Shigeaki Uchida, Henan University of Science and Technology; Kazunori Shibata, Osaka University; Kotomi Kawakami, Kitasato University; Hideki Okamura, International Christian University; Nakai Mitsuo, Fukui University of Technology; Xiao Xiao, Henan University of Science and Technology; Yin Danqing, Henan University of Science and Technology; Keke Zhang, Henan University of Science and Technology; Zhonghao Heng, Qinghai University
Keywords: debris, laser, digital hologram, removal
Abstract:
The present work deals with the removal or mitigation (collision avoidance) of small debris in near-earth orbits using a spaceborne laser system, laser ADR. Here the small debris category is defined in terms of their size in millimeters to a few centimeters. Their dangers are most characterized by the term “LNT, Lethal Non-Trackable”. As the number of debris objects in orbits increases, the possibility of encountering LNTs is no longer negligible and is expected to become a daily occurrence in the near future. Despite the serious situation, there have been few works dealing with the remediation of LNTs but they need immediate and intensive attention. The difficulties of the remediation come from the facts that (1) a lack of prior information such as orbit parameters, materials, shape, and attitude status, (2) they are very small to catch or even to acquire, and (3) there are so many.
The authors have been proposing and working on a small debris removal scheme using pulsed laser radiation where the laser system is spaceborne. In this scheme, a debris object irradiated by a laser pulse(s) diverts its course to an unstable orbit where it will eventually fall into the upper atmosphere. The course diversion or thrust is engaged by the gas emission (ablation) occurring on the surface of the debris object caused by the focused laser pulse energy.
The laser pulses are used to acquire, track, and irradiate debris objects. In the “acquire mode”, a single laser pulse is transformed into a “spherical light shell” or “spherical zone” shape with the laser satellite at the center after passing through specially designed optics. A train of laser pulses forms a series of concentric spherical light sheets. When a debris object encounters the light sheet, it scatters light and will be detected by optical sensors. A small portion of the scattered light is collected by an optical mirror with a sufficient-sized aperture and sent to the sensors. The sensors are designed to determine the position of the scattering object. The trajectory of the object can be calculated using data from multiple detections. Once the trajectory is identified, the position of the object is predicted (tracked) for irradiating a focused laser pulse to the object (tracking and irradiation modes).
In the irradiation mode, there are two important technologies; precise laser pulse focusing and laser pulse preparation for the highest energy-to-momentum conversion, so-called momentum coupling coefficient, Cm. The precise focusing is realized using digital holographic technology (DHT). In principle, the laser pulse energy is transported by the holography of the debris object itself, and automatic focusing is realized. The use of DHT gives several advantages over the conventional optomechanical steering system and optical phase conjugation technique. The benefits come from the terms “Digital” and “Hologram” separately. The hologram controls the wavefront of the light beam using optoelectronic devices such as spatial light modulators. The beam steering will be done using the wave optics principle rather than geometrical optics. In other words, holographic technology eliminates the complex optomechanical system and physical motion of massive components usually associated with another complex momentum compensation device to keep the system attitude intact in space environments.
A holographic system is composed of image sensors and a spatial light modulator (SLM) to reconstruct and manipulate the wavefront of signal light reflecting (scattered) from the target object. Like optical phase conjugation (OPC), DHT reproduces the wavefront from the target reflection with its reversed wave traveling direction and adds “digital” modification to the wavefront. However, unlike the conventional OPC, the modification takes care of the compensation of the target motion during the light round trip between the laser device and the target as well as aberration that becomes significant in the case of the extreme conditions of high relative velocity, long distance to the target, and small target size.
For thrust generation, energy and mass (of propellant) are important factors and choice between the two determines the laser parameters. In the case of spaceborne laser ADR, the energy resource is more precious and the system design needs to be done focusing on the maximum Cm realization kept in mind. For higher Cm, it is preferable to make the most use of the debris mass for thrust generation on the object while using the minimum laser energy amount. This condition is realized by controlling the focused laser intensity on the target just high enough to produce vapor and not the high-temperature plasma ionizing the material. The general rule of thumb for preparing the laser pulse conditions is to keep the power, or focusing intensity to just above the vaporization intensity of the material and yet to provide as necessary energy as possible. The latter requirement leads to the use of a long pulse duration leading to a large enough ”delta V” required by effective removal from the orbit.
It is shown that the laser and optical systems described here are within the reach of the current technology and can be implemented on the existing orbital platform such as the International Space Station (ISS). Considering the power available from the ISS (several tens kilowatts at least), With the power, LNT of up to a gram size can be deflected and removed from the low earth orbits. Demonstration experiments of the system on ISS could be proposed as a post-project after 2030. In the paper, technical developments needed to realize the spaceborne laser ADR system will be also discussed.
Date of Conference: September 17-20, 2024
Track: Space Debris