Astronauts aboard space shuttle Endeavour had to wait an uncommonly long time to board the International Space Station. NASA blamed the orbital holdup on no less than the entire planet Earth actually, its gravitational pull.
Endeavour docked at the space station Wednesday morning at 12:06 a.m. EST (0506 GMT), but it took nearly an hour for the two massive spaceships to form a secure connection using a complicated set of hooks and latches. Normally, the spring-loaded docking rings between the station and a visiting shuttle dampen out the vibrations associated with their rendezvous rather quickly, and align themselves automatically.
But not so during Endeavour's arrival. It took about 45 minutes for the relative motion to damp out. Only then could the docking systems align and pull both spacecraft together.
Space shuttle flight director Kwatsi Alibaruho said the orbital oddity was actually caused by two things: The massive bulk of the joined space station and shuttle, and the constant tug of Earth's gravity.
With Endeavour docked at the International Space Station, the complete "stack" as NASA calls it weighs more than 1 million pounds (453,600 kg).
But unlike small round satellites or, say, the moon, the docked space shuttle and station aren't symmetrical, and therein lies the problem, Alibaruho said, because external forces can affect a shuttle docking.
"One of the external forces is, really, the Earth's gravity," Alibaruho told reporters early Wednesday.
While the 11 astronauts aboard Endeavour and the space station are floating in apparent weightlessness, there is actually gravity in space.
On Earth's surface, the planet's gravitational pull is 9.8 meters per second squared (32 ft/s2). At the orbit of the space station, about 220 miles (354 km) above Earth, the force is still about 90 percent that of its strength on the surface.
Spacecraft counter the downward pull of Earth's gravity by moving horizontally fast. Really fast. They zoom around Earth at 17,500 mph, or nearly Mach 25.
The space station is a massive marvel of engineering that has the interior living space of a Boeing 747 jumbo jet and an exterior structure that is as long as a football field.
But it has modules jutting out up, down and sideways. Add a 100-ton space shuttle on the end and you've got a lack of symmetry, in which the pull Earth's gravity has a different effect on different sections based on their angle and distance from Earth.
"This creates what we call a gravity gradient," Alibaruho said. "It's a very, very small force, but with an object as massive as the space shuttle and as massive as the space station, it's significant enough to slow the process."
Typically, visiting shuttles dock in front of the station, then the entire shuttle-station stack swings around so the shuttle's fragile heat shield is not facing the direction of travel.
So while Endeavour's six astronauts had to wait a bit longer than normal to join their comrades inside the space station, the delay was one NASA knew could happen.
"It's a lot less common, but it's certainly not unexpected," Alibaruho said.
Endeavour's crew, commanded by veteran spaceflyer George Zamka, is flying a 13-day mission to deliver a new room and observation deck to the International Space Station.
The astronauts launched Monday and will spend eight days linked to the station, during which time they'll perform three spacewalks to install the new additions. Endeavour is due to return to Earth on Feb. 20.
The mission is the first of NASA's five final shuttle missions before the orbiter fleet is retired in the fall.
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SPACE.com is providing complete coverage of Endeavour's STS-130 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz based in New York. Click here for shuttle mission updates and a link to NASA TV.