Very Low Earth Orbits — Reducing orbital altitude for lower cost Earth observation and communications satellites
The global market for satellite-based Earth observation (EO) data and downstream value-added services is growing rapidly. Euroconsult have projected commercial sales of EO data to reach $2.4 billion by 2028, up from $1.5 billion in 2018, a trend is being largely driven by the demand for higher resolution imagery. Similarly, the market for value-added services is expected to grow from $3.5 billion to $6 billion over the same time period. The emergence of big data in Earth observation promises to further support and develop new markets for exploitation and provide new opportunities for innovative companies.
A further market for global connectivity using satellite-based services has also emerged. In combination with the demand for Machine-to-Machine and Internet-of-Things communications, this new market has lead to the proposed development and launch of satellite mega-constellations by the likes of SpaceX and Amazon.
Lower cost electronics and “rideshare” launch opportunities have enabled the re-emergence of small satellites (epitomised by the rise of the CubeSat). These smaller satellites can also be developed faster, allowing academic institutions, smaller companies, and developing nations to access space. Small satellites have since matured towards a mainstream market segment, providing opportunities for cost-effective in-orbit technology demonstration. When launched in greater numbers, for example by Planet and Spire Global, they are also able to provide a new class of observation data with frequent updates and global coverage.
The economic, social, and environmental impact of these space-based observation and communications products and services can be felt globally. Satellites provide value across a wide-range of application areas:
- 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.
Typical LEO spacecraft operate above 500 km to avoid the need for drag-compensation and reduce the effects of aerodynamic disturbances. The International Space Station (~400 km) is a notable exception to this, but is frequently resupplied with propellant used to “reboost” its orbit. The GOCE spacecraft was also equipped with a highly capable and efficient ion propulsion system to continually counteract drag and was able to stay in orbit below 300 km for over 4 years.
Very Low Earth Orbits (VLEO), those lower than approximately 450 km, have otherwise seen relatively little use since the early Cold War reconnaissance satellites. However, research is underway to explore the benefits of returning to these orbits and to address the challenges of operating sustainably at lower altitudes.
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.
The most significant issue is the increase in aerodynamic drag experienced by satellites operating at these altitudes. As orbital altitude is reduced, atmospheric density increases, in turn increasing the aerodynamic drag experienced by orbiting spacecraft and limiting their useful lifetime before they decay and burn up in the atmosphere.
To provide long-term operations, current satellites therefore have to be equipped with propulsion systems and carry enough fuel (propellant) to last the intended mission duration, increasing launch costs considerably. Alternative operational models include on-orbit refuelling operations, but these approaches are still subject to the issues of launch cost and aerodynamic drag.
Variations in the atmospheric density and the presence of thermospheric winds can also have a disturbing effect on the satellite stability and pointing capability. If not compensated for, these effects may have a detrimental effect on image quality or communications networks.
The residual atmosphere in low altitude orbits is also rich in highly-reactive atomic oxygen. In combination with the high orbital velocity and thermospheric temperatures, this atomic oxygen can damage the external surfaces of spacecraft through erosion. Sensitive optics, solar arrays, and antennas can also be adversely affected, reducing the mission performance and lifetime of the spacecraft.
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.
- 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.
- Atmosphere-Breathing Electric Propulsion (ABEP) systems can make use of the residual atmospheric gas as a propellant and provide effective drag-compensation without the limitation of propellant storage on lifetime. The development of such systems is underway at the European Space Agency and the Institute of Space Systems at the University of 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.
A number of these areas of technology development are being addressed within the scope of DISCOVERER, a Horizon 2020 project that aims to enable sustained and commercially viable operation of spacecraft in VLEO, principally for Earth observation applications.
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.
Designs for spacecraft that could make use of these lower altitude orbits have already begun to emerge, for example the Thales Alenia Space “Skimsat” and similar developments by newly established companies Skeyeon and EarthObservant. The DISCOVERER project is also developing novel concepts for spacecraft that could operate in very low Earth orbits. These designs benefit from the emerging technologies that are being developed within the project, for example drag-reducing materials, novel atmosphere-breathing electric propulsion systems, and aerodynamic control surfaces.
When realised, the reduction in cost of the data and services that these systems will be able to provide will increase the accessibility and democratisation of space-based information globally, with potential to have significant positive economic, social, and environmental benefits.
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The DISCOVERER project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 737183. This publication reflects only the view of the authors. The European Commission is not responsible for any use that may be made of the information it contains.
Further Reading
- The Benefits of Very Low Earth Orbit for Earth Observation Missions in Journal of Progress in Aerospace Sciences, 2020.
- RF Helicon-based Inductive Plasma Thruster (IPT) Design for an Atmosphere-Breathing Electric Propulsion system (ABEP) in Acta Astronautica, 2020.
