Low Earth Orbit (LEO)

Low Earth Orbit, or LEO, is the busy ring of space closest to our planet — the place where the International Space Station circles, where most satellites live, and where almost everything humans have launched into orbit actually flies.

Quick facts

• Altitude: about 160 to 2,000 km (roughly 100 to 1,200 miles) above the surface; most satellites stay below ~1,000 km, with the most crowding near 800 km.
• Orbital speed: about 7.8 km/s — around 28,000 km/h (~17,500 mph) — slowing slightly as altitude rises.
• Orbital period (one lap): about 90 minutes near 400 km, stretching to ~100–128 minutes higher up.
• Laps per day: roughly 14 to 16.
• Inclination (tilt of the orbit): anything from near-equatorial (~0°) to polar (~90°); the ISS flies at 51.6°.
• Eccentricity (how stretched the orbit is): usually less than 0.25, meaning nearly circular.

How it works

Anything in orbit is actually in continuous free-fall. Gravity is always pulling the spacecraft toward Earth, but it is also moving sideways so fast that the ground curves away beneath it just as quickly as it falls. The result is that it keeps “missing” the planet and circles instead of crashing.

The lower the orbit, the stronger Earth’s pull, so the spacecraft has to travel faster to stay up — about 7.8 km/s in LEO. At that speed it laps the whole planet in roughly 90 minutes, which is why a crew can see about 16 sunrises and sunsets in a single day.

Because LEO skims the very top of the atmosphere, satellites still feel a thin trace of air. This atmospheric drag slowly drains orbital energy and lowers the orbit, so LEO spacecraft need periodic reboost — short engine firings — to stay aloft. Drag gets worse during solar maximum, when the heated upper atmosphere swells: the ISS may need reboosting every few weeks then, versus only a few times a year at solar minimum. Below about 160–180 km, the air is simply too thick to hold an orbit at all.

Why it’s used

LEO’s biggest advantage is simply being close to Earth. That short distance means low signal delay and modest power and antenna needs, which makes it ideal for broadband communication constellations — many satellites flown together to cover the whole globe. Closeness also delivers high-resolution Earth imaging and science data, and the quick ~90-minute laps mean a satellite returns over the same ground often.

Polar and sun-synchronous orbits (which keep crossing the equator at the same local time) let a satellite eventually cover the entire planet and photograph each spot under consistent lighting — essential for tracking climate and change over time. LEO is also the easiest and cheapest orbit to reach, needing the least launch energy and the shortest trip. That is why it hosts crewed missions, space stations, and crew and cargo resupply: astronauts can get there fast and be returned with relative ease.

Notable missions

• International Space Station (ISS) — a crewed station at about 400 km (operating roughly 370–460 km), tilted 51.6°, with a ~90-minute period and ~16 orbits a day. Its path was chosen so it passes over about 90% of the inhabited Earth.
• Hubble Space Telescope — inclined 28.5°, with a ~96-minute period; launched near 615 km, its orbit has slowly decayed to roughly 500 km and keeps dropping. It was deployed and serviced by the Space Shuttle right here in LEO.
• NASA Aqua — a sun-synchronous polar orbit at ~705 km, ~99-minute period, watching the global climate and water cycle.
• NASA Terra — a sun-synchronous orbit with a ~10:30 AM equator-crossing time for steady, consistent-lighting Earth imaging.
• Starlink — thousands of satellites flown together at ~525–570 km for low-latency global internet.

For all its limits — crowding, drag, and the narrow slice each satellite sees at once — LEO remains the doorway to space: the nearest, cheapest, and most heavily used orbit, and the one where people actually live and work above the planet.