Gravity is one of the four forces of nature, along with the electromagnetic force and the strong and weak nuclear forces. It is an invisible force that is all-pervading. Perhaps one of the most impressive scientific breakthroughs of all time was Newton's universal law of gravitation, which is not only valid for falling apples but also for orbiting objects like the planets, moons and space vehicles. Newton concluded that there is a force of attraction between any two objects in the universe and the magnitude of this force (F) is given by:
Mass is an intrinsic property of an object which is linked to the amount of matter it contains. The mass of an object is therefore the same wherever it is in the universe. However, the weight of an object is a result of gravity and therefore is dependent on its environment. When we measure the weight of an object on Earth it is in fact the gravitational force of attraction between the object and the Earth that is being measured. The best example of the difference between mass and weight can be seen in the film footage showing astronauts walking on the Moon.
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On the Moon, the acceleration due to gravity (g) is 6 times lower than on Earth. So although astronauts have the same mass their weight was 6 times less. This explains why they were actually jumping rather than walking on the moon !
To get a better understanding of gravity and weightlessness, we must now focus our attention on what happens when you let an object fall freely. When an object is in continuous free-fall and there are no external forces acting on it, then the object can be described as being weightless. In this case, all gravity effects disappear and only the internal forces inside the object remain. Free-fall can be implemented in ground-based facilities called drop towers, in special aircraft which fly parabolic flights, as well as in sounding rockets.
From this equation, it can be seen that at an altitude of 400 km above the Earth's surface a stationary object would still have about 89% of its terrestrial weight. An adult male with mass 80 kg (or weight 785 Netwons) would appear to weigh about 71 kg (actually 699 Netwons), which is far from being weightless !
The trick to achieving weightlessness is to propel an object using a rocket engine with an initial velocity parallel to Earth's surface, while simultaneously allowing the object to fall freely. In this way, the object will be in continous free-fall but will never hit the Earth. At a particular velocity, the trajectory of the object becomes a circle, and this is known as a circular orbit around the Earth.
The diagram below shows that when our ESA astronaut jumps from the hypothetical skyscraper (i.e. above the atmosphere) with a side-velocity of about 28 000 km/h, he will be in continuous free-fall (Jump C). In the cases of Jumps A and B, our astronaut does not have sufficient velocity to make a full circle and, sadly for him, he ends up crash-landing back to Earth.
These three forces are inherent to orbital flight and cannot be totally avoided. In addition to these perturbations, there are often disturbances from the spacecraft itself, as a result of rotation and altitude re-boosting. Other perturbations, known as g-jitters, can come from the movement of mechanical parts (e.g. ventilation fans, closing doors) and crew activities (e.g. astronauts doing their exercises !).
Great care must therefore be taken, during both the design phase of orbiting spacecraft and the in-orbit operations, to minimise those accelerations which will disturb the microgravity environment. Measurements of gravity aboard the International Space Station have shown that g is approximately 1 millionth of that on Earth, which is where the term microgravity comes from (scientifically speaking micro means one millionth or 10-6). This is why astronauts are able to float around so effortlessly, as can be seen in the following movie of Ulf Merbold taken aboard the former Russian space station, Mir.
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