Research Progress in Laser Active Debris Removal of CAST

CHEN Chuan, YANG Wulin, GONG Zizheng, , LI Ming

1 Beijing Institute of Spacecraft Environment Engineering, Beijing 100094

2 National Key Laboratory of Science and Technology on Reliability and Environment Engineering, Beijing 100094

3 China Academy of Space Technology, Beijing 100094

Abstract: Based on the introduction to theresearch status and trend of international space-based laser debris removal technology, the existing problems of space-based laser debris removal technology are systematically analyzed.In view of the existing problems, the work and research progress of the Beijing Institute of Spacecraft Environment Engineering in thisfield are introduced from several aspects, such as dynamic behavior of laser-driven debris, orbit transfer model, ground simulation system, space-based removal system scheme and target selection strategy. The main research methods include laser-driven micro-impulse measurement experiment, surface triangulation three-dimensional reconstruction calculation method based on laser-material interaction theory, simulation calculation based on orbital dynamics, etc. It also looks forward to the future research direction in thefield of this technology.

Key words: debris active removal, laser ablation drive, dynamics behavior, orbit transfer model of laser-driven debris, simulation system, target selection strategy

1 lNTRODUCTlON

Increasingly space debris caused by developing aerospace activities has created a significant hazard to space assets.High-velocity debris colliding with a spacecraft could cause malfunctions in the materials, destroy some mechanisms or even totally destroy the spacecraft. For debris smaller than 1 cm, shielding could help but with additional launch weight and complexity it is not a solution. Active orbital transfer can avoid the collision with debris larger than 10 cm. Debris ranging between 1 cm and 10 cm (cm-class debris) are the most dangerous as they cannot be shielded from and have the largest impact during orbital transfer. Moreover, the amount of debris will be considerably increased if collision does occur with cmclass debris.

With the worsening space debris environment, shielding and active orbital transfer is not enough to handle the situation. It will be active debris removal that will deal with this problem in the future. In 2006, J.C Liou simulated the impact on the debris environment with active debris removal, the results showed that active debris removal can control the debris quantity, and number can be decreased with optimal removal schemes[1]. Furthermore, active debris removal is the best way not only because it controls the quantity of debris but also reduces the collision probability.

Many active debris removal technologies have been conducted in recent times such as robot arm, solar sail and harpoon[2]. These technologies are mainly intended for debris larger than meter size. It is difficult in terms of technologies and high cost for removing of cm-class debris with these existing technologies.In addition, this type of approach is not suitable for removing cm-class debris because of the long-term period, near-distance engagement and low efficiency. In contrast to the aforementioned technologies, active debris removal with a space-based laser system is preferred with its adjustable laser energy and deformable spot size. Efficiency is higher than other technologies as the distance to debris can be distant hence more debris can be engaged from a single position[3,4].

2 TECHNOLOGY STATUS OF ACTlVE DEBRlS REMOVAL USlNG SPACE-BASED LASER

Laser debris removal occurs as the surface of space debris is irradiated by a high-energy laser to melt, vaporize and ionize,forming a plasma jet plume, and the debris is driven by reverse velocity increment through an impulse coupling effect[5], as shown in Figure 1. Velocity increments change the orbit of the debris. When the perigee of its orbit is below 200 km, the debris will fall and burn down under the action of the atmosphere, thus achieving the removal of space debris[2], as shown in Figure 2.

Figure 1 Diagram of laser ablation procedures

Figure 2 Procedures for debris removal with space-based laser system

Since 1989, many countries have demonstrated the scheme aiming at laser debris removal[6-8]. The main contents of the scheme are as follows: laser system, detection and tracking system, platform, removal strategy, etc. The detection and tracking system is mainly responsible for the detection of target debris and the accurate delivery of laser energy to target debris.The laser system is responsible for providing enough laser energy to generate sufficient speed changes. The platform is responsible for the energy supply and the basic working environment of the system. The removal strategy is responsible for the specific implementation of the removal, including target selection strategy, laser driving strategy and effect evaluation.

At present, the laser debris removal system concept has been basically verified, its feasibility and designs have been demonstration. It is at the stage of key technology research and engineering application development. The approaches of different countries have been gradually unified. The spacebased platform, Nd:YAG laser andfiber laser collocation with an active and passive optical detection and tracking system in the common optical path of the laser emission system are the mainstream options.

The main obstacle to the further development of this technology, especially to the engineering advancement, lies in the limitation of the hardware of each subsystem, especially the defi ciency in the single pulse energy, repetition frequency, power and other performance parameters of the space-based high-energy laser. Due to the restriction in hardware performance, the existing removal schemes are often only paper designs, and it is difficult to achieve actual engineering applications. In the last year or two, researchers from all over the world have begun to shift from studying just the feasibility of the scheme to the application, exploration or strategy analysis based on the existing laser technology. As the previous research on the removal strategy was insufficient, all the schemes only assumed a simple calculation on the ability to lowerthe orbit[9-11], not through a full strategy design to fully tap the potential capacity of the scheme, and hence it also lead to the lack of further optimization in the design direction.

So, there are two main directions in the technology development of laser removal of space debris. Thefirst direction is to continue to improve the system scheme, develop prototypes and demonstrate on the ground using existing hardware capa-bilities, and promote the development of corresponding lasers according to the requirements of systems, including spacebased engineering applications using high-energy Nd:YAG solid-state lasers andfiber lasers with coherent beam combining technology of which the main representative is the European ICAN coherent synthesis technology[12].

The second direction is to optimize the removal strategy.On the one hand, based on the level of existing hardware technology, the capability limit of the laser removal system is explored through the research and optimization of the strategy, and the removal efficiency is maximized by exploring new removal methods and ideas. On the other hand, based on the original overall scheme, the indicators are optimized and perfected through strategy research, so as to reduce the requirement of the related hardware parameters as much as possible,and thus reduce the pressure to research breakthrough key technology.

In both cases, the debris removal process driven by laser ablation, including the process of generating an impulse and the process of debris de-orbiting after generating the impulse,needs to be mastered accurately. For thefirst process, existing methods cannot be used to calculate complex irregular shape targets and targets in a rotating state, and are limited to the case where full coverage of the target spot is possible. For the case of partial coverage of the target spot the process cannot be calculated, hence cannot meet the actual needs. This in fluences the result where only a simple plane target calculation,giving the magnitude and direction of impulse, in the second process can be calculated, hence it is difficult to simulate the real driving orbit change process.

In view of the above research status, the Beijing Institute of Spacecraft Environmen Engineering, which is affiliated to the China Academy of Space Technology (CAST), has studied the dynamic behavior law of space debris using the calculation method of dynamic behavior of irregular rotating targets driven by laser. The simulation system for laser-driven space debris removal was established based on the orbit transfer model of laser-driven debris, with which the optimization of the spacebased debris removal system scheme and the strategy of target selection were studied also.

3 DYNAMlC BEHAVlOR LAW OF SPACE DEBRlS DRlVEN BY LASER

In the process of laser ablation to drive debris, it is necessary to be able to calculate the magnitude and direction of the impulse produced by the laser ablation impluse according to the laser parameters and debris parameters[13]. For largescale debris and asteroids whose size is larger than the spot diameter, the effect of the non-full-coverage spot should also be considered. In order to solve this problem, our research team proposed a new calculation method based upon a surface three-dimensional triangulation reconstruction and an impulse coupling law of the laser ablation driving the object, hence verifying the accuracy through theoretical calculation and experiment. The in fluence on the debris characteristics with the laser driving effect was studied by this method.

3.1 New Calculation Method

The basic formula to calculate the impulse under laser irradiation is as follows:

Based on this we have the formula to calculate the impulse of an irregularly shaped target under laser irradiation[14,15]:

For the curved surfaces we have:

These formulas are hard to use because an irregularly shaped target always has many different surfaces including plane and curved surfaces, of which the geometry information is hard to obtain, including the surface area, normal vector and the equation of the curved surface especially.

Based on the idea of afinite element, the research team of our institute proposed a method to reconstruct a three-dimensional surface of an irregular object using several triangles, and then establish a general calculation method to calculate each triangle, andfinally obtain the overall driving effect. According to the vertex coordinate information of triangulation reconstruction of the target surface which can be easily obtained by radar or optical detection, the triangulation reconstruction of the target surface is made in order to convert its surface into several triangles as shown in Figure 3. Each triangle is a basic computing unit of which the impulse can be calculated by formula (1), as shown in Figure 4. Hence the formula (2) can easily be used to calculate the impulse for the whole target. To account for the spot irradiation, we need an additional step to carry out triangulation screening and spot profile reconstruction based on the spot size, as shown in Figure 5.

Figure 3 Schematic diagram of triangulation reconstruction

Figure 4 Basic computing unit

Figure 5 Triangulation screening and reconstruction

3.2 Method Accuracy Verification

3.2.1 Theoretical calculation verification

For some typical geometric targets, such as cube, sphere,cylinder, as shown in Figure 6, accurate calculation methods and formulas to calculate the impulse generated by the irradiation laser have been obtained already[16,17]. So, the accuracy of the proposed method can be verified by comparing the calculated result with that of the existing formula, as shown in Table 1.

Figure 6 Triangulation reconstruction of the typical geometric target

Table 1 Calculation result with different vertex numbers

The result shows that the surface triangulation reconstruction method proposed in this paper has no error when the target surface contains no curved surface. Also for the curved surface, there will be some error in the result due to the vertex we use to do triangulation reconstruction, which means the precision of triangulation reconstruction isn't enough. However when the density of vertices increases by interpolation,the ideal calculation accuracy can be obtained.

3.2.2 Impulse measurement experiment

Figure 7 Experimental device

The micro-impulse measurement experimental results for a laser ablation driving a sphere, cylinder, cube and cone verified the accuracy of the surface triangulation method in calculating the non-full coverage spot. A torsion pendulum measuring system was used in the experiment. The sample isfixed at one end of the torsion pendulum swing arm. The laser ablation drives the sample to rotate. The displacement of the other end of the torsion pendulum was measured by a displacement sensor, as shown in Figure 7.

The experimental samples of typical geometric structures such as spheres, cylinders, cubes and cones were designed, as shown in Figure 8. The laser wavelength was 1064 nm, with a repetition frequency of 1Hz, pulse width of 10 ns, an original spot ofФ13 mm, so the spot diameter was 1 mm after focusing, and the single pulse energy range was 50 - 250 mJ.

The experimental results were in good agreement with the calculation results, as shown in Figure 9. The calculation values fell within the experimental error range. The cube error was less than 2% in the region of energy＞110 mJ, but the maximum error was 22% when the energy was low. When the laser energy was more than 130 mJ, the difference between the calculated results and the experimental results began to increase, and the calculated results were larger than the experimental results, but the maximum error was less than 5%; the calculated results of the cylinder were within 4% above the experimental results. The error may be due to the assumption that the energy of the laser beam was uniformly distributed over the spot, whereas the energy of the actual irradiated spot was not uniform.

Figure 8 Schematic diagram of experimental samples and laser irradiation positions

Figure 9 Experimental results

3.3 In fluence of Debris Characteristics on Laser Driving Effect

Most of real space debris are irregular objects with rotating geometry. The different geometric and rotational characteristics of space debris have great in fluence on the laser irradiation driving effect. Through the work of this research team, the influence of the geometric shape and rotational state of the debris and the net laser driving effect is studied using the new calculation method.

3.3.1 In fluence of debris geometry on laser driving effect

By calculating the direction and magnitude of the impulse produced by the target with different angle laser irradiation,the in fluence of the target geometry on the laser driving effect was analyzed. The result is shown in Figure 10.

For a regular geometric shape target, the impulse magnitude comparison assumes a plane target with the same section,and the direction comparison baseline is the incident laser direction. For a cylinder the impulse loss was 22% - 44%, the de flection angle was 0 - 9.6 degree. While for a cone, the impulse loss was 16% - 80% and the de flection angle was 0 - 48.1 degree.

Figure 10 Calculation result on the target geometry

Figure 11 In fluence of the rotating state of debris on impulse angle

For asteroids with a basic contour similar to spheres, Bennu, the maximum impulse loss was 17% compared to a regular sphere with the same diameter and 43% compared to a plane target with the same cross section. For the asteroid A8567, the minimum impulse was 38.58% compared to the maximum impulse, and the impulse de flection angle was 1.07 - 12.17 degree.

3.3.2 In fluence of rotating state of debris on laser driving effect

The process of laser irradiation driving a rotating plate,cylinder, cuboid, cone and other typically shaped regular and irregular target was studied by the surface triangulation three-dimensional reconstruction method. The influence of target shape and rotating state on the laser driving effect were obtained and analyzed. The calculation results are shown in Figure 11.

For a plate, the in fluence of its rotating state on laser action was analyzed by formula, and verified by calculation using different parameters. During the laser irradiation time, the larger the turning angle of the target (nω/f) was, the smaller the maximum de flection angle of the impulse direction was. When the frequency is constant, the faster the speed of the target was,the smaller the maximum de flection angle was. When the target rotation speed was constant, the higher the pulse frequency was, the greater the angle maximum was. However it will not affect the time to reach the maximum angle.

For cylinders and cubes, because of their symmetry, the law is the same as the plate; for cones, because of their lack of 180 degree rotational symmetry, the period of their de flection changes will be 360 degrees, and for irregular objects, Bennu,except for their period of 360 degrees, they cannot be completely balanced because of the irregular shape, and tends to the minimum point around 3.5 degrees.

4 SlMULATlON SYSTEM FOR LASER-DRlVEN DEBRlS REMOVAL

4.1 Orbit Transfer Model

Figure 12 Flow chart for orbit transfer model

The model is based on the calculation method for dynamic behavior of an irregular target driven by laser, combined with an orbital dynamics algorithm. It can simulate the process of rotating irregular debris driven by a given space-based laser platform. The flow chart of the model is shown in Figure 12.

4.2 Example Analysis

Five major types of space debris (Na/K spheres, carbon-phenolic aldehydes, MLI composites, aluminum and steel)listed in the ORION program of the United States were selected to simulate the effects of velocity increment, frequency and the range of laser. The velocity increment ranges from mm/s to m/s; the velocity increment was directed to the debris along the platform; the frequency range was from 1 Hz to 100 Hz;the range of the laser was from 10 km to 1000 km.

There are two relative positions for the debris and the platform: the platform moved in front of the debris and the lasers irradiated along the direction of debris movement; the platform moved after the debris and the lasers irradiated against the direction of debris movement. The results are shown in Figure 13.

According to the calculation results, when the platform laser irradiated against the direction of debris movement, the range of the platform, velocity increment, frequency can effectively improve the debris removal efficiency. However with the increase of the parameters, the benefits will decrease, the process of which is independent of the debris material and track distribution. When the platform laser irradiated along the direction of debris movement, simple continuous drive was far less effective than against the direction. The more precise timing for the drive was needed,rather than simply driving continuously within the range of the effect.

Figure 13 Simulation results offive major types of space debris

4.3 Simulation Program

Our team developed 3D simulation software based on orbital dynamics and laser-ablation response in C++/Qt with Visual Studio 2016. The software interface enables many frames to show all aspect of the removal procedures. It can also create different scenarios and define all items to show various removal demands. The GUI for the software is shown in Figure 14.

In order to test the functions of the software, two scenarios were simulated.

1) Removal procedures

Selection of the debris and the laser which is located in different orbit. The laser was selected in a slightly higher position than the debris. One supposes the direction of additional velocity is from the laser to the debris. The procedure is shown in Figure 15.

Figure 14 Software GUI

Figure 15 Removal procedure for specific debris

Figure 16 Removal procedures for multi-debris and multi-laser

2) Debris located in specific areas

Selection of 698 debris located at an altitude of 800 km and utilizing 21 space-based laser systems. The results showed that only one debris can be removed per month. The procedure is shown in Figure 16.

5 NEW SCHEME FOR SPACE-BASED DEBRlS REMOVAL SYSTEM

5.1 Concept of Laser Driven Relay Removing Debris

Our research team designed several schemes for the removal of 2000 km, 1200 km, 800 km, 400 km orbital debris from the previous research, and verified the removal capability by simulation. However, similar to those proposed by European and American countries, the schemes have high requirements for laser, emission mirror indicators, and hence the application in space-based platforms is still very difficult.Take the 800 km orbital 10 cm scale debris as an example. The perigee of the orbital debris when reduced to 200 km, requires a velocity change of 85 m/s according to the coplanar orbit change method. The scheme parameters of space-based laser debris removal systems given by the United States, Japan and Russia are shown in Table 2[3,7,8]. We assume that the laser beam can be accurately converged on a 10 cm diameter spot at a 200 km operating distance, and the required size of the laser transmitter is 3 m. The calculated results are also listed in Table 2.

This performance is difficult to implement, especially for high-energy pulsed lasers. Even on the ground, existing high-energy pulsed lasers with single-pulse energy up to kilo level focal magnitude tend to be bulky, and it is difficult to work continuously at high frequencies due to the long charging process. There are also problems with energy supply and heat dissipation on space-based platforms. Hence there is still a long way to go for engineering requirements. In addition, although the size of the transmitter mirror can be achieved usingexisting technology, such a large aperture optical system means a large platform size with high manufacturing and launch costs.

Table 2 Parameters of international space-based laser LEO debris removal systems

If a laser with single pulse energy is smaller, the above technical obstacles could be overcome. The problem, however, is that small-energy lasers cannot induce sufficient velocity changes during a single rendezvous, requiring multiple rendezvous to successfully remove the target debris. As the debris obtains a certain velocity change during the single rendezvous,the removal platform must follow the track the change; otherwise it is difficult to rendezvous with the debris again. The removal of the debris taking into account tracking the change means that propellant consumption is high which increases platform load and impacts the removal efficiency. Therefore,in order to overcome the obstacles with high-energy laser technology and inspired by small satellite constellations, we propose a constellation system consisting of several small satellite laser platforms distributed at different orbital heights, which can drive debris through multiple intersections by means of laser-driven relay, and reduce the debris orbit layer by layer.Eventually the debris will therefore be removed.

Figure 17 Schematic diagram of active space debris

Figure 18 Schematic diagram of active space debris removal by quasi-laser-driven relay

A laser driven relay space debris removal system is shown in Figure 17. Each satellite platform in the system has a certain range in debris detection, tracking and driving capabilities.According to the working distance of the satellite platform and the orbital descent ability, the orbital altitude spacing can be set, so that the driving range can be correlated with each other,and the detection and tracking ranges can cover each other,so as to form a complete laser driving constellation system.According to the characteristics of debris tracking, the actual removal scheme can be divided into direct relay drive removal and quasi-relay drive removal.

For debris with ideal track parameters, a direct relay-driven removal method can be adopted (each platform removes the same debris). Starting from the highest orbit satellite platform to detect, track, and drive the target, the orbit of the satellite platform is reduced to enter the low orbit satellite platform tracking, debris tracking and driving range. The satellite in the constellation drives the target from a higher to lower level by level andfinally removes it successfully by de-orbiting the debris. In this process, the low orbit satellite platform can always obtain the orbital information of the target debris from the high orbit drive process because of the mutual coverage of detection and tracking range, thus adjusting its orbit in real time to obtain a better driving window.

When no debris with ideal orbital parameters are found,the debris may not be able to enter the low-level orbiting satellite platform range after being driven down by the high-level orbiting satellite platform, so an orderly relay may not be possible. In this case, a quasi-relay removal scheme (each platform removes different debris) can be used, as shown in Figure 18.That is, a satellite platform located in the nearest layer of orbit to the atmosphere first removes removable debris from its orbit. The second-level orbiting satellite platform then drives removable debris within its range of action to an orbit in the range of thefirst-level orbit. The satellite platforms in each tier are driven to track the debris within their range in this order.With this quasi-relay method, the highest layer debris can be removed without affecting the number of debris in each layer.

5.2 Key Parameters and Constellation Layout of Relay System Microsatellite

Based on an impulse coupling mechanism of laser-matter interaction, the key parameters of the system can be determined as shown in Table 3 to achieve a power density threshold I ＞ 107 I＞107W/cm2and minimize the requirement for laser energy and emission mirror. The combination of these parameters can enable the projected laser energy to reach a level of 107W/cm2power density threshold at a distance of 20 km, and obtain a higher impulse coupling coefficient at a distance of 10 km by reducing the size of the spot.

According to the laser range of each platform, the orbit distribution of the satellite constellation can be further determined, that is, a laser-driven small satellite can be set at an orbit altitude of 20 km. 20 km drive distance ensures the convergence of driving range. Taking the 800 km altitude target orbit as an example, orbital debris is reduced to 200 km by realizing a removal constellation composed of 30 small satellites using the relay drive principle.

5.3 Simulation of Removal Capability

In order to ensure that the constellation can drive debris in relay, it is necessary to require a single satellite to have enough orbital capability to drive debris to the adjacent satellites.Therefore, the orbit determination capability of the small satellite platform should be simulated and verified according to the parameters of the small satellite platform system. For this purpose, a simulation program was used to calculate and evaluate the rail descending capability.

The system parameters of each satellite platform are: pulse laser single pulse energy 10 J with a repetition frequency 1 Hz,target energy efficiency 80%, laser effective range 20 km, target debris diameter 10 cm. In order to calculate the maximum clearance capacity of the system, the optimal position was chosen as the starting position, that is, the target debris starting position is in the same orbit with the platform, located 3 km ahead of the displacement system.

The surface roughness ratio of LEO space debris is usually in the range of 0.15 - 25 cm2/g. Amongst them, the average surface-to-mass ratio of aluminum alloy material which accounts for 44% of the debris is 0.37 cm2/g, space debris of 10 cm aluminum alloy material is 200 g, the average surface-to-mass ratio of multi-layer insulation material which accounts for 37% of the debris is 25 cm2/g, and the average mass of 10 cm multi-layer insulation material is 3.14 g. Under the given input conditions above, the velocity increment and the orbit reduction impact of space debris with different materials are shown in Table 4. The velocity increment and orbital reduction of aluminum alloy circular plates with a radius of 10 cm and different thickness are calculated as shown in Table 5.

Table 3 Parameters of laser and transmitting mirror

Table 4 Speed increment and orbit reduction of different material space debris

Table 5 Speed increments and orbit reduction of aluminum alloy plate with different thickness

From the results of Table 4 - 5, it can be seen that the single satellite platform has limited orbital descent ability to target debris of different materials, only 20 - 30 km, due to the limitation of the laser single pulse energy and operating range.This distance is insufficient to remove debris successfully, but it is sufficient to bring debris down into the range of a lower orbital satellite platform in the constellation system, so as to ensure the realization of a laser relay drive removal process between satellite platforms with different orbital altitudes in the constellation system.

6 TARGET SELECTlON STRATEGY

Target selection is an important part of the removal strategy, which is of great significance to the early mission analysis of a space-based laser removal system and the actual operation of the system in orbit in the later stage of the overall scheme design. Based on the existing centimeter-scale space debris data, with our research team statistics the orbital characteristics, by providing data input and initial assumptions for the determination of debris groups, selection of different index parameters according to the different debris removal mission requirements and mission objectives, we can establish different target screeners on this basis, as shown in Figure 19.

1) Target debris group removal

Figure 19 Different target selection methods for different tasks

Targeting specific debris groups. A debris swarm may have originated from a sudden spacecraft impact or disintegration event, so it has similar orbits and its orbital parameters are changing gradually. The target debris group screening parameters are calculated in two stages. Firstly, the impact probability between the target and the known spacecraft is calculated, and the target with high impact probability is removed in priority according to this order. Secondly, the orbital parameters of the target with the same impact probability are calculated and analyzed, and the trend of detachment from the debris group is taken as a screening index. Indexes are removed to prioritize.

2) Target area debris removal

Target debris in specific areas. The region may be a region with a high risk of debris impact, a region with intensive space activities, or an important space mission area, with the objective of reducing the amount of debris passing through the region. The time of the debris threat to the orbits in the region is calculated, and the debris is sorted according to the timing of the debris threat to the orbits in the region.

3) Target spacecraft protection

Through traversing all known debris data, debris orbits are screened according to known spacecraft orbits, including orbital plane, orbital height, eccentricity, perigee, apogee and other factors to obtain debris orbits close to the target spacecraft orbit. The debris in this range was further screened using the Box method, and the debris intersecting with the target spacecraft at different distances of 10 km, 5 km and 1 km was obtained. Further, the probability of collision is calculated by probability cloud based on debris of dangerous intersection within 1 km. According to the impact probability, intersection distance and threat level, the threat ranking of debris to the target spacecraft is obtained, and the removal priority is determined to select the removal targets.

4) Target area spacecraft protection

According to the calculation method for a single target spacecraft, the probability of impacting debris, the intersection distance and the threat level of all spacecraft in the target area is obtained, and the threat rank of each spacecraft is ranked.On this basis, the total impact probability and the number of dangerous intersections for the whole target spacecraft are calculated, and the debris with the most threat to the spacecraft and the highest total impact probability are further screened out to obtain the overall threat ranking to the target spacecraft group. According to this, the removal priority screening of removal targets are set according to the order of the whole highrisk debris followed by single high-risk debris.

5) Overall space debris environmental governance

Short-term governance objectives. According to the orbital distribution of debris, the maximum distribution area is analyzed and determined, and a debris removal scheme is calculated based on the maximum decline rate of debris in the area,and the removal priority is determined to screen the removal targets.

Long-term governance objectives. Combined with the analysis of the environmental evolution model to determine the maximum growth area of debris, the debris removal scheme is calculated based on the maximum number of debris intersecting this area, and the removal priority is determined to screen the removal targets.

6) Removal platform task target screening

When the parameters of the removal platform are determined, the time window and the maximum removal capability of debris entering the removal range are calculated according to the removal platform orbit and its laser action capability(distance, frequency, energy density). The debris removal effect under different removal time sequences are obtained. According to different task indexes (total debris number decreasing,total impact probability decreasing, the fastest debris number decreasing, the fastest impact probability decreasing), the time efficiency of each sequence is calculated, and the optimal time sequence and the optimal target selection satisfying the given task index are obtained.

7 PROSPECTS

Laser-driven space debris removal technology is one of the most promising space debris environment remediation technologies with its unique technical advantages. The research team of the Beijing Institute of Spacecraft Environment Engineering has conducted extensive and in-depth research on the dynamic behavior of laser ablation-driven debris, the orbit-changing model of laser-driven debris, a ground simulation system, the space-based removal system scheme, and the removal strategies in thisfield. A series of achievements have been obtained, which is of great significance to the engineering application of the technology. According to the research results, the law of laser-debris interaction is of great significance to laser-driven debris removal technology, which needs further study; under the current conditions with the laser performance parameters limitation, it is more important to study the optimization of scheme parameters and removal strategies in depth;at the same time, the simulation system as a research tool is more important.

In order to further promote the in-depth development of this technology, further in-depth study is needed from the following aspects:firstly, it is necessary to carry out laser ablation driving experiments with a large spot to study the interaction mechanism between the laser and material and the dynamic behavior of debris under the condition of a large spot, so as to further improve the dynamic model; secondly, the simulation system needs to be further improved to realize the real-time calculation and analysis of various removal strategies;finally, it is necessary to develop a ground principle prototype, ground demonstration and verification system, to conduct ground demonstration and verification experiments, and further improve the technical maturity to promote the engineering process.

In addition, the improvement of space-based laser performance is also crucial to the development of this technology. At present, the main possible solutions are the space-based engineering applications of high-energy Nd:YAG solid-state lasers and the large number of coherent beam combining technologies forfiber lasers, which are mainly represented by the ICAN project in Europe.