NASA Shuttle Endeavour docking delay by Earth's gravitational pull

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.