Deflection Of An Asteroid By Laser Ablation And Laser Pressure Technique

The NEO deflection method employing laser ablation is an interesting and promising method to deviate the orbit of the asteroid. This technique employs trains of solar-pumped laser pulses, from a number of spacecrafts in specific formation, to ablate material from the asteroid surface. This process not only reduces the momentum of the asteroid by required amount but also creates a pressure in the opposite direction due to continuous bombardment of high pressure laser pulses. The main advantage of this technique is that it relieves the strict constraint on the proximity to the asteroid surface, thus mitigating the effects of inhomogeneous gravity field, temperature variations and damage of the spacecraft due to regolith. The solution requires no futuristic technology and would work on wide variety of asteroid types. NOMENCLATURE a Semi-major axis, m [aI ; bI ; cI ] Radial dimensions of an ellipse, m A Area, m A Matrix of Gauss planetary equation c Speed of light (299792.458 km/s) Cr Concentration ratio C20;C22 Harmonic coefficients for a second order, second degree gravity field Cineq Inequality constraint in optimisation d Diameter, m D Search space for a solution vector D Directional cosine matrix F; F Force, N i Inclination, rad H Enthalpy of sublimation, J l Length, m L Distance, or depth, from the surface of the asteroid, m J Objective function K Conductivity of a material, W/m_K m Mass, kg M Mean anomaly, rad n Mean motion, m/s or index of refraction v; v Velocity, m/s v Average velocity of particles according to Maxwell's distribution of an ideal gas, m/s θ True latitude, rad *Bachelor of Technology,Electrical Engineeering Department, 4 year κ Angular in-plane component in gravity field, rad λ Wavelength, m μ Gravitational constant ( μ⊙ =132724487690 km /s) v True anomaly, rad ξ Variable used in calculation of δr(δk) ρ Density, g/m2 ζ Variable used in calculation of δr(δk) π Variable used in calculation of δr(δk) ρ Variable used in calculation of δr(δk) SOURCES For the template and the rigorous mathematical works, I cite [1]. The plume technology has been inspirationally derived and I cite [2]. ASTEROIDSA DISCUSSION Asteroids are small rocky-icy and metallic bodies that orbit around the Sun. They are thought to be the shattered remnants of planetsimals, the bodies that never became massive enough to become planets. There are millions of these asteroids majority of them orbiting in the main asteroid belt between the orbits of Jupiter and Mars and co-orbital with Jupiter (Jupiter Trojans). Some other reservoirs where asteroids and comets are present are the Kuiper belt (Kuiper belt objects) and the trans-Neptunian orbit (Trans-Neptunians). However a significant population of asteroids with different orbital are present, some of which pass very close to the Earth’s orbit. Such asteroids are known as Near-Earth Asteroids (NEAs) or generally Near-Earth Objects (NEOs). Some these NEOs actually cross the Earth’s orbital path and calculations show that their probability of impacting the Earth is significantly higher than the other asteroid groups. These asteroids are termed Potentially Hazardous Asteroids (PHAs). Asteroids are classified by their characteristic spectra and accordingly they fall into three main categories:  C-Type Asteroids:They are carbonaceous and constitute the majority of the asteroids. They have an albedo ranging from 0.03 to 0.10.  S-Type Asteroids: They are siliceous in nature and make up 17% of the asteroids. Rahul Sengupta / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.1049-1056 1050 | P a g e They are a bit brighter than C-type with an albedo between 0.10 to 0.22.  M-Type Asteroids: They are metallic in nature, most of them made up of iron. Albedo ranges from 0.1 to 0.2. The orbits of asteroids are determined through different ways. Some orbitals are defined from observational records, while some are measured through the perturbations they cause in the nearby object as they pass close to it. Telescopes have also been used for identification and tracking of the asteroid orbit and now computational methods are used for such kind of calculations. Whatever techniques are used the orbital characteristics of an asteroid depends on some specific orbital elements. The numerical values of these elements for asteroid Apophis 99942 have been given in the table. Element Measured Values Semi-major axis aA Eccentricity eA Inclination iA Argument of periapsisωA Period TA Mean motion nA Mass mA Gravitational constantμA Dimensions a1,b1, c1 Rotational velocity wA 0.9223 AU 0.1911 0.05814 rad 2.2059 rad 323.50 days 2.2479x10 rad/s 2.7 x 10kg 1.8015993 x10km/s 191m,135m,95m 5.8177 x 10 rad/s Table 1.Some physical values of Apophis 99942. CONCEPT OF LASER ABLATION Laser ablation may be defined as a sputtering process leading to the ejection of atoms, molecules and even clusters from a surface as a result from the conversion of an initial electronic or vibrational photoexcitation into kinetic energy of nuclear motion. In laser ablation, high power laser pulses are used to evaporate matter from a target surface. As a result, a supersonic jet of particles (plume) is ejected normal to the target surface. The plume, similar to rocket exhaust, expands away from the target with a strong forward directed velocity distribution of different particles. This ejected plume, thus, provides a reaction force that produces a significant amount of force on the target object in the direction opposite to that of its ejection. To apply this technique in the case of deflection of an asteroid, the spacecrafts responsible for this operation are equipped with lasing equipments that are discussed later. The process of laser ablation has an operation time of many orders of magnitude, from the initial absorption of laser radiation to material ejection. In macroscopic level, modification occurs when the laser fluence is above the threshold fluence Ft𝑕(J/cm ) or ablation threshold. This puts a lower limit to the fluence and the intensity of the laser pulse. The fluence would vary depending on the material and optical properties of the asteroid. In general, for a successful sublimation or ablation process, the laser pulse duration must be longer than the time between electron-electron collisions and the relaxation time of the electron-phonon coupling. If the pulse duration is shorter than the thermalisation time, the electric field of the laser pulse might exceed the threshold for optical breakdown and the target material is transformed on an ultra-fast time scale into plasma. When the target area, due to ablation is converted into plasma, the entire system may face extreme nonequilibrium situations resulting in orbital perturbations and instability in the asteroid. These effects are not at all welcome as they tend to destabilize the asteroid and might even disintegrate them which are held together by mutual gravitational attraction. Hence, an upper limit to laser pulse duration is established and the lasers onboard the spacecraft has to operate within this region of laser fluence and intensity. The energy that is available for ablation is an important criterion. It depends on the laser wavelength, optical properties of the asteroid material and also on the diameter of the illuminated spot caused by the laser. This available energy is an interesting parameter that helps in designing the orbit of the spacecrafts around it. This issue has been dealt with later in the paper. The main importance of calculation of the amount of energy available for ablation purpose can be seen from the fact that the power needed to ablate required amount of material is in direct relationship with the distance of the spacecraft from the asteroid surface. The spacecrafts must be placed in such an orbit that would allow it to focus optimal power necessary for deflection. It can be calculated as follows, The electric field of a monochromatic, linearly polarized plane wave pulse (laser pulse) in a homogeneous and nonabsorbent medium is given by