Atmospheric entry is the movement of an object from outer space into the atmosphere of a celestial body such as a planet, dwarf planet, or natural satellite. Atmospheric entry can be divided into the following:
- Controlled entry or reentry of a spacecraft or projectile being navigated or following a pre-defined trajectory
- Uncontrolled entry, such as those of astronomical objects, including space debris or bolides.
The atmospheric drag and heat produced during atmospheric entry can cause the breakup or complete disintegration of objects. Uncontrolled objects generally accelerate through the atmosphere at extreme velocities due to the influence of gravity. On Earth, atmospheric entry acts as a barrier or filter preventing large cometary meteors from reaching the surface, as they often disintegrate into dust at high altitudes.
Spacecrafts experiencing atmospheric entry are designed to balance three key requirements: deceleration, heating, and the accuracy of the landing or impact site. When objects enter the atmosphere, they have to displace gas particles along their path. This creates friction, producing drag or air resistance, slowing them to safer speeds while also creating significant heat.
The majority of objects enter the atmosphere at hypersonic speeds as a result of their suborbital, orbital, or unbound trajectories. Technologies that allow for controlled atmospheric entry are collectively known as entry, descent, and landing (EDL). These include various technologies enabling atmospheric entry at high velocities and low-velocity alternatives that utilize buoyancy methods. These are typically for planetary entry with thick atmospheres and strong gravity that complicate hyperbolic high-velocity entry.
Approximately 10-40% of the mass of satellites that reenter the atmosphere likely reach the surface of the Earth. On average, roughly one cataloged object reenters the Earth's atmosphere each day. Most objects surviving reentry land in the oceans.
In 1920, Robert H Goddard published a "Report Concerning Further Developments" in space travel. The report contained the first description of an ablative heat shield:
In the case of meteors, which enter the atmosphere with speeds as high as 30 miles per second (48 km/s), the interior of the meteors remains cold, and the erosion is due, to a large extent, to chipping or cracking of the suddenly heated surface. For this reason, if the outer surface of the apparatus were to consist of layers of a very infusible hard substance with layers of a poor heat conductor between, the surface would not be eroded to any considerable extent, especially as the velocity of the apparatus would not be nearly so great as that of the average meteor.
The practical development of reentry systems began with the increase in the range of ballistic missiles. Earlier shorter-range missiles, such as the V-2, had stabilization and aerodynamic stress issues (problems unrelated to heating) that caused many to break apart during atmospheric reentry. Medium-range missiles, such as the Soviet R-5 (1200 km range), required separate reentry vehicles, as it was no longer possible for the entire rocket to survive atmospheric entry. This separate vehicle had ceramic composite heat shielding to protect the payload. The first ICBMs (Inter-Continental Ballistic Missiles), with ranges from 8,000 to 12,000 km, were made possible due to the development of modern ablative heat shields and blunt-shaped vehicles. This technology was pioneered in the USA by H Julian Allen working at Ames Research Center.
In the United States, H. Julian Allen and A. J. Eggers, Jr. discovered that a blunt-shaped object with higher drag made the most effective heat shield during atmospheric entry. They made this discovery in 1951, although it remained a military secret until 1958, when it was eventually published. The first non-munition entry vehicle launched by the US was Discover-I, a reconnaissance satellite recovery vehicle on February 28, 1959.
A number of accidents have occurred during atmospheric entry; notable examples include the following:
- Soyuz 1 April 1967—The vehicle's control system failed while still in orbit causing parachutes to entangle during the emergency landing sequence and the death of the cosmonaut on board.
- Soyuz 11 June 1971—A valve failed on reentry, causing rapid depressurization of the cabin, killing the three crew members.
- Space Shuttle Columbia February 2003—A large piece of foam fell from the shuttle's external tank, breaching the spacecraft wing and causing the vehicle to break up as it returned to Earth, killing the seven astronauts on board.
Notable uncontrolled atmospheric reentries include those below:
- Cosmos 954 1978—This crashed in the Northwest Territories of Canada after an uncontrolled reentry; the probe was nuclear-powered, leaving radioactive debris near the crash site.
- Skylab 1979—An uncontrolled reentry spread debris across the Australian outback, killing a cow and damaging multiple buildings.
- Salyut 1991—This reentered over Argentina, scattering debris over the town of Capitan Bermudez.
Multiple forces affect objects during atmospheric entry. Below is a simplified diagram (two-dimensional flight over a spherical non-rotating planet) of the forces acting on an object entering the Earth's atmosphere at a radial distance (r), with a velocity (V), at an angle (γ), and mass (m) for a planet of radius (r0) and gravitational acceleration (g).
Assuming no additional thrust from the vehicle itself, the three forces acting on it are gravity (mg), lift (L), and drag (D). Local gravitational acceleration is defined as:
Where g0 is the gravitational acceleration at the surface of the planet, 9.8 m/s for Earth. The aerodynamic force is resolved into lift (normal to the velocity vector) and drag (along the velocity vector) and represented as:
Where ρ is the atmospheric density, S is the vehicle or object reference area (the area it covers on a plane perpendicular to the motion), and CL/CD are the lift and drag coefficient, respectively. These coefficients are functions of the angle of attack and the angle from a longitudinal body axis to the velocity vector.
Spacecraft must be in a specific Earth orbit before they can initiate a return journey. For successful landings, atmospheric entry trajectories must follow a three-dimensional corridor that doesn't
- undershoot and experience too much drag, causing the vehicle to slow down too rapidly and experience greater heating or
- overshoot and experience too little drag, skipping off the atmosphere rather than entering it.
Within the "reentry corridor," the atmospheric drag on the spacecraft is large enough to let it fall to Earth instead of veering back off into space, but not so large that the resistance it receives from the surrounding air destroys it. Most reentry vehicles (notable exceptions include the Space Shuttle) have little control during atmospheric reentry. Instead, they follow a pre-calculated trajectory through the reentry corridor as they fall down to Earth. The size of this corridor is defined by three competing parameters: deceleration, heating, and accuracy. Spacecraft designers must consider how their vehicle will balance these three parameters to guarantee a practical reentry corridor and ensure the control system is capable of guiding it through without under or overshooting.
There are two types of atmospheric reentry trajectories that control the design of crewed or uncrewed spacecraft:
- Ballistic entry
- Lifting entry
Ballistic entry is an atmospheric reentry trajectory where the slowing force is always opposed to the line of flight, a "dragging" force. An example of a ballistic entry is missiles. The primary design parameter for ballistic entry is the ballistic coefficient (β):
Where W is the vehicle weight, CD is the drag coefficient, and A is the corresponding reference area used in the definition of the drag coefficient. The amount of deceleration an object experiences is inversely related to its ballistic coefficient. This means a light, blunt object (low β) slows down much more rapidly than a heavy, streamlined object (High β). Put differently, heating and deceleration are less intense for low ballistic coefficients (i.e., low weight and/or high drag and large frontal area) and more intense for high values (i.e., high weight and/or low drag and small frontal area.
Lifting entry is atmospheric reentry where the primary force is perpendicular to the flight path, a "lifting" force. While drag is present during the entry, the flight path can be adjusted continuously in terms of both vertical motion and flight direction while the vehicle's velocity slows. Non-ballistic entry introduces other forces, besides drag, in an attempt to increase the time reentry takes and spread energy dissipation over a longer period, reducing the gravitational force on the vehicle. An example of a lifting entry without high velocities and heating is a sailplane. The primary design parameter for lifting entry is the ratio of lift to drag defined by the ratio of their respective coefficients:
Low values of L/D produce moderate gravitational forces (g loads), moderate heating levels, and low maneuverability. Higher values of L/D produce lower g loads, but entries take a very long time while experiencing continuous heating. An example of this type of lifting reentry is the Space Shuttle, with an L/D value of around unity (lift and drag equal) and a total entry time of about twenty-five minutes. Although lifting entry reduces the peak temperatures compared to that of ballistic entry, due to the longer time the total heat load that must be absorbed is higher. Lateral maneuverability during entry (also referred to as "cross-range capability") dramatically increases as L/D increases.
The Earth's atmosphere is composed of mainly diatomic molecules oxygen and nitrogen. As spacecraft fall under gravity during atmospheric entry of Earth, they create significantly high densities of gas molecules. This causes a shock wave and an enormous rise in pressure in front of the spacecraft along its path. Due to the extreme speeds with which objects enter the atmosphere, even at altitudes where the pressure is low, it can profoundly affect the reentering spacecraft. The shock wave occurs when air molecules bounce off the front of the vehicle and collide with the incoming air, bending the airflow around the vehicle. The vehicle's shape defines whether the shockwave is attached or detached.
If the vehicle is streamlined, the shock wave may attach. If the vehicle is blunt, the shock wave will detach and curve in front of the vehicle. At approximately 70 to 80 km above Earth, this pressure becomes large enough to cause diatomic oxygen and nitrogen to split into single atoms, which causes further reactions. These reactions are exothermic, releasing a significant amount of heat.
The pressure built up by the spacecraft passing through the atmosphere causes it to slow down gradually. Spacecraft are designed to utilize this drag to slow down and land safely, converting kinetic energy into thermal energy. However, this heat requires major design considerations, with engineers spending significant time developing "thermal protection systems" to survive atmospheric entry. For example, attached shock waves (resulting from streamlined vehicles) result in more heat being transferred with tremendous localized heating at the attachment point. In contrast, a blunt vehicle with a detached shock wave caused it to curve in front of the vehicle, creating a boundary of air between the shock wave and the vehicle's surface.
H. Julian Allen and A. J. Eggers, Jr. first proved that the heat load experienced by an entry vehicle is inversely proportional to its drag coefficient; i.e., the larger the drag, the lower the heat load. This is due to blunt entry vehicles forcing air down quicker than it can move around the vehicle, creating an air cushion that pushes the shock wave further away from the vehicle, preventing hot gases from being in direct contact with the vehicle.
The primary method of heat transfer is convection for reentry vehicles entering Earth's atmosphere at speeds under roughly 15,000 meters per second (this speed varies for other planets depending on their atmosphere). Above this speed, heat is primarily transferred by radiation. The heating rate or rate of change of heat energy for a reentry vehicle (q) can be simplified as:
Where V is the vehicle's velocity (m/s), ρ is the atmospheric density (kg/m3) and rnose is the vehicle's nose radius (m). This produces a heating rate measured in watts per square meter (W/m2). As the vehicle passes through the atmosphere, its velocity drops but the atmospheric density increases. This means the maximum heat transfer for Earth reentry occurs around 85% of the entry velocity. Also, steeper reentry angles generate higher peak heating rates as they maximize deceleration deeper into the atmosphere, creating a trade-off for trajectory design:
- Steeper reentry angles cause high peak heat transfer but over a shorter time
- Shallower reentry angles cause lower peak heat transfer rates but over a longer period
Temperatures during reentry cause the air in the shock layer to be both ionized and dissociated. Chemical dissociation necessitates multiple physical models to describe the thermal and chemical properties of the shock layer. Aeronautical engineers typically rely on four basic physical models of gas when designing heat shields:
- Perfect gas model
- Real (equilibrium) gas model
- Real (non-equilibrium) gas model
- Frozen gas model
The two main considerations within entry vehicle design are vehicle shape and thermal protection systems (TPS).
The size and shape of the reentry vehicle define the ballistic coefficient and the amount of lift generated. This requires understanding the drag coefficient for the given shape of the reentry vehicle. Estimates can be made using vehicle models in a wind tunnel to measure the drag coefficient. However, the conditions created in a wind tunnel are roughly 25 times slower than typical reentry speeds. Therefore, reentry vehicle designers use mathematical models to predict the hypersonic flow of air and the drag coefficient. These models are a specialized area of aerospace engineering known as computational fluid dynamics (CFD).
The shapes used in the design of entry vehicles include the following:
A shape can be designed as a complete sphere or a spherical section forebody with a converging conical afterbody. Heat flux on the spherical section can be estimated analytically using Newtonian impact theory or, more accurately, using the Fay-Riddell equation. The static stability of the spherical section is assured if the center of mass is upstream from the center of curvature. Pure spheres do not generate lift unless flying at an angle of attack.
Before CFD and high-speed computers, spherical sections were used for crewed entry vehicles as their geometry could be assessed using closed-form analysis. They were used in the early Soviet Vostok and Vokshod and in the Soviet Mars and Venus descent vehicles. Apollo command and service modules used a spherical section forebody heat shield with a converging conical afterbody. They utilized a lifting entry with an angle of attack of -27o (0o would be blunt end first) to produce an average lift-to-drag ratio of 0.368. Other examples of spherical section entry vehicles include the crewed Soyuz, Zone, Gemini, and Mercury capsules. Utilizing lift entry with these shapes reduces the peak g-force felt by the crew. Even small amounts of lift can significantly reduce the forces felt by the crew compared to a purely ballistic entry.
A sphere-cone shape is a spherical section with a frustum or blunted cone attached. The sphere-cone typically offers improved dynamic stability than that of a spherical section. Designing a sphere-cone entry vehicle with a sufficiently small half-angle and a properly placed center of mass can provide aerodynamic stability from Keplerian entry to surface impact. The "half-angle" is defined as the angle between the cone's axis of rotational symmetry and its outer surface.
The first sphere-cone aeroshell used by the US was the Mk-2 RV (reentry vehicle), developed in 1955 by the General Electric Corp. The design was derived from blunt-body theory and used a radiatively cooled TPS based upon a metallic heat shield. A newer RV, the Mk-6, used a non-metallic ablative TPS. Reconnaissance satellite recovery vehicles and space exploration missions also used a sphere-cone shape, such as NASA's Viking missions. The shape allows for lower mass TPS.
Biconic entry vehicles have a sphere-cone shape with an additional frustum attached, offering a significantly improved L/D ratio. The higher L/D makes biconic shapes better suited for transporting people to Mars due to the lower peak deceleration. Biconic designs for Mars aerocapture typically have an L/D of approximately 1.0.
Crewed entry vehicles have also used non-axisymmetric shapes, such as the winged orbit vehicle that uses a delta wing to maneuver during descent. This approach has been implemented by the US (Space Shuttle) and the Soviets (Buran spacecraft).
The TPS protects the vehicle from the tremendous heat produced during atmospheric reentry. Multiple approaches are possible, including ablative heat shields, heat sinks, and radiative cooling.
The first method used to deal with atmospheric entry heating was to create a heat sink using extra material. This material would absorb the heat transferred to the vehicle and lower the peak temperature. This form of TPS was utilized for ICBMs in the 1950s, in particular for those with high reentry angles. Building entry vehicles with a heat sink significantly increases the mass, reducing the mass available for the payload.
Ablative heat shields function by lifting the hot shock layer of gas away from the heat shield's outer wall to create a cooler boundary layer. This layer comes from blowing gaseous reaction products from the heat shield material in order to protect against heat flux. The overall process of reducing the heat on the shield's outer wall is called blockage. Ablation occurs at two levels in an entry vehicle's TPS:
- The outer surface of the TPS material melts and sublimes (changes directly into a vapor)
- The bulk TPS material undergoes pyrolysis, expelling gases
The gas produced by pyrolysis drives blowing, blocking the convective and catalytic heat flux. Ablation can also provide blockage against radiative heat flux, introducing carbon into the shock layer, making it optically opaque. Ablation is used on the warheads of ICBMs and crewed entry vehicles such as the Apollo capsule. The main drawback of ablation is the degradation of the outer layer of the heat shield, meaning the vehicle cannot be reused without a complete refurbishment.
A hot enough object radiates heat back to its environment through emission until it is in thermal equilibrium (the heat being absorbed is equal to the heat emitted). The energy emitted is a function of temperature and the surface's emissivity (dimensionless property between 0 and 1). High emissivity means a surface will emit almost as much energy as it absorbs, meaning it reaches thermal equilibrium sooner (a lower temperature). This process, reducing equilibrium temperatures by emitting heat before a vehicle's structure can absorb it, is known as radiative cooling.
Even for materials with very high emissivity, the equilibrium temperatures produced during atmospheric entry can exceed the melting point of aluminum (standard material for spacecraft). This poses problems for radiative cooling, and engineers must utilize surface-coating material, such as ceramic with both high emissivity and melting point. Next, the surface coating has to be isolated from the aluminum vehicle via efficient insulation. "Shuttle tiles" combine a special
with a special coating, with an emissivity of around 0.8, and extremely efficient insulation made up of highly refined silicate.
Each Space Shuttle had more than 21,000 of these lightweight tiles for thermal protection.