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May 06, 2012

Laser Ablation for the Effective Deflection, Exploration and Exploitation of Asteroids

Proposed for the deflection and mitigation of asteroids, laser ablation is caused by illuminating the surface of the asteroid with a laser light source. The laser beam heats the surface material, enabling it to ablate, transforming directly from a solid to a gas. The ablated material then forms into a small and extended ejecta plume. This action is considered to be similar to the formation of a rocket exhaust. The exhaust material will induce a small and continuous force onto the asteroid. This force can then be used as a low thrust deflection technique to nudge an asteroid away from an Earth impacting event. Previous proposals considered using a single spacecraft, mounted with a nuclear reactor and mega-watt laser. However, this paper investigated using a swarm of small spacecraft, each equipped with an kilo-watt solar pumped laser. This provides a much lighter and more adaptable concept. Each spacecraft simultaneously hits the asteroid, ablating a small portion of its surface. By superimposing their laser beams, the required surface power density can be achieved. Multiple ablation spots can also be created. This is considered to increase the flexibility and the overall redundancy of the deflection mission.

Laser Ablation Experiments at the University of Strathclyde


Asteroids 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. Potential methods of asteroid mitigation and deflection have therefore been addressed by numerous authors. Amongst the many possibilities to deflect Near Earth Asteroids, laser ablation has been shown to be theoretically one of the most effective cases.

Laser ablation is achieved by irradiating the surface of an asteroid by a laser light source. The resulting laser light is targeted onto the asteroid. This enables the surface rock to sublimate, transforming directly from a solid to a gas. Material subsequently expands into a debris cloud of gas, dust and ejecta. This action provides a continuously low thrust that, over an extended period of time, can deflect asteroid(s) away from a potentially threatening trajectory.




Each laser is powered by the Sun, either directly or in-directly. Light deployable mirrors are therefore needed, but can become susceptible to the degrading effects of the ejecta plume. Within the vicinity of the ejecta plume it is currently assumed that any ejected particle will immediately re-condense and stick onto any exposed surface. Degradation is considered to follow the Beer–Lambert–Bouguer law. Significant degradation will affect the intensity of the laser, its operational lifetime and its overall efficiency of the ablation technique.

Therefore to fully understand and examine laser ablation, a series of experiments have been performed. Within a vacuum chamber, a 90 watt continuous wave laser has been used to initiate the ablation response of a range of asteroid analogue target material. Measurements have included the composition, divergence, mass flow rate and density of the ejecta plume. Direct comparison to the existing model and its defining assumptions has then been made. The ablated ejecta have also been collected and assessed. The experiments have shown a considerable reduction in the size, height and mass distribution of the collected ejecta, and its overall contamination and degradation effects of the ejecta plume. The experiment also highlighted the applicability of laser ablation as an effective method for the extraction and exploration surface and subsurface material. Confirmed by microscopic analysis the ablated ejecta plume is chemically identically to the original pre-abated target material. However, the absorptive properties of the ejecta is considerable different. Laser ablation results in the extraction of deep and previously inaccessible material. This can be used for remote sensing, in-situ and/or sample return missions. This would enable scientists to further characterise the composition, formation and evolution of asteroids, and other rocky bodies. However, unlike conventional exploration based missions, the material can be collected remotely. This eliminates the risk of the spacecraft having to physically land and/or attach itself to the given body.

Within this paper the results gained from the laser ablation experiment will be presented. Direct comparison has been made to the current modelling technique and its associated assumptions. Where necessary current assumptions have been updated, eliminated or replaced. Not only does laser ablation present itself as an effective method to deflect asteroids, but also as a novel technique for the analysis and exploitation of asteroids, and other small planetary & celestial bodies.

Laser surface ablation has been theoretically demonstrated to be an advantageous method in the potential mitigation and deflection of Near Earth Asteroids. However to fully verify this approach a series of experiments were performed that examined the development of the ejecta plume induced by each ablation event. This included the flow rate, velocity and dispersion as a function of the target material’s composition

The rate of the ablated material will ultimately define the modulus and direction of the imparted force exerted onto the asteroid. The evolution of the ejecta will also have a significant effect on the contamination and operations (i.e. endurance) of any optical surface. Therefore the rate of degradation onto optical surfaces was also assessed. These issues are fundamental to the asteroid ablation technique. Therefore to successfully assess the effectiveness and efficiency of the laser ablation technique a detailed understanding of these parameters is required.

A 90 W CW fibre-coupled semiconductor laser (LIMO 90-F200-DL808), is being used as the laser source for all experiments. This provides an approximate surface power density, accounting for losses, of 37 kW/cm2 at the focus. All experiments occur within a sealed and self contained glass test chamber. Under standard atmospheric conditions, the environment within the test chamber is purged with nitrogen gas. This is to reduce the occurrence of atmospheric combustion to negligible levels. Any innate material combustion still occurred. The utmost care and attention was taken to ensure that the experiment was designed to be an appropriately scaled analogue of the asteroid deflection event(s).

Within the test chamber each asteroid analogue target material was mounted on a raised pedestal. This was located at a pre-determined location, relative to the focal point of the laser beam. In-situ monitoring systems surrounded the test chamber. This included two CMOS high resolution, high speed digital cameras (Panasonic HDC-SD60), and a third dedicated FLIR thermal camera (SC7000). The thermal camera was used to measure the temperature of the spot and the ejecta plume. The experiment’s configuration is shown below.


Outcomes
The results, thus far, have demonstrated the sensitivity of the ablation process to the specific laser characteristics and properties of the chosen target material. This is relative to the focal point of the laser, the volumetric removal of the ejected material, the material phase changes within the ablation volume and the dispersion of the ejecta plume. An example of the ablation process is given below:



Cone Angle Development of the Solid (left, middle) and High Composite Samples (right)
However work is still required to fully investigate and confirm the ablation process. However work is still required to fully investigate and confirm the ablation process. For future experimental campaigns all ablation events will occur within a vacuum chamber. This will enable a near-space environment to be created.

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