Gravity in Space

Gravity is the architect of the universe. Stars, planets, moons, asteroids and galaxies all owe their shapes and orbits to gravity. Without gravity, the universe would be a cold, empty fog of gas. With gravity, that gas has clumped into stars and planets, planets gather moons, stars gather planets, galaxies gather stars and galaxies themselves gather into clusters. Almost everything you see in a clear night sky exists because of gravity.

  • What it does in spaceBuilds stars and orbitsPulls gas into clumps
  • Earths orbit speed107,000 km/hAround the Sun
  • Moons orbit speed3,683 km/hAround the Earth
  • Suns mass99.8% of solar systemIts gravity rules
  • Galaxy spinBillions of years per turnGravity holds stars in
  • Dark matter5x more than visible matterExtra gravity, unknown stuff

What is an orbit?

An orbit is the curved path one object follows around another, held in place by gravity. It is essentially a constant balance between two things:

  • The pull of gravity, trying to drag the orbiting object inward.
  • The objects sideways motion, trying to carry it onwards in a straight line.

If the sideways motion is too slow, gravity wins and the object falls in. If the sideways motion is too fast, gravity loses and the object flies away. At just the right speed, the two balance: the object keeps falling around the central body forever, in an orbit.

Building the solar system

The solar system formed about 4.6 billion years ago from a giant cloud of gas and dust. Gravity slowly pulled the cloud into a clumping disc. Most of the matter fell into the centre to make the Sun. The rest formed the planets, moons, asteroids and comets, all orbiting the Sun in roughly the same plane.

The same process is still happening around young stars today. Telescopes have spotted hundreds of protoplanetary discs: spinning rings of dust and gas around new stars, slowly clumping into new planets. Some of these systems have already produced young planets we can directly photograph.

Fact The Sun contains about 99.8 per cent of all the mass in the solar system. The other 0.2 per cent is shared between the 8 planets, hundreds of moons, asteroids, comets, and everything else. Most of that 0.2 per cent is in Jupiter alone. We live on a tiny rocky speck.

How fast do you need to orbit?

Orbit speed depends on how close you are to whatever you are orbiting:

  • Low Earth orbit (where the International Space Station is, about 400 km up): around 28,000 km/h. Each orbit takes 90 minutes.
  • Geostationary orbit (about 36,000 km up): around 11,000 km/h. Each orbit takes exactly 24 hours, matching Earths spin, so the satellite hangs over one spot.
  • Moons orbit (about 384,400 km away): 3,683 km/h. Each orbit takes 27.3 days.
  • Earths orbit around the Sun: 107,000 km/h, taking 365.25 days.

Tides

The Moons gravity pulls the Earth, and stretches the oceans slightly into a bulge on the side facing the Moon (and a matching bulge on the opposite side). As Earth spins, different parts of the world rotate through these bulges, causing the regular rise and fall of the sea we call the tides. See the tides for more.

Slingshots and gravity assists

Space probes use the gravity of planets to speed up or change direction without using fuel. The Voyager probes, launched in the 1970s, used the gravity of Jupiter and Saturn to fling themselves on to Uranus, Neptune and out of the solar system. The Cassini probe used the gravity of Venus, Earth and Jupiter to reach Saturn faster. This is called a gravity assist or gravitational slingshot.

Did you know? Astronomers can detect planets around distant stars by watching the tiny gravitational wobble they cause in their host star. As a planet orbits, the star itself moves slightly back and forth around their common centre of gravity. Highly sensitive telescopes can pick this up. This technique discovered the very first planets around other stars in the 1990s and continues today.

Galaxies

Galaxies are gigantic gatherings of stars (and gas, dust, planets, dark matter) held together by gravity. Our own Milky Way contains around 100 to 400 billion stars. They all orbit around the centre, which contains a supermassive black hole (4 million times the mass of the Sun) called Sagittarius A*. Our solar system takes about 225 million years to complete one orbit around the galaxy.

Galaxies themselves often orbit each other in groups and clusters. The Milky Way is on a collision course with the Andromeda Galaxy: in about 4 billion years they will merge into one giant galaxy under the relentless pull of gravity.

Dark matter

When astronomers measure how galaxies rotate, they find they spin faster than visible matter alone can explain. To stop them flying apart, there must be much more gravity than the stars and gas produce. The extra mass is called dark matter: an unknown form of matter that does not give off light. About 27 per cent of the universe is dark matter, against just 5 per cent visible matter (and 68 per cent dark energy, even more mysterious). What dark matter is made of remains one of the biggest mysteries in physics.

Try this Watch a video of the International Space Station as it passes overhead (you can find tracking apps online). It moves quickly across the sky, going from horizon to horizon in just a few minutes. Each pass shows you something orbiting Earth in real time, kept in flight by exactly the balance we have described: enough sideways speed to keep falling around the planet without ever landing.
Deeper dive: how Kepler worked out the laws of orbits

Long before Newton wrote down his law of gravity, a German astronomer called Johannes Kepler worked out three precise laws for how planets orbit the Sun. He did this by patient calculation, using decades of careful observations made by the Danish astronomer Tycho Brahe. The results, published between 1609 and 1619, were:

  • First law: every planet orbits the Sun in an ellipse (a slightly squashed circle), with the Sun at one focus point of the ellipse.
  • Second law: a line connecting a planet to the Sun sweeps out equal areas in equal times. This means a planet moves faster when it is closer to the Sun, slower when further away.
  • Third law: the square of the orbital period (how long one orbit takes) is proportional to the cube of the orbits average size.

Kepler had no idea what caused these patterns. He suspected a force from the Sun but could not work out what it was. Half a century later, Isaac Newton showed that all three of Keplers laws are predictions of his single law of universal gravitation. The story is one of the great triumphs of science: careful observation by Brahe, mathematical pattern-finding by Kepler, then a deep theoretical explanation by Newton. Together they replaced the old Greek idea of perfect circular crystal spheres with a clear, calculable picture of how the solar system really works.

For more, see what is gravity and weightlessness.