Why Don't We Float Away?

Earth is a giant ball spinning at over 1,600 km per hour at the equator, and zooming around the Sun at 107,000 km per hour. So why dont we get flung off into space? The simple answer is gravity. Earth has so much mass that its gravitational pull holds us, our atmosphere, our oceans and everything else firmly to the surface. The faster we try to move, the harder gravity pulls back, keeping us comfortably stuck to our planet.

• Earths mass5.97 x 10^24 kgAbout 6 thousand billion billion tonnes
• Earths spin speed1,670 km/h at equatorSlower nearer the poles
• Orbital speed107,000 km/hAround the Sun
• Earths gravity9.8 m/s2Strong enough to hold an atmosphere
• Escape velocity11.2 km/sSpeed needed to leave Earth forever
• Centripetal effect at equatorTinyCuts gravity by just 0.3%

Earths grip on us

Earth contains an enormous amount of matter: about 6,000,000,000,000,000,000,000,000 kilograms. That huge mass produces a gravitational field that pulls everything nearby (including you, the air, the oceans and the Moon) towards the centre of the Earth. The force is strong enough that even an Olympic high-jumper or a rocket needs serious help to break free.

Why spinning does not throw us off

You might think that because Earth is spinning so fast, we should be flung outwards (like clothes in a spin dryer). There is indeed a tiny outward effect, called the centrifugal effect, but it is very small.

At the equator, where Earths spin is fastest, this outward effect reduces your apparent weight by only about 0.3 per cent. So a 50 kg person at the equator weighs only about 150 grams less than the same person at the North Pole, where there is no spin effect.

Earths gravity is more than 300 times stronger than the outward spin effect. You stay firmly on the ground.

Fact Even though we are spinning at over 1,600 km/h, we do not feel it because the air, the ground and everything else around us is spinning at the same speed. There is no relative motion, so no breeze. You only notice the Earths spin in subtle ways: the way storms swirl (the Coriolis effect), the way a Foucault pendulum slowly rotates and the way the stars seem to wheel across the sky at night.

Why we dont float into space

For you to leave Earth permanently, you would need to reach the escape velocity of about 11.2 km/s (40,000 km/h). That is roughly 25 times faster than a passenger jet. Walking, running, jumping or even most aircraft come nowhere close. Only a rocket can build up enough speed to escape.

Even when you jump, gravity quickly pulls you back down before you can climb very high. Olympic high jumpers reach only about 2.4 metres at their peak. World-record long jumpers cover about 9 metres horizontally before gravity wins.

What about astronauts in orbit?

If gravity is so strong, how can astronauts on the International Space Station float around? The answer is that they are NOT floating because there is no gravity (Earths gravity at the ISS is still about 90 per cent as strong as at the surface). They are floating because they are in free fall.

The ISS moves so fast sideways (about 28,000 km/h) that as it falls towards Earth, Earth curves away beneath it at the same rate. The result: the ISS keeps falling forever, never landing. The astronauts inside are also falling, so they have no sense of being pulled. This is weightlessness, and it is the same feeling you would briefly have at the top of a big drop on a roller coaster.

Did you know? If Earth suddenly stopped spinning, anything not attached to the ground at the equator would be flung sideways at over 1,600 km/h: stronger than the strongest hurricane. The atmosphere would keep spinning even after the ground stopped, producing massive winds. Fortunately, Earths spin is rock-steady (it slows only by about 1.7 milliseconds per century), so we are perfectly safe.

Why does the atmosphere stay too?

The air around you is made of trillions of fast-moving gas molecules. They would happily fly off into space, except that Earths gravity keeps them gently pulled back. Hot molecules at the very top of the atmosphere occasionally have enough energy to escape (especially the very lightest, hydrogen), but the heavier ones like nitrogen and oxygen cannot.

Smaller worlds with weaker gravity have lost their atmospheres or never built them up. The Moon has almost no atmosphere. Mars has a thin one. Mercury has almost none. Only worlds at least as massive as Earth tend to hold onto a substantial atmosphere.

The oceans too

The same gravity holds down all the worlds water. The oceans cover 70 per cent of Earths surface to an average depth of 3.7 km. They stay in place because every drop of water is being pulled down by gravity. The tides rise and fall by only a few metres because of the Moons gentle additional pull (see the tides).

Try this Stand on a set of bathroom scales and read your weight. That is the force of Earths gravity pulling on you. Now jump up and (carefully) check the scales again as you land: for the briefest moment, the reading shoots up because you are accelerating downwards. Astronauts experience this effect on take-off, when the engines push them up at multiple times Earths gravity. On the way back down, they feel the opposite at re-entry: their organs feel like they weigh many times their normal amount.

Deeper dive: what if Earth was much smaller or much bigger?

If Earth had been much smaller (say the size of the Moon), its weaker gravity would not have held onto a substantial atmosphere. Air molecules would have leaked out into space within a few million years. Life as we know it could probably not exist; we need air to breathe and a thicker atmosphere to keep liquid water on the surface.

If Earth had been much bigger (say the size of Jupiter), gravity at the surface would be many times stronger. Walking would feel like carrying a heavy backpack. Bones and muscles would have to be much stronger. Large animals like elephants and dinosaurs would never have evolved: they would collapse under their own weight. Building tall structures would be much harder. Rockets would need to be many times more powerful to escape into space.

If Earth had been the size of a giant gas planet like Jupiter, the gravity would have crushed any rocky surface long ago, and the planet would consist mostly of hydrogen and helium gas. There would be no solid ground to walk on.

Astronomers searching for habitable planets around other stars look very carefully at the planets size. A world too small loses its air; a world too big has gravity too strong for life. Earth sits in a sweet spot for life, much as it sits in a sweet spot of distance from the Sun. We are very lucky to live where and when we do.

For more, see what is gravity and weightlessness.