Spacecraft that are no longer needed for planetary exploration can either be reused for another mission, or returned back towards Earth in an effort to burn up on their descent and avoid spreading debris into space. It is an effective way of disposing of dead satellites without sending debris hurtling off into orbit.
As it reenters Earth’s atmosphere, a spacecraft reenters like an enormous glider. Guided by small steering rockets but mostly gravity on its journey through the upper atmosphere.
Heat Shields
One of the greatest hurdles to spacecraft reentry is dealing with the tremendous heat generated when approaching hypersonic speeds. To mitigate this kinetic energy, skilled missile men and women use an inflating lightweight shield system that helps mitigate atmospheric friction’s melting effects and protect from melting effects of atmospheric friction.
Thermal protection systems (TPSs) on Discovery orbiter, for instance, consist of thousands of pieces of high-temperature reusable insulation tiles designed to withstand temperatures reaching 1,260 degC (2,300 degF) during reentry. This material allows it to endure temperatures which reach most surfaces during reentry and allows Discovery’s crew to survive its return home without incident.
Thermal capabilities of tiles can be attributed to their emissivity, thermal stability and porous structure; using this combination, the tiles dissipate excess heat by absorbing it and radiating it away. Furthermore, the TPS also contains inflatable rings similar to those used by fighter jets or found on bulletproof vests that can be deployed quickly to defend vulnerable areas like nose cones.
Atmospheric Braking
Atmospheric braking (also referred to as aerocapture) can reduce a spacecraft’s velocity over time and be used to make orbital adjustments without using propulsion burns. Atmospheric drag can also be used to lower an elliptical orbit’s apoapsis by making several passes through its atmosphere, and can even lower its apoapsis altogether.
Air-braking is a complex science, but its basic principle can be reduced to this: A space vehicle can slow its entry speed and circularize its orbit by creating drag through air compression waves. Shock layers generated in such compression waves heat primarily due to frictional losses between gas molecules in them and air molecules as they contract and expand, but some heating can come from isentropic expansion of compressed and expanded molecules as they compress/expand again.
To achieve this, the spacecraft must bank so its lift vector is almost vertical during reentry, moving into higher levels of atmosphere where air density is thicker and slowing it down more effectively.
Thermal Radiation
Spacecraft reentry movies and books often depict fast moving objects becoming extremely hot during atmospheric entry. While this may be partly true – as hypersonic speeds cause air molecules to pass by at hypersonic speeds and rub together, creating friction which heats them up – the physics involved are far more complex.
Air dissipates heat through convection (movement of molecules across space in bulk) and conduction (touch between molecules touching each other), but at hypersonic speeds it transforms into an incandescent gas, radiating away heat via radiation; this explains why spacecraft and meteors leave trails of glowing plasma as they reenter Earth’s atmosphere.
Thermal radiation is the conversion of thermal energy into electromagnetic waves, including visible light. All matter at temperatures above absolute zero emits this radiation that strikes other surfaces and may be absorbed, reflected, or transmitted depending on their temperatures: Phe (emitted radiation); Phi (incident radiation); Pr (reflected radiation); and Pht (transmitted radiation).
Entry Angle
Spacecraft entering Earth’s atmosphere must follow an exact trajectory determined by three factors – vehicle trajectory, rate of deceleration, and aerodynamic heating – when entering through its narrow corridor of atmosphere known as reentry corridor.
Before reentry, a vehicle’s shape must be optimized for maximum stability and control, which explains why shuttles typically resemble giant spheres or at least have a spherical section forebody and converging cone afterbody designs to optimize airflow while increasing cross-range capability (the area over which reentry trajectory predictions can be made).
Reentry angles are critical. Skylab’s 1979 reentry proves this point; had it entered at an excessively steep angle, its fuel would not burn off and instead bounce off of Earth’s atmosphere further than expected, potentially landing on land or populated areas or busy shipping lanes. Therefore, close to reentry Space-Track issues TIP Messages with estimated longitude/latitude where debris may land.