Friday, March 25, 2011

Falling vs. Gravity

When you launch something into orbit … you have launched it, via rocket thrust, so powerfully fast and high and far that when gravity’s pull finally slows the object’s forward progress enough that it starts to fall back down, it misses the Earth. It keeps on falling around the Earth rather than to it. As it falls, the Earth’s gravity keeps up its tug, so it’s both constantly falling and constantly being pulled earthward. The resulting path is a repeating loop around the planet.

I don’t get it.

I mean, “falling” in this instance is something different from gravity’s “tug”? If I jump out of an airplane I will fall to Earth, right? I thought I was falling because of gravity’s tug, because I was being drawn toward the center of greatest mass. The orbiting body is falling away from the Earth (“around the Earth”?) yet being constantly tugged back toward it? “It starts to fall back down,” Mary Roach says. What prevents the object from finishing what it started?

If gravity is strong enough to slow the launched object’s forward progress, why isn’t it strong enough to pull that object back home?

How can an object “miss” the Earth? That’s a pretty big target, especially right up close. Has a barn door beat by orders of magnitude.

Mary Roach is attempting an explanation in layman’s language, avoiding math, which, admittedly, I wouldn’t understand either, but what does “falling” mean here? Although a gravity-free experience is often termed “free fall,” what does falling mean if there is no destination for the fall? If there were no atmosphere (with all its buffeting) would your fall toward and ultimately onto Earth be a different experience from gravity-free falling? If you weren’t looking toward it would you know you were falling toward anything?

source: Packing for Mars: the curious science of life in the void by Mary Roach


Art Durkee said...

Basically, it's like this:

If you throw a ball with sufficient enough force, velocity overcomes gravity, and the ball goes into orbit.

An orbit is a balance-point between gravitational attraction and velocity.

The velocity needed to stay in orbit changes with altitude, because gravitational attraction is subject to the inverse-square law. The force of attraction is the inverse of the square of the distance. in other words, the gravity you feel is a lot less 3000 miles above the surface of the planet than it is 300 miles above the planet. A more distant orbit will be longer and slower than a near-earth orbit, but it will also use power to maintain its balance and position.

Art Durkee said...

Should read, it will use LESS power to maintain. . . .

Glenn Ingersoll said...

thanks, Art ... and you didn't use the word "falling" once!

the velocity that gets you off planet does not dissipate, right? in frictionless outer space, once pushed, an object will keep going forever ... whether it's falling, rising or following its nose.

if you had a tall enough staircase could you walk into orbit?

Art Durkee said...

Near-Earth space is not actually 100 friction-free. There's the upper fringes of the atmosphere, and also the solar wind. Both of these, although very tenuous, do have a cumulative effect over long periods of time. So satellites that have been in orbit a long time can have their orbits degrade after some time, because of the small forces involved.

There is a critical threshold called escape velocity, which is the speed required to escape Earth's gravitational field entirely. To get our astronauts to the moon, they had to reach escape velocity, while setting their vector for the Moon, so that when they arrived they could insert into orbit. Then return. Remember, NASA did all these calculations with room-size computers that had less computing power than your current cellphone or laptop!

If you achieved escape velocity in a direction that had no objects to run into, yes, you would indeed keep going with basically no further velocity changes. The further you ride out from the Sun, the thinner the solar wind gets, and the purer the vacuum becomes, reducing friction effectively to zero. There is also a velocity threshold for the solar system (solar escape velocity), and you just keep going for a very, very long time. The two Voyager probes have now passed outside the limits of our solar system, and are now in interstellar space.

As for walking into orbit, there is in fact a proposed construction called a space elevator. It's basically a cable run between the ground and a satellite in geosynchronous orbit. Very good SF novels written around space elevator technology have been written by Arthur C. Clarke and Charles Sheffield; both of these SF writers are "hard SF" writers, and you can count on the science in their books being completely authentic, if extrapolated into the future from existing science.

Basically, you would ride a cable car up the cable into space. The thing with space elevators is, they're energy-efficient. They don't require the fossil fuel expense of rocket launches, or re-entry vehicles. Re-entry isn't a problem of brining up when entering the atmosphere, because it's a slow entry, not a quick fall.

You see, there are advantages to being a science fiction geek. :)