# Why is there no gravity in space

## weightlessness

### Isaac Newton and the principle of gravity

Science falls into the lap of some people, or more precisely: just off the mark. At least if one can trust the following legend: According to this, Isaac Newton made one of the most important discoveries in physics around 1666 when he was sitting lazily under a tree in the garden.

Suddenly a falling apple startled him out of his thoughts. This experience is said to have inspired the then 23-year-old to his law of gravitation, which he presented years later in his "Philosophia Naturalis Principia Mathematica".

Newton had recognized that it is the same principle that drops the apple to the ground and keeps the moon in its orbit: all bodies attract each other. But this is only noticeable when a body has cosmic dimensions. The greater the mass of a body, the greater its gravitational pull (also called gravity). For example, the earth has six times the gravity of the moon.

The fact that the moon, as a smaller celestial body, does not crash onto the larger earth is due to its rapid movement around the earth. As a result, it develops strong centrifugal forces that counteract gravity: centrifugal force and gravity are balanced, the moon remains in its orbit.

### Gravity keeps us on the ground

In daily life we ​​feel gravity by staying on earth and not flying away. In contrast, all bodies that fall freely and without any self-acceleration (i.e. without rocket propulsion) under the influence of gravity are weightless: for example Newton's notorious apple - or we when we jump in the air. However, the duration of a jump in the air is too short for us to be aware of it.

Newton's law of gravity says that the further we get away from the earth, the earth's gravity gets weaker. A body that weighs 100 kilograms on earth would therefore weigh just 15 kilograms 10,000 kilometers from Earth.

But even astronauts at an altitude of 200 kilometers move weightlessly in their space capsule. At this height the gravitational pull of the earth is still 94 percent. The secret, in turn, lies in the satellite's fast circular orbit around the earth: The centrifugal force that arises in the process cancels the earth's attraction, the satellite and astronaut are weightless.

### Weightlessness on earth: trap towers and parabolic flights

Basically, the following applies: Gravity cannot be switched off because it is a property of all masses. The scientists can only compensate for the effects of gravity - by creating situations of free fall and thus simulating weightlessness.

In Bremen, for example, there is a 110 meter high drop tower that tests the reactions of various materials to the weightless state. The samples to be analyzed fall freely in an evacuated shaft and are weightless for a good four seconds.

However, many experiments require more time. In such cases, the scientists resort to sounding rockets. During a parabolic, propulsionless flight phase, the built-in samples experience weightlessness for up to 13 minutes. The experiments can be observed during the flight using video microscopes.

Scientists can also simulate some of the properties of weightlessness underwater. Space travelers therefore often train in the water before flying into space. The astronaut is balanced by small weights. The hydrostatic pressure on the body surface creates a balance: the astronaut floats.

### Happy but weakened: people in weightlessness

After their return to Earth, astronauts often report the tremendous feeling of happiness they felt during their weightlessness.

Basically, however, the human body is not made for floating: muscles and bones are so little stressed in weightlessness that many astronauts can hardly stand on their feet when they land on the ground. In order to prevent massive muscle wasting, the residents of a space station have to go on the treadmill every day.

In the first few days in space, many astronauts suffer from what is known as space sickness. Your sense of direction is massively disturbed: In the organ of equilibrium of the inner ear, tiny crystals exert pressure on the sensory hairs, which thus indicate the direction of gravity.

In weightlessness, the crystals can no longer exert this pressure: there is no longer any above or below. The organism reacts with vomiting and sweating.

However, after a few days, the body adapts - the symptoms disappear. From now on, the astronauts orientate themselves primarily visually, as their sense of balance no longer provides any useful information.

### Microchips and Tadpoles: Research in Weightlessness

Weightlessness affects not only people, but also a wide variety of materials. Scientists achieve certain effects by melting metal samples in space and mixing them with others: Lighter and warm materials no longer rise to the top. Metal parts with very different densities, for example aluminum and lead, can be alloyed.

So researchers can study the effects of intermingling that is impossible on Earth. This is particularly interesting for industrial processes. In weightlessness, for example, crystals grow faster and more regularly, which the computer industry could take advantage of: purer crystals result in better microchips.

Biologists also do research with plants and animals in space. For example, a group of scientists tested the development of balance in tadpoles in weightlessness.

### Doctors hope for inspiration from space

Doctors are interested in space research in two ways: On the one hand, it is about the treatment of astronauts in space. Heart massage or artificial respiration, for example, pose problems, as the patient and the treating person must first be fixed in the correct position.

And the difficulties increase with the duration of the expeditions: the crew of a Martian expedition would be on the road for more than two years without the possibility of an early return.

On the other hand, the doctors hope to gain knowledge about diseases on earth: many of the body's reactions to weightlessness are similar to diseases.

Take muscle wasting, for example: because the human skeleton needs regular vibrations, astronauts have to go on a shaking board. Perhaps one day it will be taken for granted that people who are particularly at risk of developing osteoporosis will climb such a board for a short time every day in order to prevent the disease.