Thanks to technological advancements, the market for high-performance/low-cost ultra-small satellites (1 to 100 kg) is expanding. However, most small satellites currently do not carry any propellant and therefore cannot actively maintain orbit or position. As a result, it is impossible to extend their service life or withdraw them from their orbit. The latter issue leads especially to the accumulation of space debris, which is becoming a serious problem.
Installing a propulsion system on a small satellite can solve these problems. However, the propulsion systems currently installed on large satellites are difficult to use with small satellites due to capacity, weight, and cost. They also use high-pressure gas or toxic substances as propellant, which raises issues for the environment and sustainability.
A solution gathering some attention is a small propulsion system using water as propellant, developed by the start-up Pale Blue, which is a spin-out from the University of Tokyo under CEO Jun Asakawa. It abandons the conventional high-pressure or toxic propellants and instead uses water, which can be stored at low pressure and is safe, non-toxic, and easy to obtain. This approach solves the issues above, while leading to overwhelming reductions in size and cost at the same time.
Small water-based propulsion systems for satellites have been researched by other companies as well, but most of them focused on systems to heat a two-phase gas-liquid flow inside a channel and separate the gas and liquid. These systems, however, sometimes could not achieve sufficient separation and were not operational in orbit. In addition, the amount of latent heat (the heat needed for vaporization) made it hard to install them on small satellites with tight, strict electric power restrictions.
During his PhD program at the University of Tokyo, Asakawa developed a water-resist jet propulsion system by using a water vapor supply mechanism with a gas-liquid separation space called the “vaporizing chamber” that was able to solve the problems mentioned above. The vaporizing chamber functions in low-pressure and room temperature conditions and supplies vapor by making use of the waste heat from heating elements inside the satellite as latent heat. In 2019, the demonstration satellite “AQT-D” carrying this propulsion system, made small enough to fit into a 10cm square, was launched as the world’s first ultra-small satellite with a propulsion system allowing for orbit transfers and was successfully dispatched from the International Space Station.
In April 2020, Asakawa decided to use this research to contribute to society, and together with members of his research team at the University of Tokyo founded Pale Blue. The company does both business and research through joint demonstration experiments with private businesses and demonstration programs in space with the Japan Aerospace Exploration Agency (JAXA). The “water-propelled ultra-small integrated propulsion system” developed by Pale Blue was selected by JAXA as a demonstration project for its “Innovate Satellite Technology Demonstration Aircraft 3” to be launched in 2022. This propulsion system integrates a water ion propulsion system, which can make large orbit changes, and a water-resist jet propulsion system, which can make quick and small orbit changes and propel in multi-axis directions, into a single component.
Asakawa says, “Small propulsion systems have been the limiting factor for the use of small satellites. By bringing innovation in this field, we hope to enable business based on small satellites and deep space exploration. We want to use scientific technology to maximize human happiness and contribute to civilization.”