Very Low Earth Orbit (VLEO)

Very Low Earth Orbit, or VLEO, is the band of orbits that hugs the very top of the atmosphere, where a satellite skims so close to Earth that the thin air around it shapes almost everything about how it is built and flown.

Quick facts

• Altitude: typically about 200-350 km, and by definition below roughly 450 km (the European Space Agency, or ESA, describes VLEO as below about 450 km). The record for the lowest sustained Earth-observation orbit is JAXA’s SLATS/Tsubame at 167.4 km.
• Orbital speed: about 7.8 km/s (roughly 17,500 mph) — slightly faster than higher orbits, because a lower circular orbit has to move quicker to balance Earth’s stronger pull.
• Orbital period: roughly 88-93 minutes (about 90 minutes at ~300 km), so a craft circles Earth about 16 times a day.
• Inclination: chosen by the mission, not fixed by altitude; Earth-observation craft often fly Sun-synchronous near-polar orbits (around 96-98 degrees).
• Air density: far higher than at typical higher Low Earth Orbit, and it climbs steeply as you drop lower. Down here as much as ~96% of that faint leftover air is atomic oxygen, a highly reactive gas that eats away at exposed surfaces.

How it works

Like any orbit, a VLEO satellite is in continuous free-fall. It moves sideways so fast — about 7.8 km/s — that the curve of its fall matches the curve of the Earth, so it keeps “missing” the ground. The catch is that this low, the atmosphere has not fully thinned out. The faint leftover gas pushes back like a constant headwind, a force called aerodynamic drag, which steadily saps the orbit’s energy and would otherwise spiral the craft down to burn up on re-entry. Because air density rises sharply as you drop lower, the drag — and the fuel needed to fight it — climbs steeply too.

To stay up, VLEO spacecraft thrust back. Most use high-efficiency electric or ion propulsion — engines that fling out charged gas at very high speed — either continuously or in periodic burns. ESA’s GOCE, for example, ran a xenon ion thruster throttled in real time between about 1 and 20 millinewtons to exactly cancel the drag it measured, holding a “drag-free” orbit. A newer idea, air-breathing electric propulsion, scoops up the thin atmosphere itself as fuel. The air down here is also chemically harsh: highly reactive atomic oxygen erodes exposed surfaces, so these craft need oxygen-resistant coatings and slender, streamlined shapes — a bit like a badminton shuttlecock — to keep drag low.

Why it’s used

Flying closer to Earth pays off directly. For Earth observation, the shorter distance gives much sharper images from a given camera, or the same sharpness from a smaller, cheaper one. For communications, the shorter path means lower latency (less delay) and a stronger signal for the same power — which is why VLEO interests broadband constellations. Launches can be cheaper too, since payloads do not have to be pushed as high. And the same drag that is a nuisance doubles as a built-in cleanup crew: when a VLEO satellite dies, drag pulls it down quickly, so it does not linger for years as space debris. The region is, in effect, self-cleaning.

Notable missions

• GOCE (ESA, 2009-2013) — the Gravity field and steady-state Ocean Circulation Explorer flew a drag-free orbit at roughly 255 km, then was lowered toward about 224 km late in the mission, using a continuously throttled xenon ion thruster to map Earth’s gravity in unprecedented detail.
• SLATS / Tsubame (JAXA, 2017-2019) — the Super Low Altitude Test Satellite stepped down through several altitudes and set a Guinness World Record for the lowest Earth-observation orbit at 167.4 km, using an ion engine plus gas-jet thrusters and measuring atomic-oxygen erosion.
• Aeolus (ESA, 2018-2023) — a wind-profiling mission carrying a Doppler wind lidar at about 320 km, using its propulsion to push back against drag, and ending with a first-of-its-kind assisted re-entry in 2023.
• Tiangong (China) — a crewed station in the lower band of LEO, roughly 340-450 km, showing the drag-maintenance needs of large crewed structures down here.
• Commercial concepts — proposed sub-300 km imaging and broadband satellites are driving work on air-breathing electric propulsion to sustain flight below 200 km.

VLEO comes down to a clear bargain: you gain resolution, lower latency, cheaper launches, and a tidy disposal path, but you pay for it in propellant, careful propulsion and materials engineering, and the extra effort of keeping a craft aloft where the air still bites.