The interaction of an electromagnetic wave with the shock-induced plasma flow field generated around a blunt body capsule as it encounters the planetary atmosphere of Mars is examined. From two preliminary engineering models of the Martian atmosphere and the estimated entry-trajectory characteristics of the capsule, it appears that the communication links to and from the capsule may be blacked out during entry because of a high free-electron concentration in the wake region of the capsule. In the worst case of a frozen flow field behind a normal bow shock in local thermodynamic equilibrium, estimates of the free-electron concentration in the wake region indicate peak values between 7.9 × 1010 e−/cc and 7.7 × 1011 e−/cc. Both of these values are above the critical electron concentration of 6.53 × 1010 e−/cc for the S-band direct transmission link to Earth, and 1.80 × 109 e−/cc for the indirect UHF transmission link through a relay bus. Also, atmospheric reflection and refraction of the telemetry signals are considered near blackout. The reflection and refraction coefficients are shown to satisfy Riccati equations, which are solved numerically to yield the desired coefficients. Estimates of the reflection and refraction coefficients for several possible electron concentration and collision frequency combinations are cited for linear and exponential plasma wake distributions.
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