A great deal has been written and broadcast about the 'weightless' environment of space; much of it misleading and some of it just plain wrong. This has lead to the common belief that in space there is no gravity. If that is the case though, what makes satellites orbit the Earth and the Earth orbit the Sun?
One of the commonest images of space flight is of astronauts floating freely inside the International Space Station, or of them hanging in space whilst they work on the outside of the station. The general public and popular science often refer to this as ‘weightless’; scientists and engineers prefer to use the term ‘microgravity’
Put simply, microgravity is a local environment where gravity does not seem to act. There are many different ways available to scientists to produce microgravity and the durations and qualities vary a great deal. All of them however rely on letting gravity take hold of the local environment and allowing it to fall freely. The duration of microgravity can vary between just a few seconds and several months or even years. The quality can vary between around a thousandth and a millionth of the normal strength of gravity experienced in everyday life. Another important question for scientists is whether they can work with their equipment and samples shortly before, during and shortly after the microgravity period.
In short, microgravity is an environment where we can limit the everyday effects of gravity for scientific purposes.
Albert Einstein touched on microgravity experiments when he was working on his theory of General Relativity. He said that, so long as the volume we are dealing with is small enough, there would be no way of distinguishing between being in a rocket resting on the launch pad and the same rocket flying between the stars, with the engines running, accelerating it at a rate of 1 g (9.8 m s-2).
On Earth the rocket and the astronaut are both attracted towards the centre of the planet. Each and every atom is attracted separately, with a force that is proportional to the atom’s mass. This force is of course its weight. Both the rocket and the astronaut would be accelerated towards the centre of the planet at a rate of 9.8 m s-2 if were not that the rocket were sitting on the surface of the planet and so unable to move. The reaction from the ground acts only on the atoms of the astronaut’s feet, these atoms then support the next layer of atoms above them which in turn support the atoms above them and so on. The two sets of forces acting in different directions compress the astronaut and it is this squashing that is perceived as the effects of gravity.
In space the rocket is being accelerated forwards at 9.8 m s-2. As a result, each atom of the astronaut is accelerated forwards as well but only because it is pushed forwards, by the layer of atoms below them. The effect is the same as sitting on the launch pad; the astronaut experiences a vertical squashing that they perceive as the effects of (in this case artificial) gravity.
Let’s change the situation. Now the two astronauts are in spacecraft that are in free fall. The first rocket is in deep space, far away from the gravitational influence of any planet, star or galaxy. Other than for the tiny gravitational forces between the atoms of the space craft and the astronaut the situation is truly weightless. The astronaut does not feel any compression and so they correctly assume that gravity has no influence over them.
The second rocket is above the Earth’s atmosphere and is falling towards the planet, being accelerated at 9.8 m s-2. All of the atoms of the astronaut and the rocket are being accelerated at the same rate and so the astronaut does not experience any of the squashing that they did on the launch pad. Without looking out of a window they have all the experience of being ‘weightless’ and so they falsely assume that they are beyond the reach of gravity.
The message is simple; we can remove the effects of gravity on an object either by placing it in freefall or by recreating freefall conditions.