تجزیه و تحلیل تجربی از پلوم قطع شده لیزر برای بهره برداری و انحراف سیارک

It has been theoretically demonstrated that laser ablation is effective in the potential deflection and mitigation of asteroids. However, there have been few experimental studies to support this claim. The theoretical models are currently based on assumptions regarding the laser beam diameter, the power requirement, the formation of the ejecta plume, and the potential for ejecta to contaminate and otherwise degrade any exposed surface. Recent proposals suggesting the use of a solar pumped laser, in particular, can be deeply affected by the re-condensation of the ejecta. To either validate, amend and/or eliminate these assumptions a series of laser ablation experiments have been performed. Using a 90 W, continuous-wave laser operating at 808 nm, a rocky magnesium iron silica based material – olivine – has been ablated. These experiments were used to examine the validity of the theoretical model and the experienced levels of contamination. It will be shown that the current model correctly predicts the ablated mass flow rate for rocky based asteroids, but overestimates the contamination rate and the degradation of the optics.

Near Earth Asteroids (NEAs) represent both an opportunity and a risk. Their pristine environment captures the early formation of the solar system; while their impact potential could result in the mass extinction of life. The Earth has been, and will continue to be, the subject of many other ground and air impacting events. Amid the observed population, there are at least between 2000 and 200,000 objects that could impact the Earth [1]. On average, an asteroid with a diameter greater than 100 m impacts the Earth once every 10,000 years. This can cause local damage, earthquakes and tsunamis. Asteroids that impact the Earth with a diameter larger than 1 km are considered to be global killers. Such an impact event is considered to catastrophically annihilate 90% of all life, resulting in a nuclear winter, with little chance of recovery within the near term [1]. This is thought to have happened, once before, approximately 65 million years ago, with the impact of a 10 km diameter asteroid at 12 km/s [23]. Therefore potential methods of asteroid mitigation and deflection have been addressed by numerous authors [2], [3] and [4]. Amongst the many possibilities to deflect NEAs, ablation has been shown to be theoretically one of the most effective methods [5]. Work conducted in 2009 by Sanchez et al. [5] compared the effectiveness of six different asteroid deflection techniques. Through a multi-criteria, quantitative comparison the nuclear interceptor, kinetic impactor, mass driver, low thrust tug, ablation and the gravity tractor were assessed. Assessment was made relative to the achievable miss distance at Earth, the warning time, the total mass into orbit and the current technology readiness level. With both a relatively short warning time and a low mass into space, ablation can provide significantly higher and more controllable rates of deflection. The technique is also advantageous as it avoids the catastrophic fragmentation of the asteroid. It also eliminates the need of having to physically land and/or attach a system onto the surface of the asteroid [5]. Ablation is achieved by irradiating the asteroid with a light source. This can either be collected and focussed solar radiation or with a laser light source. Within the illuminated focal point, the absorbed energy increases the temperature of the asteroid, enabling it to sublimate. This transforms the exposed material directly from a solid to a gas. The ablated material then expands to form an ejecta plume. Over an extended period of time, the resultant thrust, induced by the ejecta plume and acting on the asteroid can be used to push the asteroid away from its original threatening trajectory [2]. This increases the minimum orbit interception distance between the Earth and the asteroid, otherwise preventing the Earth impacting event [5], [6], [7] and [8]. Previous proposals for the initiation of laser ablation considered using either a ground-based or space-based facility [1] and [19]. For a ground-based facility an average power level of several giga-watts would be required to deflect a small, 40–80 m in diameter asteroid [19]. This was considered to be a substantial investment in infrastructure and resources. Therefore an alternative option was to mount a mega-watt laser onto a large single spacecraft. The laser would be powered by a nuclear reactor [20]. However, manoeuvring and operating such a large structure, at close proximity to the asteroid, under an irregular gravity field was considered to be very difficult. This is further coupled with developing a nuclear reactor for space-based applications, and the associated political ramifications. Therefore an alternative concept was proposed. Instead of a large single structure, a swarm of small spacecraft, each equipped with an identical kilo-watt solar-pumped laser could be used [10]. This provides a much lighter and more adaptable concept. By superimposing each laser beam, the cumulative surface power density would be used to initiate the ablation process [10]. Singular or multiple ablation spots can also be used. This increases the flexibility and overall redundancy of the deflection mission. As required, more spacecraft can be added or removed from the existing configuration, eliminating the need to develop and design new spacecraft(s) [[6], [7] and [9]]. The potential for deflection is therefore dependent on the number of spacecraft located within the vicinity of the asteroid, their combined laser power and the material properties of the asteroid. Within this configuration, each laser would be powered by the Sun, either directly or indirectly. For direct pumping the solar radiation is collected and concentrated directly onto the laser medium. For indirect pumping, the incoming solar radiation is first focussed onto a set of highly efficient solar cells. This immediate step is used to convert the incoming solar radiation into electrical energy that is then used to power the laser. For both cases – direct and indirect pumping – a light, deployable primary mirror and a smaller secondary (both known as the solar concentrators) are needed to collect the freely available solar radiation of the Sun. A steering mirror is also required to target the laser beam onto the surface of the asteroid. Large radiators are needed to keep the laser within its operational limit and to cool the spacecraft. However, any exposed surface within the vicinity of the ejecta volume, including the steering mirror and solar concentrators, would be subjected to the continual contaminating effects of the condensing ejecta. It is currently assumed that once the ejecta plume comes into contact with any given surface that the particles will immediately re-condense and stick. The continued accumulation of the ejecta will decrease the transmittance and increase the absorbance of the exposed surface. The degradation caused by the ejecta is considered to follow the Beer–Lambert–Bouguer law and is dependent on the absorptivity of the condensed material. The laser beam is also expected to be attenuated by the ablated plume of ejecta. This paper will show that the effect of the ejecta contamination, according to the current model, has a major impact on the ability of the laser ablation process to produce a significant deflection action. Therefore to examine the actual operational and environmental constraints of laser ablation, a series of experiments have been performed. Within a vacuum chamber, and using a 90 W continuous-wave laser, the ablation response of a magnesium iron silicate (olivine) sample has been assessed. This assessment included the development of the ejecta – cone angle divergence, mass flow rate, and plume density – and the affects of the condensing ejecta. All results have been compared to the numerical model. This paper therefore investigates the effect of laser irradiation on a rocky based asteroid simulate. Providing experimental data and model validation is an important step towards the realisation of a laser asteroid deflection system. This paper details the current ablation and asteroid deflection models; highlighting the conditions placed upon the assumed physical parameters and the response of the ablation process. The laser ablation experiment is then presented. The results from the experiment are then given and placed within the context of a revised deflection simulation. Conclusions have been drawn and areas of future work addressed.

The experiments performed within this paper enabled the current modelling technique for laser ablation to be assessed. While the experimentally measured and theoretically predicted mass flow rates of the target material and deposition rate of the ejecta compared well to each other, the nature of the deposited ejecta was significantly different. The experimentally measured absorptivity values were two orders of magnitude lower than assumed in the model. The density of the deposited ejecta was also reduced. The model therefore assumed an incorrect growth of the deposited material, density and absorptivity of the condensed material. It also assumed that all the ejected material would bind with any exposed surface. This was determined not to be the case. These parameters therefore represent a current inaccuracy within the existing modelling technique. This will otherwise affect the endurance, efficiency and overall response of any ablation based asteroid deflection and mitigation mission. Ejecta contamination will always affect any laser ablation based mission. However the experiment also demonstrated that the effect of the contaminating deposited ejecta was far less than predicted in the model. This results in a comparatively larger level of achievable deflection, where there is no immediate saturation of any exposed surface. However, further work is still required to assess the laser ablation process. A wider range of asteroid analogue target material intends to be tested. This includes a highly porous sample and a collection of meteorites. The inhomogeneous nature of the target material, and that of the asteroid, must also be accounted for. Different compounds will ablate at a lower and higher sublimation temperature. The model also needs to be developed to account for the three dimensional thermal diffusivity and the temperature dependence on the optical and thermal properties of the target material. This includes the emissivity, heat capacity, density and thermal conductivity. The non-thermal emissions created by the ablated gas particles also needs to be considered. More experiments are therefore required to further explain and develop the mathematical model and existing discrepancies. Once proven successful for a range of compositions present within the asteroid, and small body population, the role of laser ablation could be extended for a number of space-based applications. The laser ablation process itself effectively tunnels into the illuminated material, extracting material in the form of an ejecta plume. This results in the extraction of deep and previously inaccessible material that could not otherwise be retrieved through conventional in-situ and sample return based missions. The material extracted from laser ablation could be collected externally by the spacecraft flying through the plume. The collected ablated material could then be used as part of a sample return mission and/or for resource exploration and exploitation. In-situ spectra analysis of the ablated ejecta plume could also be performed. Laser ablation therefore provides a novel technique for the interaction and collection of material from a small, rocky body. It is also considered to be advantageous as it provides a contact-less and remote method of analysis that eliminates any mission requirement to physically land and/or attach itself to the asteroid. For asteroid deflection purposes it also avoids any possible fragmentation of the asteroid. Laser ablation could enable scientists and engineers to further characterise the composition, formation and evolution of asteroids and other small bodies.