What goes up must come down. And of you are a low flying satellite, you come down even quicker. Despite these low orbits being especially good for ultra high-resolution observation of the Earth, or fast data transmission for commercial networks, the problem of higher drag in lower orbits has always shortened the time such an expensive and high value asset could be used. Until now, that is.
Companies associated to ESA have now shown proof of concept of an electric engine that can keep low flying objects in orbit indefinitely. It even has exciting applications around MARS, Titan, Venus or any object with an atmosphere. These all become refueling stations on DEEP SPACE missions.
Falling down
Objects put in Low Earth Orbit experience residual drag from the atmosphere around Earth. The ISS, even flying at 400 km above Sea Level, loses around 50 meters of altitude per day. To compensate the NASA Shuttle, European ATV and the Russian Soyuz were or are used to Reboost the ISS. The ISS also has its own thrusters. Refueled by docking spacecrafts they are fired about once every month depending on the varying drag encountered.
Geostationary satellites, which experience no such drag, often stay on task for fifteen years or more. Equipped with very reliable and redundant electronics, their life is only limited by the amount of station keeping propellant they can take on-board. Earlier this decade, the much lower flying aerodynamic and expensive GOCE satellite (***), with much effort managed a little under 5 years before it decayed, while otherwise being in perfect health. Again, the higher amount of drag and limited propellant were the main culprit for its fast decay.
It would be preferable if we no longer have to worry about drag or propellant budgets, and could replace the launch weight and cost of the (reboost-) propellant – a cost said to be around 100 million/year for the ISS – with high value mass like science equipment, food and crew. We would end up with a longer scientific or economic life. An engine technology that makes all this possible would be revolutionary. ESA is testing just such a revolutionary engine.
Jets in space
While jet fighter turbines need to compress dense air and heat it up to produce thrust, ESA developed an electric equivalent that can suck up rarefied air molecules in the upper atmosphere, compress it and pushes it through an ion thrusters. Suddenly simple air electrified with energy from solar panels becomes a rocket propellant.
The idea is not new. Engineers have always dreamt of simply scooping up air from the lower and denser atmosphere to fill the propellant tanks of an orbiting spaceship. But the maneuver was always considered to be too risky as one could end up burning up in the atmosphere or returning to the surface much sooner than anticipated. Also the net thrust one could gain from such an engine obviously must be bigger than the drag to be overcome.
A proof of concept engine able to attract and compress a sufficient amount of molecules electrically without hair-raising maneuvers has now been demonstrated on Earth. The resulting device is called an ELECTRIC RAMJET (*) and the device looks a lot like the collector dish (the bussard scoop) we see on the ENTERPRISE ships in the STAR TREK Franchise, albeit on a far smaller scale. The device also does not need nuclear power, as some old ideas proposed. Solar power will do just fine.
Purple haze
Besides the onboard Xenon, the device can shift to using a nitrogen–oxygen air mixture which it can scoop up from the layers in the upper atmosphere. As such it becomes a versatile, multi-propellant or flex-propellant design.
The intake of the engine was devised by Quintescience of Poland (****). One of the tasks was to prevent the energetic molecules from simply bouncing off. The company Sitael of Italy integrated it in a dual-stage thruster design, which ensures better charging and acceleration of the incoming air, a harder challenge than in other electric propulsion engines. They tested the device in their vacuum chamber test facilities simulating operation of the device at 200 km.
The engineers were elated when they saw the traditional blue haze from Xenon engines turn into a purplish color, a sign the device was generating thrust using nitrogen and oxygen. If they can one day prove the design in orbit, the need to trash satellites a long time before the end of their useful life will become a thing of the past.
Refueling in orbit around MARS before we go to Titan
But the use cases go further. An up-scaled version could help the ISS or any space station stay in orbit for longer without refueling. Further along, the technique is useful, not only around Earth but around any planet with a gaseous atmosphere. The more the orbit decays, the easier it becomes to scoop up air.
Because satellites and scientific probes have to worry less about the propellant they carry, more exotic mission scenario’s can be envisioned.
Imagine a satellite able to simultaneously fill its propellant tanks, while using the available air molecules to keep its altitude. This would imply that any object with an atmosphere becomes a refueling station. No need to ever land on the surface. Satellites flying on fumes could spiral inwards to the denser parts of the atmosphere, scoop up propellant, and spiral outwards again to return to their station. And if we would like to refuel in orbit around MARS before we venture out deeper into space, we could.
A satellite similar to the NASA DAWN satellite, which spent years in the asteroid belt circling around VESTA and CERES, and which now has largely depleted its ion and hydrazine propellant, could decide to return to MARS, scoop up propellant from its atmosphere, and return to study other objects in the asteroid belt. The mass saved would have been used to double or triple the science package.
Probes could do this on a Mission around Titan, Saturn, Mars, Venus, Earth, before they sets course to another deep space destination. We could extend the life of science satellites as long as they remain healthy and have them visit far more destinations.
Comparing apples to apples
It also solves the curse of specialization. Mass limited scientific satellites can only send a handful of specialized instruments, usually prototypes, on one satellite to one or a handful of destinations. It often takes years before similar instruments are used at different objects in our solar system making comparison of data and characteristics , beyond educated guesses, virtually impossible. But with this type of engine, several specialized satellites, each with their own strengths, could be sent in succession to a large variety of celestial objects far apart. Space exploration would become much more efficient and the amount of acquired data would grow exponentially. Just imagine what you would do with this revolutionary capability.
Footnotes:
(**)Esa news article, dated 5 March 2018,World-first firing of air-breathing electric thruster.
(***) GOCE: Gravity Field and Steady-State Ocean Circulation Explorer. The measurements required flying in a very low orbit. Launched 2009 it decayed in 2013, despite all design decisions and precautions being taken to lengthen its life as long as possible. Further introductory reading, wikipedia: GOCE
(***) Quintescience website (link) and a relevant paper (.pdf) on their engine intake design: , Conceptual design of an air-breathing electric propulsion system, in Proc. 34th International Electric Propulsion Conference, Paper No. 2015-271, Kobe (Japan), 2015
