Very Low Earth Orbits — Reducing orbital altitude for lower cost Earth observation and communications satellites

Projected Earth observation data and value-added services (VAS) sales to 2028 [Images by courtesy of Euroconsult]
  • Infrastructure monitoring and asset tracking
  • Environmental monitoring
  • Precision agriculture, crop monitoring, and food security
  • Defence, intelligence, and security
  • Maritime surveillance and anti-piracy
  • Disaster monitoring, management, and response
  • Energy and natural resources (exploration and monitoring)
  • High-bandwidth, low-latency, global communications

Typical Orbits

Historically, satellite orbits have been classifiable by their altitude and the type of mission they are typically used for:

  • Geostationary orbits (GEO, 35,786 km) are frequently used for fixed communications (e.g. broadcasting) and observations (e.g. weather) due to their synchronised rotation with the Earth and ability to achieve global coverage with only a small number of spacecraft.
  • Medium Earth Orbits (MEO, 2000 km to 35,786 km) are popular for navigation constellations such as GPS, GLONASS and Galileo, that require a balance between global coverage and diversity (number of satellites visible from the ground at the same time).
  • Low Earth Orbits (LEO, < 2000 km) are the primary choice for observation missions as they are closer to the Earth’s surface and can therefore obtain higher resolution images. They are also more accessible for manned missions and orbiting space stations. Recently, LEO has also become popular for communications constellations due to increased bandwidth and reduced latency and power requirements.
ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) spacecraft [Image Credit: ESA–AOES-Medialab]

Benefits of Very Low Earth Orbits

Reducing the orbital altitude of satellites in LEO below 450 km can provide a number of benefits that could drive the development of a new generation of satellites that can provide higher performance at a lower cost.

  • For Earth observation spacecraft, reducing the distance to the ground allows smaller and less expensive payloads to provide equivalent or improved resolution and quality.
  • For communications, the shorter distance reduces latency (time-delay) and also the required power for transmission.
  • Lower altitude orbits are naturally resilient to a build-up in debris due to the effects of drag and therefore have a lower risk of on-orbit collision.
  • The same effect of drag ensures that spacecraft are naturally disposed of quickly after their mission is complete or if they suffer a catastrophic failure.
  • Launch vehicles can deliver a larger mass into lower altitude orbits, reducing the specific (per unit mass) launch cost.
  • Mapping errors are reduced, improving the accuracy of ground imagery and location-based services.
  • The radiation environment may be less aggressive and therefore more favourable to standard electronic components, reducing cost and the need for redundancy.

Challenges

Despite these wide-ranging benefits, there are still many challenges in operating in the denser regions of the upper atmosphere that mean lower altitude orbits are yet to see commercial exploitation.

Left: Atomic oxygen erosion [Image Credit: ESA — CC BY-SA IGO 3.0], Right: Combined atomic oxygen erosion and ultraviolet degredation [Image Credit: NASA Langley Research Center]

Technology Development

In order to address these challenges and enable the exploitation of lower altitude orbits, several lines of active research and technology development are underway:

  • Experiments to understand the erosion characteristics of atomic oxygen are being performed at facilities around the world, for example at the European Space Agency.
  • Materials that have resistance to atomic oxygen erosion and can also reduce the aerodynamic drag experienced by satellites in orbit are being searched for and developed. A novel experimental facility is currently being commissioned at The University of Manchester that will provide new insight into the gas-surface interactions that occur in orbit, driving the future search for novel drag-reducing materials.
The Rarefied Orbital Aerodynamics Research (ROAR) Facility at The University of Manchester [Image Credit: Vitor Oiko]
  • Data collected in-orbit, for example on-board the Materials ISS Experiment-Flight Facility (MISSE-FF) and by the JAXA “TSUBAME” satellite is supporting ground-based investigations.
  • In 2021, an aerodynamics test CubeSat called SOAR will be launched to investigate the gas-surface interactions in very-low Earth orbit and test candidate drag-reducing materials.
The Satellite for Orbital Aerodynamics Research [Image Credit: Alejandro Macario Rojas]
Atmosphere-Breathing Electric Propulsion (ABEP) thruster jet with nitrogen propellant [Image Credit: IRS-Stuttgart]
  • The increased atmospheric density in lower altitude orbits also presents the opportunity to perform a wide range of novel aerodynamics-based orbit and attitude control. The SOAR test satellite will demonstrate some of these ideas, for example contributing to pointing capability or reducing the requirements on alternative attitude control actuators such as magnetorquers and reaction wheels.

Outlook

Sustained operations in lower altitude orbits could be realised by combining the technological developments discussed above. This would result in a new class of satellites that have equivalent or better performance than those at higher altitudes whilst simultaneously being cheaper to develop and launch. The use of lower altitude orbits also has a role to play in the sustainability of space for future generations by ensuring that spacecraft are disposed of responsibly after their useful lifetime.

“Skimsat” concept [Image Credit: Thales Alenia Space]

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Nicholas Crisp

Nicholas Crisp

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Research Associate at The University of Manchester. Orbital aerodynamics, spacecraft design, and systems modelling.