constant there is enough experimental evidence to show that absorbtivity increases during the time of irradiation [15, 16]. This means that energy requirements for damage could be less than what is calculated; • the approximation of the generalized Fourier heat flow model by equation (1) i.e. a one-dimensional heat flow model, is valid only approximately. This however is not a major problem since with appropriate boundary conditions the general Fourier equation can be solved to get temperature distribution within the object as a function of depth x and time t, i.e. a solution of the form T (x, t) can be found for the general Fourier equation. The absence of convection, radiation and plume absorption effects (arising from the vapour created by laser radiation) is assumed. This also assumes no transverse heat transfer (uniformally heated thin slab transferring heat by conduction with no heat transfer across boundaries). • further approximation of equation (1) by equation (2) is valid only for small At and small Ax. 3. Sensitive Spacecraft Elements The effects of laser radiation are likely to be felt most of all by: • electronic circuitry. Very Large Scale Integrated circuits may malfunction unless protected suitably from laser radiation; • solar panels; • optical solar reflectors used in thermal control of spacecraft; • coatings on optics (mirrors and lenses); • attitude sensors; • exposed mechanical components especially relating to the Attitude and Orbit Control Systems. Most electronics are housed inside the spacecraft structure which provide some measure of protection. Adequate shielding may also afford some measure of protection. Exposed elements of the AOCS such as hydrazine tanks and plumbing are normally made of materials such as titanium, stainless steel and would have damage criteria similar to those for missiles. We will therefore mainly consider only the other elements. 3.1 Methodology Let us consider the melting of a target subjected to laser irradiation. Let the area of the target illuminated be A. Let Cp be specific heat, Tm the temperature of melting. T, initial temperature. Let the thickness of the target be 3. Let the power intensity of the laser be I, pulse duration of the laser be At and the absorptance of the target a. Then as per the very simple calculations outlined in Appendix A the energy required from the laser to melt target is:
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