Reentry
Reentry or re-entry is the return to Earth 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 e.g. the Moon, which has no atmosphere. The synonymous term atmospheric reentry is sometimes used for clarity (while "reentry" has the obvious advantage of brevity.) For manned missions reentry has until now only applied to returning to Earth.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?
However, to avoid a fast descent, our orbital craft could arguably direct its rocket motor partially towards Earth, using it to brake and hover. This however would require an extreme amount of fuel, and (with current technology) most of the fuel would have been used up while climbing to orbit in the first place, unless the spacecraft were refulled in orbit. (Hypothetically, a spacecraft with a mass fraction of less than 0.5 could take off, reach orbit and conduct a powered landing without using any atmospheric braking or refuelling.)
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.
Notable reentry mishaps occurred during the following missions:
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 heat (and at best result in the meteor-style landing we wanted to avoid).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
In theory, it would be imaginable to accomplish this mostly by active, powered braking, by firing the craft's rocket engine in the opposite direction. The more the craft slowed down however, the less centrifugal force would there be to counter Earth's gravity: The spacecraft would plunge into lower layers of the atmosphere pretty fast, which isn't an option (see above). 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 high atmospheric friction into thermal energy, which gets dissipated mainly as infrared radiation. This again generates 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 large impact upon the mission. Death and/or mission failures have occurred in this flight regime. Nevertheless, the use of strong heat shields has so far been regarded as the only possible approach and all orbital returning spacecraft have been equipped with such.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
The highest velocity 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.See also