Analysis of the Impact of Large Constellations on the Space Debris Environment and Countermeasures

CHEN Chuan ,YANG Wulin ,GONG Zizheng, ,ZHANG Pinliang ,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:Large constellations have developed rapidly in recent years because of their unique advantages,but they will inevitably have a major negative impact on the space debris environment,leading to its deterioration.The key to mitigate the impact is the success rate and duration of the post-mission disposal (PMD) process.Aiming at solving this problem,this paper further studies the impact of large constellations on other space assets under different PMD strategies through simulation,and proposes corresponding strategies and suggestions for mitigation.According to OneWeb's large constellation launch plan,the dangerous intersection of the large constellation with existing space assets at different stages of the constellations life cycle is calculated by simulation.Based on this,the influence of the large constellation operation on existing space assets at different times and strategies of PMD is analyzed.The conclusion shows that in the PMD stage,large constellations have the greatest impact on existing space assets; the PMD duration and number of satellites performing PMD at the same time are key factors to the degree of negative impact.The faster the PMD is,the less threat it poses to other spacecraft.More results and conclusions are still being analyzed.

Key words:large constellations,space debris environment,long-term environmental evolution,impact probability,dangerous intersection,coping strategies

1 INTRODUCTION

Due to the characteristics of their size enabling fast response,flexible applications and low cost,the small satellite market has developed rapidly since 1995.From 2015 to now,small satellite applications have been enriched and expanded,and have begun to enter a systematic application stage[1,2].The great value of large scale small satellite constellations in a series of new applications,such as space-based global communications and remote sensing,has been recognized.These applications provide new opportunities for economic development,global education,rural health care,remote services and environmental science.At the same time,small satellites have also made the cost of entering and utilizing space drop rapidly,paving the way for many new participants who are trying to devote themselves to commercial space applications.In 2017 alone,the number of small satellites under 500 kg launched worldwide reached 312,accounting for more than 80% of the total number of spacecraft launched in that year.The world's major space powers and commercial companies have also proposed extremely large scale small satellite constellation launch plans,as shown in Table 1[1].According to these plans,the number of satellites to be launched into orbit in the next few years will surge from more than 2,000 to over 20,000.China has also proposed several small satellite constellation plans deployed at different orbital altitudes.

Don Kessler pointed out that the deterioration of the space debris environment is far from being curbed even without considering the impact of large scale satellite constellations.Based upon calculation,if there is no effective control,the debris density will reach a critical value after 70 years,leading to a chain reaction due to the cascading impact of debris (Kessler syndrome),which would make the near-Earth space completely unavailable[4,5].A large number of successful small satellite launching plans will make the satellite density in typical orbit areas increase sharply,which will not only make the orbit and frequency resources more limited,but also lead to a significant increase in the probability of a space collision,which will undoubtedly further aggravate the deteriorating trend of the space debris environment[6].Existing space debris mitigation guidelines and the related space traffic management policies do not take into account the explosive development and deployment of large scale small satellite constellations,and it is difficult to meet this enormous challenge[7].Therefore,in order to cope with the rapid development and deployment of large scale small satellite constellations,it is urgent to introduce new measures to regulate their development.

Table 1 Small satellite constellation launch plan

This paper investigates the existing research results on the debris environmental impact of large scale small satellite constellations,analyses their impact on the long-term evolution of the space debris environment and the impact risk faced during each phase as of the large scale small satellite constellations during deployment,operation and deorbit.According to One-Web's constellation launch plan,the threat during deployment,operation and deorbit of the constellation to existing space assets has been analyzed.On the basis of this research,the countermeasures for large scale small satellite constellations are proposed.

2 IMPACT OF LARGE SCALE SMALL SATELLITE CONSTELLATIONS ON THE LONG-TERM SPACE DEBRIS ENVIRONMENT

Up until now,according to most debris environment evolution studies,the frequency of human space launches,one of the important factors,is generally 50-70 times per year,which is based on past average data.Many debris mitigation policies are also based on this.However,large scale small satellite constellation projects have caused human space launch activities to increase in step,and the additional increased number of launches now reaches about 100 per year,see Figure 1,which will undoubtedly have a very significant impact on the long-term evolution of the debris environment[8].

Figure 1 LEO launch number [8]

In 2016,ESA studied the issue of the impact of large scale small satellite constellations on the long-term space debris environment.Based on the space debris model,the long-term impact of a typical large scale small satellite constellation (1080 satellites of 200 kg each distributed in 20 orbital planes with an inclination of 80 degrees at an altitude of 1100 km) with a life span of 50 years (2021-2071) on the space environment was simulated[9].The mission process includes a two-year constellation construction stage (20 launches per year,18 satellites each launch),a 50-year maintenance operation stage (12 launches per year,18 satellites each time) and a post-mission disposal (PMD) processing stage.According to the results,the launch and operation of large constellations will lead to a sharp increase of the number of LEO orbital objects and a reduction in potential impact times.When the PMD success rate reaches more than 90%,the number of orbital objects and impact times will slowly return to normal level after the decommissioning of constellations.With Less than 90% of PMD success rate it will lead to the continuous growth of orbital objects,and the space environment will continue to deteriorate irreversibly,as shown in Figures 2 and 3.

They also further analyzed the impact of large constellations on debris environment at 90% PMD success rate at different times.According to the current regulations,the longest deorbit time for a satellite in a constellation after its mission can be 25 years.If the satellite deorbit time is shortened,the number of objects in LEO and the peak number of impact times will be reduced,and the speed of environmental recovery will be increased accordingly,as shown in Figures 4 and 5.

Figure 2 The impact of constellations on the number of debris above 10 cm in LEO orbit at different execution rates of PMD [9]

Figure 3 The impact of constellations on collision times of LEO orbital objects at different execution rates of PMD [9]

According to the research results,launch plans for large scale small satellite constellations will undoubtedly greatly aggravate the deterioration of the space debris environment,and a major impact in the short term is inevitable.The impact on the debris environment can be minimized by shortening the deorbit time after the mission and improving the PMD success rate.However,due to the fact that the satellite deorbit time can be up to 25 years in accordance with the current mitigation guidelines and the actual PMD implementation rate is far below the ideal value of 90%,the impact of large constellations on debris environment evolution will be catastrophic.

Figure 4 The impact of constellations on the number of debris above 10 cm in LEO orbit at different PMD times [9]

Figure 5 The impact of constellation on collision times of LEO orbital objects at different PMD times [9]

3 IMPACT RISK OF LARGE SCALE SMALL SATELLITE CONSTELLATIONS

According to the conclusion obtained from the long-term debris evolution model,shortening the PMD time after a large scale small satellite constellation mission can reduce the impact on the short-term debris environment by reducing the peak number of orbital objects caused by the launch and operation of a constellation.Improving the PMD success rate of a satellite in a constellation can greatly reduce or even eliminate the medium-term and long-term impact on the debris environment.However,satellites in a constellation are also facing the risk of debris impact during launching,deploying and deorbiting.In case of failure due to impact,not only the function of a satellite,including post-mission deorbiting,may be lost,but a large number of debris will also create greater environmental hazards.

In view of this situation,CNES and NASA have taken One-Web's constellation plan as an example to analyze and calculate the impact risk of large scale small satellite constellations during the phases of launch and deployment,operation and post-mission deorbit.

According to OneWeb's constellation plan,720 satellites will be evenly deployed in 18 orbital planes in a circular orbit with an inclination of 87.9° at an altitude of 1200 km.The launch and deployment time of the constellation is 1 year,the operation time is 5 years,and the orbit descent time is 5 years after the mission[10].

From the research of CNES,through the debris data of each orbit under the MASTER debris model,the possible impacts can be classified into three categories:penetrating impact,fatal impact and disintegration impact[11].The penetrating impact is judged by the impact limit equation according to the size and velocity of debris; the fatal impact is judged by the size of the debris larger than 1 cm in the penetrating impact results; and the disintegration impact is judged by the impact energy density greater than 40 J/g.According to the statistical results of several calculations,the number of penetration impacts,fatal impacts and disintegration impacts occuring during the whole operation period was 11-38,1 and 0.06 respectively.Among them,the impact ratio is 5% in launch and deployment phase,35-60%in operation phase and 40-60% in deorbit phase.

In the case of NASA's research,the fragmentation data provided by the ORDEM 3.0 model was used to calculate the probability of a catastrophic disintegration impact (impact energy density greater than 40 J/g)[12].The calculated results are shown in Figure 6.

According to the results of CNES and NASA,the main impact risk of large constellations during their life cycle comes during the post-mission orbital phase (with more than 60% impact and fatal impact probability).During the launch phase,the probability of collision is relatively low because the satellites are put into orbit in batches and the duration is short.Due to the selected orbit height of 1200 km this avoids the dense orbital height distribution of other debris and spacecraft,hence the impact probability is not high.Due to its long duration of deorbit,the impact probability during the descent stage is much higher than that of the first two stages because it will traverse the densely distributed area of space objects.

Figure 6 Probability distribution of a disintegration impact at different stages for the OneWeb constellation [12]

4 THREATS OF 4 LARGE SMALL SATELLITE CONSTELLATIONS TO EXISTING SPACE ASSETS

According to OneWeb's constellation launch and deployment plan and the analysis and calculation methods of NASA and CNES,the lifetime of a constellation is divided into three stages:launch and deployment,on-orbit operation and deorbit after mission.The dangerous potential intersect times of spacecraft with distances less than 10 km from other space assets in the three stages were calculated to analysis the impact on other space assets during the constellation life cycle,including spacecraft in orbit and space debris larger than 10 cm which have been tracked and coded.

According to the simplified calculation method used during NASA research,the orbital parameters calculated at different stages are established.In the ascending stage,the orbital inclination is 87.9°,the perigee altitude is 475 km,and the apogee altitude is 600 km,840 km and 1080 km respectively.The ascending time of a single satellite is 60-70 days,and the total launch and deployment time is 1 year.Each orbit has 40 satellites at the same time according to the launch period and batch.The operation stage involves the circular orbit of 1200 km with an inclination of 87.9°.Satellites operate in 18 orbital planes,each of which has 40 satellites,and their operation time should be 5 years.In the PMD stage,the satellites in each orbital plane descend in turn according to the perigee altitude of 1000 km,800 km,600 km,400 km and 200 km,and all satellites descend synchronously for 5 years.

Figure 7 Number of intersections within 10 km at different stages of large scale small satellite constellations

The calculation results are shown in Figure 7.It can be seen that in the stage of constellation launch and deployment,due to the short time and batch deployment of satellites,the number of close-range rendezvous occasions with existing space assets is less.In the operational stage,the orbit is located at 1200 km altitude with fewer space targets,but the number of rendezvous occasions increases due to the increase of constellation satellite density.In the PMD stage,because of the large number of satellites passing through the LEO region with dense spatial assets at the same time,and the long duration of this stage,the number of potential intersections is obviously greater than the other two stages.

When the PMD time is five years according to OneWeb's plan,the proportion of PMD time in this stage accounts for 60.64% of the total number of PMD time in the whole life cycle,which is basically consistent with the calculation results of NASA and CNES.As shown in Figure 7,the number of possible intersections can be significantly reduced by increasing the speed of deorbit and thus shortening the PMD time of tasks;when the deorbit time is less than three years,the number of possible intersections in this stage reduces to only 44% of the total number of intersections,less than the number during the operational stage; when the deorbit time is shortened to one year,the number of possible intersections decreases to only 20.94% of the total number of intersections,which is close to the number of single-year intersections in the operational stage.

5 CONCLUSIONS AND SUGGESTIONS FOR MEASURES

The results show that the deployment of large scale small satellite constellations will inevitably cause adverse impacts on the space debris environment,mainly in the increase of the number of orbital objects and the number of possible collisions.The key to mitigate and control the adverse impact of large scale small satellite constellations lies in PMD processing,improving the PMD success rate can greatly reduce or even eliminate the medium-and long-term impact on the space debris environment.Shortening PMD time can not only reduce the short-term impact of constellations on the debris environment,accelerating the recovery of debris from the environment,but also reduce the risk on satellites in constellations as well as the impact threat on other space assets.Based upon the current technology level,shortening the PMD time means higher satellite development costs,so it is urgent to study new technologies in this field.

Looking at the above conclusions,this paper proposes the following suggestions for future large scale small satellite constellation plans from the perspective of debris environmental governance:

1) The deployment orbit has been fully communicated and coordinated beforehand.The large number of satellites in a large constellation will greatly increase the number of space assets and potential targets deployed in the region,thus increasing the complexity of satellite operation and collision risk.New large constellations should avoid the existing high-density orbits and other large constellation deployment orbits.

2) Satellite orbital positions for constellations should be fully open and transparent,including detailed launch plans,operational orbits,and deorbit paths.Angular reflectors,automatic transponders and other devices need to be equipped to report in real-time the position of satellites in order to reduce the difficulty of early warning processing for dangerous intersection and to minimize the impact on other spacecraft.

3) Satellites in constellations should have timely and reliable PMD capabilities.Post-mission deorbit time shall not be 25 years as stipulated in the current mitigation guidelines,but should be shortened to 5 years or even shorter.The PMD processing capability should be reliable enough to ensure that all satellites in a constellation have an over 90% PMD success rate.

4) It should be ensured that the reentry process of satellites is safe and reliable.Through reasonable design and appropriate material selection,a satellite can fully disintegrate and burnt during the re-entry process,avoiding any residual debris from reaching the ground,or the remaining debris can be confined to an area that does not pose a threat to life or property as far as possible.This is particularly important for large-scale systems where hundreds or even thousands of satellites may re-enter the atmosphere every year.