Very low Earth orbit

Very low Earth orbit is a range of orbital altitudes below 400 km (250 mi), and 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, radar, infrared, weather, telecommunications, and rural internet access among others.

Interest
In 2009 governments started showing interest in VLEO satellites, such as the European Space Agency's scientific satellite "Gravity Field and Steady-State Ocean Circulation Explore" (GOCE), designed to take accurate measurements of Earth's gravitational field. It demonstrated a sustained orbit of between 250 and 300 km (155-186 mi ) for three years from 2009 to 2013.

The Chinese Space Agency launched the Tiangong-1 space station into VLEO in 2011 orbiting at an average of 355 km (220 mi), and the Tiangong-2 in 2016, both which have since deorbited. The Tiangong launched in 2021, operates at a distance of roughly 350–450 km (217-280 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 (391 mi) to operate at seven different altitudes, from 271 km (168 mi) to a final altitude of 167.4 km (104.0 mi).

Companies took note of this increased interest in VLEO. In 2016 Skeyeon filed the first VLEO satellite patent describing commercial satellite operation in orbits from 100 to 350 km (62-218 mi), with plans to put satellites into VLEO orbit. Companies such as Albedo, EOI Space, Thales Alenia Space and others announced plans at later dates.

In June 2021, the “1st International Symposium on VLEO Missions and Technologies”, with almost 200 registered 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.

Benefits
The benefits of satellites operating in VLEO are many fold, including; inherently higher satellite performance; 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.

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 both gravitational pull and atmospheric drag are 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, smaller vehicles will burn up on reentry, while larger ones will burn up and/or break up, potentially creating hazards to the Earth and inhabitants below.

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 and higher gravitational forces.

Fuel consumption
Fuel consumption increases exponentially the closer to Earth the orbit is. The International Space Station (ISS) originally orbited at an average of 350 km (217 mi) from Earth, but was boosted to an average of 400 km (248 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. Moving the ISS an average of 50 km (21 mi) higher, from 350 to 400 km, resulted in the reduction of annual fuel usage by 58%. The ISS now requires re-boosting only a few times a year due to orbital decay.

Atmospheric drag
There is the residual atmosphere in VLEO creating significant drag on satellites planning to maintain orbit. Unlike the ISS on the border of VLEO/LEO orbits that is resupplied with fuel to counteract gravity and drag, once most larger LEO and GEO spacecraft achieve orbit, they require little or no propulsion to maintain orbit. Smaller satellites such as micro and nano satellites are not built for fuel resupply, and must carry their own, or develop it from available resources. To this end a number of companies and governments are developing engines utilizing different concepts for propulsion in VLEO. The company Kreios Space is developing an air breathing propulsion system for VLEO satellites, whereas Skeyeon has a patent on an propulsion system using a self-sustaining ion engine. The drag of the residual atmosphere also calls for improved aerodynamic spacecraft design. Current designs of a large square object in orbit with huge solar panels attached, characteristics of many higher altitude orbital space craft, will not function in VLEO.

Exposure to oxygen
Additionally, satellites in VLEO are exposed to very high levels of elemental oxygen, also known as atomic oxygen (AO), a highly reactive form of oxygen that corrodes most substances quickly. This requires the use of special coatings to protect objects and equipment in this orbit. In VLEO orbit, estimates are that up to 96% of the atmosphere is AO. At VLEO altitudes, the total amount of O atoms rises exponentially the lower the altitude, and is orders of magnitude greater than that encountered by the ISS at 400 km altitude. Any vehicle spending more than a month in a VLEO orbit will require special coatings and protection, or corrode quickly. Materials have been developed for VLEO use that simultaneously provides two key benefits: protection from AO damage, and an atomically smooth outer surface that scatters AO atoms elastically, resulting in half the drag of traditional materials that promote diffuse scattering of incident oxygen and other atoms. Such materials can extend the lifetime of a VLEO satellite by reducing corrosion and reducing drag.

Deorbiting requirements
Due to plans by several companies to create large satellite constellations, some with over 10,000 satellites, on September 29, 2022, the Federal Communications Commission (FCC) adopted a new rule. New satellites placed in orbit must either deorbit or be placed in a graveyard orbit five years after mission life, reduced from a previously generally accepted 25 years after mission life. This applies to US-licensed satellites as well as those from other countries that seek to access the US market. Satellite owners are now required to submit deorbit plans with their launch proposals, to help eliminate additional space junk. This most likely will include reserving propellant to initiate deorbit, increasing either launch costs to carry more fuel, or decreasing mission life.

Satellites in VLEO will also require deorbiting plans when a mission is completed. However, due to the atmospheric drag in VLEO being orders of magnitude higher, the satellites attitude control system that normally ensures that the vehicle's roll, pitch, and yaw directions are maintained, can be instructed to rotate the satellite to its highest surface area facing forward. Smaller vehicle design, with a low drag orientation during operation and a high drag orientation after mission completion, can potentially be an optimal vehicle choice for VLEO orbits, and take advantage of the inherent self-cleaning properties of atmospheric drag on re-entry.