An Introduction To Laser Ablation For Debris Removal
This article does not review space debris in depth — you can find out more about space debris here.
In low earth orbit and geosynchronous orbit are millions of pieces of metals and paint that we refer to as space debris. Thousands of these objects are trackable and therefore avoidable, like dead satellites and rocket body parts. Millions are also very small (< 1 cm in diameter), but can be shielded from using technology like whipple shielding.
However, it’s estimated that there are 600-900k untrackable and unavoidable pieces of space debris ranging from 1–10 cm in diameter. These pieces of debris travel up to 7 times the speed of a bullet — which is enough to cause significant damage to operating satellites and the ISS.
Today, most research for space debris removal is geared towards the removal of large debris, like using nets, harpoons, large robotic arms, etc.. Although it is critical to remove large debris since they can break up, removing small 1–10 cm debris is equally important, since those are the pieces causing the large debris to break up. In the end, it is the small debris that perpetuates the problem.
One of the most popular methods of small debris removal is laser ablation, which is the process of repeatedly shooting laser pulses at a small piece of debris until its orbital velocity slow down, causing it to fall back into the Earth’s atmosphere and burn up.
Ground-Based VS Space-Based Systems
Existing research for laser ablation tends to be divided between ground-based and space-based systems, although newer research is shifting more towards space-based systems.
This was spurred by the multiple challenges faced by ground-based systems. To start, the lasers would need to travel a significant distance — 350–1000 km — to focus on a small debris particle with a diameter of a few centimeters. In addition, the system would need to meet the particle with extremely high accuracy to send pulses in the proper direction, which would require an extremely advanced target detection and acquisition system.
The laser would also loose energy traveling such a long distance, so a very large beam director mirror would be needed to create sufficiently powerful laser pulses. The ground-based system would also need to wait over multiple orbits because it is only in one location, meaning it would take a longer time to lower the debris.
Lasers on space-based systems, on the other hand, would only need to travel 500 km at most, allowing the system to have much better accuracy and use less energy. It would also be near the debris, so it could send pulses in real time and therefore reduce the debris’ orbital perigee much sooner.
There are two major subsystems in a space-based laser ablation system. The first subsystem is for target detection. First, a large FOV telescope is used to detect reflected sunlight from debris fragments. Then, a second smaller telescope uses low energy laser pulses to track and predict the movement of the object, which helps confirm whether or not the object is a piece of debris.
Once the debris is detected, the laser subsystem ablates the debris and changes its velocity. When the laser impulses hit the debris, the laser energy actually turns into thermal energy, and the spots of irradiation on the debris melt to produce plasma (although the plasma is harmless).This causes the perigee of the object’s orbit to decrease, allowing it to fall back into the Earth’s atmosphere and burn up. The amount of time it takes to fall into the atmosphere varies, ranging from half a revolution to an entire year.
Laser parameters vary a lot across different studies since many different models have been designed. In one model from TU Delft, the optimal laser parameters were a 20 kW laser that shot 600 J energy pulses at a repetition frequency of 33.33 Hz (when the system was positioned in a 800 km sun-synchronous orbit).
In another model from the NASA Langley Research Center, the optimal laser was a 5.38 MW laser the shot a continuous beam of energy at the debris. This model reviewed laser vaporization, but the research is similar to laser ablation.
In the end, laser parameters mostly depend on the:
- Space-based system’s distance from the debris, since the longer the laser travels, the more energy it loses
- And solar panel efficiency, since some lasers, like UV wavelength lasers, require a lot of energy, and would therefore require extremely efficient solar panels to work.
A critical part of creating and studying laser ablation systems is understanding the targets themselves. Space debris come in a wide variety of shapes, orientations, and materials, which all influence how laser impulses affect the debris.
For simplicity, many studies don’t account for the geometrically complex shapes and random orientations of debris. However, these parameters are critical in the effectiveness of laser systems — slight variations in their values can yield large differences in the debris’ modified trajectories.
Some recent studies have acknowledged this, leading to the creation of software like EXPEDIT (Examination Program For Irregularly Shaped Debris Targets), which investigates laser ablation while accounting for parameters such as those mentioned previously. One of the primary findings from EXPEDIT is that it may be better to view laser ablation systems from a probabilistic standpoint rather than a deterministic one, which may influence how future laser systems are developed.
Why Hasn’t It Been Implemented?
Currently, laser ablation is only hypothetical, as are many other space debris removal proposals. Only a few missions have been approved so far to remove space debris, and they are generally geared towards removing large pieces of debris or preventing the abandonment of future satellites. Up to this point, there’s been little action towards the removal of space debris because:
- Nothing catastrophic has happened regarding the destruction of vital satellites, although many researchers have argued that we’re on the brink of such an event (i.e. Kessler Syndrome)
- Since operating satellites and the ISS are still surviving using shielding and emergency maneuvers, there is less urgency to remove space debris.
- There is no economic incentive to remove space debris. The space industry is infamously expensive, with launches costing thousands of dollars per kilogram. Until many operating satellites are destroyed by space debris, it’s difficult to see the economic gain from spending millions of dollars removing space debris.
- Policy surrounding space property has developed very little since the 1960s, despite the industry developing a lot. This makes removing debris legally complicated, since its difficult to identify who owns a centimeter-sized piece of metal.
If you want to learn more about laser ablation, check out these papers:
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