A micro/nano joint satellite design of high maneuverability for space debris removal

1 Introduction

According to the prevailing concept, rapid space debris removal is defined as a crucial way to ensure the safe operation of in-orbit spacecraft. By means of a removal mechanism, a satellite catches debris and tows it to other orbits to prevent severe space security threats (Clean Space 2022, ESA Space Debris Office 2017). Nevertheless, there are difficulties for the satellite in identifying the debris and maneuvering quickly to it as well as remaining flexible when the debris is being removed (Liou and Johnson 2006).

Since the European Space Agency’s (ESA) “Clear Space” initiative was launched in 2012, countries have been actively encouraged to take measures to remove space debris. The initiative has three branches named ecodesign, cleansat, and e.deorbit. For e.deorbit, there are six steps: launch into space; perform launch and early orbit phases and commissioning; transfer and phase to target orbit; rendezvous with debris; capture debris and deorbit debris (Biesbroek et al. 2017).

Since the launch of the first artificial earth satellite in 1957, more than 8,000 spacecraft have been sent to earth’s orbit, but merely approximately 2,000 of them are operated at present. The remaining craft have either expired from service or have been shut down, becoming space junk and forming an artificial environment filled with debris (Rossi et al. 1994, Lanouette et al. 2015). According to statistics, as early as 2018, space debris larger than 10 cm reached 23,000 pieces; fragments with a scale of 1–10 cm numbered approximately 750,000; fragments with a scale of 1–10 mm numbered over 100 million; and fragments below 1 mm numbered in the tens of billions (Krag et al. 2017). In low earth orbit, which is the orbit most heavily used by humans, debris travels at 7.8 km/s (the first cosmic velocity), and the average speed for entering the atmosphere is 10 km/s (Pardini and Anselmo 2017).

Internationally, up to 30 satellite maneuvers need to be performed annually to prevent debris impacts (Li et al. 2018). Debris less than 1 cm causes spatial impact and results in functional loss and failure of components, subsystems, or even the entire spacecraft (Kurihara et al. 2015). This kind of debris cannot be monitored and cataloged and can be avoided passively only by installing a protective structure. In addition to minor debris, there are also different debris sizes. Debris larger than 10 cm, can lead to explosion, disintegration and complete failure of spacecraft. No protection could be taken, but the debris could be accurately detected, cataloged, and managed. Spacecraft avoid these debris actively, and debris removal is available (Gong et al. 2018).

A more severe situation is that a large amount of space debris is generated because of innovative programs carried out by various countries (Johnson-Freese and Burbach 2019, Martin, 2019). For example, Mission Shakti conducted by India in 2019 generated approximately 6,587 debris fragments larger than 1 cm and 7.22 × 10⁵ debris larger than 0.1 cm. The impact of other debris with the debris fragments created by the Indian anti-satellite test led to secondary ejecta of debris, creating more debris fragments with a wider distribution in space (Schonberg 2001, Jiang 2020).

Three challenges in space debris removal are as follows: the first difficulty is to recognize, approach, and move synchronously when the debris is rotating after it enters a hover state; the second task is to maintain safe and reliable operation when removing the debris with no new debris generated; and the third challenge is safe and controllable orbit transferring and deorbiting the debris and removing the debris (Yan et al. 2018).

Almost all countries apply the technique of flexible removal by casting nets with only one tether connected to the removal spacecraft (Aslanov and Yudintsev 2013). This net limits maneuvering potential, especially when the net contacts rapidly rotating debris (Vadali 2002).

The present article proposes a design of a micro/nano joint satellite (joint satellite for short), which enables the satellite to maneuver toward space debris at high speed and to clean up it when removal is necessary. The joint satellite is combined by four homogeneous micro/nanosatellites for remote sensing, and each micro/nanosatellite (node satellite or node for short) is a node of the joint satellite. Before removal, the joint satellite is composed of four node satellites with high mobility and high removal performance; when removing debris larger than 10 cm, the joint satellite transforms into four node satellites, and the dynamic removal mechanism for each node recognizes and propels the debris. All four nodes in combination can follow and remove the rotating debris.

2 Space debris removal based on a micro/nano joint satellite

The existing technology involves the three-step removal of debris: “capture-push/pull-deorbit.” The removal mechanism connects with the satellite by only one connection, so removal from multiple dimensions cannot be realized. Compared with the prevailing technology (Rubenchik et al. 2014, Qi et al. 2017, Vallduriola et al. 2018), the joint satellite has distinct features due to its unique structure.

First, to improve image and debris recognition, the joint satellite performs networking data fusion calculations by making use of the remote sensing data of the four nanosatellites. Second, each nanosatellite is equipped with a set of large engines for improving high maneuverability joined with nine minor-thrust engines for enhancing small-range fast maneuverability and well-regulated attitude. Finally, the four nodes are installed with visual navigation processing and independent propulsion. Therefore, when the joint satellite deploys the removal mechanism, its speed is fast, and its configuration is adjustable. Ultimately, after unfolding, the removal mechanism is controlled by the four nodes for spinning up and moving synchronously against the debris, effectively avoiding a coiled net and removal failure caused by debris rotation in the course of deorbiting.

The RemoveDebris satellite, developed by researchers at the University of Surrey in UK, was the first to demonstrate the removal technology. The satellite, designed with a debris collection net, was launched into space on a SpaceX Falcon 9 on April 2, 2018. When it spotted floating debris, it launched a large net, enclosed the debris, and pulled it back to the Earth with cables. On September 16, 2018, the media confirmed the technology for its successful application. After the net was launched, it rapidly unfolded into a hexagon with six sharp corners spreading out to increase the “intercept area” for removal. The debris contacting the net was immediately entangled with it for disposal by RemoveDebris (Yan et al. 2018). However, the technology proposed in the present article differs from the aforementioned technology in the following four aspects.

1. Low cost for the joint satellite composed of four node satellites. Regardless of whether the nodes operate jointly or separate as an independent node, they are the same type of payload, so that they share the same designed platform, on-orbit system, and on-orbit processing software for remote sensing images.

2. The four node satellites provide remote sensing data of different spectral and optical characteristics. Meanwhile, the four satellites can carry out on-orbit data fusion for remote sensing images, which is conducive for joint maneuvering to debris quickly and accurately in high-mobility trajectories.

3. Even when the four node satellites separate for removing the debris, each satellite is available for high maneuverability and remote sensing as well as near-field communication, which is helpful for them to control their motion attitude and to manipulate the removal net attitude and rotation.

4. Since all the four nodes feature high maneuverability and remote sensing, they are all sensitive to control the net to make dynamic changes when removing the debris.

3 Micro/nano joint satellite design of high maneuverability for space debris removal

As shown in Figure 1, four node satellites are constructed in a “bisected quadrangle” to form the joint satellite. When carrying out observation and identification tasks targeted at the debris, the joint satellite, because of its high mobility and superior removal performance, maneuvers rapidly to one or multiple suspected debris to fulfil the task. After the debris has been identified, the four nodes start the removal task by expanding in the diagonal direction. Subsequently, a dynamic removal net that has been tied to the 4 node satellites is unfolded for removal. The four nodes operate with flexibility in a cooperative manner for propulsion and debris removal.

Figure 1

A micro/nano joint satellite.

3.1 Single micro/nanosatellite

Figure 2 shows the front view of the micro/nanosatellite, which is made up of a communication antenna, small-thrust engine, large-thrust engine, optical remote sensing payload, micro/nanosatellite joint connection mechanism (which functions as a connection at the normal state and transforms into a large-thrust engine after the four nodes separate when removing), and solar panel.

1. Communication antenna (A): The antenna is used for TT&C communication and payload data transmission; when removing debris, it exchanges data with the satellites at close range.

2. Small-thrust engine (SE): The small-thrust engine adjusts the attitude of a single micro/nanosatellite; when the net catches the debris, the small-thrust engines of the four node satellites cooperate with each other to jointly control the structure and motion posture of the net.

3. Large-thrust engine (LE): The large-thrust engine is used for routine orbital transferring. When the micro/nanosatellites are combined in the joint satellite, only the engines on the outside of the joint satellite can be activated. When removal is achieved by the four nodes, the horizontally oppositely placed engines operate in conjunction to achieve time-shared propulsion, reducing vibrations while tightening the net. In addition, the oppositely placed engines provide the four nodes with strong maneuverability to control the net for pulling and pushing debris.

4. Joint connection mechanism (J): This mechanism works in conjunction with a large-thrust engine with insertable ports on its structure for connecting with adjacent micro/nanosatellites. The oppositely placed engine in a satellite not only works as an engine but also functions as a connection. In daily operation, it does not provide propulsion, and the node satellite exchanges data via a mounted-on plug with the neighboring node satellites. During removal, the micro/nanosatellites communicate with each other through antennas, and the connection mechanism functions as a large-thrust engine.

5. Remote sensing payload of the joint satellite (P): Four cameras with different spectral and optical properties are installed for the four node satellites as their payloads. When observing the debris, the four groups of payload exchange data, maintain networked computing via the joint connection mechanism for further processing and analysis, and finally conduct remote sensing survey of debris.

6. Removal net (N): A four-square removal net has a mesh density related to the debris size, satellite mass, and satellite propulsion. As the debris are of different masses and sizes, the design emphasizes fuel efficiency of the joint satellite and the net in terms of the structure, size, and mesh density in addition to propulsion thrust.

7. Depositary cabin of the removal net (C): The removal net is tightly compressed and stored in the cabin. In addition, it unfolds when removing.

8. Solar panel (S): The solar panel powers the satellite and its payload.

9. Hook system for the removal net and node satellite (H): After the four node satellites separate, the hook system connects the net and the node satellite body.

Figure 2

Front view of the micro/nanosatellite. A: communication antenna; SE: small-thrust engine; LE: large-thrust engine; P: remote sensing payload; J: joint connection mechanism (functions as a connection under normal states and transforms into a large-thrust engine when removing); and S: solar panel.

Figures 3–5 give different views of the micro/nanosatellite.

Figure 3

Bottom view of the micro/nanosatellite. H: hook system for the removal net and node satellite; SE: small-thrust engine; and J: joint connection mechanism (functions as a connection under normal states and transforms into a large-thrust engine when removing).

Figure 4

Rear view 1 of the micro/nanosatellite. LE: large-thrust engine; A: communication antenna; SE: small-thrust engine; P: remote sensing payload; H: hook system for the removal net and node satellite.

Figure 5

Rear view 2 of the micro/nanosatellite. SE: small-thrust engine; H: hook system for the removal net and node satellite; LE: large-thrust engine; and P: remote sensing payload.

4 Typical application design for the joint satellite

The joint satellite has high maneuverability for debris removal, which entails the two operating states below.

4.1 Normal remote-sensing warning and high maneuvering

In Figure 6, four node satellites, combined into the joint satellite, are shown on top left, top right, bottom left, and bottom right. P refers to the remote sensing payload of the joint satellite. Four cameras with different properties for each node satellite are used to collect data and conduct a remote sensing survey of debris on the basis of comprehensive analysis. The four nodes exchange remote sensing payload data, maintaining four-satellite network computing, transmitting data directly, and executing dynamic control via the connection mechanism. When performing removal, the explosive ignites, and the four satellites expand in a rectangle and spread out the removal net in four directions.

Figure 6

Normal state for the micro/nano joint satellite of high maneuverability. P: remote sensing payload; LE: large-thrust engine; C: depositary cabin of the removal net, and J: joint connection mechanism.

C is a depositary cabin for storing the removal net, where it is tightly compressed. It unfolds as shown in Figure 7. LE refers to the main propeller. Two groups of propellers for each node satellite are located symmetrically outside the satellite, as shown in Figure 6, which shows the normal state.

Figure 7

Space debris removal by the micro/nano joint satellite (unfolding of the net).

5 Satellite features

The features include superior data discrimination of identified debris by the four remote sensing cameras of the nodes with great accuracy.

They also include two pairs (four) of oppositely placed engines that maneuver rapidly toward the debris when the net is ready for removal.

When the joint satellite separates and moves into four corners of a rectangle, it quickly sets free the net stored in the depositary cabin of the joint satellite to remove the debris.

Rotating debris can be removed effectively. Four node satellites separate from the joint satellite and pull the net with tethers. Since the four satellites are all available for remote sensing and maneuvering, the net dynamically rotates, and its shape can be changed at any time under the pulling of the node satellites.

Because the joint deorbits the debris by reopening the net, the net is reusable as long as no damage arises from the debris in the removal process.

6 Conclusion and key difficulties

This article explores a micro/nano joint satellite with high maneuverability for space debris removal. The joint satellite is composed of four node satellites that are available for debris recognition and strong maneuverability. The joint satellite can be used in daily operation and is responsible for observing the debris. When removing the debris, the joint satellite separates and unfolds the net for the task. Compared with a normal satellite, a joint satellite is favored for its faster maneuverability and higher flexibility.

The focus of planned future studies is to target a more complicated removal net and a joint satellite that may consist of 6, 8, 10, or 16 satellite nodes. The modularization design for micro/nanosatellite should be taken into account to reduce the production cost and shorten the production cycle. Demanding observation and changeable debris call for modularization of the interfaces interlinking 6–16 satellite nodes. Intersatellite data transmission is challenging, possibly requiring distributed computation by interspersing a robust data bus in the removal net.

In terms of joint dynamic orbit transformation and the removal net shape, four degrees of freedom will be explored for integrated six and even eight degrees of freedom control of the joint satellite (Liu et al. 2012).

Small debris is of millimeter-level size, and the explosion plume generated by the explosion of the explosive device is limited; however, the explosion plume affects the explosion in a limited manner. When the joint satellite separates, the net expands because of node satellite engine thrust. The connection functions as a physical link for fixing the net, and it can be disconnected by less destructive force. In addition, to reduce the debris and the impact on the satellite and space environment, a follow-up simulation and experiment will be included in the studies of micro/nanosatellite maneuvering separation combined with locking mechanism separation.

When the joint consists of more node satellites, more tethers are needed to pull the removal net. When one or more unstable debris with various shapes, speeds, and incident directions contact the net, the force analysis and motion characteristic analysis of the whole joint satellite become complicated (Jiang et al. 2020).