Orbital debris is a significant concern among those in the space industry, especially as satellites orbiting the Earth become increasingly critical infrastructure and exponentially more numerous. Risk reduction focuses on large debris objects, ignoring smaller, often equally as deadly debris. I devised a preliminary spacecraft concept to mitigate the threat posed by this class of debris.
Overview
Sentinel is a conceptual in-situ orbital debris measurement and mitigation device. It is low cost and scalable for various mission concepts. Current debris risk mitigation technologies are very nascent, though the emerging solution for risk mitigating is Space Situational Awareness (SSA) and collision avoidance maneuvering. Active debris removal strategies are risk generating more debris, expensive, and typically remove debris that can be well tracked anyways. SSA can only be as good as the data available, and data leveraged by government and commercial operators alike is dangerously incomplete. Sentinel will mitigate orbital debris by completing this dataset and moving to clear out orbits at needed altitudes of <10cm debris by sustaining hypervelocity impacts.
Background & Heritage Study
NASA characterizes the remaining <10 cm debris using the HUSIR, HAX, and Goldstone radar systems. These are ground-based systems with the capability to identify at limited inclinations, and not the fidelity to track, debris down to 1 cm below 2000 km and about 0.5 cm at ISS altitudes [1].
Debris <10 cm in diameter is extremely difficult to measure using ground-based systems and is orders of magnitude more common than well characterized debris.
The distribution of <10 cm debris is 2+ orders of magnitude larger than >10cm debris, untracked, and often just as dangerous [2]
It poses the highest penetration risk to most robotic missions operating in LEO and poses an ongoing risk to even the ISS's critical components [3]. NASA ORDEM models predict by 2030, >1mm debris flux in LEO orbits will be roughly 1 particle/m^2/yr, practically guaranteeing some sort of damage [4]. It is important to note that the <10 cm, particularly <5mm debris environment is not well characterized and the US Government currently has no dedicated sensor for doing so. The LDEF impacts provided a sampling of the environment in the '80s and the Shuttle radiators and windows provided reliable sample space for the environment up until 2011. More recently, the 1 m^2 DRAGONS space debris sensor (SDS) was deployed on the ISS using a novel acoustic sensor to provide proof of concept for characterizing the ISS orbit's debris environment. Designed to operate for 2 years, it became unresponsive after 26 days.
Competitive Landscape
In early 2022, IARPA put out an orbital debris detection and tracking RFI to "detect and track currently undetectable orbital space debris." In recent years, the in-situ debris characterization and SSA market has become quite crowded, with proposed products from Northstar, Scout, Odin, Privateer, and Spaceable, to name a few. Many concepts involve launching satellite constellations with extremely sophisticated sensors, while others involve mounting hardware or leveraging the sensor suites of existing satellites. The space debris mitigation company Astroscale designed the first in-situ debris observation satellite, IDEA-OSG 1, a 22 kg cubesat built to monitor the debris environments in LEO using conductive stripes. It experienced a launch failure in 2017.
Notably, ground based debris observation and tracking continues to improve. SSA provider LeoLabs, recently expanded its catalogue of phased-array radars with S-band capability, which it advertises allows it to track debris down to 2mm, the lower limit of existing technology [5]. Not to be outdone, Lockheed Martin is operating the $2bn Space Fence, an S-band radar system designed to track up to 200,000 objects (on roughly 2/5 of all objects >1cm in LEO) [6].
System Design
The Sentinel system will be built to detect, characterize, and mitigate <10 cm debris to submillimeter debris over a large surface area for specific orbits on demand using a novel sensor suite in tandem with debris shields. It will be capable of maneuvering to sample specific orbits and rendezvous with satellite systems in LEO several times within its lifetime. Debris data will be conveyed to ground or in-space networks for real-time response. The system aims to be inexpensive, simple, robust, and maintain EOL de-orbit capability throughout operation – a "brute-force" debris measurement device with the functionality to protect another satellite.
Objectives
As a system, the Sentinel satellite will act as a shield to extend the operational life and mitigate catastrophic risk of MMOD for other satellite systems while simultaneously gathering valuable data on uncharacterized orbital debris in-situ. The functional objectives of the Sentinel satellite system are outlined in the block diagram below.
Sentinel Functional Decomposition for debris measurement and mitigation missions
Spacecraft Mass & Power budget
To be effective, Sentinel must be a relatively low-cost and simple system while prioritizing robustness. The satellite can meet mission objectives weighing under 400kg, as outlined in the table below. System communications will be simplified, outfitted with an S-band transceiver. In future designs, intersatellite communication arrays could be added instead to allow for low-latency debris warning direct to a target from Sentinel or a network of satellites. A ~3m^2 array of photovoltaic cells such as Spectrolab's XTJ Prime can provide 4 kW for electric propulsion, system controls, and data processing.
Spacecraft Mass Current Best Estimate
Optical Detector
Sentinel will utilize a novel optical technique for detecting and characterizing debris, which allows for the low power active sensing of debris. The sensor, based on a 2014 concept by the Naval Research Laboratory [7], uses a wide-angle camera to detect light scattering of a light sheet projected using a low power laser and conical mirror. If two lasers generate offset light sheets of different frequencies, position and velocity data of particles down to 0.1mm can be detected with as little as 10mW. Using 4 sensor suites at 50 W, an area of up to 5m^2 can be constantly monitored for 0.3mm debris, an area 5x larger than the largest in-situ measurement arrays. Because the optical power source is variable, the fidelity of debris detected and detection area can theoretically be modified from 10cm at 20m distance down to 0.01cm at 2cm distance.
Optical debris sensor detection limits for power and debris size. 50 W contour highlighted red.
The optical sensor suite will consist of two lasers mounted near a fish-eye camera lens with a boom that extends out from the s/c and constrains two axially offset conic mirrors. The optical sensor suite will be mounted on each of the four sides of the Sentinel satellite, mounted on a linear actuator and hinge that will allow a variable angle. This sensor suite will detect debris opposite of the shielding sensor as to measure un-mitigated MMOD.
Optical Sensor Detection FOV at 30° angle and 1m detection distance. Shield not pictured on opposite side of s/c
Shielding & Modularity
Sentinel will utilize a modular, lightweight, deployable shield for debris field characterization and mitigation. The shield will be located on the leading side of the s/c, equipped with a conductive Ni-Au coated Cu thin wire mesh for debris detection and a hard backstop to ensure robust operation throughout lifetime. The thickness and count of wire stripes can be tuned for cost and debris size to be detected, but they can be formed with micrometer separation on a nonconductive polyamide material and then attached to a shield element inexpensively. The shield element material will consist of a 4cm Lexan backstop plate, which NASA has tested extensively with hypervelocity impacts [8], and a 1cm steel plate for additional protection. To mitigate and measure debris, once the s/c has maneuvered, it will attenuate its orientation according to the customer satellite's orbit, effectively clearing orbital debris from its trajectory while collecting valuable data that can be used to statistically characterize <10cm debris at its altitude.
If payload volume becomes a constraint, the deployable shield size can be modular and modified based on mission requirements by extending the s/c body and slotting in or removing shield elements. Once the maneuvering rendezvous is complete, the shield will deploy. The deployment will be based on a spring-loaded telescoping system that locks out at steps in deployment to mitigate partial deployment failures. Each shield element is about 50cmx50cm in area, about 10kg each.
Rough sketch of stowed & deployed modular shield elements, as well as individual shield element face with extension options for increased surface area.
Operations & Trajectory Design
Sentinel must first maneuver to a desired altitude and trajectory, rendezvousing just ahead of a customer satellite if desired, in order to perform the desired debris measurements and mitigations there. It will be designed to complete several orbital trajectory changes, primarily orbit phasing and raise, throughout its lifetime using electric propulsion, enabling on-demand use of its services in LEO.
Orbital Maneuvers
Sentinel can use a gridded ion thruster with around 4000s Isp supplied by power from a roughly 3 kW triple junction solar array, which must deploy to about 9m^2. The spacecraft trajectory can be designed to maneuver on-sun to limit battery size requirements. The low thrust capability of the ~400kg satellite will allow for a total dV budget of roughly 11 km/s with 100kg of propellant. This will allow for up to 10 phasing maneuvers below 1000km altitude, including station keeping and small (~1deg) inclination adjustments.
Low thrust capability selected for simplicity and robustness to orbital debris hazards
Deorbit
Deorbit of Sentinel will be done using the direct re-entry method described by NASA Debris Mitigation Standards. The s/c will have heavily shielded propulsive and attitude control that will enable this. If necessary, the s/c can utilize cold-gas RCS for attitude control, which could be used as an emergency de-orbit mechanism.
Summary
Sentinel is a highly viable and technologically ready system that will fill in the potentially catastrophic gaps in the industry's primary debris mitigation strategy, SSA & collision avoidance. It can do this at a larger scale and faster than competitors using cheap, reliable sensors and large detection areas. In the near term, the sensor suite (shield elements and optical sensor package) could be tested and demonstrated as mounted payloads on a LEO pathfinder or space mobility vehicle.
References
[1] NASA Orbital Debris Program Office write up on current state of mitigating small debris.
[2] Debris distributions have changed notably in the past decade as mass to orbit has gone up significantly and tracking becomes increasingly important, Horstmann et al.
[3] NASA Astromaterials Research & Exploration Science on everything you want to know about debris
[4] International Orbital Debris Conference & current work with ORDEM is interesting and far from bleak.
[5] LeoLabs has a ground-based approach, beefing up heritage technology and compute instead of trying something new. I suspect it will work well for them but <2cm debris remains difficult.
[6] I dug up some archives about the Space Fence specs.
[7] Optical Orbital Debris Spotter Concept
[8] NASA's DRAGON Space Debris Sensor lasted almost a month. Here is how it was tested.