Reentry is the process of returning a spacecraft to Earth or another planet through atmospheric friction, whereby its velocity slows and heats up due to atmospheric friction.
Spacecraft that aren’t positioned correctly may overheat and break apart. Engineers have developed various methods to enhance capsule design to prevent this.
Atmospheric drag
Atmospheric drag is the force that opposes an object moving through fluid and causes it to slow down, and is an essential factor influencing spacecraft reentry and astronaut safety. Additionally, atmospheric drag plays an essential role in designing spacecraft and orbital vehicles.
Atmospheric drag aids reentry vehicles by dissipating energy and decreasing their chance of colliding with Earth, enabling spacecraft to slow faster while decreasing heat generation and therefore the need for thermal protections.
Space agencies and organizations have developed numerous standards and guidelines designed to reduce ground casualties over time, with consensus among them reaching an expectation of 10-4 casualties for any uncontrolled re-entry (Fig 13). Casualty areas corresponding to this threshold are determined by orbital inclinations of objects entering orbit (see “Impact Analysis: Results and Trends for Reentry Targeting Systems (TARPS).
Entry angle
How a spacecraft enters Earth’s atmosphere has an immense effect. Most small probes reenter at steep angles to minimize overall heating; manned capsules need to be capable of withstanding decelerative g-forces upon reentry and keep their passengers warm through body heat alone. Apollo Command Module crew managed this feat thanks to their shallow angle entry; using their body heat alone as heat preservation.
Entry angle also influences the amount of gas and debris released into the atmosphere, with aerothermodynamics model simulations providing mass loss maps which are then used to calculate amounts ablated and related by-products that then feed back into coupled chemistry-climate models to identify environmental impacts.
As objects near reentry, SpaceTrack issues TIP messages providing estimated times and locations of impact (Window). While their accuracy may be affected by uncertainties in aerothermodynamics and atmospheric chemistry-transport modelling, their accuracy can also be affected by winds; particularly for objects with high eccentric orbits.
Heat shield
As soon as a spacecraft enters an atmosphere at high speeds, it experiences tremendous amounts of heat due to friction between air molecules and its surfaces, as well as from adiabatic heating (when it compresses its surrounding atmosphere). Spacecraft designers rely on thermal protection systems in order to protect crewmembers and payloads from such extreme temperatures.
Heat shields help dissipate heat to keep spacecraft from overheating and potentially incurring structural damage, and help the vehicle maintain an ideal internal temperature during descent. Multiple types of shields have been utilized on space missions; advanced materials are even still in development for use as shields.
National Oceanic and Atmospheric Administration’s LOFTID mission will test whether inflatable thermal protection systems, known as LOFTID systems, can safely slow large payloads such as crewed spacecraft and rocket components during their reentry back to Earth’s atmosphere.
Altitude
Spacecraft that enter Earth’s atmosphere to slow down create considerable heat during their entry. This heat results from friction between their spacecraft and Earth’s more-compressed air mass; temperatures may reach 4,500 degrees Fahrenheit depending on their design.
This study investigates the effects of non-equilibrium atmospheric chemistry within the shock layer on post shock relaxation conditions for reentry trajectory reentry, as well as estimating generated chemical by-products according to ablation at stagnation points using GEM-based thermochemical models.
Results show that ORSAT and SCARAB are in close agreement in their predictions of altitude versus relative velocity; however, there are discrepancies when it comes to altitude versus time predictions for similar trajectories due to differences between models using different initial conditions and rotating spherical Earth models.