How low orbit can satellites go?

Europe and Japan are both working to have long duration satellites operating two to three times closer than regular low orbit satellites. This would allow for closer and more detailed pictures from optical cameras and various other instruments could provide more details.

In a world-first, an ESA-led team has built and fired an electric thruster to ingest scarce air molecules from the top of the atmosphere for propellant, opening the way to satellites flying in very low orbits for years on end.

ESA’s GOCE gravity-mapper flew as low as 250 km for more than five years thanks to an electric thruster that continuously compensated for air drag. However, its working life was limited by the 40 kg of xenon it carried as a propellant – once that was exhausted.

Replacing onboard propellant with atmospheric molecules would create a new class of satellites able to operate in very low orbits for long periods.

Air-breathing electric thrusters could also be used at the outer fringes of atmospheres of other planets, drawing on the carbon dioxide of Mars, for instance.

“This project began with a novel design to scoop up air molecules as propellant from the top of Earth’s atmosphere at around 200 km altitude with a typical speed of 7.8 km/s,” explains ESA’s Louis Walpot.

There are no valves or complex parts – everything works on a simple, passive basis. All that is needed is power to the coils and electrodes, creating an extremely robust drag-compensation system.

The challenge was to design a new type of intake to collect the air molecules so that instead of simply bouncing away they are collected and compressed.

The molecules collected by the intake designed by QuinteScience in Poland are given electric charges so that they can be accelerated and ejected to provide thrust.

Sitael designed a dual-stage thruster to ensure better charging and acceleration of the incoming air, which is harder to achieve than in traditional electric propulsion designs.

“The team ran computer simulations on particle behaviour to model all the different intake options,” adds Louis, “but it all came down to this practical test to know if the combined intake and thruster would work together or not.

Japan heading below 200 kilometers to 160 and maybe even 146 kilometers

Japan Tsubame, Super Low Altitude Test Satellite, is a JAXA satellite intended to demonstrate operations in very low Earth orbit (below 200 km), using ion engines to cancel out aerodynamic drag and equipped with sensors to determine atomic oxygen density, an exposure facility to measure material degradation in the 200 km orbit, and a small camera.

Initial designs had conventional, though slightly canted, solar panels (compare to the aerodynamic shape and on-body solar panels of GOCE, which flew in a 255 km orbit).

Low Earth Orbit” (LEO), where many satellites live, goes from 160km (100 miles, 525,000 feet) to 2,000km (1,240 miles, 6.5 million feet). In LEO, we have some sample objects to look at.

Our own Project Calliope satellite will be 230km up (143 miles, 755,000 feet). The International Space Station (ISS) cruises higher up, from 278km (173 miles, 912,000 feet) to 460km (286 miles, 1.5 million feet).

Starting above the ‘space’ limit but a bit before LEO, the inner Van Allen Belts, which magnetically shield the Earth’s surface from high energy particles, extend from 100km (62 miles, 33,000 feet) up to 10,000km (6,200 mil, 3.3 million feet).

If there was more powerful propulsion it could be possible to sustain orbit at about 140 kilometers.

A constant 0.2 Newtons of thrust could allow a satellite to operate at 146 kilometers of altitude.


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