And they don't. 'Gravity' totally distorts gravity. One of the key tension element in the movie is a marauding pile of space junk, left over from an explosion, that whips by every ninety minutes, threatening to kill the stranded astronauts. From a cinematic perspective, I get it that you need tension - but this would not happen. Here's why:
The orbit (direction, and altitude) of a spaceship , or anything else in orbit is the geometric sum of two components (vectors);
- Gravity, is causing it fall toward the Earth
- Inertia, that drives it forward in it’s current direction
In a stable orbit, the vectors sum to the shape of the orbit. This can be visualized as a rectangle, with one side as the thrust vector, the other side as the gravity vector, and the diagonal as the actual direction (vector sum).
The spaceship cannot control the magnitude of the gravity vector, but blasting the engines increases the inertia vector.
Accelerating, changes the relative effects of inertia and gravity, increasing the length of the rectangle, relative to the width … in effect raising the craft to a higher orbit (bigger circle, less curvature)
If two spacecraft had been orbiting side by side, then one accelerates, and moves to a higher orbit.
Because the rate of falling (gravity) is less, relative to the inertia,the time required for each orbit is longer, and (counterintuitively),from the perspective of the other craft, the faster craft will appear to fall behind. Taken to the extreme, if one of the craft accelerated enough, it would rise to the same orbit as the moon, and only circle the Earth once every 28 days.
Because the rate of falling (gravity) is less, relative to the inertia,the time required for each orbit is longer, and (counterintuitively),from the perspective of the other craft, the faster craft will appear to fall behind. Taken to the extreme, if one of the craft accelerated enough, it would rise to the same orbit as the moon, and only circle the Earth once every 28 days.
Unless something happens to slow the green spacecraft, it will remain at this higher orbit, well out of the way of the slower craft.
The movie ‘Gravity’ gets this all
wrong.
In order to add tension, the story has a
mass of space junk
whipping around every ninety minutes, and slamming into
the stranded astronauts (Presumably, the director chose a
ninety-minute cycle time because we baby boomers vaguely
remember that each of John Glenn’s orbits took about
ninety minutes).
whipping around every ninety minutes, and slamming into
the stranded astronauts (Presumably, the director chose a
ninety-minute cycle time because we baby boomers vaguely
remember that each of John Glenn’s orbits took about
ninety minutes).
This would not happen. If a nearby spaceship explodes,
and one is fortunate to avoid the initial blast, the danger is past.
Whatever detritus blows off in the same direction as the orbit
will accelerate, and move to a higher orbit, where it will not
be a problem. Anything that blows off in the opposite direction
from its orbit will descend to a lower orbit, consistent with its
net post-explosion velocity. And, anything that blows out sideways,
just goes off its own way.
and one is fortunate to avoid the initial blast, the danger is past.
Whatever detritus blows off in the same direction as the orbit
will accelerate, and move to a higher orbit, where it will not
be a problem. Anything that blows off in the opposite direction
from its orbit will descend to a lower orbit, consistent with its
net post-explosion velocity. And, anything that blows out sideways,
just goes off its own way.
The only way these every-ninety-minute
collisions could occur
would be if a giant scaffolding were erected to hold the stranded
astronauts in place, in the path of the space junk.
And this scaffolding would have to be moved about 1,000 miles
east every hour to compensate for the rotation of the Earth
around its axis.
would be if a giant scaffolding were erected to hold the stranded
astronauts in place, in the path of the space junk.
And this scaffolding would have to be moved about 1,000 miles
east every hour to compensate for the rotation of the Earth
around its axis.
