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).