Cutting-Edge Space Exploration Technology Maturity Level Facilitation with Support of Space Debris Removal Missions

Hamed Ahmadloo and Jingrui Zhang

(School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China)

Abstract: Considering current space debris situation in outer space environment, different methods for debris removal missions are proposed. In addition, advanced technologies are needed to be demonstrated for future human space exploration programs. The main issue regarding to these missions is high mission cost for both debris removal missions and space environmental tests to achieve high maturity level for new space-usable technologies. Since, these missions are unavoidable for future of human space activities, a solution which can tackle these challenges is necessary. This paper will address to an idea which has the possibility to give a solution for facilitating technology readiness level (TRL) maturity tests by debris removal mission platform consideration.

Key words: space debris removal missions; technology demonstrator missions; technology readiness level; drag augmentation methods

Increment of space debris rotating around the Earth, currently become a critical issue for future space exploration programs. Different outer space environments such as low earth orbit (LEO) and geostationary orbit (GEO) are suffering the most from the left behind space objects such as non-functional satellites and rocket bodies. The number of space junks in LEO is dramatically higher than GEO, but danger of collision in GEO can come from big communications satellites which has been failed to be removed from valued positions in this orbit. Studies are predicting uncontrolled collision incidents (Kessler Syndrome) that can destroy all operational satellites and spacecraft, and also make human space access limited due to unpreventable future collision impacts[1]. Regardless of debris’ sizes which are rotating around the Earth, any impact can be fatal with respect to high orbital velocities (7-10 km/s in LEO). Since few years ago, mitigation guidelines had been proposed to decrease the risk and number of space objects after their mission time[2], although, debris removal missions are needed to be considered for at least five big debris to be removed annually from 2020[3]. Now, there are some hazard debris which are necessary to be removed as soon as possible, because of their critical orbital position with high collision impact risk with other catalogued object in the same orbit.

For debris removal missions, different concepts have been proposed and even developed to demonstrate their usage possibility potential. Passive and active debris removal missions are considered with respect to cheapest, simplest and the most efficient method for tackling space junks’ problem in different regions. For most of proposed methods, except the complexity of those missions, mission cost is an important issue to be taken to account for any nation trying to clean outer space environment. Even simplest and cheapest passive method’s cost can be quite high and also challenging to find the sufficient budget to cover whole mission cost.

In addition, human space exploration program is required to reach advanced technologies for future space missions. The process of adequate any technology for space programs, should pass certain levels of existing standards. Technology readiness level (TRL) is defined in 9 levels to reach sufficient integrity of being able to use a specific technology in space[4]. Any technology required to pass long term and costly steps to reach final level (TRL 9) which can add delay to some missions with many technologies needed to be verified beforehand. Based on defined TRL standards, the main difficulty comes from the necessity of technology testing in real environment of outer space. For these purposes, some specific missions called “Technology Demonstrator Missions” has been designed and launched for new advanced space technologies demonstration[5].

This paper will address to a potential solution for both space debris removal and technology demonstrator missions to firstly remove the space junks simpler and cheaper and also make the advanced technology tested for further space exploration programs. Space debris population and available debris removal methods have been explained in this paper, in addition to some examples of required space technologies of NASA and ESA. The key solution proposed to consider the possibility of carrying out both type of these missions at the same time and trying to look for a common platform to fulfill requirements of them in a single missions.

1 Debris Affection of Outer Space

From beginning of human space activity up until now, there are about 100 000 objects between 10 cm to 30 cm, 11 000 objects over 30 cm rotating with high velocity around the Earth. Hundreds of thousands of smaller objects are also in the same situation, 50 million fragments of debris sized 1 mm and greater, orbiting the Earth as well. The total mass of debris in outer space reached 19 000 tons, with average orbital velocity of 7 km/s to 13 km/s in LEO. Highest concentration of space junks are in LEO with 2/3 of total orbital debris in outer space, which only 6% of total objects in LEO are operational satellites[6].

Some non-functional satellites like Envisat in LEO from ESA have 150 years of natural decay time and currently it has high chance of colliding with 2 other catalogued space objects[7]. Sun-synchronous orbit(SSO) is high value economical region for Earth observations satellites with almost full Earth coverage which has highest collision possibility in LEO. Annual collision probability for satellite with cross section of 10 m2in SSO with 1 cm debris is 0.8%[8]. Simulations are illustrating with current mitigation guidelines and compliance rate of 50% for next 100-200 years, the number of debris sized 10 cm or greater can be doubled[8].

Apart from LEO region, geostationary orbit (GEO) also suffers from occupying valued positions with non-functional satellites. GEO is a limited resource with high valued which there is utilization competition among nations and commercial space sector to be used by their own satellites. Huge number of non-functional satellites filled this orbit with variety of ages (around 45 years). Observation estimations show that GEO is filled by more than 500 unknown, un-catalogued debris objects with visual magnitudes of 18.5. Basically, large and expensive telecommunication satellites are occupying this orbit with their millions of dollar mission cost. Presently, 95% of insured satellites are in GEO (18.3 billion dollar) which shows the importance of this critical orbit and any collision accident can be fatal especially for their operating commercial companies[8].

2 Debris Removal Mission Methods

Several methods have been proposed for active debris removal missions; never the less, still the risk of successful de-orbiting maneuver is moderately high, due to challenges of accurate and precision tracking of the debris in orbit, rendezvous maneuver, low maturity level of required technologies and also political costs which can be hard to overtake from single country.

Currently different scenarios are possible to response in case of having collision danger with space debris[8]:

① Ignore with no mission modification;

② Dodging close approach debris as response to the warning;

③ EOL mitigation guidelines compliance;

④ Active debris removal (ADR) operation.

For the fourth option which can be the most affective one for future space programs several ADR techniques are presented and investigated such as[7-9]:

① Robotic re-orbiting;

② Drag augmentation;

③ Electrodynamic tethers;

④ Laser ablation;

⑤ Ion shepherd methods;

⑥ Tethered tugging of large LEO debris;

⑦ Harpoons or nets to capture debris;

⑧ Electrostatic tractors.

Still none of above mentioned methods had been developed for real in-space practice. The risks of collision accidents in ADR missions are relatively high which can ended up to increment of number of debris and fragments. No space agency in the world currently has the full confidence in ADR mission and also did not financially support an ADR mission.

3 Requirements for Space Exploration

Any technology to reach sufficient maturity level to be able to perform in outer space environment needs to be certified through a certain process defined by space agencies called technology readiness level(TRL). TRLs are a systematic measurement system which can assess the maturity level of a particular technology and also provide understandable comparison methodology between different types of technology. Selection of technologies for different space agencies can vary from perspective point of view of each space agency. NASA Strategic Space Technology Investment Plan, considers the horizon of 20 years for future space exploration programs[10]. Innovative technologies required for challenges of human missions to deep space, technologies affordability, safety, and feasibility for each mission and enabling human to travel to never before visited destinations are the key features of NASA for technology development selection. These criteria are different for other space agencies for instance ESA considers overall vision of space science programs with addressing number of science questions from proposed technologies and selection goes through a competitive process before beginning to invest on their maturity level[11]. The basic research in new technologies and new technologies potential for higher number of applications are important for future technologies selection.

Fig. 1 explain the TRLs step by step, considering reaching TRL 6 to TRL 9, it is required first demonstrate the full scale prototype and test it in real environment. These steps are done through costly technology demonstration missions[5]. Basically, whole TRL process is long term evaluation and progress for each technology, which can add delay to project in need of these type new technologies.After selection process, 1 to 2 years for finalizing the development of new technologies is necessary and having the demonstrated technology patented and start the marketing up to 10 years is required[10].

Fig.1 Technology readiness levels(TRL)

4 Required Technologies for Space Exploration

For the case of this paper study, two space agencies of NASA and ESA future technology development programs are considered. Although both NASA and ESA use different approach to select technologies for their future missions, the fields of technology development has some similarities. NASA is looking for technology development activities which quickly make access to critical capabilities with potential to revolutionize space exploration, discovery and work. The key factors for selection fields should address to[10]:

① Human presence and activities in space;

② Structure, origin, and evolution of the solar system;

③ Earth and the universe;

④ Extend benefits of space for the Nation.

And based on the fields that they are looking for development, the core technologies are defined as:

① Launch and in-space propulsion;

② High data rate communications;

③ Light weight space structures and materials;

④ Robotics and autonomous systems;

⑤ Environmental control and life support system;

⑥ Space radiation mitigation;

⑦ Scientific instruments and sensors;

⑧ Entry,descent, and landing.

On the other hand, ESA critical technologies are identified for missions and candidate missions for the science programs. Level size of the missions divided to S, M and L-Class missions and more importantly, L and M-Class missions with key objective of reaching TRL≥5-6 before entering the implementation phase. Some examples of the ESA future missions including their technology development activities are presented in Tab.1[11].

Tab.1 Selected ESA technologies for future space missions

5 Debris Removal Missions: A Platform for Technology Demonstration

The big issue revolving around debris removal missions is that a number of debris with especially big sizes directly lead to huge financial pressure to make the LEO environment stable for near future.In addition, debris removal missions should be repetitive to reach suitable number of debris to be removed annually (5 large critical debris/per year) or multiple debris to be removed in one single mission. In terms of the multiple debris removal in a single mission, selecting different debris in the same orbital plane can be more efficient but it is not the most optimal way for targeting the debris with highest danger of collision.

On the other hand, the missions with the aim of technology demonstration before reaching their final maturity level are needed to be tested in operational environment which is outer space. These tests can be costly and also difficult to reach their final TRLs in a short time. These limitations will finally lead to long technology demonstration process and less chance for more number of technologies to be tested in space. This problem easily can affect the future space exploration programmes of human and brings down the speed of achieving more advanced technologies for complex space missions.

Considering at the same time, both problems of space debris removal missions and technology demonstration missions; it is possible to look for a solution that debris removal missions which are necessary in near future to facilitate the new technologies to be tested in space environment. With this idea, instead of having missions with only objective of removing space debris, also they can be used as a platform which examines the desired technologies as well. This idea may lead to more complexity to the mission and may do not give the platform for all required advanced technologies, but in some cases it will be quite useful.

The space agencies which are looking for both type of missions are needed to be driven the mission proposals in a way that the technology for removing debris, are selected based on the required future advanced technologies or another option is designing the mission with both goals of debris removal and at the same time testing one or more advanced technologies. This idea at the beginning may looks difficult to implement, the problem comes from the classical point of view which always experts look at these two types of missions separately. The most optimal method for benefiting both types of missions is trying to use the most the extra capacity of any debris removal mission to test new advanced technologies for future space exploration programs.First step can be detailed mission design from initial studies of any program to lead to combinations of both types of missions.

From the list of necessary advanced technologies which are needed to be demonstrated both from NASA and ESA, there many potential fields which can be combined with debris removal missions, for instance, In-Space Propulsion Systems, Robotics and Autonomous Systems, Light Weight Structures, Scientific Instruments and Sensors, Space Radiation Mitigation and Re-Entry Tests.

6 In Space Fabrication Demonstraion

One of the new fields which different space agencies are now trying to demonstrate is in space fabrication with 3D printing. The advantage of this method is giving the possibility of producing large scale structure such as solar panel arrays with no need of sending the compact structure to space and deploy it in orbit. By using 3D printing and in-orbit manufacturing, instead of having compact version of deployable mechanism, 3D printer extruders can give the possibility of generating required structure. Currently, 3D printing is tested inside International Space Station for producing small sized components, but still no test of autonomous usage of 3D printing in space environment has been done.

In this paper, combination of 3D printing extruders which are the basic step of large scale complex structure manufacturing and drag sail fabrication for debris removal missions in LEO is presented. This system is working under control of a mother spacecraft and it provides the possibility to produce drag sails with desired sizes. This idea has the advantage of targeting multiple debris to be removed in a single mission with in-space 3D printing manufacturing technology. In addition to debris removing from the LEO region, it can provide the basic level demonstration of using 3D printing extruders in space. This mechanism is designed to support the main idea of this paper which focuses on the advantages of combining debris removal with advanced technology demonstrator missions.

The mechanism itself contains two main parts of system box and subsystem box, subsystem box includes material container for extruders and also the laminate spool for covering the inner part of drag sail frame (Fig.2 and Fig.3).

Fig.2 Systems container

The system box has the capability to expand to the required size of drag sail and the upper part of system box, can attach the connection device to grab debris in orbit. The connection device uses the shooting net to attach to the debris for removal phase (Fig.4).

The system box contains the sets of expandable transversal beams which they are connected to the four corners of the booms. This system by using the first set of transversal beams and putting it in front of the extruders can start to generate the first drag sail frame (Fig.5).

Fig.4 System box expansion directions

Fig.5 First set of transversal beams

Fig.6 Initializing frame manufacturing

It is important to have the beams aligned with the extruders in the first step and then extruded beams can be produced by the extruders to the desired length.By having the longitudinal beams of the frame, the front transversal beam can be moved to the end part of the extruded beams. In this step, the main structure of the fame can be produced in optimized size for the selected debris (Figs.6-8).

Fig.7 Beams generated with 3D printing extruders

Fig.8 Frame structure being completed

The final step is covering the inner part of the frame with laminate material and by keeping the size of the laminate part bigger it can generate the parachute effect (Fig.9). For fixing the frame itself with the laminate surface, the pressure welding on sides of the frame can be used (Fig.10).

Fig.9 Inner sail being added to the frame

Fig.10 Heat and pressure used for final attachments

After generating the drag sail, the connection device can shoot the net to attach to the debris. By having the debris attached the generated drag sail can be released from the spacecraft and

de-orbiting phase will be started Figs.11-13.

This process can be repeated for multiple debris and the mother spacecraft is responsible for orbital manoeuvre for the next debris (Fig.14). Based on mission designed requirements, the drag sail sizing and manufacturing phase can be optimized to increase the total efficiency of each mission.

Fig.11 Approaching selected debris

Fig.12 Net being deployed to attach the debris

Fig.13 Doors of system box being open

Fig.14 Drag sail for de-orbiting being released

7 Conclusion

Regarding to current issues with both debris removal missions and advanced space technology demonstrator missions, this paper presented a new idea which can facilitate these two types of mission in the same platform. The main idea is finding potential possibilities of using debris removal missions as the platform which can provide space access for TRL space environmental tests to reach required maturity level of the advanced space technologies for future human space exploration. In this paper with brief introduction of required advanced space technologies from ESA and NASA, combination of in-space fabrication with 3D printing extruders for drag sail in-space manufacturing is presented as an example for possibility of 3D printers large scale fabrication in space.

Acknowledgments

The designed mechanism itself submitted as a patent under Beijing Institute of Technology Patent Office (Patent No. 201611018438.8).