John Pike of Globalsecurity.org, say the 2010 deadline seems arbitrary and easily moved if shuttle problems come up. Griffin calls that idea "naive" and says the deadline is based on strict budgets.

The current space mission is considered vital to getting the station built and working following the long lapse of shuttle equipment and crew deliveries after shuttles were grounded following Columbia.

This flight will be the first time the station will have a third crew member since the accident that killed seven astronauts. And it will bring the first piece of equipment designed to allow the housing of six people. More manpower would enable scientific experiments in orbit, something that has been at a minimum with the tiny crew.

"This launch is the one that opens the door to resuming space station assembly," said Logsdon. "The pressure here is to finish assembly by 2010. You've got a firm back end."