How Satellites Stay in Orbit: Gravity and Motion

Every day, hundreds of satellites sweep above Australia, relaying ABC broadcasts to remote stations in the outback, guiding Qantas pilots across the Tasman, and helping a farmer near Dubbo decide when to water a crop. They look calm, quietly tracing arcs in the night sky, yet each one is in a constant free fall. The dance between that fall and the curvature of Earth is what keeps them circling endlessly.

For a Year 11 or 12 physics student in Brisbane, Perth, or Hobart, this dance is one of the most elegant examples of motion in the curriculum. Newton's description of gravity explains why a piece of metal hundreds of kilometres up never comes crashing down. The atmosphere is thin at orbital heights and physics takes over.

The same ideas also illustrate a wider truth about mechanics. From biological and cellular systems to coordinate transforms and clock readings, the rules of motion behave consistently. Understanding orbit is a window into how the universe really works.

The Invisible Tug of Gravity

Gravity is the only force a satellite truly needs to obey in the vacuum of space. Its source is Earth's mass, which pulls on every object with a strength that weakens with distance. Isaac Newton showed that the same force that drops an apple in Adelaide also pulls a communications relay 36,000 kilometres above the equator.

Because the force falls off as the square of the distance, satellites farther from Earth feel less tug, while those in lower orbits feel more. This inverse-square rule makes the high geostationary arc practical for weather watchers, broadcasters, and Sky Muster operators delivering internet to Northern Territory homes.

Students often ask whether gravity is "weaker" up there. It is, yet still strong enough to bend a satellite's straight-line travel into a curved path. Without that pull, the spacecraft would simply drift off into deep space.

Falling But Never Landing

A satellite stays up by constantly falling toward Earth while moving sideways fast enough that the curved surface of the planet drops away beneath it. Imagine skimming a stone across Lake Burley Griffin in Canberra. The trick is to throw it so that it is forever falling just at the rate the Earth curves.

In one second, Earth curves by about 4.9 metres. If a satellite's sideways speed produces that exact drop, it circles the globe indefinitely. This is the essence of free fall kept in check by angular momentum, and it is why vacuum matters: any gust of wind would steal energy and drag the craft lower.

This principle is sometimes called weightlessness, though more accurately it is continuous microgravity. Astronauts aboard the International Space Station are not floating because they lack weight. They are floating because they and the station are both plummeting together, in the same unending fall.

Why Speed Keeps Satellites Aloft

The link between velocity and orbit is direct. At low altitudes of about 200 kilometres, a craft must barrel along near 28,000 kilometres per hour. Lower the altitude and the required speed rises slightly; raise it and the speed drops, because gravity weakens.

At about 35,786 kilometres up, a satellite moves at roughly 11,000 kilometres per hour and completes one lap of Earth in 24 hours. From the ground it appears motionless above a fixed longitude, which is exactly what the Optus fleet and many meteorological satellites rely on, sitting above Australia and communicating with dishes from Cairns to Albany without a tracking motor.

This balance of speed and altitude is called centripetal balance. The satellite's velocity wants to carry it off in a straight tangent line; gravity tugs it inward, bending that line into a circle. Neither wins, and so the orbit persists.

Height and the Shape of an Orbit

Altitude does more than change speed. It decides the lifetime of a satellite, the kind of view its instruments get, and the radiation it must endure. Low Earth orbit, between 160 and 2,000 kilometres, suits Earth observation. It gives crisp images of bushfires in the Blue Mountains or algal blooms near the Great Barrier Reef, but it also brushes against traces of air that slowly drag a satellite down.

Medium Earth orbit, between 2,000 and 35,786 kilometres, hosts navigation satellites used by Pilbara mining trucks and freighters crossing New South Wales. Higher than that, communications satellites circle in step with Earth's rotation, and above them the graveyard orbit collects retired spacecraft.

Above the atmosphere there is almost nothing to slow a craft, so an orbit can persist for millions of years. The Moon itself is a natural satellite with this kind of permanence, held for billions of years by the same balance of motion and gravity that keeps a CubeSat from the University of Melbourne spinning along.

Air, Drag, and Staying Power

The atmosphere does not stop at a clean line. Layers thin out for hundreds of kilometres, and a low satellite still bumps into occasional molecules. Each bump saps a little energy, shrinking the orbit. Engineers at the Australian Space Agency in Adelaide design satellites with enough fuel to compensate, sometimes using drag sails.

Sun activity also matters. During solar storms, the upper atmosphere puffs up, increasing drag and costing satellites altitude. Operators of the NBN's Sky Muster constellation pay close attention to space weather forecasts because their birds, positioned over Australia for remote internet, must occasionally fire thrusters to stay in place.

Even small disturbances matter. When a satellite re-enters, it does so because its energy has slowly bled away. Some pieces burn up over the Southern Ocean; larger fragments are tracked by the Tidbinbilla Deep Space Communication Complex outside Canberra before they descend.

Australia in the Age of Orbital Science

Australia sits in a remarkable position for orbital science. The European Space Agency's tracking station at New Norcia, north of Perth, manages deep-space missions. Antennas near Canberra link to Voyager, the Mars rovers, and craft beyond the asteroid belt. Closer to home, the Australian Space Agency, headquartered in Adelaide, is funding local launch capability and student-built satellites from UNSW, ANU, and RMIT.

Even younger pupils are getting involved. Australian school teams have built experiments flown on suborbital rockets, and ABC's Curious Kids answers questions like "why doesn't the Moon fall down?" using these very physics ideas. From classroom chalkboards to antennas sweeping the skies near Hobart, the story of orbit is a shared one.

When a teacher links this science to regional geography and historical milestones in Australian learning, the idea of forces in balance suddenly feels less abstract. The mathematics used to compute satellite tracking times is the same mathematics used to plot train lines from Sydney to Perth.

When you next look up at a steady moving light after dusk above Melbourne, remember you are watching an object whose only tether is gravity and whose only dance partner is its own momentum. Understanding that dance is among the most satisfying things a science student can learn, and it carries over into how enzymes function in digestion, where precise motion and fit do their own quiet work.