The European Space Agency is funding 12 research projects to develop technologies that will enable satellites to operate for longer in very low Earth orbits, at altitudes below 450 kilometres.
The projects focus on innovative electric propulsion systems, corrosion-resistant materials and designs that reduce air resistance, as well as new applications for Earth observation and communications. The need for these technologies arises from the conditions faced by satellites operating very close to Earth.
At an altitude of no more than 150 kilometres, a satellite can travel at more than 28,000 kilometres per hour and complete an orbit around the planet roughly once every 88 minutes, but it gradually loses altitude as it collides with scattered air particles in the upper atmosphere. Satellites do not all operate in the same orbital range, as the choice of altitude is linked to the nature of the mission.
Different orbits determine satellite functions
Low Earth orbits, known as LEO, are at altitudes below about 2,000 kilometres and host the International Space Station and a large number of imaging and communications satellites. Medium Earth orbits, or MEO, are used to operate navigation satellites, including satellites in the Global Positioning System, or GPS.
At an altitude of nearly 36,000 kilometres, satellites operate in geostationary orbits, which are used for television broadcasting, communications and weather monitoring. These satellites move above the equator at the same angular speed as the Earth's rotation, so they appear stationary to an observer above a specific location, despite continuing to orbit the planet.
Satellites closer to Earth need a higher orbital speed than those farther away. At an altitude of 150 kilometres, orbital speed reaches about 7.81 kilometres per second, and completing a full orbit takes roughly 87.5 minutes. At an altitude of 400 kilometres, speed falls to about 7.67 kilometres per second, while the orbital period extends to about 92 minutes.
By contrast, a geostationary satellite travels at a speed of about 3.07 kilometres per second and completes its orbit in about 23 hours and 56 minutes. These calculations concern ideal circular orbits and do not mean that conventional satellites can remain for long at an altitude of 150 kilometres without engines to compensate for atmospheric drag.
Atmospheric drag lowers orbital altitude
Satellites in normal orbital conditions do not need to run their engines continuously to keep orbiting, because their horizontal speed keeps them in continuous free fall around Earth. But the atmosphere does not end at a sharp boundary; its density gradually decreases with increasing altitude, and scattered particles remain even in the regions where low-orbit satellites operate.
Satellites close to Earth collide with these particles and experience drag that drains part of their orbital energy, gradually lowering their altitude. The effect becomes stronger as a satellite gets closer to Earth, and also varies with atmospheric density, which is itself affected by solar activity, making it necessary to monitor the orbit and make corrections when required.
For this purpose, navigation and control systems track the satellite's path and use small engines to replace the energy it has lost. When a satellite drops below the altitude set for its mission, its engine can be fired in the direction of travel to increase its orbital energy and return it to the required level, rather than allowing it to continue descending towards the denser layers of the atmosphere.
The European Space Agency previously applied this approach to its GOCE scientific satellite, which operated in a very low orbit at an altitude of about 250 kilometres. The satellite used an ion engine to compensate for atmospheric drag during its mission. However, keeping the engines running in this way normally requires carrying propellant, and once the available supply runs out, the mission's lifespan becomes limited.
Air-breathing propulsion addresses propellant depletion
Conventional propellers are not a suitable solution to this problem, although using them may seem straightforward. A propeller needs air of sufficient density to push backwards, while the atmosphere is extremely thin at altitudes of hundreds of kilometres.
The operating conditions for a propeller facing particles flowing at high orbital speed also differ fundamentally from the environment in which aircraft propellers operate. The agency is therefore studying a more complex technology called atmospheric air-breathing electric propulsion.
It involves collecting the scattered air particles encountered by the satellite as it moves, then feeding them into an electric engine that uses energy to accelerate and expel them backwards, generating thrust equal to the drag acting on the satellite and maintaining its altitude. In this way, the air particles causing the satellite to descend become propellant that can be used to stabilise its orbit, rather than relying solely on a supply carried from launch.
But the engine cannot operate without a source of energy. It needs electricity, which can be generated by solar panels, as well as a highly efficient system for collecting and accelerating the particles. The European Space Agency has tested the principle experimentally on Earth, but building an integrated, reliable system capable of operating for long periods in orbit remains an engineering challenge.
Low orbits improve imaging and communications
The difficulties are not limited to the engine itself, but also include ensuring that the satellite's components can withstand the surrounding environment and the impact of particles with which it continually collides during its rapid movement. Very low orbits offer benefits beyond testing propulsion technologies.
A satellite's proximity to Earth's surface allows it to capture more detailed images using imaging equipment similar to that operating at greater altitudes, while also reducing signal transmission times, which could improve the performance of communications and internet services. One European team is developing a group of satellites capable of providing near-continuous monitoring of Earth's surface, producing images with a resolution of up to one metre and transmitting data with low latency.
The funded projects also include developing sensors to measure the effects of atomic oxygen, a highly reactive form of oxygen capable of damaging some materials and coatings used in spacecraft. Researchers are also studying more streamlined structural designs to reduce air resistance and slow the loss of orbital energy.
These designs are particularly important at very low altitudes, because reducing the frontal area and improving the satellite's shape could ease the burden on the propulsion system and improve its ability to remain in orbit. One potential advantage of these orbits is that, once a satellite stops operating, it naturally loses altitude because of atmospheric drag, then enters denser layers and burns up, rather than remaining in space for many years.
This could help reduce the accumulation of space debris, although satellites operating in these orbits would remain exposed to collision risks. The projects form part of the European Space Agency's technology vision through 2040, but they do not mean that satellites using atmospheric air are ready for widespread commercial deployment.
A move towards practical use depends on the successful development of propulsion and particle-collection systems, materials and designs capable of operating efficiently and reliably for long periods in the very low-orbit environment.
