28 October 2005

Gravity in space

This is an old one I started a while ago but didn't finish...

China's Shenzhou VI spacecraft is not orbiting exactly as planned and will have to be restored to its original trajectory, state-run media say.
The "orbit maintenance operation" would take place early on Friday morning, said official news agency Xinhua.

Gravity has drawn Shenzhou VI too close to earth, the agency said.

Shenzhou VI, which has two astronauts on board, is in a low enough orbit to be affected by the Earth's gravitational pull.


Gravity acts over infinite distance. It does get weaker as you get further from something, but it never goes away entirely. For the geeks, the formula is F=GMm/r^2, where F is the force of gravity, G is the gravitational constant, M and m are the masses of two objects (typicall M is the bigger, like Earth, and m is the smaller, like Shenzhou VI), and r is the distance between the two. Note that as r gets bigger and bigger, F gets smaller and smaller, but you can never get to F=0.

Astronauts in the International Space Station, or onboard a Space Shuttle aren't floating because there's "no gravity," they're actually falling towards the Earth as fast as they're going around it. Think of it like if you're in an elevator and the cable was cut. As both you and the elevator fall, you're "floating" inside the box, but gravity is DEFINITELY pulling you. This state is called free fall.

As for Shenzhou VI, they probably meant either it wasn't in a stable orbit to start, or else the tenuous bits of atmosphere are causing some friction which slows it down and decays the orbit.

1 comment:

Bones of Time said...

The expansion of the gravitational force is the inverse square thought, but the origin of this force is in the core point of a mass, which is why the inverse square concept works. You can chect out the thought on timebones.blogspot.com