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More objects are being launched into space than ever, and most are headed for low Earth orbit. This region of space has become increasingly crowded with launches from SpaceX and others that have doubled the number of Earth satellites in just a few years. We talk about rockets and low Earth orbit (LEO) a lot, but we rarely explain where it is and why it’s essential. Let’s figure it all out.
How High Is Low Earth Orbit?
There’s nothing innate about the Earth or its atmosphere that marks the boundaries of low Earth orbit. This is a distinction created by humans to break up all the ways an object might orbit our planet. How it’s defined can vary somewhat, but we are talking about space immediately outside the atmosphere.
According to NASA, low Earth orbit is considered any orbital trajectory below 1,200 miles (2,000 kilometers). However, the surface of Earth is not entirely smooth, and even a perfectly circular orbit can vary in altitude. An object is said to be in low Earth orbit if it completes a revolution every 128 minutes or less. A 128-minute orbit, according to Kepler’s third law, works out to a semi-major axis of 8,413 km (5,228 mi). Such an object with less than 0.25 eccentricity (a roughly circular orbit), therefore, would have an average altitude of 2,042 km (1,269 mi).
The ISS is in a low Earth orbit.
Credit: NASA
Despite this, most objects in low Earth orbit are in the first 100-200 miles of space, so some NASA sources simply consider LEO to be the space occupied by LEO orbits. Yes, the reasoning is a bit circular, but we are talking about orbits here. Things are further complicated because sub-orbital objects can reach the space we consider low Earth orbit. However, they cannot be said to be in low Earth orbit because they don’t complete a circuit before falling back to Earth.
Why Is LEO Important?
As mentioned above, most objects we send into space are headed for low Earth orbit. This is where you’ll find the International Space Station (210 miles), the Hubble Space Telescope (370 miles), and the more than 4,000 SpaceX Starlink satellites (around 340 miles) launched thus far.
Most satellites can do their jobs just fine from LEO, so there’s no reason to boost them higher. Every ounce sent into space costs a surprisingly large amount of money, and the costs go up when you need to use more fuel to get a spacecraft farther from Earth. And it’s not just fuel—reusable vehicles like the SpaceX Falcon 9 can land after sending a payload off to LEO. Still, they are often completely expended when blasting a satellite into a higher orbit, raising costs.
Starlink satellites being deployed in LEO.
Credit: SpaceX
Some satellites, like GPS nodes, are launched into higher “geostationary” orbits around 18,000 miles (30,000 kilometers). This allows them to remain fixed over a specific part of the globe. These satellites, as well as anything else leaving Earth, needs to pass through LEO. In the past, that was of no concern, but this region of space is getting worryingly crowded. Adding 4,000 new Starlink satellites has more than doubled the number of satellites in orbit, and that’s just the start.
Many scientists are concerned that we are sending too many objects into space without a way to deorbit them later. Even tiny pieces of debris zipping around in LEO can be hazardous to spacecraft, making low Earth orbit inaccessible if the problem spirals out of control. There’s even a name for that: Kessler syndrome. It’s a chain reaction where space junk pulverizes one piece of equipment after another until LEO is filled with tiny impactors. There will be disagreements on how to manage LEO in the coming years as some of the richest people in the world try to realize their visions for satellite megaconstellations.