Comets are small celestial bodies orbiting around the Sun.They are remnants left over from the formation of the solar system;their interiors store original material of the planetary disk within the solar system.They a...Comets are small celestial bodies orbiting around the Sun.They are remnants left over from the formation of the solar system;their interiors store original material of the planetary disk within the solar system.They are thus"fossils"for studying the early solar system.According to their orbital periods,comets are classified as long-period comets(orbital period P>200 years)and short-period comets(P<200 years).Long-period comets originate from the Oort Cloud.Compared to short-period comets,they enter the inner solar system less frequently and contain more primitive materials.Studying long-period comets helps us understand the origin of the solar system and reveals characteristics of the Oort Cloud.This paper begins with a summary of our extensive survey of the literature regarding methods of observing comets and foci of comet studies.We introduce systematically the main parameters currently used to assess the activity of long-period comets,including gas production rate,dust production rate,dust properties,morphological characteristics,etc.Subsequently,we discuss in depth the activity mechanisms of long-period comets,covering not only the water ice sublimation-driven mechanism(similar to that of short-period comets)but also various mechanisms that may dominate the activity of long-period comets in the low-temperature environment at the aphelion.These mechanisms include the sublimation of CO or CO_(2) gas ice,the polymerization reaction of cyanides,the crystallization of amorphous water ice,the annealing process of amorphous water ice,the thermal decomposition effect,and the electrostatic supercharge phenomenon.We then summarize the evolving activity of long-period comets as they travel from the Oort Cloud to the vicinity of their perihelions.We analyze unique properties of long-period comets,including such special phenomena as changes in dust color,coma structure,and tail structure.Finally,we summarize currently unresolved scientific questions,and then the entire paper.展开更多
One of the detection objectives of the Chinese Asteroid Exploration mission is to investigate the space environment near the Main-belt Comet(MBC,Active Asteroid)311P/PANSTARRS.This paper outlines the scientific object...One of the detection objectives of the Chinese Asteroid Exploration mission is to investigate the space environment near the Main-belt Comet(MBC,Active Asteroid)311P/PANSTARRS.This paper outlines the scientific objectives,measurement targets,and measurement requirements for the proposed Gas and Ion Analyzer(GIA).The GIA is designed for in-situ mass spectrometry of neutral gases and low-energy ions,such as hydrogen,carbon,and oxygen,in the vicinity of 311P.Ion sampling techniques are essential for the GIA's Time-of-Flight(TOF)mass analysis capabilities.In this paper,we present an enhanced ion sampling technique through the development of an ion attraction model and an ion source model.The ion attraction model demonstrates that adjusting attraction grid voltage can enhance the detection efficiency of low-energy ions and mitigate the repulsive force of ions during sampling,which is influenced by the satellite's surface positive charging.The ion source model simulates the processes of gas ionization and ion multiplication.Simulation results indicate that the GIA can achieve a lower pressure limit below 10-13Pa and possess a dynamic range exceeding 10~9.These performances ensure the generation of ions with stable and consistent current,which is crucial for high-resolution and broad dynamic range mass spectrometer analysis.Preliminary testing experiments have verified GIA's capability to detect gas compositions such as H2O and N2.In-situ measurements near 311P using GIA are expected to significantly contribute to our understanding of asteroid activity mechanisms,the evolution of the atmospheric and ionized environments of main-belt comets,the interactions with solar wind,and the origin of Earth's water.展开更多
Sent out at 1:31am GMT+8 on May 29 by a Long March-3B carrier rocket from the Xichang Satellite Launch Center in Sichuan province,China,Tianwen-2,the second mission of China’s Planetary Exploration Program,correctly ...Sent out at 1:31am GMT+8 on May 29 by a Long March-3B carrier rocket from the Xichang Satellite Launch Center in Sichuan province,China,Tianwen-2,the second mission of China’s Planetary Exploration Program,correctly entered the transfer trajectory toward an asteroid named 2016HO3 after flying for 18 minutes.Its solar wings unfolded properly,signaling a successful start,and primed for the next stage of its mission.展开更多
基金supported by the Operation,Maintenance and Upgrading Fund for Astronomical Telescopes and Facility Instruments,budgeted from the Ministry of Finance of China(MOF)and administrated by the Chinese Academy of Sciences(CAS)the National Natural Science Foundation of China(grant Nos.12173093)science research grants from the China Manned Space Project with No.CMS-CSST-2021-B08.
文摘Comets are small celestial bodies orbiting around the Sun.They are remnants left over from the formation of the solar system;their interiors store original material of the planetary disk within the solar system.They are thus"fossils"for studying the early solar system.According to their orbital periods,comets are classified as long-period comets(orbital period P>200 years)and short-period comets(P<200 years).Long-period comets originate from the Oort Cloud.Compared to short-period comets,they enter the inner solar system less frequently and contain more primitive materials.Studying long-period comets helps us understand the origin of the solar system and reveals characteristics of the Oort Cloud.This paper begins with a summary of our extensive survey of the literature regarding methods of observing comets and foci of comet studies.We introduce systematically the main parameters currently used to assess the activity of long-period comets,including gas production rate,dust production rate,dust properties,morphological characteristics,etc.Subsequently,we discuss in depth the activity mechanisms of long-period comets,covering not only the water ice sublimation-driven mechanism(similar to that of short-period comets)but also various mechanisms that may dominate the activity of long-period comets in the low-temperature environment at the aphelion.These mechanisms include the sublimation of CO or CO_(2) gas ice,the polymerization reaction of cyanides,the crystallization of amorphous water ice,the annealing process of amorphous water ice,the thermal decomposition effect,and the electrostatic supercharge phenomenon.We then summarize the evolving activity of long-period comets as they travel from the Oort Cloud to the vicinity of their perihelions.We analyze unique properties of long-period comets,including such special phenomena as changes in dust color,coma structure,and tail structure.Finally,we summarize currently unresolved scientific questions,and then the entire paper.
基金Supported by the National Natural Science Foundation of China(42474239,41204128)China National Space Administration(Pre-research project on Civil Aerospace Technologies No.D010301)Strategic Priority Research Program of the Chinese Academy of Sciences(XDA17010303)。
文摘One of the detection objectives of the Chinese Asteroid Exploration mission is to investigate the space environment near the Main-belt Comet(MBC,Active Asteroid)311P/PANSTARRS.This paper outlines the scientific objectives,measurement targets,and measurement requirements for the proposed Gas and Ion Analyzer(GIA).The GIA is designed for in-situ mass spectrometry of neutral gases and low-energy ions,such as hydrogen,carbon,and oxygen,in the vicinity of 311P.Ion sampling techniques are essential for the GIA's Time-of-Flight(TOF)mass analysis capabilities.In this paper,we present an enhanced ion sampling technique through the development of an ion attraction model and an ion source model.The ion attraction model demonstrates that adjusting attraction grid voltage can enhance the detection efficiency of low-energy ions and mitigate the repulsive force of ions during sampling,which is influenced by the satellite's surface positive charging.The ion source model simulates the processes of gas ionization and ion multiplication.Simulation results indicate that the GIA can achieve a lower pressure limit below 10-13Pa and possess a dynamic range exceeding 10~9.These performances ensure the generation of ions with stable and consistent current,which is crucial for high-resolution and broad dynamic range mass spectrometer analysis.Preliminary testing experiments have verified GIA's capability to detect gas compositions such as H2O and N2.In-situ measurements near 311P using GIA are expected to significantly contribute to our understanding of asteroid activity mechanisms,the evolution of the atmospheric and ionized environments of main-belt comets,the interactions with solar wind,and the origin of Earth's water.
文摘Sent out at 1:31am GMT+8 on May 29 by a Long March-3B carrier rocket from the Xichang Satellite Launch Center in Sichuan province,China,Tianwen-2,the second mission of China’s Planetary Exploration Program,correctly entered the transfer trajectory toward an asteroid named 2016HO3 after flying for 18 minutes.Its solar wings unfolded properly,signaling a successful start,and primed for the next stage of its mission.
