Weight, mass and weightlessness

What is weightlessness?

On board the International Space Station (ISS), astronauts appear to ‘float’: this is known as weightlessness or microgravity. However, at the ISS’s altitude (around 400 km), Earth’s gravitational field g is still close to 90 per cent of its value at ground level! Gravity has therefore not disappeared.

The explanation: perpetual free fall

Both the ISS and the astronauts on board are in free fall around the Earth, at the same orbital speed. As the astronaut and the station are falling ‘together’ at the same speed, the astronaut feels no contact force (no support) from the station: it is this absence of a sensation of weight that is called weightlessness, even though weight (the force of gravity) still actually exists.

Comparison: Earth, Moon, space

Situation	g (N/kg)	Sensation
Earth’s surface	9.8	Normal weight
Moon’s surface	1.6	Reduced weight, higher jumps
On board the ISS (~400 km)	~8.7	Weightlessness (free fall)

A Common Misconception

Weightlessness is not caused by an absence of gravity in space: this is a widespread but incorrect belief. It results from the fact that the object and its support are both in free fall at the same speed, and therefore no force acts between them. Far from any mass (very far from any planet or star), gravity would indeed become negligible, but this is not the case in low Earth orbit such as that of the ISS.