Very low Earth orbit (VLEO) is a range of geocentric orbits with lowest altitudes (at perigee) below 400 km (250 mi). It is of increasing commercial importance in a variety of scenarios and for multiple applications, in both private and government satellite operations. Applications include Earth observation (especially gravity and magnetic fields), telecommunications, and rural internet access among others. Spacecraft may be put into a highly elliptical orbit around Earth with a perigee as low as 80 to 90 km (50 to 56 mi), surviving for multiple orbits. Sub-orbital flight and near space is sometimes considered to be the case up until 160 km of altitude above Earth.
Interest In the Eighties, space agencies started showing interest in VLEO satellites, leading to the European Space Agency's Earth observation satellite "Gravity Field and Steady-State Ocean Circulation Explore" (GOCE), designed to take accurate measurements of Earth's gravitational field. It demonstrated a sustained and precisely controlled orbit at between 224 and 300 km (139 and 186 mi) for almost four years from 2009 to 2013. The Chinese Space Agency launched the Tiangong-1 prototype space station into VLEO in 2011, orbiting at an average of 355 km (221 mi), and the Tiangong-2 prototype in 2016, both of which have since deorbited. The Tiangong space station, launched in 2021 (permanently crewed since 2022), operates at a distance of roughly 350–450 km (220–280 mi). The Japanese Space Agency, JAXA, launched its Super Low Altitude Test Satellite, or SLATS (“Tsubame”), in 2017, whose orbit slowly decreased from an initial altitude of 630 km (390 mi) to operate at seven different altitudes, from 271 km (168 mi) to a final altitude of 167.4 km (104.0 mi). In 2016, Skeyeon filed a VLEO satellite patent describing commercial satellite operation in orbits from 100 to 350 km (62 to 217 mi). Companies such as Albedo Space, EOI Space, Thales Alenia Space and others have also announced plans. In June 2021, the “1st International Symposium on VLEO Missions and Technologies”, registered almost 200 attendees from industry, academia, space agencies and government. In April 2022 DARPA issued a proposal to study VLEO for HF transmissions, and in December 2022 the benefits of VLEO are mentioned as a possibility for future 6G communication technology, using a constellation of small satellites in VLEO. Project 200 by Bellatrix Aerospace aims to build an Ultra-Low Orbit satellite that will orbit at a height of less than 200 kilometers. By 2026, it hopes to launch its first satellite.
Benefits The benefits of satellites operating in VLEO are manifold, including: ability to sense variations of the gravitational and magnetic fields of the Earth at high spatial resolution; substantially lower launch and operating costs; communication payloads with significantly better link budgets; and creating self-cleaning orbits, essentially solving the significant problem of space debris. Earth observation at ultra-high resolution is a potential benefit too, but it has to be traded off with a much reduced field of view or swath and a reduced capacity to image the desired targets compared to satellites in higher orbits. Hypothetically, the thin air can also be harvested as propellant to counteract atmospheric drag of the lower orbit.
Link budget
Roughly defined as a measurement of all power gains and losses in a communication system. With satellites, the process of transmitting from the Earth to the satellite is known as the uplink, and from the satellite to the Earth as the downlink. The difference between the power sent at one end and received at the other end is known as transmission loss. Since the power density of the radio waves decreases with the square of distance between the transmitter and receiver, primarily due to spreading of the electromagnetic energy in space according to the inverse square law, the closer the satellite is to Earth, the less power required to get a signal to either Earth or satellite, and the better the link budget. This improved link budget can be used for either lower power at the same data rate, higher data rate at the same power, or a combination of both. Smaller and/or more powerful transmitters can be either ground based, satellite based, or both.
Self-cleaning orbits If VLEO orbits are sufficiently low, they are essentially self-cleaning, solving the significant problem of space debris. Because atmospheric drag is greater in VLEO than in higher orbits, vehicles in VLEO will remain there until either their propulsion runs out, or they are resupplied with fuel. Once propulsion ends, vehicles made with demisable materials will burn up on reentry, while in general satellites with non-demisable units will partly burn up and break up, potentially creating hazards to the Earth' inhabitants below, although at present (2026) the casualty risk due to meteoroids and similar natural falling bodies remains larger.
Challenges There are several challenges for keeping satellites operating in VLEO that higher orbits do not have. Orbits below about 450 km (280 mi) require the use of novel technologies for satellites to operate in, such as frequent bursts of propulsion, or even continuous propulsion (e.g., GOCE), to counteract the atmospheric drag.
Fuel consumption Air drag and therefore fuel consumption increase exponentially the closer to Earth the orbit is. The International Space Station (ISS) originally orbited at an average of 350 km (220 mi) from Earth, but was boosted to an average of 400 km (250 mi) in 2011. This allowed the ISS to go from an average fuel use of 8,600 kg per year to 3,600 kg per year. The ISS now requires re-boosting only a few times a year due to orbital decay.
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