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. Here, we’ll address both—and more.
How High Is Low Earth Orbit?
According to NASA, a low Earth orbit (singular) is any orbital trajectory that stays within 1,200 miles (2,000 kilometers) of Earth’s surface. There’s also a practical inner boundary to LEO space below which spacecraft can’t orbit, imposed by atmospheric drag. In another sense, the term “LEO” is also frequently used as a collective noun denoting the region of space within which low Earth orbits take place, i.e. “in LEO.”
To stay in orbit, you must move at ludicrous speeds: For example, the ISS orbits at a velocity of more than 17,000 miles per hour, or just shy of 5 miles per second. At those speeds, even at LEO altitudes, a satellite will make a complete circuit around the planet once every 128 minutes or less.
The ISS is in a low Earth orbit.
Credit: NASA
There’s nothing innate about the Earth or its atmosphere that marks the boundaries of low Earth orbit. The names are distinctions created by humans to break up all the ways an object might orbit our planet. How they’re defined can vary somewhat, but ultimately, space begins immediately outside the atmosphere.
Fast Facts About Low-Earth Orbit
Gravity in LEO isn’t much different from that on Earth. So why does everything float on the ISS? Astronauts and spacecraft experience weightlessness, more accurately known as microgravity, because to stay in orbit is to be in constant freefall.
Here are a few facts about LEO, by the numbers:
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Other than the Apollo missions, no human being has ever gone farther from the planet than LEO.
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The Starlink megaconstellation and the International Space Station are both in low Earth orbit, at a little over 200 miles up.
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Low Earth orbits are technically in outer space because space officially “starts” at the Kármán line, about sixty miles from Earth’s surface. (Sorry, Sir Richard Branson, but you’re still not an astronaut.)
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LEO is the innermost region of outer space, a “candy shell” extending up to 1,200 miles from the surface. That’s about one-third of the planet’s radius—scaled down, it’s a little thinner than the candy coating on a peanut M&M.
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A spacecraft in LEO must move at a minimum orbital velocity of about 17,000 miles per hour.
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More than 12,000 objects are in orbit around the Earth, and more than 10,000 are in LEO.
Credit: Mars Inc.
Why Is LEO Important?
LEO is important because we want to do things in space. Pretty much anything in a rocket is going to, or through, LEO. So if LEO is too hazardous to make it worthwhile—or even possible—to keep satellites in orbit, everyone loses.
On a geopolitical level, LEO matters because if you control space, you have control of the surface. Think “air superiority” plus “panopticon.” Satellite weapons controlled by “the enemy” are the kind of real-life boogeyman that keeps presidents and generals up at night. Mostly, the international community treats control of space like a bigger version of mutually assured destruction. The International Space Station has been the great olive branch in the sky since its 1998 launch, with dozens of stakeholder countries doing research under the sheltering aegis of joint operations between the US, Canada, the European Union, and Russia. However, all good things must come to an end, and we can’t keep the ISS in orbit forever. But what happens once it’s gone? Right now, the majority position stateside is that the US should keep some sort of major presence in LEO, in hopes of keeping an adversarial power (that is, spacefaring nations led by autocrats, such as Russia or China) from opening the door for a hostile takeover by stepping into the dreaded “space gap.”
NASA is Commercializing LEO, and That’s Probably a Good Thing
Space is huge—and it’s also really expensive. Every ounce sent into space costs an exorbitant amount of money, and the costs go up when you need to use more fuel (and/or a big, expendable rocket) to get a spacecraft farther from Earth. Love it though we might, the Space Shuttle is just not practical. The eye-watering cost of the Artemis Project, hand in hand with the fallout from the delayed Psyche mission, has shaken NASA to its foundation. Meanwhile, private space companies can put a kilogram in orbit for pennies on the dollar. SpaceX Falcon launches start at a bottom-end sticker price of $20M, and you can snag a spot on a Smallsat Rideshare for about $325K. Thus, it came as some relief when NASA debuted its Commercial Crew and Commercial Resupply Services program, in which the agency contracts with private companies to fulfill its needs for crewed ISS launches and orbital logistics.
So far, NASA has held fast to its stated plan of commercializing low Earth orbit and retiring the ISS, eventually transitioning its LEO operations into a hybrid public/private model where the agency is “one of many customers of a low-Earth orbit commercial human space flight enterprise.” In 2021, NASA awarded over $400 million to three companies to develop three proposed commercial LEO “destinations.” One, a joint project by Boeing, Blue Origin, and Sierra Nevada, is called the Orbital Reef, intended as an orbiting “mixed-use business park.” Then there’s Starlab, a mostly-commercial endeavor by Starlab Space (a collaboration between Nanoracks, its parent company Voyager Space, and Lockheed Martin). The third award went to Northrop Grumman, which TechCrunch reports is “working with Dynetics to deliver a modular design based around its Cygnus spacecraft.”
It’s Crowded In Space
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, must pass through LEO. In the past, that was of no concern, but today, NASA has to track thousands of satellites and thousands of known pieces of potentially dangerous debris.
Starlink satellites being deployed in LEO.
Credit: SpaceX
A growing number of experts are concerned that humans are sending too many objects into space without a way to deorbit them later. Why? Space is huge, but things still manage to collide. There are already thousands of satellites and a dangerous amount of space debris (also known as MMOD, for micrometeoroid and orbital debris) in orbit around Earth. Even tiny pieces of debris zipping around in LEO can be hazardous to spacecraft. According to NASA, “averaging speeds of 10 km/s (22,000 mph), a 1-centimeter paint fleck is capable of inflicting the same damage as a 550-pound object traveling 60 miles per hour on Earth. A 10-centimeter projectile would be comparable to 7 kilograms of TNT.”
It’s a tough life up there if you’re a satellite, what with the solar weather and the cosmic sandblaster. MMOD impacts create great swathes of splintered metal and grit, all traveling at orbital velocities. Perversely, solar flares also contribute to the debris problem. Solar flares can heat Earth’s atmosphere enough to make it expand, raising the functional floor of LEO by expanding the friction threshold of the atmosphere. A 2022 solar flare resulted in the loss of dozens of Starlink satellites, when they failed to climb high and fast enough to “take shelter from the storm.”
If accidental satellite collisions and MMOD strikes weren’t enough of a problem, there’s also the very real threat of their deliberate destruction by anti-satellite weapons (ASAT for short). ASAT tests in the past—our own and those of other nations—have created hundreds of thousands of new pieces of debris, some of which may ironically threaten our very presence in space. You just can’t get away from the sandblaster of orbital debris.
Hypothetically—mathematically—as long as you can coordinate their orbits, you can put a staggering number of satellites into space, all circling the planet in an intricate dance. But the real world is messy, and so is LEO. The Starlink constellation alone has more than 7,600 satellites in LEO (out of a planned total of 12,000). China’s Guowang megaconstellation will add another 13,000. So what happens if the debris problem spirals out of control?
Kessler Syndrome
If the space debris problem snowballs, it could choke off the whole LEO altitude band with deadly debris, making LEO inaccessible and potentially threatening—or altogether revoking human access to space. There’s even a name for this disastrous cascade: Kessler syndrome. NASA scientist Donald Kessler calculated in the 1970s that by 2000, orbital debris from human activity in space would outpace MMOD as the greatest ablative risk to spacecraft in orbit. (It did.)
Kessler syndrome is what you get when you accrue space debris faster than it can burn up in the atmosphere. It’s a chain reaction where space junk pulverizes one piece of equipment after another until LEO is volumetrically filled with tiny impactors, and you can’t use it for satellites at all.
Credit: Mark Garlick/Science Photo Library/Getty Images
There’s no known numerical threshold of objects near-Earth space can hold before it gets resentful and kicks over into Kessler syndrome. Kessler himself published a 2009 paper explaining that his eponymous syndrome should be understood as a thing that built up over decades, not a sudden, cinematic collapse à la Gravity. Still, scientists are at least starting to have an idea of where the limits may lie. A 2023 study calculated a maximum safe carrying capacity of some 72,000 satellites, which you may wish to take with a grain of salt. Some projections have us breaking 100,000 by 2030—a figure that may also be in need of some seasoning.
An extreme case of Kessler syndrome has been proposed as a Great Filter: one answer to the Fermi paradox, a longstanding scientific question that asks, basically, where is everybody else? So far, the number of known star systems with intelligent life—or any sign of having ever harbored life—is one. If there’s intelligent life out there, why can’t we see it? We got this far, so clearly it’s possible to get this far, tautologically speaking. But what if there’s something waiting for us in the future, just like it waits for everybody else, to scuttle our ships just as we try to make our way into the broader Universe? What if it’s us, humanity, ourselves?
What if it’s not us?
At least we’ve got a running start.