Universe Today
If the Roman Empire had been able to launch a satellite in a relatively high Low Earth Orbit – say about 1,200 km (750 miles) in altitude – only now would that satellite be close to falling back to Earth. And if the dinosaurs had launched a satellite into the furthest geostationary orbit - 36,000 km (23,000 miles) or higher -- it might still be up there today.
While we've really only launched satellites since 1957, those examples show how long objects can stay in orbit. With the growing problem of accumulating space junk in Earth orbit, many experts have stressed for years that satellite operators must figure out how to responsibly dispose of derelict satellites at the end of their lives.
The European Space Agency (ESA) and the United Nations Office for Outer Space Affairs (UNOOSA) have collaborated for a new infographic to show how long it would take satellites at different altitudes to naturally fall back to Earth.
While the natural de-orbit process can be relatively fast for satellites flying at low altitudes -- taking less than 25 years -- for satellites launched into orbits tens of thousands of kilometers away, it can be thousands of years before they return.
Gravity has little effect on a satellite's return to Earth. The biggest factor in satellites decreasing their orbit is the amount of drag they encounter from Earth's atmosphere. A satellite can remain in the same orbit for a long period of time as the gravitational pull of the Earth provides a balance to the centrifugal force satellites experience in orbit. For satellites in orbit outside the atmosphere, there is no air resistance, and therefore, according to the law of inertia, the speed of the satellite is constant resulting in a stable orbit around the Earth for many years.
"If we look at our statistics, we have about 300 objects per year returning to Earth, burning up in the atmosphere," said Francesca Letizia, a space debris engineer at ESA, in a podcast on space debris. "Below 500 km, the effect of the atmosphere, the spacecraft can reenter within 25 years. At 800 km above Earth, it will take about 100-150 years to fall back to Earth."
Letizia said the biggest risk for old satellites that aren't currently operating is the risk they pose for exploding and creating more fragments, or for colliding with other satellites and either causing damage or destruction and also creating additional objects in Earth orbit.
In Depth: This is What Happens to Spacecraft When They Reenter Earth's Atmosphere
This means that as we launch satellites to space we must consider how they will be removed at the end of their lives, or else the area around Earth will be filled with old, defunct spacecraft at risk of collision, explosion, and the near-certain creation of vast amounts of space debris.
Further reading: ESA
