At what point to astranaughts become 'weightless' when flying directly away from Earth (ie not orbiting)

Question

This is the second part of my previous question.

N.B. weight and mass are two different things JeffE

Answer 1

You're mistake here is assuming that when the astronauts go to the moon (and Mars in the future) they are not in orbit... They actually are...

When in orbit around the Earth the orbit is relatively circular (doesn't have to be, but most satellites are, as these are the most stable), however, when going to the Moon or elsewhere, a burn is needed to enter into a much more elliptical orbit... They are still in orbit around Earth until they reach the Moon, where they burn to circularize into a Lunar orbit.

Gravity is everywhere, and all satellites and spacecraft use orbits about some central body to reach destinations. For inter-system missions satellites enter Heliocentric (sun-centered) orbits and use other planets' gravity in some cases for "gravity-assist" fly-bys, where they do not actually enter the planets orbit. Instead, they use the planets gravity to increase the speed of the craft without using fuel. These "gravity-assists" are actually orbits themselves... however they are called hyperbolic orbits as they simply "curve" the orbit around the planet. This is because the spacecraft is usually already going fast enough such that it exceeds the "escape velocity" of that planet.

escape velocity is defined as

Ve = sqrt(2*mu/r)

where, mu = Gm (gravitational constant * mass of the central body)
r = distance at point of closest approach

Answer 2

OK, we're not orbiting; just flying straight away from Earth and stopping here and there to check our weight. The Earth's radius is about 4,000 miles. At an ALTITUDE of 4,000 miles (assuming the spaceship has stopped moving momentarily), the astronauts will weigh 1/4 as much--g, the force of gravity, falls off as the square of the distance from the center of gravity of the Earth. At a height of 12,000 miles, they will weigh 1/16th as much (total of 16,000 miles to the Earth's center). And so on. I'm ignoring the gravitational pull from the moon, sun, etc., which would only slightly affect the numbers when we're close to Earth.

Keep in mind that to perform these "weighings," the rocketship will have to act as a "hover craft," doing a burn to keep it at a fixed distance from Earth (or constant velocity) and not starting to fall back. The amount of force required to hover will be exactly the weight of the ship and its contents AT THAT ALTITUDE.

Einstein said that experiencing acceleration was indistinguishable from feeling the force of gravity. So anytime the spaceship is accelerating, the astronauts are going to "feel" some extra weight.

There IS gravity in a vacuum. Gravity is the weakest force at "close range" but the strongest over long distances, and cannot be blocked or shielded.

There are points in space where there is gravitational pull in opposite directions which effectively "cancels" most of the gravity. If the astronauts can find one of those spots, they'll be about as weightless as possible. Keep in mind that they are still exerting gravitational pull on each other, the spaceship, packets of Tang and dried beef, etc.

See also - Lagrange Points, and the Solar and Heliospheric Observatory Satellite

Answer 3

Contrary to previous answers, weightlessness isn't related to "leaving the Earth's gravitational field", which reaches far, far beyond orbital height (the body couldn't orbit otherwise!). An orbiting object is effectively free-falling round the earth, and when falling, you offer no resistance to the gravitational pull, and hence feel no "weight". You can also experience weightlessness much closer to earth; a plane flies to a great height and then nose-dives in free-fall. People in the plane start to float - astronauts get initial experience this way.

So to answer your initial question, you don't experience weightlessness until the spaceship goes into orbit. This doesn't have to be orbit round the Earth; going to the Moon is also "falling", so astronauts were weightless on the way.

Answer 4

Astronauts are never truely 'weightless' since they are still well within the earth's gravity well. It's just that as they 'fall' around the earth, everything else is falling at the same rate, so it appears to be unaffected by gravity. Jump up on a trampoline and just as you reach the maximum height release a baseball from your hand. The ball will fall at the same rate you do, thus appearing to be weightless RELATIVE TO YOU. By the way, in this solar sytem you are always in the gravity influence of some planet. The solar system truly ends not at the orbit of Pluto but at the heliopause, that place where our sun's solar wind is cancelled out by the solar wind from other stars. That's a long, long, ways out there.

Answer 5

They are never weightless but just like you can do on earth in a free falling plane or when your are going down on a roller coaster you are weightless. When the astronauts are in Space they are weightless because they are in free fall. They would not be able to orbit the earth if there was no gravity, they are using the earths gravity to stay in orbit, therefore not having to fire there OMS Engines at all times to stay on orbit.

Answer 6

By definition they are never actually 'weightless' the earth's gravitational field extends out into space infinately, as does the gravitational field of every object in the universe. However, in reality they begin to 'feel' weightless once they're about 80-100 miles away from the earth's surface

Answer 7

They become weightless at the very moment when their spaceship stop using it's enginess i.e. stops accelerating. This can be at 100 km or at 1000000000000 km. When the engines are turned off, the spaceship start "falling" together with the astronauts to what ever direction the gravitional fields are pulling them.

Though they are weightless, they still have their mass, which means they have their inertia.

Answer 8

Gravity is just a force applied to the body. Other forces have the same effect, like acceleration (+ve or -ve).

So you begin to feel 'weightless' once you reach a constant velocity, and are not subject to either gravity or acceleration.

Answer 9

Someone becomes weightless only when no other forces than his weight acts on him and I'm sure for that..Why do you want to know the exact position anyway??=p

Answer 10

They never become weightless. They just leave a gravitational field.