Satellites orbit Earth at a wide range of heights, depending on their purpose. Some are only a few hundred kilometres above the surface, while others are tens of thousands of kilometres away.
Earth's atmosphere doesn't suddenly end at a particular altitude. Even at 400–500 km, there are still traces of atmosphere, creating enough drag to gradually slow satellites down.
From a few hundred kilometres above our heads to almost 36,000 kilometres into space, thousands of satellites are travelling around Earth at extraordinary speeds.
But why do they orbit at different heights, how do they stay up there, and what actually happens when something goes wrong?
Look up at the night sky and you might occasionally spot a satellite passing overhead. It appears as a small, steadily moving point of light, crossing the darkness before disappearing from view.
What is less apparent is that this seemingly insignificant object could be travelling at approximately 28,000 kilometres per hour, hundreds of kilometres above the surface. Even more remarkable is that it might complete an entire journey around Earth in roughly 90 minutes.
But not all satellites occupy the same region of space. Some orbit relatively close to Earth, others travel tens of thousands of kilometres away, and a handful follow extraordinarily elongated paths that take them between vastly different altitudes.
These aren't arbitrary distances. Each orbit is selected according to what the satellite needs to accomplish.
So, how high are satellites actually orbiting, and what determines where they go?
Low Earth Orbit: The Busy Neighbourhood
Altitude: approximately 160–2,000 kilometres above Earth's surface.
Low Earth Orbit (LEO) is where much of today's satellite activity takes place.
The International Space Station (ISS), for example, operates approximately 400 kilometres above Earth. The Hubble Space Telescope orbits at approximately 540 kilometres, while many of the increasingly familiar Starlink satellites operate at around 550 kilometres.
This region is particularly useful because satellites can observe Earth in considerable detail without being excessively far away.
Applications include:
- Weather observation
- Environmental monitoring
- Military surveillance and reconnaissance
- Scientific research
- Telecommunications
- Internet connectivity
- Disaster monitoring and emergency response
The relatively short distance also means that signals can travel between satellites and ground stations with comparatively little delay.
However, there is a considerable price to pay for operating so close to Earth. Satellites in LEO encounter traces of atmospheric gases. Although the atmosphere is extremely thin at these altitudes, it is not entirely absent.
The resulting resistance, known as atmospheric drag, gradually reduces orbital energy. Without occasional adjustments, satellites can descend towards Earth's atmosphere and eventually burn up during re-entry.
Solar activity can make this problem considerably worse by heating and expanding the upper atmosphere, increasing the drag experienced by satellites.
It is a reminder that even hundreds of kilometres above us, Earth has not entirely released its atmospheric grip.
Medium Earth Orbit: The Navigation Network
Altitude: approximately 2,000–35,786 kilometres.
Further out, we encounter Medium Earth Orbit (MEO).
This region is particularly important because it accommodates many of the satellites responsible for global navigation systems.
The American Global Positioning System (GPS), for example, operates satellites at approximately 20,200 kilometres above Earth's surface. Europe's Galileo navigation system operates at approximately 23,200 kilometres.
These satellites transmit precisely timed signals that receivers on the ground use to calculate their positions.
Your smartphone, car navigation system and numerous other technologies depend on this infrastructure.
Unlike satellites in low Earth orbit, navigation satellites operate much further away. Their orbital periods are consequently longer, with GPS satellites taking approximately 12 hours to complete an orbit.
The additional distance allows individual satellites to cover much larger geographical areas, although several satellites are needed to provide reliable global positioning.
MEO demonstrates how orbital altitude is not simply about getting closer to Earth. Sometimes, being considerably further away is precisely what makes a technology practical.
Geostationary Orbit: The Satellites That Never Seem to Move
Altitude: 35,786 kilometres above Earth's equator.
Perhaps the most fascinating commonly used satellite orbit is the geostationary orbit (GEO).
At this altitude, something remarkable happens. A satellite travelling in a circular orbit directly above the equator, moving in the same direction as Earth's rotation, takes exactly one sidereal day to complete its journey.
A sidereal day is approximately 23 hours, 56 minutes and 4 seconds.
Because the satellite and Earth rotate at matching angular speeds, the satellite appears to remain permanently above the same geographical location.
From the ground, it looks stationary.
This has enormous practical advantages. Television broadcasting, telecommunications and meteorological observation all benefit from satellites that remain apparently fixed in the sky.
Weather satellites operating in geostationary orbit can continuously monitor enormous areas of the planet, making it possible to observe developing storms, cloud formations and other atmospheric changes. The European Meteosat weather satellite programme is one example.
There is, however, an important distinction between geostationary and geosynchronous orbits.
A geosynchronous satellite has the same orbital period as Earth's rotation, but if its orbit is inclined or elliptical, it will appear to move across the sky.
Only a circular, equatorial geosynchronous orbit is truly geostationary.
Polar and Sun-Synchronous Orbits: Watching the Entire Planet
Not every satellite travels around Earth in the same general direction as the planet's rotation. Some follow polar or near-polar trajectories, passing approximately over the North and South Poles.
As the satellite circles Earth, the planet rotates beneath it. This means that successive orbital passes can cover different geographical areas, eventually allowing the satellite to observe almost the entire planet.
These orbits are particularly useful for:
- Mapping and geographical surveys
- Monitoring deforestation
- Tracking agricultural activity
- Measuring sea temperatures
- Monitoring ice sheets and glaciers
- Observing environmental changes
A particularly useful variation is the sun-synchronous orbit. Satellites in these orbits pass over particular locations at approximately the same local solar time on successive days.
Why does this matter?
Consistent lighting conditions make it easier for scientists to compare satellite images and identify changes on the ground.
For example, an environmental researcher examining deforestation over several years benefits enormously from images taken under similar lighting conditions.
It is an elegant example of orbital mechanics being used to improve scientific accuracy.
Highly Elliptical Orbits: The Long Way Around
Not all satellites follow neat, circular paths. Some occupy highly elliptical orbits, travelling considerably closer to Earth at one point and much further away at another. One well-known example is the Molniya orbit, historically developed by the Soviet Union.
These orbits are particularly useful for communications and observation over high-latitude regions.
Geostationary satellites have a significant limitation: because they sit above the equator, their viewing angle becomes increasingly shallow towards the polar regions.
For countries situated far north, a highly elliptical orbit can provide a more useful alternative.
A Molniya satellite takes approximately 12 hours to complete an orbit, spending much of that time relatively far from Earth, where it can maintain a useful view of northern regions.
The satellite accelerates considerably as it approaches Earth and slows as it moves further away.
This behaviour is a direct consequence of gravity and the conservation of orbital energy.
How Do Satellites Stay Up There?
Perhaps the biggest misconception about satellites is that they remain in space because they have somehow escaped Earth's gravity.
They haven't. Gravity is actually what keeps them in orbit.
Imagine throwing a ball horizontally.
It travels forward while simultaneously falling towards the ground. Throw it harder and it travels further before hitting the surface.
Now imagine throwing it so fast that, as it falls, Earth's curved surface continually drops away beneath it.
The ball would keep falling, but never reach the ground.
That is essentially what happens to an orbiting satellite.
It is continuously falling towards Earth while travelling sideways fast enough to keep missing it.
At approximately 400 kilometres altitude, the International Space Station travels at around 27,600 kilometres per hour. At this speed, it completes approximately 16 orbits of Earth every day.
The satellite doesn't need to keep firing its engines to maintain its basic orbit. Once correctly positioned and moving at the required velocity, gravity provides the necessary continuous acceleration. However, small adjustments are sometimes needed to compensate for atmospheric drag and other disturbances.
Is There Actually an End to Earth's Atmosphere?
This raises another interesting question. Where does Earth's atmosphere end and space begin?
There is no universally agreed physical boundary where the atmosphere suddenly disappears.
For practical purposes, the Kármán line, approximately 100 kilometres above sea level, is widely recognised as the boundary of space. But atmospheric gases extend considerably further.
Even at the altitude of the International Space Station, approximately 400 kilometres above Earth, traces of the atmosphere remain. The density is extraordinarily low compared with conditions at sea level, but it is sufficient to produce measurable drag.
This is why satellites in lower orbits gradually lose altitude unless their trajectories are corrected. It also explains why the ISS occasionally needs reboost manoeuvres to maintain its orbit.
Space is not quite as empty as we might imagine.
The Growing Problem of Space Debris
There is another consequence of placing thousands of objects into orbit: space is becoming increasingly crowded.
Satellites share their orbital environment with discarded rocket stages, fragments from previous collisions, abandoned spacecraft and countless smaller pieces of debris.
Some objects are tracked, but not everything can be monitored with equal precision.
The danger is that objects travelling at orbital velocities can cause catastrophic damage even when they are relatively small.
In low Earth orbit, debris commonly travels at approximately 7–8 kilometres per second. A collision between two objects can produce thousands of additional fragments, potentially creating further collision risks.
NASA has identified particularly significant concentrations of orbital debris around certain altitudes, including approximately 750–800 kilometres.
This creates a growing challenge for satellite operators and space agencies.
As commercial satellite constellations expand, particularly those providing global internet connectivity, responsible orbital management becomes increasingly important.
End-of-life disposal, collision avoidance and the removal of existing debris are all becoming central issues in the future of space operations.
The question is no longer simply how to put satellites into orbit. It is how to ensure that those orbits remain usable.
Just How Far Away Are These Satellites?
To put the distances into perspective, consider the following:
Object| Approximate altitude or distance
Kármán line| 100 km
International Space Station| 400 km
Hubble Space Telescope| 540 km
Starlink satellites| 550 km
Typical sun-synchronous satellites| 600–800 km
GPS satellites| 20,200 km
Galileo satellites| 23,200 km
Geostationary satellites| 35,786 km
Moon| 384,400 km from Earth's centre, on average
One particularly interesting comparison is the Moon. Even geostationary satellites, which are almost 36,000 kilometres above Earth, are still relatively close compared with the Moon's average distance.
The Moon is approximately 384,400 kilometres from Earth's centre, more than ten times further away than the altitude of a geostationary satellite.
Our planet's immediate orbital neighbourhood is therefore only a small part of the space surrounding it.
What Does the Future Hold?
Satellite technology is evolving rapidly. The expansion of commercial satellite networks, improvements in Earth observation and increasingly sophisticated scientific instruments are changing how humanity uses the space immediately surrounding our planet.
Low Earth orbit is likely to become even more important, particularly for communications, environmental monitoring and commercial space operations.
Meanwhile, new technologies are being developed to improve satellite propulsion, extend operational lifetimes and reduce the amount of debris left behind.
There is also growing interest in orbital servicing: the possibility of repairing, refuelling or upgrading satellites rather than simply abandoning them when their useful lives end.
The challenge is balancing technological progress with the long-term sustainability of Earth's orbital environment.
Space may appear limitless, but useful orbital positions are not.
The Bigger Picture
The next time you use satellite navigation, watch a weather forecast or connect to a satellite-based communications service, consider what is actually happening above you. Somewhere between 400 and 36,000 kilometres away, an extraordinary network of machines is travelling around our planet at thousands of kilometres per hour.
Some are photographing forests, oceans and cities. Others are monitoring weather systems, providing navigation signals or transmitting information across continents.
They occupy different heights because each mission has different requirements. And while we often think of space as an enormous, distant frontier, much of the infrastructure supporting modern civilisation is operating in a relatively narrow region immediately surrounding Earth.
We have effectively constructed an invisible technological highway around our planet. The question is whether we can continue expanding that highway without making it increasingly difficult, or even dangerous, for future generations to use.
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