Question

Where the International Space Station orbits, the gravitational acceleration is just \(11.4 \%\) less than its value on the surface of the Earth. Nevertheless, astronauts in the space station float. Why is this so?

Short answer

Answer: Astronauts float inside the ISS because they are in a state of free fall, experiencing the same gravitational acceleration as the space station itself. This causes them to float and feel weightless relative to their environment, even though the gravitational acceleration in the ISS orbit is slightly less than on Earth's surface (8.68 m/s² compared with 9.81 m/s²).

Step-by-step solution

Review the concept of gravitational acceleration

Gravitational acceleration is the acceleration of an object due to the force of gravity. On Earth, the gravitational acceleration (often represented as g) is approximately 9.81 m/s². The value of gravitational acceleration varies depending on where an object is in relation to Earth, such as greater distance leading to a lower acceleration.

Define free fall

Free fall is a state in which an object experiences only gravity's pull and no other forces. In this state, all objects fall at the same rate, regardless of their mass. When an object is in free fall, people inside it will float as they are subject to the same acceleration, making them weightless relative to the object. Astronauts in the ISS experience free fall, which makes them float.

Calculate the gravitational acceleration on the surface of Earth

The gravitational acceleration on Earth's surface is known as a standard value, approximately: g = 9.81 m/s²

Calculate the gravitational acceleration in the ISS orbit

The gravitational acceleration in the ISS orbit is given as 11.4% less than on Earth. Therefore, we can calculate the gravitational acceleration in the ISS orbit (g_iss) using the following equation: g_iss = g - 0.114 * g = 0.886 * g Now, substitute the value of g: g_iss = 0.886 * 9.81 m/s² g_iss ≈ 8.68 m/s²

Explain why astronauts float in the ISS

As calculated previously, the gravitational acceleration in the ISS orbit is slightly less than on Earth's surface (8.68 m/s² compared with 9.81 m/s²). Although there is gravitational acceleration present, the ISS and its astronauts are continuously falling towards Earth in a circular path - they are experiencing a free fall. In this state of free fall, astronauts on the ISS feel weightless relative to their environment because they are subject to the same gravitational acceleration as the ISS itself. This ensures that they float inside the space station despite the presence of gravity.

Key concepts

Free Fall

Free fall is an intriguing concept that occurs when an object is solely under the influence of gravitational force, with no other forces acting upon it. This state causes all objects, irrespective of their mass, to fall at the same rate.
In a free fall scenario, the only force acting on the object is gravity, creating a sensation of weightlessness for anything within the object itself, such as astronauts aboard a spacecraft.
This doesn’t mean gravity is absent, but that its effects, such as pulling objects toward Earth, are not perceived because both the object and the observer fall together.

• Free fall means gravity is the sole force on an object.
• All objects in free fall accelerate at the same rate.
• Weightlessness is felt because the object and observer are falling together.

The International Space Station (ISS) experiences this concept continuously as it orbits Earth, allowing astronauts the unique experience of floating as everything within the station is in constant free fall around the planet.

International Space Station

The International Space Station (ISS) is a marvel of human engineering, orbiting Earth approximately every 90 minutes. Despite being 400 kilometers above Earth (248 miles), it is still under the influence of Earth's gravitational pull.
The ISS orbits the Earth at such a speed that it is in continuous free fall toward the planet but also moving forward fast enough to keep missing it. This specific balance of speed and altitude makes maintaining an orbit possible.

• The ISS orbits Earth every 90 minutes.
• It remains under Earth's gravity, despite being in space.
• Being in continuous free fall gives rise to the sensation of weightlessness.

The ISS provides a unique environment for conducting scientific research in a microgravity environment. This allows for experiments that would be impossible on Earth, such as observing how materials behave outside the influence of strong gravity.

Weightlessness

The phenomenon of weightlessness is fascinating and often misunderstood. It occurs when objects appear to have no weight, which is precisely what happens when something is in free fall, like in the International Space Station.
This sensation comes from the fact that both the object and the surroundings, like the ISS, are subject to the same gravitational acceleration, typically creating what we call a 'microgravity' environment.

• Weightlessness is felt when objects are in free fall.
• Gravity is present, but everything falls at the same rate.
• A 'microgravity' environment is created, making it ideal for scientific experiments.

Surprisingly, while the gravitational pull on the ISS is only slightly less than on Earth, this microgravity environment is perfect for scientific research, enabling astronauts to perform tasks and experiments that would be challenging under Earth's stronger gravitational forces.