Weightless in a “Zero G” Environment?
With Commander Chris Hadfield returning home from the ISS this week, there has been a lot of talk about astronauts getting “used to gravity” all over again after being “weightless” in a “zero G” environment. Astronomers and space scientists use these terms in a specific way, and not necessarily the way in which they are used in the popular press.
First – I almost hesitate to mention this because it’s pretty obvious, except that, well, one reads this all the time – it’s just plain wrong to say that there is no gravity in space. Yes indeed there is gravity in space. That’s what keeps objects – such as the Moon and spacecraft, for example – in orbit around planet Earth, that keeps the planets in orbit around the Sun, and that keeps the Sun and other stars in orbit around the centre of our Milky Way galaxy. If there were no gravitational force in space, all moving objects – such as the Moon, spacecraft, planets, comets, stars, etc. – would move in straight lines according to Newton’s first law of motion: “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.” But such objects don’t move in straight lines; they follow curved paths under the combined force of gravity exerted by other objects in their vicinity (Einstein preferred to describe massive bodies as causing a curvature of space, rather than as exerting a “force” that we call gravity, but that’s beyond my little pea brain to comprehend).
The force of gravity diminishes as the square of the distance from the centre of mass of the attracting body. At the altitude of the ISS, about 410 km above Earth’s surface, the force exerted by Earth’s gravity is about 12% less than we experience here on the surface. By comparison, at the top of Mt. Everest the force of gravity is about 0.3% less than at sea level, and at the distance of the Moon the gravitational force exerted by Earth – which keeps the Moon in orbit around Earth – is only 0.028% as strong as on Earth’s surface.
So yes, there certainly is gravity in space. What people mean – or should mean – is that an orbiting body, such as a spacecraft or a person floating inside a spacecraft, feels weightless (i.e. is freely floating). This is because the object or person is constantly being pulled – or “falling”, as we call it – toward the attracting body, such as Earth, at a rate that is exactly matched by the orbital speed of the body or person. So “weightlessness” occurs when there is nothing preventing a body from freely moving toward an attracting body under the influence of its gravity. By contrast, when we are standing on the surface of our home planet, it’s the surface itself that exerts a force on us and that prevents us from falling toward the centre of the planet under the influence of gravity. The feeling of force on our bodies when we are standing on Earth’s surface is the sensation that we call “one G“.
Another example of weightlessness under the force of gravity was last year’s jump by Felix Baumgartner out of his capsule at the so-called “edge of space”. When he jumped, what caused him to fall? Well, it was Earth’s gravitational force. But until the atmosphere through which he was being pulled became dense enough to cause measureable resistance, he felt weightless, even as gravity was accelerating him to greater speeds. When someone jumps off a diving board into a pool, or off a chair onto a floor, the person briefly feels weightless, because s/he is freely falling under the force of gravity.
So when we say “one G“, what we mean is the feeling that we experience when we are standing on Earth’s surface, when gravity is exerting a force on us, but we are not moving because the surface of the planet is exerting an equal force against our bodies in the opposite direction. When astronauts talk about “zero G“, they do NOT mean that there is no gravity acting on them. What they mean is that they feel weightless because they are freely falling in orbit around Earth, under the influence of Earth’s gravity.
So “zero G” does not mean the absence of gravity! It means the feeling of weightlessness that is the result of being pulled, or “falling” freely, because of gravity.
Here is a decent short article discussing this in slightly different words: