Progress of Planetary Defense Research in China*

LI Mingtao WANG Kaiduo

(National Space Science Center, Chinese Academy of Sciences, Beijing 100190)

(University of Chinese Academy of Sciences, Beijing 100049)

Abstract Near-Earth Asteroids (NEA) impose potential major disaster to humanity.Planetary defense is an inevitable requirement for the survival of human civilization.In recent years,China has made rapid progress in planetary defense research,which has won the attention of the government and attracted more and more scholars and organizations.This paper summarizes the research progress in planetary defense in China in recent years,including the fireball events in China,academic activities and policy planning,monitoring and warning technology,onorbit defense technology,impact hazard assessment,international cooperation and science popularization.

Key words Near-Earth asteroids,Planetary defense,Kinetic impact,Space-based observation,Hazard assessment

1 Introduction

Near-Earth Asteroid (NEA) impact is a major potential threat to human society.NEA have impacted our planet Earth numerous times in history,profoundly shaping the evolution of the Earth’s climate and ecological environment,and triggering the extinction and evolution of species.66 million years ago,an NEA with a diameter of about 10 kilometers impacted the Gulf of Mexico in North America,resulting in the extinction of more than 70% of the global species,including dinosaurs[1].It is also because of this impact that human beings have the opportunity to dominate the Earth and develop today’s highly developed human civilization.If we do not want to repeat the fates of dinosaurs,we must pay attention to the threat of NEA impacts.

Even if a small-scale NEA impact will bring significant disaster.On 15 February 2013,an NEA with a diameter of about 18 m exploded above the Chelyabinsk region of Russia,causing damage to nearly 1500 people and 3000 houses[2].If it happened in densely populated regions,the consequences will be even more serious.

It has become a consensus all over the world to strengthen the response to NEA impact risk.After the Chelyabinsk impact in 2013,two organizations,IAWN(International Asteroid Warning Network) and SMPAG(Space Mission Planning Advisory Group),were established under the promotion of the United Nations Office for Outer Space Affairs (UNOOSA).IAWN aims to coordinate global organizations to discover and warn potential threat NEA that may impact the Earth;SMPAG aims to research and develop defense means against NEA impact.At present,the United States and Europe are leaders in global planetary defense. The United States has discovered 98% of the total discovered NEA,and has launched the DART (Double Asteroid Redirection Test) mission on 24 November 2021.It is expected that it will impact the moonlet of the Didymos double asteroid system by the end of September 2022.This is also the world’s first dedicated planetary defense mission.As part of the AIDA (Asteroid Impact Deflection Assessment) program of the US Europe cooperation,Europe will launch the Hera mission in 2024 to conduct a detailed rendezvous survey of the Didymos double asteroid system to better evaluate the impact effect.

In recent years,China has made rapid progress in planetary defense. The Chinese government joined IAWN and SMPAG[3]in 2018.On the China Space Day in 2021,ZHANG Kejian,director of the China National Space Administration(CNSA),pointed out in his speech that China would build a NEA defense system.Chinese scholars have also made significant progress in NEA monitoring and warning,on-orbit defense means,risk assessment,etc.This report will provide an overview of the above progress,focusing on the achievements since 2020.

2 Recent Fireball Events

From 2017 to 2021,there were five eye-catching fireball events,including the Shangri-La event in 2017,the Xishuangbanna event in 2018,the Songyuan event in Jilin in 2019,the Yushu event in Qinghai in 2020 (seeFig.1),and the Henan event in 2021.

Fig.1 Yushu fireball event in 2020

These fireball events did not cause any threat or property damage,but they attracted public attention and were widely spread on the internet.At the same time,these fireball events also attracted the attention of scientific research experts and governments.Scientists studied and interpreted the fireball event.The government has gradually paid attention to the threat of NEA impacts as a serious scientific issue and potential disasters that may occur in the future.

3 Academic Activities and Policy Planning

In the past few years,Chinese academia has paid more and more attention to the issue of NEA defense.

In 2018,the China Aerodynamics Research and Development Center (CARDC) organized a seminar on“ Hypervelocity Problem of Asteroid Impacting the Earth”.The seminar later evolved into a National Symposium on Planetary Defense (NSPD).The 2nd NSPD was held in Beijing in 2019,also organized by CARDC;and the 3rd NSPD was held in Nanjing in 2020 by Purple Mountain Observatory,Chinese Academy of Sciences (PMO,CAS).In 2021,the fourth NSPD,also the 1st China Planetary Defense Conference (CPDC) was held in Guilin,Guangxi province. Academician WU Weiren of the Chinese Academy of Engineering served as the chairman of the conference,with more than 300 participants.

In 2018,the 634th Xiangshan Science Conference was held,with the theme of frontier scientific issues and key technologies in NEA monitoring and warning,defense and resource utilization[4].More than 40 experts and scholars from nearly 30 organizations attended the meeting.China National Space Administration (CNSA)also had representative in the meeting.

In terms of government activities and policy planning,there is also much progress.

In 2018,CNSA officially joined IAWN and SMPAG,marking that the government officially paid attentions to NEA impacts.In 2019,“Investigation,Defense and Utilization of Small Bodies” were selected as one of the 20 major scientific issues and key technical problems issued by the China Association for Science and Technology (CAST)[5].

On China space day in 2021,ZHANG Kejian,director of the CNSA,pointed out in his speech that China would build an NEA defense system.This was the first time that NEA defense system was announced by Chinese government.

On China Space Day in 2022,WU Yanhua,deputy director of CNSA,pointed out that a NEA impact mission would be implemented in 2025 or 2026.This was the first time that a NEA impact mission was announced by Chinese government.

In 2022,the Information Office of the State Council issued the white paper “China’s Space Activities in 2021”,pointing out that the NEA defense system would be constructed in future.

In 2022,the Deep Space Exploration Laboratory(DSEL) was kicked off in Hefei,Anhui province.It was mentioned in the press release that DSEL would be responsible for leading the feasibility study and implementation of major projects such as the NEA defense system.

In 2022,academician WU Weiren published an article in Strategic Study of CAE journal[6],presenting the strategic thoughts on NEA impact risk mitigation,including monitoring and warning,on-orbit defense and hazard assessment,and making a prospect for future development.

4 Monitoring and Warning Technology

In 2018,the 2.5 m large field of view sky survey telescope jointly operated by the University of Science and Technology of China (USTC) and PMO was kicked off in Lenghu and was expected to be completed in 2022.In 2021,the National Astronomical Observatory,Chinese Academy of Sciences (NAO,CAS) and other organizations discovered that Lenghu is a world-class optical observatory site,laying the foundation for the construction of optical sky survey telescopes in China.

China is developing the Chinese Space Station Telescope (CSST),which is expected to have powerful capability to make follow-up observations of NEA.The CSST also has the potential to discover NEA if the survey strategy is dedicatedly designed.The commercial space enterprise “Origin Space” launched the “Yangwang 1st” space-based telescope in 2021,which claimed to have performed the survey and observation of NEA.Optical satellites of other commercial space enterprises are also said to have the capability to observe brighter NEA.

In terms of NEA discovery,China now has only one dedicated ground telescope,the Chinese Near-Earth Objects Survey Telescope (CNEOST) of PMO.CNEOST is a 1.2 m aperture telescope in Xuyi,Jiangsu Province.Using CNEOST,PMO has discovered 5,5 and 1 NEA in 2020,2021 and 2022 respectively.PMO,Yunnan Observatory,and NAO also studied the NEA characteristics observation,including spectral observation and light curve observation.

Qianxuesen Space Technology Laboratory (Qian-Lab)[4],National Space Science Center,Chinese Academy of Sciences (NSSC,CAS)[7],PMO[7],NAO[8]and other organizations studied space-based monitoring mission concepts.QianLab proposed Constellation of Heterogeneous Wide-field Near-Earth Object Surveyors(CROWN) mission concepts[4],a hybrid constellation including a main satellite telescope and several micro satellite telescopes in the Venus-like orbits.

The NSSC and PMO have cooperated to propose the Earth-leading orbit telescope mission concept[7].The telescope is proposed to be deployed about ten or twenty million kilometers in front of the Earth.Simulation shows that it has a good warning efficiency on NEA approaching the Earth from the direction of the Sun.It is of great significance to compensate for the blind sky region of ground-based optical observatories and improve the completeness and integrity of the NEA monitoring and warning system.Fig.2showed Earth leading orbit telescope mission concept.

Fig.2 Earth leading orbit telescope mission concept

5 On-orbit Defense Technology

NSSC[9,10],Beijing Institute of Spacecraft Environmental Engineering (BISEE)[11],CARDC[12,13]and other organizations have researched on on-orbit defense technology of NEA.Progress has been made on the evaluation of applicable scenarios of typical planetary defense means,and new concepts on-orbit defense mission concepts.

In view of the disadvantages of the traditional kinetic impact means that is difficult to deal with largescale NEA under short-term warning time,NSSC proposed two new mission concepts,namely the “ Enhanced Kinetic Impactor (EKI)”[8]and “Assembled Kinetic Impactor (AKI)”[9],the orbital deflection distance of NEA can be increased to 3–10 times,providing innovative solutions for dealing with large-scale NEA under short-term warning time.

The EKI mission captures a stone with the mass of about 200 tons to form an enhanced kinetic impactor,the orbit deflection distance can be increased to 10 times compared with the traditional kinetic impactor (seeFig.3).The AKI mission will impact the asteroid with a satellite in combined with the final stage of the launch vehicles. The orbital deflection distance of the AKI launched by a single Long March 5 launcher to the asteroid is equivalent to that of three traditional impactors launched by three Long March 5 launchers.

Fig.3 EKI mission concept

6 Impact Hazard Assessment

National Institute of Natural Hazard of the Ministry of Emergency Management (NINH,MEM),NSSC[14],PMO[15],CARDC[16,17],Beijing Institue of Technology(BIT) and BISEE[18,19]and other organizations have researched on NEA hazard assessment.Significant Progress has been made in NEA atmospheric entry process.

In 2021,NSSC revealed 5 typical atmospheric entry modes of NEA impacts[10],including the asteroids entering the atmosphere layer at a small angle,which may be temporarily captured by the gravity of the Earth and finally impact the Earth,resulting in a particularly long flight range of asteroids,which poses a challenge to the prediction of asteroid impact and impact sites.

In 2022,PMO analyzed the Aletai meteorite super long strewn field in Xinjiang,and confirmed that asteroids can impact the Earth by skipping[11].Numerical modeling suggests that the stone skipping-like trajectory(seeFig.4) associated with a shallow entry angle is responsible for Aletai’s exceptionally long strewn field if a single-body entry scenario is considered.The stone skipping-like trajectory would not result in the deposition of large impact energy on the ground but may lead to the dissipation of energy during its extremely long-distance flight.

Fig.4 Skipping-like trajectory of Aletai’s meteorite (Source:Science Advances)

7 International Cooperation and Science Popularization

In 2018,CNSA officially joined IAWN and SMPAG.The PMO and NAO have been participating in the international joint NEA observation activities organized by IAWN.

In 2021,CARDC,as a co-organizer,participated in the organization of the 7th IAA Planetary Defense Conference (PDC),which was the first time that China participated in the organization of PDC.

In 2022,a group of graduate students of NSSC won the graduate student grant of the 8th PDC through professional review,and was invited to present the study on the conference.

In terms of science popularization,NSSC[20],PMO and other organizations have created a large number of science popularization works.A special WeChat platform “Planetary Defense and Utilization” has been established to spread science popularization works and news related to planetary defense.

8 Conclusion

In the past few years,China’s planetary defense has made rapid progress.Chinese scholars have successfully held the first China Conference on Planetary Defense,attracting more and more researchers to engage in planetary defense research. The Chinese government announced to build an NEA defense system and planned the implementation of the on-orbit demonstration mission of NEA defense.In terms of monitoring and warning,world-class optical observation site has been found,new observatories will soon be put into operation,and innovative space-based monitoring and warning missions have been proposed.In the aspect of on-orbit defense technology,the applicable scenarios of typical onorbit defense technologies are analyzed,and new concepts on-orbit defense means are proposed.In the aspect of hazard assessment,the impact hazard evolution chain is analyzed,and a new mechanism of NEA entry into the atmosphere is analyzed and verified.China has more extensively participated in international planetary defense cooperation and has made remarkable progress in science popularization.

It is expected that in the next five years,China’s planetary defense will make greater breakthroughs and make greater contributions to safeguarding the security of the Earth life and building a community with a shared future for mankind.