What is Low Earth Orbit? A basic explainer

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More objects are being launched into space than ever before, and most of them are headed for low Earth orbit. This region of space has become increasingly crowded with launches by SpaceX and others that have doubled the number of Earth satellites in just a few years. We talk a lot about low Earth orbit (LEO), but we rarely explain where it is and why it is essential. Let’s find out everything.

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How high is low earth orbit?

There is 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 could orbit our planet. How it is defined may vary slightly, 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 Earth’s surface 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, corresponds to a semimajor axis of 8,413 km (5,228 mi). Such an object with an eccentricity of less than 0.25 (a roughly circular orbit), therefore, would have an average altitude of 2,042 km (1,269 mi).

International Space Station

The ISS is in low Earth orbit.
Credit: NASA

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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’re talking about orbits here. Things are further complicated because suborbital objects can reach the space we consider low Earth orbit. However, they cannot be said to be in low Earth orbit because they do not complete one circuit before falling back to Earth.

Why is Leo important?

As mentioned earlier, most of the objects we send into space are headed for low Earth orbit. Here you’ll find the International Space Station (210 miles), the Hubble Space Telescope (370 miles), and the more than 4,000 SpaceX Starlink satellites (approximately 340 miles) launched to date.

Most satellites can do their job well as LEOs, so there is no reason to raise them higher. Every gram sent into space costs a surprisingly large amount of money, and costs rise when more fuel needs to be used to get a spacecraft farther from Earth. And it’s not just reusable fuel-like vehicles like the SpaceX Falcon 9 that can land after sending a payload to LEO. However, they are often completely depleted when launching a satellite into a higher orbit, adding to costs.

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Starlink satellites being deployed

Starlink satellites deployed in LEO.
Credit: SpaceX

Some satellites, such as GPS nodes, are launched into “geostationary” orbits above approximately 18,000 miles (30,000 kilometers). This allows them to remain fixed on a specific part of the globe. These satellites, as well as anything else leaving Earth, must pass through LEO. In the past, this wasn’t a concern, but this region of space is getting worryingly crowded. The addition of 4,000 new Starlink satellites has more than doubled the number of satellites in orbit, and that’s just the beginning.

Many scientists worry that we are sending too many objects into space without a way to deorbit them afterwards. Even tiny bits of debris hurtling into LEO can be hazardous to spacecraft, making low-Earth orbit inaccessible if the problem gets out of control. There’s even a name for it – 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 over how to handle LEO over the next few years as some of the richest people in the world try to realize their visions for satellite mega-constellations.

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