Launch Rockets are the only means currently capable of reaching orbit or beyond. Other
non-rocket spacelaunch technologies have yet to be built, or remain short of orbital speeds. A
rocket launch for a spaceflight usually starts from a
spaceport (cosmodrome), which may be equipped with launch complexes and
launch pads for vertical rocket launches and runways for takeoff and landing of carrier airplanes and winged spacecraft. Spaceports are situated well away from human habitation for noise and safety reasons.
ICBMs have various special launching facilities. A launch is often restricted to certain
launch windows. These windows depend upon the position of celestial bodies and orbits relative to the launch site. The biggest influence is often the rotation of the Earth. Once launched, orbits are normally located within relatively constant flat planes at a fixed angle to the axis of the Earth, and the Earth rotates within this orbit. A
launch pad is a fixed structure designed to dispatch airborne vehicles. It generally consists of a launch tower and flame trench. It is surrounded by equipment used to erect, fuel, and maintain launch vehicles. Before launch, the rocket can weigh hundreds of tons. The
Space Shuttle Columbia, on
STS-1, weighed 2030 metric tons (4,480,000 lb) at takeoff.
Reaching space The most commonly used definition of
outer space is everything beyond the
Kármán line, which is above the Earth's surface. (The United States defines outer space as everything beyond in altitude.)
Rocket engines remain the only currently practical means of reaching space, with planes and
high-altitude balloons failing due to lack of atmosphere and alternatives such as space elevators not yet being built. Chemical propulsion, or the acceleration of gases at high velocities, is effective mainly because of its ability to sustain thrust even as the atmosphere thins.
Alternatives Many ways to reach space other than rocket engines have been proposed. Ideas such as the
space elevator, and
momentum exchange tethers like
rotovators or
skyhooks require new materials much stronger than any currently known. Electromagnetic launchers such as
launch loops might be feasible with current technology. Other ideas include rocket-assisted aircraft/spaceplanes such as
Reaction Engines Skylon (currently in early stage development),
scramjet powered spaceplanes, and
RBCC powered spaceplanes. Gun launch has been proposed for cargo.
Leaving orbit On some missions beyond
Low Earth orbit, spacecraft are inserted into parking orbits, or lower intermediary orbits. The parking orbit approach greatly simplified Apollo mission planning in several important ways. It acted as a "time buffer" and substantially widened the allowable
launch windows. The parking orbit gave the crew and controllers time to thoroughly check out the spacecraft after the stresses of launch before committing it for a long journey to the Moon. was the first known artificial object to achieve escape velocity from the Earth (replica pictured). Robotic missions do not require an abort capability and require radiation minimalization only for delicate electronics, and because modern launchers routinely meet "instantaneous" launch windows, space probes to the Moon and other planets generally use direct injection to maximize performance by limiting the boil off of
cryogenic propellants. Although some might coast briefly during the launch sequence, they do not complete one or more full parking orbits before the burn that injects them onto an Earth escape trajectory. The escape velocity from a celestial body decreases as the distance from the body increases. However, it is more fuel-efficient for a craft to burn its fuel as close as possible to its
periapsis (lowest point); see
Oberth effect.
Astrodynamics Astrodynamics is the study of spacecraft trajectories, particularly as they relate to gravitational and propulsion effects. Astrodynamics allows for a spacecraft to arrive at its destination at the correct time without excessive propellant use. An
orbital maneuvering system may be needed to maintain or change orbits. Non-rocket orbital propulsion methods include
solar sails,
magnetic sails,
plasma-bubble magnetic systems, and using
gravitational slingshot effects.
Transfer energy The term "transfer energy" means the total amount of
energy imparted by a rocket stage to its payload. This can be the energy imparted by a
first stage of a
launch vehicle to an upper stage plus payload, or by an upper stage or spacecraft
kick motor to a
spacecraft.
Reaching space station In order to reach a
space station, a spacecraft would have to arrive at the same
orbit and approach to a very close distance (e.g. within visual contact). This is done by a set of orbital maneuvers called
space rendezvous. After rendezvousing with the space station, the space vehicle then docks or berths with the station. Docking refers to joining of two separate free-flying space vehicles, while berthing refers to mating operations where an inactive vehicle is placed into the mating interface of another space vehicle by using a
robotic arm.
Reentry Vehicles in orbit have large amounts of kinetic energy. This energy must be discarded if the vehicle is to land safely without vaporizing in the atmosphere. Typically this process requires special methods to protect against
aerodynamic heating. The theory behind reentry was developed by
Harry Julian Allen. Based on this theory, reentry vehicles present blunt shapes to the atmosphere for reentry. Blunt shapes mean that less than 1% of the kinetic energy ends up as heat reaching the vehicle, and the remainder heats the atmosphere.
Landing and recovery The
Mercury,
Gemini, and
Apollo capsules
splashed down in the sea. These capsules were designed to land at relatively low speeds with the help of a parachute. Soviet/Russian capsules for
Soyuz make use of a big parachute and braking rockets to touch down on land.
Spaceplanes like the
Space Shuttle land like a
glider. After a successful landing, the spacecraft, its occupants, and cargo can be recovered. In some cases, recovery has occurred before landing: while a spacecraft is still descending on its parachute, it can be snagged by a specially designed aircraft. This
mid-air retrieval technique was used to recover the film canisters from the
Corona spy satellites. == Types ==