# Weightlessness

Why are you weightless in space? I know that you are weightless in orbit because you are in free fall. On a trip to the moon, are you weightless because you are in free fall relative to the moon, or because you are too far from a sufficiently massive body?

Rob

On a trip to the moon for most of the time you are in free fall because there are no forces besides gravity that are controlling your motion. Only at the beginning and the end are the rockets used to control the motion of the space craft.

A version of that is my favorite question to make people look stupid.

Me: Why do astronauts float around when they are in space?

Them: Because there isn’t any gravity

Me: What hold the moon in orbit around the earth?

Them: Gravity, ummmm.

It is a good question. What if we had a rocket that flew as directly as possible to where the moon would be when it arrived? What would the gravity aboard the ship feel like?

Imagine you were inside a baseball. And I happened to be really really strong. I pick up the baseball and throw it hard enough that it hits the moon. Once the baseball leaves my hand you’ll be weightless until you hit the moon, disregarding some teensy marginal effects from being slowed while leaving the Earth’s atmosphere. And assuming you aren’t mashed to paste from the initial throw.

Unless there’s something stopping you, you’re ALWAYS in free fall no matter where you are in the solar system. You’re falling towards the center of the Earth, except you’re stopped by the Earth’s crust. Or, if you’re on an airplane, by the floor of the airplane. But any time you’re in a ballistic path, you’re in freefall whether you’re going up or down. So whenever you’re in orbit, you’re in freefall, whether that orbit is circular, elliptical, parabolic or hyperbolic.

As far as being in a rocket, if the rocket is thrusting you’ll feel “gravity” from the acceleration of the rocket. If you turn off the rocket motor you’ll be in free fall and won’t feel any gravity relative to the spaceship…you’ll both be falling at exactly the same rate. The only way you’d feel gravity is if the spaceship was falling at a lower speed than you were…possible if the atmosphere were really really thick, I suppose.

You only feel gravity here on earth because the surface of the earth is breaking your free-fall and “pushing back” on you (and then all the parts of your body are “pushing back” on those that lie directly above them, or are “pulling back” on those that are dangling, like your arms).

If your space capsule was near earth, but not in an actual orbit or not in free-fall, you’d feel the earth’s gravity in much the same way you do in an airplane once it reaches cruising altitude. To give you some prespective, the earth is about 6,400 km in radius. An airplane flies at about 11 km cruising altitude, so the % difference in gravity isn’t going to be noticeable. The International Space Station orbits at about 350 km above the earth’s surface, which is still only about 5% further away from the center of the earth. But gravity falls off like the inverse square of the distance, so you’d feel noticeably lighter on the ISS if you could somhow stop it and hold it still.

If you were floating out in space, without any thrusters on, then you’d always be in freefall towards something. But as soon as you hit the thrusters and started accelerating, you’d lose the weighless feeling and you’d be pushed against the aft of the ship, in exactly the same way that you are pushed against the back of your seat when you hit the accelerator in your car.

Okay. But at the risk of highjacking, why is it that things burn up when they come back through the atmosphere, but they don’t burn up when they leave the atmoshere? I’m thinking of things like Mir, etc. Are they protected on the way up?

I’m sure there’s an obvious answer, but it doesn’t come to me at the moment.