# Why do astronauts float in ISS? |

Before answering the question why astronauts float in the International Space Station (ISS), it is necessary to specify certain terms of physics, in a simple way.
**Gravity** is the force of the gravitational field that pulls us towards the center of the Earth, this term refers to the planet Earth. it is also the force that gives weight to any physical body.
**Gravitation** more generally refers to this force or interaction between any two massive celestial bodies. But these two terms are identical.
**Weightlessness** is the almost complete absence of the sensation of weight in contrast to gravity.
**Microgravity** characterizes a very weak gravitation felt by an object located very far from any massive body or in a corner of space where the gravitational influences of several massive bodies cancel each other out as on Lagrange points.
**Zero Gravity** or **Zero G** does not exist because gravitation pervades all of space up to the large-scale structures of the universe. The experience of "zero gravity" is an illusion that gives the impression that gravity disappears. But in reality it is the feeling of weight that disappears when one is in free fall. Indeed, if you fall in free fall in an elevator whose cable has been cut, the two bodies fall at the same speed and neither is resting on the other. If the two bodies fall, it is precisely because of gravity. What is the physical phenomenon that allows astronauts to float in the international space station? Gravity measures the acceleration of an object in free fall on the Earth's surface. | | Newton's second law which is expressed by F=ma says that the force of gravity (f) is equal to the mass (m) by the acceleration (a). In this equation, a force of 1 newton acting on a mass of 1 kg, accelerates by 1 m/s^{2}. Thus the force of gravity is about 9.81 newtons for a mass of 1 kg. However, gravitation and the acceleration are two aspects of the same force (F=ma and F=mg ⇒ g=a). Gravity is expressed by g_{0}=9.81 m/s^{2} or an acceleration due to gravity (at sea level at 45° latitude). Contrary to the weight, the mass of a body (expressed in kg) that which intervenes in the calculation of the gravitational force g, does not change according to the star where the body is located. It is g which decreases as a function of the square of the distance which separates two bodies. Our international space station is traveling in low orbit located approximately 415 km from the Earth's surface. This distance is very small and the gravity at this altitude (g=9 m/s^{2}) is practically the same as on the ground (g=9.81 m/s^{2}). Thus, the astronauts on board the International Space Station find themselves in a state of weightlessness but not of microgravity. ISS and astronauts are in free fall around Earth. This is why astronauts float on the International Space Station.
*NB: The Earth is a reference for other celestial objects where the gravitation is different. Thus the terrestrial gravity is equal to 1 concerning the other objects, this is why we confuse the mass and the weight (p=mg). * | | ### Image: The International Space Station (ISS) is placed in low Earth orbit, at an altitude of approximately 415 km, it circles the Earth in 90 minutes at a speed of 7.7 km/s or 27,700 km/h. The international crew, which is dedicated to scientific research in the space environment, sees about 16 sunrises and sunsets every day. Astronauts aboard the International Space Station are in a state of weightlessness but not microgravity. ISS and astronauts are in free fall around Earth. This is why astronauts float on the International Space Station. |