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Japan Aerospace Exploration Agency

Research and development

Research and development of an ion engine for a super-low-altitude satellite

Research and development of an ion engine for a super-low-altitude satellite

Space Power System Group

Space Power System Group

(From left)
Katsuhiro Miyazaki, Hiroshi Nagano, Kenichi Kajiwara, Yasushi Okawa

Increasing visual acuity of a satellite by lowering its orbit

There are various types of "earth observation satellites", which observe every part of the globe from space. JAXA has launched earth observation satellites such as the greenhouse gases observing satellite "Ibuki", which monitors carbon dioxide, regarded as the principle cause of global warming, as well as the advanced land observing satellite "Daichi", which is focused on mapping and resource surveying.
Many earth observation satellites observe Earth by traveling in an orbit that passes the North and South Poles at a right angle to the equator; this is at an altitude between 600 and 800 km. From that height, Earth is observed with an optical telescope. If the altitude of the satellite could be lowered, the "resolution", i.e., the visual acuity of the telescope, could increase since Earth would be closer. For example, if the altitude of a satellite equipped with an optical telescope having a resolution of 2.5 m was lowered from 800 km to 200 km, an object of about 60 cm could be distinguished. (Fig. 1)
In addition, there is the problem that the laser, when it is used for observation, becomes diffused because of the high altitude. However, by lowering the satellite altitude, this diffusion can be suppressed, enabling high-accuracy observation. This is another advantage of lowering the orbit altitude.

Fig.1 Concept of super-low-altitude satellites

Fig.1 Concept of super-low-altitude satellites

How can atmospheric drag be overcome?

Since 2006, JAXA has been studying development of an artificial satellite that orbits at a low altitude of about 200 km. During this time, we have identified two problems that must be resolved. One is "atmospheric drag". Space is generally defined as beginning from an altitude of 100 km, but, in fact, it is very unclear at what point the sky ends and space begins. By definition, the target altitude of a super-low-altitude satellite, 200 km, is in space, but there is still some small measure of atmosphere. Since this atmosphere creates resistance, a satellite would fall down to Earth in a few days without any thrust. Therefore, a super-low-altitude satellite needs an engine that produces thrust.
Unlike when launching an artificial satellite, flying a satellite at a low altitude does not require a large thrust. It would be enough to generate a thrust that counteracts the atmospheric drag. However, since the satellite will be used for a long time on a specific orbit, a fuel-efficient engine that can generate thrust for a long period of time is ideal. There is an engine suited for this task. The "ion engine" is used to maintain the north-south orbit by geosynchronous orbiters such as the Engineering Test Satellite VIII, "Kiku-8".
The ion engine produces thrust through the reaction force obtained when ions accelerated by an electrical force are expelled. It consists of an "ion generator" (generates ions from a propellant such as xenon), an "ion acceleration system" (accelerates those ions), a "neutralizer" (emits electrons for neutralizing the ions expelled into space), seven "power sources" (operate the above three sections), and a "controller" (controls the power sources). (Fig. 2)

Fig.2 Ion engine

Fig.2 Ion engine

The problem with oxygen when it becomes an atom

Fig.3 Super-low-altitude test satellite 'SLATS'

Fig.3 Super-low-altitude test satellite "SLATS"

At a high altitude, oxygen in the atmosphere becomes atomic oxygen, which occurs when the two atoms separate. Atomic oxygen is an extremely strong oxidizing agent and may cause malfunctions in satellites. Since there is nearly 1,000 times more atomic oxygen at an altitude of 200 km than at 600 km, its effects are a concern. However, the reality of how it affects the satellite surface or interior is not clearly known.
Resolving the two problems of atmospheric drag and atomic oxygen is the key to realize a super-low-altitude satellite. Therefore, we are researching and developing the "super-low-altitude test satellite (SLATS)" (Fig. 3) with the objective of demonstrating how to keep a super-low-altitude orbit with the ion engine and obtaining distribution data for atomic oxygen. In order to reduce the atmospheric drag as much as possible, the ion engine mounted onto SLATS has been designed to be compact by building the operation controller into the power source as well as using the newest materials for parts. In addition, it is equipped with various new developments enabling flight at a super-low altitude, for example, functions that allow autonomous operation of the ion engine through the on-board use of detected orbit data, since the communication time with the ground station is reduced. For maintaining the orbit, we have found that a circular orbit can be stably maintained with thrust on/off operation for each revolution, and the satellite is expected to operate in orbit with that principle applied. The orbit keeping test period for SLATS is scheduled to be about 100 days. We will conduct tests of 7 to 50 days each at altitudes between 250 and 180 km and obtain the necessary data for practical use in addition to photographing Earth using optical sensors. Focusing on a launch in 2013 (JFY), we are currently conducting performance tests of the ion engine hardware. Moreover, with an eye toward practical satellites after SLATS, we are conducting research to extend the service life over 20,000 hours.

Fig.4 SLATS testing schedule (planned)

Fig.4 SLATS testing schedule (planned)

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