The key payload of the space gravity experiment - the self-developed satellite electrostatic levitation accelerometer.

Produced by: Science Popularization China

Produced by: Space Application Engineering and Technology Center of the Chinese Academy of Sciences, Gravity Laboratory of Huazhong University of Science and Technology, China Science Museum Online

Supervised by: Computer Network Information Center of the Chinese Academy of Sciences

Editor's Note:

The recently successfully launched Tianzhou-1, in addition to docking with Tiangong-2, refueling in orbit, will also carry out a series of space science experiments and technical tests.

The Space Application Engineering and Technology Center of the Chinese Academy of Sciences is the overall unit for the space application system of the manned space program. The Chinese Academy of Sciences is responsible for the planning, implementation, and promotion of results for manned space science and application missions. It specifically undertakes the organizational management of engineering development, system design, integration, testing, reliability assurance, in-orbit technical support, operational control management of payloads, data acquisition, and promotion of application results, among other technical support, support, safeguard, and service work.

Special thanks go out to the Space Application Engineering and Technology Center of the Chinese Academy of Sciences for their support!

Although Tianzhou-1 is a cargo spacecraft, valuable space experiment opportunities should not be wasted, and it still carries a number of scientific experiment payloads.

Among them, the Gravity Center of Huazhong University of Science and Technology has undertaken the project of "Key Technology Verification for Testing Non-Newtonian Gravity Experiments." The "Testing Non-Newtonian Gravity Experiments" aims to use the microgravity environment to test whether the gravitational force between objects at micrometer distances still satisfies Newton's law of universal gravitation. This is a purely fundamental physics experiment that is of great significance for unifying the four fundamental interactions and exploring new interactions. This experiment must first develop high-precision weak force measurement technology. To this end, the project team has developed an accelerometer based on picometer-level capacitive sensing and microvolt-level electrostatic control technology, known as an electrostatic levitation accelerometer. This accelerometer is the technical foundation for non-Newtonian gravity experiments. The purpose of this space experiment is to use the space environment of the Tianzhou-1 cargo spacecraft to conduct in-orbit testing of the high-precision electrostatic levitation accelerometer. Typically, the gravitational acceleration at the Earth's surface is 1g (9.8m/s²). The resolution of the electrostatic levitation accelerometer verified in this project reaches the order of 10⁻¹¹g. Compared to the gravitational acceleration at the Earth's surface, it can resolve acceleration changes to the tenth decimal place, which is extremely precise.

Compared with traditional spring-connected accelerometers, electrostatic levitation accelerometers have higher precision but greater technical difficulty. Currently, only a very few countries, such as France, have mastered all its technologies, which remain strictly closed to our country. Starting from the year 2000, the Gravity Center of Huazhong University of Science and Technology has conducted systematic research on theoretical mechanisms, key technologies, development processes, testing, and calibration over three consecutive five-year plans (10th, 11th, and 12th Five-Year Plans). They have solved all the technologies and ultimately developed a series of electrostatic levitation accelerometers with independent intellectual property rights and different measurement accuracies. During this period, the project team successfully completed flight experiments for SJ-8 and XX-5 satellite-borne accelerometers in 2006 and 2013, respectively, gradually raising China's satellite-borne accelerometer development level to the order of 10⁻⁷g and 10⁻⁹g. The accelerometer on Tianzhou-1 has been improved to the order of 10⁻¹¹g, raising China's satellite-borne accelerometer development capabilities to an internationally advanced level. The high-precision electrostatic levitation accelerometer device verified this time is shown in the figure, consisting of two devices. The probe box achieves high-precision acceleration measurement, while the electronic control box provides power supply to the probe box and realizes data communication functions with the satellite platform.

[Image 1: Probe box and electronic control box of the non-Newtonian gravity experiment test device]

During its flight, Tianzhou-1 is not always in a very stable state, and the electrostatic levitation accelerometer will be interfered by the vibration noise of the spacecraft. Fortunately, this experiment has an excellent partner. Tianzhou-1 also carries an active vibration isolation device developed by the Space Application Engineering and Technology Center of the Chinese Academy of Sciences. This device can isolate spacecraft vibration interference and provide a quiet experimental platform for the electrostatic levitation accelerometer. At the same time, this vibration isolation device is also part of this space experiment, and the electrostatic levitation accelerometer will provide testing and evaluation for this vibration isolation device. The electrostatic levitation accelerometer and the active vibration isolation device complement and support each other.

High-precision space accelerometers are one of the essential key payloads for space gravity experiments as weak force measurement sensors or inertial references. For example, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) working group reported twice in 2016 detecting gravitational waves emitted during the merging of two black holes, attracting worldwide attention. Gravitational wave detection and gravitational wave astronomy are poised to become one of the new scientific and technological commanding heights. Space gravitational wave detection will utilize multiple spacecraft in formation spanning hundreds of thousands to millions of kilometers to detect gravitational waves. This wavelength has a richer source of gravitational waves and has become one of the research hotspots in this field. In space gravitational wave detection, higher precision is needed to measure and control the acceleration disturbance of spacecraft, reaching 10⁻¹⁵m/s² or even higher. Inertial sensors based on capacitive displacement sensing and electrostatic feedback control have become development solutions.

This flight experiment will further lay the foundation for conducting experiments such as "testing non-Newtonian gravity," "testing the equivalence principle in space," and "detecting gravitational waves in space" on the space station.

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