Strategic study on the development of space science in China and proposals for future missions

In recent years, China's space science has entered the fast lane of innovation and development. With the implementation of a batch of space science missions, a series of original scientific results have been achieved. Chang’E-4 realized the world's first soft landing and roving on the far side of the moon, Chang’E-5 returned China's first sample from an extraterrestrial body, and Tianwen-1 left the imprint of a Chinese spacecraft on Mars for the first time. China Space Station has completed its in-orbit assembly and entered a new era of application and development, with the deployment of scientific experiment cabinets and extravehicular exposure platforms, as well as the Chinese Survey Space Telescope (CSST).

Since 2011, SPP, led by CAS, has been successfully implemented. The scientific achievements of the missions in the program, such as DAMPE, QUESS, ShiJian-10 (SJ-10) and Hard X-ray Modulation Telescope (HXMT), Taiji-1, the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM), ASO-S, EP, etc., have significantly improved our understanding of the Universe.

To foster the high-quality development of space science, the Chinese space science community has conducted strategic research, resulting in a series of outcomes that lay a foundation for planning future missions.

Section 2 introduces China's space science development strategy. Since 2022, CAS has organized scientists across the country to conduct a new round of study on the medium and long-term development planning for space science activities in China. It has sorted out the major scientific frontiers in space science and, and identified five major scientific themes for China to make breakthroughs in the future.

Major space science frontiers are summarized. The major scientific frontiers in areas of space astronomy, heliophysics, lunar and planetary science, Earth science, microgravity physics and space life sciences can be summarized as “One Black, Two Dark, Three Origins and Five Characterizations”.

The aim of “One Black” is to explore dense celestial bodies, as represented by black holes, and the physical laws under extreme conditions in the universe. “Two Dark” refers to dark matter and dark energy, the natures of which are poorly understood. “Three Origins” focus on the origin of the universe, the solar system and life. “Five Characterizations” explores the characteristics and working laws of the near-Earth system, Earth-Moon system, the solar system, and the extra-solar system, as well as the laws governing matter movement and life activity in the space environment.

China’s main scientific themes are presented. China's space science will focus on five scientific themes, namely, “Extreme Universe”, “Space-Time Ripples”, “the Panoramic View of the Sun and the Earth”, “Habitable Planets”, and “Biological & Physical Science in Space”.

The theme “Extreme Universe” explores the origin and evolution of the universe, and reveals the physical laws of the universe under extreme conditions. The main scientific questions to be addressed include the nature of dark matter particles, the sources of high-energy radiation in the universe, the nature of dark energy, the physical mechanisms of dynamic cosmic detections and transient sources, the dark age of the universe, and the history of reionization.

The theme “Space-Time Ripples” detects low and medium -frequency gravitational waves and primordial gravitational waves in order to reveal the nature of gravity and space-time. The scientific questions to be addressed include the formation of supermassive black holes and their co-evolution with host galaxies, as well as the testing of early cosmological models.

The theme “the Panoramic View of the Sun and the Earth” explores the Sun, Earth and the heliosphere, and reveals the physical processes and laws governing the complex system of the Sun-Earth and the Sun-Solar system connections. The main scientific questions to be addressed include the structure and coupling processes of the solid Earth's inner layers, the characteristics of the solar magnetic field and the mechanism of the origin of the solar magnetic cycle, and the processes and mechanisms of solar wind and interstellar medium interaction.

The theme “Habitable planets” explores the habitability of solar system planets and exoplanets, and searches for extraterrestrial life. The main scientific questions to be addressed include the mechanism of interaction between human activities and the natural environment, the composition of the Moon’s deep matter and its layer structure, the origin and evolution of asteroids and comets, the evolution of habitable environments and potential life signals on Mars, and the habitability and life signatures of exoplanets.

The theme “Biological & Physical Science in Space” reveals the laws of matter movement and life activities under space conditions, and deepens the knowledge of fundamental physics such as quantum mechanics and general relativity. The scientific questions to be solved mainly include fire safety in manned spaceflight, preparation of high-performance materials and the behavior of materials in extreme space environments, quantum effects in the gravitational field, basic medical problems of medium- and long-term spaceflight and residence, and research on special biotechnology in space.

Section 3 is about proposals for China’s near future space science missions. Based on the China's development strategy study for space science, a series of new space science satellite missions have been proposed for the near future, which will focus on major scientific issues in the category of "origins", such as the origins of the universe, the origins of space weather, the origins of life, etc. It is proposed that by 2030, in-orbit scientific exploration is expected to yield major discoveries and original results in the following directions, such as the dark age of the universe, the solar magnetic activity cycle and high-speed solar wind, Earth-like exoplanet, the physical laws of the universe under extreme conditions, and the nature of gravity and space-time.

Discovering the Sky at the Longest wavelength (DSL). With 1 mother satellite and 9 daughter satellites operating in a 300 km circular lunar orbit, DSL takes advantage of the highly clean electromagnetic environment on the far side of the Moon to conduct ultra-long-wave radio astronomy observations, which will provide key observational evidence to open a new window to image the dark age of the universe with MHz radio waves.

Solar Polar Orbit Observatory (SPO). With the implementation of the solar polar orbit observation system above the ecliptic plane, SPO aims to achieve the first-ever frontal imaging of the solar polar region, so as to make crucial breakthroughs in research on the origin of solar magnetic activity cycles and the origin of high-speed solar wind.

The Earth 2.0 (ET). A space observatory at the Earth-Sun L2 point conducts a long-term, large-scale survey of exoplanets, to explore habitable Earth-like planets outside our solar system, and target candidates for extraterrestrial life search.

Enhanced X-ray Timing and Polarimetry (eXTP). The mission will conduct research on the detection of dense celestial bodies, such as black holes and neutron stars, with high time resolution, high energy resolution and high-precision polarization, so as to explore the physical laws and principles under the extreme conditions in the universe, including extreme gravity, magnetism, and density.

Taiji-2. A three-satellite constellation in a 60-million-kilometer heliocentric orbit, constituting a 3-million-kilometer ultra-long baseline space-based gravitational wave observation system, detects millihertz (0.1mHz～1.0Hz) gravitational waves in space.

In addition, with the successful implementation of the Chang’E-6 lunar sample-return mission, the future missions are also being developed, such as the Chang’E-7 Lunar Polar Regions Exploration, Tianwen-2 near-Earth asteroid sample return and main-belt comet exploration mission, the International Lunar Research Station (ILRS), and the China Space Station for Space Science and Applications.

International cooperation is introduced in Section 4. The peaceful use of outer space is a matter of concern for the well-being of all humankind. Since space exploration entails high investment and risk, a country cannot and should not be alone in its own endeavors. In the 21^st century, international cooperation in space science has received increasing attention, and almost all flagship space science missions incorporate elements of international cooperation, which not only reduces a country's space investment and risks, but also multiplies scientific outputs, facilitates the country's scientific and technological diplomacy, and has become an important part of international foreign policy.

Missions such as the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) and the Einstein Probe (EP) involve in-depth international cooperation. Missions such as the planned DSL, ET, and eXTP will also involve specific international cooperation in scientific research. Chang’E-4, Chang’E-5, and Chang’E-6 missions of the Lunar and Planetary Exploration Project have carried onboard international payloads from other countries, and have produced influential scientific results. The China Space Station has also invited other countries to participate in international cooperation, and projects from Switzerland, Germany, Italy, and other countries have been selected.

In the future, China will actively initiate, propose and participate in international cooperation projects. Through mutual cooperation across missions, joint observations and scientific experiments, joint payload development, testing and calibration, or flying piggyback payloads on each other’s platform, open access of scientific data, and joint scientific teams, China will engage in multi-level international cooperation to fully leverage the platforms such as China Space Station and ILRS, to make significant contributions to answer fundamental scientific questions.

Space science in China is advancing from the initial stage into a new era of accelerated development. The release of the first national medium and long-term plan for space science is imminent, and a series of new space science satellite missions are about to be approved for implementation. China will continue to promote international scientific and technological cooperation to explore the vastness of the universe, and contribute more Chinese wisdom and solutions to the peaceful use of outer space.

See the article：

Strategic Study on the Development of Space Science in China and Proposals for Future Missions

https://doi.org/10.11728/cjss2024.04.2024-yg19

https://www.cjss.ac.cn/cn/article/doi/10.11728/cjss2024.04.2024-yg19