Orbital Flight
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An orbital spaceflight (or orbital flight) is a
spaceflight Spaceflight (or space flight) is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such ...
in which a
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
is placed on a trajectory where it could remain in
space Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...
for at least one
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
. To do this around the Earth, it must be on a free trajectory which has an
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
at
perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
(altitude at closest approach) around ; this is the boundary of space as defined by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
, the
US Air Force The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
and the FAA. To remain in orbit at this altitude requires an
orbital speed In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or ...
of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires greater
delta-v Delta-''v'' (also known as "change in velocity"), symbolized as and pronounced , as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or l ...
. The
Fédération Aéronautique Internationale The World Air Sports Federation (; FAI) is the world governing body for air sports, and also stewards definitions regarding human spaceflight. It was founded on 14 October 1905, and is headquartered in Lausanne, Switzerland. It maintains worl ...
has established the
Kármán line The Kármán line (or von Kármán line ) is a conventional definition of the Outer space#Boundary, edge of space; it is widely but not universally accepted. The international record-keeping body Fédération Aéronautique Internationale, FAI ( ...
at an altitude of as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about , as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient
aerodynamic lift When a fluid flows around an object, the fluid exerts a force on the object. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force paral ...
from the atmosphere to support itself. Due to
atmospheric drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
, the lowest altitude at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately . The expression "orbital spaceflight" is mostly used to distinguish from
sub-orbital spaceflight A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the surface of the primary (astronomy), gravitating body from which it was launched. Hence, it will not complete one orbital ...
s, which are flights where the apogee of a spacecraft reaches space, but the perigee is too low.


Trajectory

There are three main "bands" of
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
around the Earth:
low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...
(LEO),
medium Earth orbit A medium Earth orbit (MEO) is an geocentric orbit, Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level.
(MEO), and
geostationary orbit A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular orbit, circular geosynchronous or ...
(GEO). According to
orbital mechanics Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to rockets, satellites, and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and the law of universal ...
, an orbit lies in a particular, largely fixed plane around the Earth, which coincides with the center of the Earth, and may be inclined with respect to the equator. The relative motion of the spacecraft and the movement of the Earth's surface, as the Earth rotates on its axis, determine the position that the spacecraft appears in the sky from the ground, and which parts of the Earth are visible from the spacecraft. It is possible to calculate a
ground track A satellite ground track or satellite ground trace is the path on the surface of a planet directly below a satellite's trajectory. It is also known as a suborbital track or subsatellite track, and is the vertical projection of the satellite's ...
that shows which part of the Earth a spacecraft is immediately above; this is useful for helping to visualise the orbit.


Launch

Orbital spaceflight from Earth has only been achieved by
launch vehicle A launch vehicle is typically a rocket-powered vehicle designed to carry a payload (a crewed spacecraft or satellites) from Earth's surface or lower atmosphere to outer space. The most common form is the ballistic missile-shaped multistage ...
s that use
rocket engine A rocket engine is a reaction engine, producing thrust in accordance with Newton's third law by ejecting reaction mass rearward, usually a high-speed Jet (fluid), jet of high-temperature gas produced by the combustion of rocket propellants stor ...
s for propulsion. To reach orbit, the rocket must impart to the payload a
delta-v Delta-''v'' (also known as "change in velocity"), symbolized as and pronounced , as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or l ...
of about 9.3–10 km/s. This figure is mainly (~7.8 km/s) for horizontal acceleration needed to reach orbital speed, but allows for
atmospheric drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
(approximately 300 m/s with the
ballistic coefficient In ballistics, the ballistic coefficient (BC, ''C'') of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration: a high number indicates a low negative acceleration—the ...
of a 20 m long dense fueled vehicle), gravity losses (depending on burn time and details of the trajectory and launch vehicle), and gaining altitude. The main proven technique involves launching nearly vertically for a few kilometers while performing a gravity turn, and then progressively flattening the trajectory out at an altitude of 170+ km and accelerating on a horizontal trajectory (with the
rocket A rocket (from , and so named for its shape) is a vehicle that uses jet propulsion to accelerate without using any surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely ...
angled upwards to fight gravity and maintain altitude) for a 5–8-minute burn until orbital velocity is achieved. Currently, 2–4 stages are needed to achieve the required delta-v. Most launches are by
expendable launch system An expendable launch system (or expendable launch vehicle/ELV) is a launch vehicle that can be launched only once, after which its components are destroyed during reentry or impact with Earth, or discarded in space. ELVs typically consist of s ...
s. The
Pegasus rocket Pegasus is an Air launch to orbit, air-launched launch vehicle, multistage rocket developed by Orbital Sciences Corporation (OSC) and later built and launched by Northrop Grumman. Pegasus is the world's first privately developed orbital laun ...
for small satellites instead launches from an aircraft at an altitude of . There have been many proposed methods for achieving orbital spaceflight that have the potential of being much more affordable than rockets. Some of these ideas such as the space elevator, and rotovator, require new materials much stronger than any currently known. Other proposed ideas include ground accelerators such as launch loops, rocket-assisted aircraft/spaceplanes such as Reaction Engines Skylon,
scramjet A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully b ...
powered spaceplanes, and RBCC powered spaceplanes. Gun launch has been proposed for cargo. From 2015
SpaceX Space Exploration Technologies Corp., commonly referred to as SpaceX, is an America, American space technology company headquartered at the SpaceX Starbase, Starbase development site in Starbase, Texas. Since its founding in 2002, the compa ...
have demonstrated significant progress in their more incremental approach to reducing the cost of orbital spaceflight. Their potential for cost reduction comes mainly from pioneering propulsive landing with their reusable rocket booster stage as well as their Dragon capsule, but also includes reuse of the other components such as the payload fairings and the use of
3D printing 3D printing, or additive manufacturing, is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer ...
of a superalloy to construct more efficient rocket engines, such as their SuperDraco. The initial stages of these improvements could reduce the cost of an orbital launch by an order of magnitude.


Stability

An object in orbit at an altitude of less than roughly 200 km is considered unstable due to
atmospheric drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
. For a satellite to be in a stable orbit (i.e. sustainable for more than a few months), 350 km is a more standard altitude for
low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...
. For example, on 1 February 1958 the
Explorer 1 Explorer 1 was the first satellite launched by the United States in 1958 and was part of the U.S. participation in the International Geophysical Year (IGY). The mission followed the first two satellites, both launched by the Soviet Union duri ...
satellite was launched into an orbit with a
perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
of . It remained in orbit for more than 12 years before its
atmospheric reentry Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. Atmospheric entry may be ''uncontrolled entry ...
over the Pacific Ocean on 31 March 1970. However, the exact behaviour of objects in orbit depends on
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
, their
ballistic coefficient In ballistics, the ballistic coefficient (BC, ''C'') of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration: a high number indicates a low negative acceleration—the ...
, and details of
space weather Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ion ...
which can affect the height of the upper atmosphere.


Orbit maintenance


Orbital maneuver

In
spaceflight Spaceflight (or space flight) is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such ...
, an orbital maneuver is the use of
propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...
systems to change the
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
of a
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
. For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a ''deep-space maneuver (DSM)''.


Deorbit and re-entry

Returning spacecraft (including all potentially crewed craft) have to find a way of slowing down as much as possible while still in higher atmospheric layers and avoiding hitting the ground ( lithobraking) or burning up. For many orbital space flights, initial deceleration is provided by the retrofiring of the craft's rocket engines, perturbing the orbit (by lowering
perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
down into the atmosphere) onto a suborbital trajectory. Many spacecraft in
low Earth orbit A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...
(e.g.,
nanosatellite A small satellite, miniaturized satellite, or smallsat is a satellite of low mass and size, usually under . While all such satellites can be referred to as "small", different classifications are used to categorize them based on mass. Satellites c ...
s or spacecraft that have run out of station keeping fuel or are otherwise non-functional) solve the problem of deceleration from orbital speeds through using atmospheric drag ( aerobraking) to provide initial deceleration. In all cases, once initial deceleration has lowered the orbital perigee into the
mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define limits: it be ...
, all spacecraft lose most of the remaining speed, and therefore kinetic energy, through the atmospheric drag effect of aerobraking. Intentional aerobraking is achieved by orienting the returning space craft so as to present the heat shields forward toward the atmosphere to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at
hypersonic In aerodynamics, a hypersonic speed is one that exceeds five times the speed of sound, often stated as starting at speeds of Mach 5 and above. The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since i ...
speeds. The thermal energy is dissipated mainly by compression heating the air in a shockwave ahead of the vehicle using a blunt heat shield shape, with the aim of minimising the heat entering the vehicle. Sub-orbital space flights, being at a much lower speed, do not generate anywhere near as much heat upon re-entry. Even if the orbiting objects are expendable, most space authorities are pushing toward controlled re-entries to minimize hazard to lives and property on the planet.


History

*
Sputnik 1 Sputnik 1 (, , ''Satellite 1''), sometimes referred to as simply Sputnik, was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program ...
was the first human-made object to achieve orbital spaceflight. It was launched on 4 October 1957 by the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
. *
Vostok 1 Vostok 1 (, ) was the first spaceflight of the Vostok programme and the first human spaceflight, human orbital spaceflight in history. The Vostok 3KA space capsule was launched from Baikonur Cosmodrome on 12 April 1961, with Soviet astronaut, c ...
, launched by the Soviet Union on 12 April 1961, carrying Yuri Gagarin, was the first successful human spaceflight to reach Earth orbit. * Vostok 6, launched by the Soviet Union on 16 June 1963, carrying
Valentina Tereshkova Valentina Vladimirovna Tereshkova (born 6 March 1937) is a Russian engineer, member of the State Duma, and former Soviet cosmonaut. She was the first Women in space, woman in space, having flown a solo mission on Vostok 6 on 16 June 1963. S ...
, was the first successful spaceflight by a woman to reach Earth orbit. * Crew Dragon Demo-2, launched by
SpaceX Space Exploration Technologies Corp., commonly referred to as SpaceX, is an America, American space technology company headquartered at the SpaceX Starbase, Starbase development site in Starbase, Texas. Since its founding in 2002, the compa ...
and the United States on 30 May 2020, was the first successful human spaceflight by a private company to reach Earth orbit.


See also

* Levels of spaceflight: Suborbital, orbital, interplanetary, interstellar and intergalatic *
List of orbits This is a list of types of gravitational orbit classified by various characteristics. Common abbreviations List of abbreviations of common Earth orbits List of abbreviations of other orbits Classifications The following is a list of t ...


References

{{authority control Spaceflight concepts 1957 introductions