In space technology , reentry describes the critical phase of a missile's entry into the atmosphere of the planet from which it took off. The entry into the atmosphere of another celestial body is generally not called re- entry, but called atmosphere entry . In the following, the re-entry is related to earth.
When it reenters, the atmosphere slows the missile from its typically high orbit speed and a lot of kinetic energy is converted into heat in a short time . Objects without a heat shield will be destroyed. The hot plasma generated by the compression of the air in front of the object and the frictional heat also interrupts a radio connection. A previous deceleration to a less critical speed would, according to the rocket equation, require a large amount of energy and thus large amounts of fuel. So far, this has ruled out such a procedure.
The term is not only used for manned spacecraft , but also for space probes , warheads of ICBMs , capsules with sample material, as well as for objects that may or should burn up , such as burnt-out rocket stages or disused satellites. Often the object is in orbit beforehand and the descent begins with the ignition of the brake against the direction of flight. The re-entry does not include the later phases of the descent, in which the thermal load is low. For the same reason, the term is not used for objects that have only reached a small fraction of the orbital velocity.
In manned space travel , it is return capsules ( Apollo , Soyuz , Shenzhou ) or reusable space shuttles (e.g. space shuttles ) that have to survive re-entry without damage in order not to endanger the astronauts. With MOOSE , a particularly small and light re-entry system has been developed to rescue astronauts in an emergency.
Every launch of a multi-stage rocket leaves burned-out upper stages behind, which, once the task has been completed, enter the atmosphere and partially burn up. Likewise, (disused) satellites are completely or largely destroyed in a controlled crash in order to avoid further space debris. The entry path is chosen so that large parts that could survive re-entry fall into the sea. Spectacular example of such an operation was the Russian Mir - Space Station . The Hubble space telescope could also be brought to a controlled crash after the end of its operating life, since its recovery no longer appears in NASA 's plans due to the crash of the Space Shuttle Columbia and would become too costly by other means.
In the case of probes that do not enter the same atmosphere as at the start, we do not speak of a re-entry but of an atmosphere entry. These include landings of planetary probes ( Cassini-Huygens , Mars-Rover ) and the so-called atmosphere braking or atmosphere capture .
The warheads of intercontinental ballistic missiles (ICBM) or ballistic missiles launched by submarines (SLBM) , which move over large areas in space and then - protected by a re-entry vehicle - enter the atmosphere at high speed, also re- enter.
Conditions for safe re-entry
As incident angle in which is space , the angle referred to, among which a spacecraft relative to the horizontal in the denser layers of the atmosphere of a celestial body occurs. The height of this point is determined arbitrarily. For example, NASA specifies an altitude of 400,000 feet (approx. 122 km) for entry into the earth's atmosphere (entry interface).
When re-entering, high demands are placed on the materials used and the structure of the spaceship cell. The temperature on the heat shields reaches more than a thousand degrees Celsius when it enters the earth's atmosphere, and the flight speed is quickly reduced, so that severe delays occur.
If the missile is to withstand the heat load undamaged, heat-resistant materials with a low thermal conductivity such as ceramics in heat protection tiles are usually used in reusable spaceships , which ensure adequate insulation. In addition, the heat has to be radiated again; Ceramic materials are just as suitable for this as metallic materials. By using materials with a low melting point, it is possible to use an ablative heat shield for cooling . The material used in the heat shield sublimates or pyrolyses . The resulting relatively cool boundary layer isolates the underlying layers and transports a large part of the heat away. An ablative heat shield is technically simpler and cheaper than a reusable heat shield; With an appropriate design, (even) higher entry speeds (more kinetic energy that has to be converted) are possible. If an ablative heat shield is to be used on a reusable spaceship, it must be replaced after each flight.
The entry angle and speed of the missile must be precisely calculated if a controlled, safe descent and landing in the intended landing area is to be guaranteed. The entry angle is usually between 6 ° and 7 °. If the entrance is too shallow, the spacecraft leaves the atmosphere again (after every further entry into the atmosphere it would be decelerated further, but the target area would be missed), if the entrance is too steep, the thermal load and the deceleration of the spacecraft are too great. When the Apollo spacecraft re-entered after returning from the moon, the ideal angle of entry was 6.5 °, with a tolerance of plus / minus 0.5 °.
Calculation of the flight path
Since the beginning of space travel , it has been an important task to reliably calculate the re-entry and, in particular, to determine the time and place of the burn-up or the landing site. Depending on how it comes to re-entry, different difficulties arise or occurred. The Apollo space capsules had no fuel to slow down before re-entering a low orbit , which would then have been precisely measured. Orbit corrections had to be made at a great distance before the command module capsule was cut off and had to be carried out with a very high level of precision for the conditions at the time.
When descending from a low orbit, it must be possible to precisely meter the brake ignition . For example, the American space shuttle used the weak OMS engines to reduce the orbit speed by 1% within three minutes. This delta v of only 90 m / s is sufficient to enter the atmosphere on an elliptical orbit on the other side of the earth - again rotated in the direction of flight. The shape and angle of attack of the space glider generate lift that flattens the initially steeper descent before the greatest load occurs . The power distribution becomes more compact in terms of time, which reduces heat absorption.
Particular difficulties when calculating very flat paths are / were, among others:
- insufficient knowledge of the current air density along the runway. This problem was still completely unsolved around 1960 and has led to forecast errors of up to 2 days. The ionosphere also varies regionally with solar activity .
- changing air resistance of the tumbling and rotating missile - not completely resolved to this day
- Modeling the disintegration of the missile (smaller parts are braked more strongly)
With heavy or regularly shaped bodies, the calculations are more reliable than with light satellites with different arms. Individual crashes could be calculated to within a few minutes and the track to within a few kilometers.
Spacecraft that are supposed to safely land a payload again are therefore shaped accordingly. The return capsule thus assumes an aerodynamically stable position in flight, so that the missile with the heat shield first dips into the atmosphere ( Soyuz spacecraft , Mercury spacecraft ).
Until the 1970s there was a separate network of visual observers called Moonwatch , which was overseen by the US Smithsonian Astrophysical Observatory (SAO) and comprised several hundred volunteer teams worldwide. The support of the satellite cameras (especially the Baker / Nunn stations) by relatively simply equipped amateur astronomers was necessary because, despite the technical effort, the cameras do not aim much under certain conditions in which visual observers can react much more flexibly.
Such problem areas are among others
- Measurements in twilight (missiles only in sunlight, but long exposure times impossible)
- very deep trajectories
- Inaccuracy of the forecast just before re-entry, which makes programming the cameras difficult.
In general, the take-off and landing of a (rocket-propelled) spaceship are the critical phases of the flight for which there is an increased risk of accidents.
In the case of the US space shuttle, it is known that the heat protection system used (consisting mainly of reinforced carbon-carbon panels and ceramic tiles) can withstand very high temperatures, but is very sensitive to mechanical influences. In February 2003, NASA's Columbia Space Shuttle partially burned up when it reentered at the end of the STS-107 mission because at least one of the most heavily stressed parts of the heat protection system on the left wing leading edge was damaged by a piece of foam the size of a briefcase. Since this damage was not discovered during the mission (some warnings from NASA employees were ignored or trivialized by flight control), when re-entering the wing, the plasma penetrating the wing could affect its aluminum structure to such an extent that the left surface and then the entire shuttle were destroyed.
Landings on Mars are more difficult to carry out due to the low density of the Mars atmosphere, so that landing probes can hit the surface at too high a speed and be damaged. For the same reason, there are limitations in the landing heights on the Martian surface, so currently probes can only be landed at heights of less than 2 km, which means that some of the interesting Mars regions cannot be reached. Landings on Venus or on Titan, on the other hand, are much easier to carry out due to the dense atmosphere, but the high pressure and temperature of the Venus atmosphere pose a further danger for the landing vehicles.
- List of space disasters
- Multiple independently targetable reentry vehicle
- Maneuverable reentry vehicle
- Nelson Hayes: Trackers of the Skies . Howard Doyle Publ. 1968 and Academic Press 1975 * Moonwatch Newsletters 1965-1975 (SAO Moonwatch Center)
- Günter Seeber , Satellite Geodesy: Basics, Methods and Applications. De Gruyter, Berlin 1989, ISBN 3-11-010082-7 .
- Roger D. Launius, Dennis R. Jenkins: Coming Home: Reentry and Recovery from Space , NASA e-Book 2012.