Why Do Astronauts Float in Space? (It’s Not Zero Gravity)

Short answer: astronauts do not float because there is no gravity in space. At the altitude of the International Space Station, about 400 km up, gravity is still about 90% as strong as on the ground. Astronauts float because they are falling, all the time, together with their spacecraft. The station moves sideways so fast, about 28,000 km/h, that as it falls toward the Earth, the curved surface of the planet keeps falling away beneath it. Falling without ever hitting the ground: that is what an orbit is.

This is one of the most common misconceptions in science, and the real explanation is far more interesting than "zero gravity". It goes back to an idea Isaac Newton had more than 300 years ago.

Is there gravity in space?

Yes, everywhere. Gravity gets weaker with distance, but it never switches off. It decreases with the square of the distance from the centre of the Earth. The Earth's radius is about 6,370 km, and the space station orbits about 400 km above the surface, so it is only about 6% farther from the Earth's centre than you are. Do the calculation and gravity there is still roughly 89 to 90% of what you feel standing on the ground.

In fact, if gravity were absent, the station would not orbit at all. It would fly off in a straight line into deep space. It is gravity that keeps it, and the Moon, circling the Earth. To understand what gravity really is, read The Great Cosmic Trampoline: How Einstein Reimagined the Force of Gravity.

Weightlessness is what falling feels like

Here is the key idea. What you call your "weight" is not gravity itself. It is the push of the floor, or your chair, against your body, stopping you from falling. Take that support away, and you feel weightless, even though gravity is pulling on you as strongly as ever.

You have probably felt a hint of this already: the stomach-lifting moment at the top of a roller coaster, or when a lift starts going down quickly. Now imagine you are in a lift whose cable snaps. You, the floor, and everything around you would fall at exactly the same rate. Your feet would no longer press on the floor. A pen released from your hand would hang in the air beside you. Inside the lift, you would be floating, for a few seconds at least.

This is why physicists prefer the term microgravity, or "free fall", to "zero gravity". Astronauts and their station are falling together, so nothing presses on anything. Albert Einstein called the realisation that a falling person does not feel their own weight "the happiest thought of my life". It became the starting point of his theory of general relativity.

Newton's cannonball: why the station never hits the ground

If the space station is falling, why doesn't it crash? Isaac Newton answered this with a famous thought experiment.

Imagine a cannon on top of a very high mountain, firing horizontally. A slow cannonball curves down and lands not far away. Fire it faster, and it lands farther away. Now remember that the Earth is round: its surface curves downward by about 5 metres for every 8 kilometres you travel. If you fire the cannonball fast enough, about 7.9 kilometres per second near the surface, it falls by 5 metres during each 8 kilometres of flight, exactly as much as the ground curves away beneath it. It keeps falling, but it never gets any closer to the ground.

That is an orbit. The space station travels at about 7.66 km/s, roughly 27,600 km/h (17,100 mph), and goes around the Earth in about 92 minutes. Its astronauts see around 16 sunrises and 16 sunsets every day. They are not escaping gravity; they are falling around the planet, endlessly.

Can you experience weightlessness without going to space?

Yes, for short periods. Space agencies train astronauts on special aircraft that fly in giant arcs called parabolas. The plane climbs steeply, then cuts its thrust and follows the same curved path a thrown ball would. During each arc, the plane and everyone inside are in free fall, and they float for about 20 to 25 seconds. A single flight repeats this dozens of times. The aircraft was nicknamed the "Vomit Comet", and the name tells you how many passengers feel.

Many film scenes set in weightlessness were shot this way, including scenes from the film Apollo 13, filmed in real free fall aboard such an aircraft.

What weightlessness does to the human body

Our bodies evolved under constant gravity, and months of free fall change them in surprising ways:

• Bones weaken. Without the load of body weight, astronauts can lose about 1 to 1.5% of the bone mass in their hips and spine per month.
• Muscles shrink, especially in the legs and back, which no longer have to hold the body up. Astronauts exercise about two hours a day on special treadmills, bikes and resistance machines to limit the loss.
• Fluids move upward. On Earth, gravity pulls blood and other fluids toward your feet. In orbit, they spread toward the head, giving astronauts a puffy face and thinner legs, sometimes called "bird legs".
• Astronauts grow taller. Without weight compressing the spine, they can gain up to about 3% of their height, around 5 cm, during a mission. They shrink back after returning to Earth.
• The sense of balance is confused. The inner ear relies on gravity to tell up from down, so many astronauts feel motion sickness during their first days in orbit.

Is it really "zero" gravity anywhere?

Not quite. Even far from the Earth, the gravity of the Sun, the Moon, the planets and the galaxy is still there. A spacecraft travelling to Mars is not free of gravity: it is in free fall around the Sun. Any object in space that is not firing its engines is falling along some path, so its occupants feel weightless. That is why "microgravity" is the more accurate word: tiny residual effects, such as a little air drag or the station's own small gravity, remain.

Frequently asked questions

Why do astronauts float if there is gravity in space?
Because they are in free fall together with their spacecraft. Nothing pushes against their bodies, so they do not feel their weight, even though gravity is about 90% as strong as on Earth's surface.

How fast is the International Space Station moving?
About 7.66 km per second, or around 27,600 km/h (17,100 mph). It completes one orbit around the Earth in about 92 minutes.

How high do you have to go to escape gravity?
There is no altitude where gravity stops. It weakens with distance but never reaches zero. To leave the Earth for good, a spacecraft needs to reach the escape velocity, about 11.2 km/s from the surface.

Do astronauts feel weightless on the Moon?
No. Standing on the Moon, astronauts feel their weight because the ground pushes back on them, but lunar gravity is only about one-sixth of Earth's. That is why they could jump and bounce so easily.

Would you float in a falling lift?
Yes, while it falls freely. You and the lift would fall at the same rate, so you would float inside it. Unfortunately, the landing would end the experiment.

The bottom line

Space is not free of gravity. Astronauts float because they are falling, and they never land because they are moving sideways fast enough to keep missing the Earth. Newton imagined it with a cannonball, Einstein built a whole theory on it, and every astronaut on the space station lives it every day. It is one of the most beautiful ideas in physics: an orbit is simply a fall that never ends.

Curious about another surprise of life in space? Read How Cold Is Space? Why Space Is Freezing Even Though the Sun Is Hot.

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Written by Imad El Bojaddaini, university professor of physics. Physics Voice explains the strangest ideas of physics in plain language.