Reentry
Reentry or re-entry is the return to the Earth's atomosphere from space. By extension the term is used to refer to the transition from the vacuum of space to the atmosphere of any planet or other celestial body, even if not entering again. The term is not used for landing on bodies which have no atmosphere e.g. the Moon. The synonymous term atmospheric reentry is sometimes used for clarity (while "reentry" has the obvious advantage of brevity.)Reentry mostly occurs at very high speed. Because a major difference between sub-orbital and orbital spaceflights is the greater speed of the latter, atmospheric reentry poses much more of a technical challenge with orbital flights than with suborbital flights. This article will focus on orbital reentry, though the same considerations apply with sub-orbital flights, only to a lesser extent. Also note that the below only really applies to flights where the vehicle needs to return to Earth intact. If the vehicle is, say, a satellite that is ultimately expendable, then there naturally is no need to worry about deceleration and non-destructive reentry.
Deceleration and aerobraking
The main challenge with reentry is deceleration from orbital speeds. As we've said before, orbital speeds are very high. To avoid our orbital spacecraft performing a meteor-style "landing", it has to slow down. An obvious way of slowing down is through atmospheric friction and drag (ie. using wind resistance) . This is called aerobraking. Atmospheric friction however can rapidly generate a destructive amount of heat (think running the gauntlet naked though a corridor full of blowtorches). Many smaller meteoroids burn up through such friction and never reach the Earth's surface. So is there a better way?
Of course, some active breaking is required in order to enter the atmosphere, until then no atmospheric drag is available. In theory, it would be imaginable to accomplish all or practically all the deceleration by active, powered braking, by firing the craft's rocket engine in the opposite direction. This however would require a large amount of fuel, this fuel would have had to be lifted into orbit in the first place and therefore the size of the orbiting space craft relative to the launch vehicle would have to be much smaller, unless the spacecraft were refuelled in orbit.
These four shadowgraph images represent early re-entry vehicle concepts. A shadowgraph is a process that makes visible the disturbances that occur in a fluid flow at high velocity, in which light passing through a flowing fluid is refracted by the density gradients in the fluid resulting in bright and dark areas on a screen placed behind the fluid.H. Julian Allen pioneered and developed the Blunt Body Theory which made possible the heat shield designs that were embodied in the Mercury, Gemini and Apollo space capsules, enabling astronauts to survive the firey re-entry into Earth's atmosphere. A blunt body produces a shockwave in front of the vehicle--visible in the photo--that actually shields the vehicle from excessive heating. As a result, blunt body vehicles can stay cooler than pointy, low drag vehicles.
SpaceShipOne has what has been described as a pair of flipping wings; the spacecraft itself changes shape for reentry.
One the one hand this increases drag, as the craft is now less streamlined. This results in more atmospheric gas particles hitting the spacecraft at higher altitudes than otherwise. The aircraft thus slows down more in higher atmospheric layers (which is the very key to efficient reentry, see above). It should also be noted that SpaceShipOne, in its "wings flipped" configuration, will automatically orient itself to a high drag attitude (see below).
One the other hand, it is assumed that SpaceShipOne's flipping wings also start producing lift early, in very thin and high layers of the atmosphere. This would enable the craft to stay in the higher, less friction intensive (and thus less heat-inducing) layers of the atmosphere for longer (ie. until it has slowed down much more than ordinary craft would at that height). Normally, fully inside the atmosphere, a configuration such as SpaceShipOne's flipped wings, where the aft part of the wings (the elevator part) is folded sharply upwards, would result in a stall. However, in a very thin atmosphere (as present near the edge of space), even such a configuration will likely not stall the craft, because of the very low atmospheric density. Instead, it will likely keep the spacecraft oriented at a very high attitude and thus result in producing the maximum lift possible under these exotic aerodynamical conditions. This in turn could help keeping the craft at a higher altitude for longer — and that would allow for maximum deceleration in higher atmospheric layers. This latter aspect itself isn't strictly aerobraking as it is about producing lift (to enable higher altitude aerobraking) — which is why these issues are not discussed in the aerobraking article.
It is assumed that this alternative approach to efficient aerobraking and reentry can be applied to orbital space craft design and may result in a markedly reduced need for strong heat shields in the future. How widely similar solutions will get adopted remains to be seen.
The highest speed reentry so far was achieved by the Jupiter atmosphere probe aboard the Galileo spacecraft, which reached 170,700 km per hour and a temperature of 14,000 °C.
Notable reentry mishaps occurred during the following missions:
More than 100 metric tons of man-made objects reenter in an uncontrolled fashion each year. The vast majority burn up before reaching earth's surface. On average, about one cataloged object reenters per day. Approximately, one-fourth of all objects are of U.S. origin.Why superfast reentry isn't an option
For our orbital spacecraft, one might consider minimizing the "exposure time" by passing through the atmosphere faster. However, going faster would again mean more friction, reaching deeper and denser layers of the atmosphere at higher speeds (yet more friction) and thus more temperature (and at best result in the meteor-style landing we wanted to avoid). The kinetic energy of the spacecraft has to be dissipated. It can only be dissipated by conversion to heat. Quicker conversion means higher temperatures. The enemy is high temperature and therefore it is a slower passing through the atmosphere which is required - not a faster one.The key to successful reentry
The key challenge with successful reentry then is to brake as much as possible while still in higher atmospheric layers and avoid plunging downwards too quickly.Why active braking cannot be solely relied upon
Aerobraking currently the only option
Thus, the only currently known and feasible way of decelerating from orbital speeds is mainly through aerobraking.Reliance on heat shields
Conventional wisdom dictates that aerobraking is best achieved through orienting the returning space craft to fly at a high drag attitude coupled with ultra strong heat shields on the spacecraft, to convert the craft's high kinetic energy into thermal energy (heat) by atmospheric friction. This unavoidably rapid conversion of a large amount of kinetic energy to heat results in extremely high temperatures, so the heat shield needs to be extremely strong and reliable. Relying mainly on the heat shield (and possibly a high drag attitude) makes reentry a critical time. Any errors in this portion of the flight profile are difficult to recover from and will probably have serious impact upon the mission. Death and/or mission failures have occurred during re-entry. Nevertheless, the use of strong heat shields has so far been regarded as the only practical approach and all orbital returning spacecraft have been equipped with such.Blunt body heat shields
A better future approach?
However, maverick aircraft designer Burt Rutan has recently (as of 2004) demonstrated the feasibility of an alternative or complementary approach to atmospheric reentry with the suborbital SpaceShipOne flight 15P:Related info
Uncontrolled reentry
