This book covers the latest research efforts in modeling hypersonic flights of spacecraft in low-density atmospheric layers of Earth, Mars, Venus, and other planets and their moons. The systematic studies of hypersonic gas flows inside the underexpanded jets in vacuum chambers reveal various unique kinetic and quantum phenomena that could be used in planning the planetary missions and in technological industrial processes. The role of classical similitude parameters (Knudsen and Reynolds numbers, temperature factors, and the ratio of medium specific heats) is evaluated in studies of aerothermodynamic characteristics of space vehicles under the conditions of hypersonic flights and wind-tunnel experiments with helium, argon, air, carbon dioxide, and hydrogen. Effective methods of spacecraft heat protection systems by using reactive gas injections, catalytic surface materials, and adaptive spacecraft configurations are reviewed in detail. Experimental data correlate well with numerical solutions of the modified Navier-Stokes equations and the Monte-Carlo direct simulations.