Orbital spaceflight
Encyclopedia
An orbital spaceflight (or orbital flight) is a spaceflight
in which a spacecraft
is placed on a trajectory where it could remain in space
for at least one orbit
. To do this around the Earth
, it must be on a free trajectory which has an altitude
at perigee
(altitude at closest approach) above 100 kilometres (62.1 mi) (this is, by at least one convention, the boundary of space). To remain in orbit at this altitude requires an orbital speed
of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires higher delta-v
.
The expression "orbital spaceflight" is mostly used to distinguish from sub-orbital spaceflight
s, which are flights where apogee of a spacecraft reaches space but perigee is too low.
Orbital spaceflight from Earth has only been achieved by launch vehicle
s that use rocket engine
s for propulsion. To reach orbit, the rocket must impart to the payload a delta-v
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 (approximately 300 m/s with the ballistic coefficient
of a 20 m long dense fuelled 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 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
s.
The Pegasus rocket
for small satellites instead launches from an aircraft at an altitude of 12 km.
Other techniques, such as use of a launch loop
, have been proposed for non-rocket spacelaunch
. These techniques are theoretical: no attempts have been made to orbit a vehicle using any of them.
An object in orbit at an altitude of less than roughly 200 km is considered unstable due to atmospheric drag. 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
. For example, on 1958-02-01 the Explorer 1 satellite was launched into an orbit with a perigee
of 358 kilometres (222.5 mi). It remained in orbit for more than 12 years before its atmospheric reentry
over the Pacific Ocean on 1970-03-31.
However, the exact behaviour of objects in orbit depends on altitude
, their ballistic coefficient
, and details of space weather
which can affect the height of the upper atmosphere.
(LEO), medium Earth orbit
(MEO) and geostationary orbit
(GEO).
Due to Orbital mechanics orbits are in a particular, largely fixed plane around the Earth, which coincides with the center of the Earth, and may be tilted with respect to the equator. The Earth rotates about its axis within this orbit, and the relative motion of the spacecraft and the movement of the Earths surface determines the position that the spacecraft appears in the sky from the ground, and which parts of the Earth are visible from the spacecraft.
By dropping a vertical down to the Earth's surface it is possible to calculate a ground track
which shows which part of the Earth a spacecraft is immediately above, and this is useful for helping to visualise the orbit.
NASA provides real-time tracking of the over 500 artificial satellites maintained in orbit around Earth. For the position of these satellites see NASA satellite tracking.
) 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
down into the atmosphere) onto a suborbital trajectory. Many spacecraft in low-Earth orbit (e.g., nanosatellites 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
) provide initial deceleration. In all cases, once initial deceleration has lowered the orbital perigee into the mesosphere
, 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 to fly so as to present the heat shields forwards towards the atmosphere so as to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at hypersonic
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 vehicle is a satellite that is ultimately expendable, most space authorities are pushing towards controlled re-entry techniques to avoid issues of space debris
reaching the ground and causing a hazard to lives and property. In addition, this minimises the creation of orbital space junk.
Spaceflight
Spaceflight is the act of travelling into or through outer space. Spaceflight can occur with spacecraft which may, or may not, have humans on board. Examples of human spaceflight include the Russian Soyuz program, the U.S. Space shuttle program, as well as the ongoing International Space Station...
in which a spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
is placed on a trajectory where it could remain in space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
for at least one orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
. To do this around the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, it must be on a free trajectory which has an altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
at perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
(altitude at closest approach) above 100 kilometres (62.1 mi) (this is, by at least one convention, the boundary of space). To remain in orbit at this altitude requires an orbital speed
Orbital speed
The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body...
of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires higher delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
.
The expression "orbital spaceflight" is mostly used to distinguish from sub-orbital spaceflight
Sub-orbital spaceflight
A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution....
s, which are flights where apogee of a spacecraft reaches space but perigee is too low.
Orbital launch
| Orbital Human spaceflight | |||||||
| Name | Debut | Launches | |||||
|---|---|---|---|---|---|---|---|
| Vostok Vostok programme The Vostok programme was a Soviet human spaceflight project that succeeded in putting a person into Earth's orbit for the first time. The programme developed the Vostok spacecraft from the Zenit spy satellite project and adapted the Vostok rocket from an existing ICBM design... |
1961 | 6 | |||||
| Mercury Project Mercury In January 1960 NASA awarded Western Electric Company a contract for the Mercury tracking network. The value of the contract was over $33 million. Also in January, McDonnell delivered the first production-type Mercury spacecraft, less than a year after award of the formal contract. On February 12,... |
1962 | 4 | |||||
| Voskhod Voskhod programme The Voskhod programme was the second Soviet human spaceflight project. Two manned missions were flown using the Voskhod spacecraft and rocket, one in 1964 and one in 1965.... |
1964 | 2 | |||||
| Gemini Project Gemini Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966.... |
1965 | 10 | |||||
| Soyuz Soyuz programme The Soyuz programme is a human spaceflight programme that was initiated by the Soviet Union in the early 1960s, originally part of a Moon landing project intended to put a Soviet cosmonaut on the Moon... |
1967 | 110 | |||||
| Apollo | 1968 | 15 | |||||
| Shuttle Space Shuttle The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons... |
1981 | 135 | |||||
| Shenzhou Shenzhou program The Shenzhou program is a manned spaceflight initiative by the People's Republic of China. The program put the first Chinese citizen, Yang Liwei, into orbit on 15 October 2003.... |
2003 | 3 | |||||
Orbital spaceflight from Earth has only been achieved by launch vehicle
Launch vehicle
In spaceflight, a launch vehicle or carrier rocket is a rocket used to carry a payload from the Earth's surface into outer space. A launch system includes the launch vehicle, the launch pad and other infrastructure....
s that use rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s for propulsion. To reach orbit, the rocket must impart to the payload a delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
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 (approximately 300 m/s with the ballistic coefficient
Ballistic coefficient
In ballistics, the ballistic coefficient 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. BC is a function of mass, diameter, and drag coefficient...
of a 20 m long dense fuelled vehicle), gravity losses
Gravity drag
In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field...
(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
Gravity turn
A gravity turn or zero-lift turn is a maneuver used in launching a spacecraft into, or descending from, an orbit around a celestial body such as a planet or a moon. This launch trajectory offers two main advantages over a thrust-controlled trajectory where the rocket's own thrust steers the vehicle...
, and then progressively flattening the trajectory out at an altitude of 170+ km and accelerating on a horizontal trajectory (with the rocket angled upwards to fight gravity and maintain altitude) for a 5-8 minute burn until orbital velocity is achieved. Currently, 2-4 stages
Multistage rocket
A multistage rocket is a rocket that usestwo or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or...
are needed to achieve the required delta-v. Most launches are by expendable launch system
Expendable launch system
An expendable launch system is a launch system that uses an expendable launch vehicle to carry a payload into space. The vehicles used in expendable launch systems are designed to be used only once , and their components are not recovered for re-use after launch...
s.
The Pegasus rocket
Pegasus rocket
The Pegasus rocket is a winged space launch vehicle capable of carrying small, unmanned payloads into low Earth orbit. It is air-launched, as part of an expendable launch system developed by Orbital Sciences Corporation . Three main stages burning solid propellant provide the thrust...
for small satellites instead launches from an aircraft at an altitude of 12 km.
Other techniques, such as use of a launch loop
Launch loop
A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle...
, have been proposed for non-rocket spacelaunch
Non-rocket spacelaunch
Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...
. These techniques are theoretical: no attempts have been made to orbit a vehicle using any of them.
Stability
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
. For example, on 1958-02-01 the Explorer 1 satellite was launched into an orbit with a perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
of 358 kilometres (222.5 mi). It remained in orbit for more than 12 years before its atmospheric reentry
Atmospheric reentry
Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...
over the Pacific Ocean on 1970-03-31.
However, the exact behaviour of objects in orbit depends on altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
, their ballistic coefficient
Ballistic coefficient
In ballistics, the ballistic coefficient 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. BC is a function of mass, diameter, and drag coefficient...
, and details of space weather
Space weather
Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...
which can affect the height of the upper atmosphere.
Orbits
There are three main 'bands' of orbit around the Earth: low Earth orbitLow Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO), medium Earth orbit
Medium Earth Orbit
Medium Earth orbit , sometimes called intermediate circular orbit , is the region of space around the Earth above low Earth orbit and below geostationary orbit ....
(MEO) and geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
(GEO).
Due to Orbital mechanics orbits are in a particular, largely fixed plane around the Earth, which coincides with the center of the Earth, and may be tilted with respect to the equator. The Earth rotates about its axis within this orbit, and the relative motion of the spacecraft and the movement of the Earths surface determines the position that the spacecraft appears in the sky from the ground, and which parts of the Earth are visible from the spacecraft.
By dropping a vertical down to the Earth's surface it is possible to calculate a ground track
Ground track
A ground track or ground trace is the path on the surface of the Earth directly below an aircraft or satellite. In the case of a satellite, it is the projection of the satellite's orbit onto the surface of the Earth .A satellite ground track may be thought of as a path along the Earth's surface...
which shows which part of the Earth a spacecraft is immediately above, and this is useful for helping to visualise the orbit.
NASA provides real-time tracking of the over 500 artificial satellites maintained in orbit around Earth. For the position of these satellites see NASA satellite tracking.
Deorbit and re-entry
Returning spacecraft (including all potentially manned craft) have to find a way of slowing down as much as possible while still in higher atmospheric layers and avoid hitting the ground (lithobrakingLithobraking
Lithobraking is a technique of descent by an unmanned space vehicle to the surface of a body by which the vehicle is slowed by impact with the body's surface....
) 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
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
down into the atmosphere) onto a suborbital trajectory. Many spacecraft in low-Earth orbit (e.g., nanosatellites or spacecraft that have run out of station keeping
Orbital stationkeeping
In astrodynamics orbital station-keeping is a term used to describe the orbital maneuvers made by thruster burns that are needed to keep a spacecraft in a particular assigned orbit.For many Earth satellites the effects of the non-Keplerian forces, i.e...
fuel or are otherwise non-functional) solve the problem of deceleration from orbital speeds through using atmospheric drag (aerobraking
Aerobraking
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...
) provide initial deceleration. In all cases, once initial deceleration has lowered the orbital perigee into the mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
, all spacecraft lose most of the remaining speed, and therefore kinetic energy, through the atmospheric drag effect of aerobraking
Aerobraking
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit . The resulting drag slows the spacecraft...
.
Intentional aerobraking is achieved by orienting the returning space craft to fly so as to present the heat shields forwards towards the atmosphere so as to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at hypersonic
Hypersonic
In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
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 vehicle is a satellite that is ultimately expendable, most space authorities are pushing towards controlled re-entry techniques to avoid issues of space debris
Space debris
Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion...
reaching the ground and causing a hazard to lives and property. In addition, this minimises the creation of orbital space junk.
History
- Sputnik 1Sputnik 1Sputnik 1 ) was the first artificial satellite to be put into Earth's orbit. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957. The unanticipated announcement of Sputnik 1s success precipitated the Sputnik crisis in the United States and ignited the Space...
was successfully launched on October 4, 1957 by the Soviet Union - Vostok 1Vostok 1Vostok 1 was the first spaceflight in the Vostok program and the first human spaceflight in history. The Vostok 3KA spacecraft was launched on April 12, 1961. The flight took Yuri Gagarin, a cosmonaut from the Soviet Union, into space. The flight marked the first time that a human entered outer...
, launched by the Soviet Union on April 12, 1961, carrying Yuri GagarinYuri GagarinYuri Alekseyevich Gagarin was a Soviet pilot and cosmonaut. He was the first human to journey into outer space, when his Vostok spacecraft completed an orbit of the Earth on April 12, 1961....
, was the first successful human spaceflight to reach Earth orbit.
See also
- OrbitOrbitIn physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
- List of orbits
- Ground trackGround trackA ground track or ground trace is the path on the surface of the Earth directly below an aircraft or satellite. In the case of a satellite, it is the projection of the satellite's orbit onto the surface of the Earth .A satellite ground track may be thought of as a path along the Earth's surface...
- Orbital mechanics
- Project HARPProject HARPProject HARP, short for High Altitude Research Project, was a joint project of the United States Department of Defense and Canada's Department of National Defence created with the goal of studying ballistics of re-entry vehicles at low cost; whereas most such projects used expensive rockets, HARP...
was a failed attempt, and a ram acceleratorRam acceleratorA ram accelerator has the same function as a gun; i.e., it is a device for accelerating projectiles; however, it is entirely different in that jet-engine-like propulsion cycles utilizing ramjet and/or scramjet combustion processes are used to accelerate a projectile to extremely high speeds...
is another design, to launch an object into orbit with a gunGunA gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol,... - Rocket launchRocket launchA rocket launch is the takeoff phase of the flight of a rocket. Launches for orbital spaceflights, or launches into interplanetary space, are usually from a fixed location on the ground, but may also be from a floating platform such as the San Marco platform, or the Sea Launch launch...
- Non-rocket spacelaunchNon-rocket spacelaunchNon-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...
- SpacecraftSpacecraftA spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
- SpaceflightSpaceflightSpaceflight is the act of travelling into or through outer space. Spaceflight can occur with spacecraft which may, or may not, have humans on board. Examples of human spaceflight include the Russian Soyuz program, the U.S. Space shuttle program, as well as the ongoing International Space Station...
- SpaceportSpaceportA spaceport or cosmodrome is a site for launching spacecraft, by analogy with seaport for ships or airport for aircraft. The word spaceport, and even more so cosmodrome, has traditionally been used for sites capable of launching spacecraft into orbit around Earth or on interplanetary trajectories...
, including a list of sites for orbital launches