Aerobraking

Aerobraking

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Aerobraking is a spaceflight
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...

 maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 at the low point of the 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...

 (periapsis). The resulting drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 slows the 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....

. Aerobraking is used when a spacecraft requires a low orbit after arriving at a body with an atmosphere, and it requires less fuel than does the direct use of a 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...

.

Method


When an interplanetary vehicle arrives at its destination, it must change its velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 to remain in the vicinity of that body. When a low, near-circular orbit
Circular orbit
A circular orbit is the orbit at a fixed distance around any point by an object rotating around a fixed axis.Below we consider a circular orbit in astrodynamics or celestial mechanics under standard assumptions...

 around a body with substantial gravity (as is required for many scientific studies) is needed, the total required velocity changes can be on the order of several kilometers per second. If done by direct propulsion, the rocket equation
Tsiolkovsky rocket equation
The Tsiolkovsky rocket equation, or ideal rocket equation is an equation that is useful for considering vehicles that follow the basic principle of a rocket: where a device that can apply acceleration to itself by expelling part of its mass with high speed and moving due to the conservation of...

 dictates that a large fraction of the spacecraft mass must be fuel. This in turn means the spacecraft is limited to a relatively small science payload and/or the use of a very large and expensive launcher. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements. The use of a relatively small burn allows the spacecraft to be captured into a very elongated elliptic orbit
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...

. Aerobraking is then used to circularize the orbit. If the atmosphere is thick enough, a single pass through it can be sufficient to slow a spacecraft as needed. However, aerobraking is typically done with many orbital passes through a higher altitude, and therefore thinner region of the atmosphere. This is done to reduce the effect of friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

al heating, and because unpredictable turbulence effects, atmospheric composition, and temperature make it difficult to accurately predict the decrease in speed that will result from any single pass. When aerobraking is done in this way, there is sufficient time after each pass to measure the change in velocity and make any necessary corrections for the next pass. Achieving the final orbit using this method takes a long time (e.g., over six month
Month
A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...

s when arriving at Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

), and may require several hundred passes through the atmosphere of the planet or moon. After the last aerobraking pass, the spacecraft must be given more kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 via rocket engines in order to raise the periapsis above the atmosphere—unless, of course, the intent is to land the spacecraft.

The kinetic energy dissipated by aerobraking is converted to heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

, meaning that a spacecraft using the technique needs to be capable of dissipating this heat. The spacecraft must also have sufficient surface area and structural strength to produce and survive the required drag, but the temperatures and pressures associated with aerobraking are not as severe as those of 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,...

 or aerocapture
Aerocapture
Aerocapture is a technique used to reduce velocity of a spacecraft, arriving at a celestial body with a hyperbolic trajectory, in order to bring it in an orbit with an eccentricity of less than 1. It uses the drag created by the atmosphere of the celestial body to decelerate. Only one pass in the...

. Simulations of the Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...

 aerobraking use a force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 limit of 0.35 N per square meter with a spacecraft cross section of about 37 m², and a maximum expected temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 as 340 °F (170 °C). The force density, of roughly 0.2 N (0.04 lbf) per square meter, that was exerted on the Mars Observer
Mars Observer
The Mars Observer spacecraft, also known as the Mars Geoscience/Climatology Orbiter, was a 1,018-kilogram robotic space probe launched by NASA on September 25, 1992 to study the Martian surface, atmosphere, climate and magnetic field...

, during aerobraking is comparable to the force of a 40 mph (60 km/h) wind on a human hand at sea level on Earth.

Related methods


Aerocapture
Aerocapture
Aerocapture is a technique used to reduce velocity of a spacecraft, arriving at a celestial body with a hyperbolic trajectory, in order to bring it in an orbit with an eccentricity of less than 1. It uses the drag created by the atmosphere of the celestial body to decelerate. Only one pass in the...

 is a related but more extreme method in which no initial orbit-injection burn is performed. Instead, the spacecraft plunges deeply into the atmosphere without an initial insertion burn, and emerges from this single pass in the atmosphere with an apoapsis near that of the desired orbit. Several small correction burns are then used to raise the periapsis and perform final adjustments.
This method was originally planned for the Mars Odyssey orbiter, but the significant design impacts proved too costly.

Another related technique is that of aerogravity assist
Aerogravity assist
An aerogravity assist, or AGA, is a spacecraft maneuver designed to change velocity when arriving at a body with an atmosphere. A pure gravity assist uses only the gravity of a body to change the direction of the spacecraft trajectory. The change in direction is limited by the mass of the body,...

, in which the spacecraft flies through the upper atmosphere and utilises aerodynamic lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. 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 surface force parallel to the flow direction...

 instead of drag at the point of closest approach. If correctly oriented, this can increase the deflection angle above that of a pure gravity assist, resulting in a larger 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....

.

Spacecraft missions


Although the theory of aerobraking is well developed, utilising the technique is difficult because a very detailed knowledge of the character of the target planet's atmosphere is needed in order to plan the maneuver correctly. Currently, the deceleration is monitored during each maneuver and plans are modified accordingly. Since no spacecraft can yet aerobrake safely on its own, this requires constant attention from both human controllers and the Deep Space Network
Deep Space Network
The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...

. This is particularly true near the end of the process, when the drag passes are relatively close together (only about 2 hours apart for Mars). NASA has used aerobraking four times to modify a spacecraft’s orbit to one with lower energy, reduced apoapsis altitude, and smaller orbit.

On 19 March 1991, aerobraking was demonstrated by the Hiten
Hiten
The Hiten Spacecraft , given the English name Celestial Maiden and known before launch as MUSES-A , part of the MUSES Program, was built by the Institute of Space and Astronautical Science of Japan and launched on January 24, 1990...

 spacecraft. This was the first aerobraking maneuver by a deep space probe. Hiten
Hiten
The Hiten Spacecraft , given the English name Celestial Maiden and known before launch as MUSES-A , part of the MUSES Program, was built by the Institute of Space and Astronautical Science of Japan and launched on January 24, 1990...

 (a.k.a. MUSES-A) was launched by the Institute of Space and Astronautical Science
Institute of Space and Astronautical Science
is a Japanese national research organization of astrophysics using rockets, astronomical satellites and interplanetary probes. It is a division of Japan Aerospace Exploration Agency .- History :...

 (ISAS) of Japan. Hiten flew by the Earth at an altitude of 125.5 km over the Pacific at 11.0 km/s. Atmospheric drag lowered the velocity by 1.712 m/s and the apogee altitude by 8665 km. Another aerobraking maneuver was conducted on 30 March.

In May 1993, aerobraking was used during the extended Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

ian mission of the Magellan spacecraft. It was used to circularize the orbit of the spacecraft in order to increase the precision of the measurement of the gravity field. The entire gravity field was mapped from the circular orbit during a 243 day cycle of the extended mission. During the termination phase of the mission, a "windmill experiment" was performed: Atmospheric molecular pressure exerts a torque via the then windmill-sail-like oriented solar cell wings, the necessary counter-torque to keep the sonde from spinning is measured.

In 1997, the Mars Global Surveyor
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...

 (MGS) orbiter was the first spacecraft to use aerobraking as the main planned technique of orbit adjustment. The MGS used the data gathered from the Magellan mission to Venus to plan its aerobraking technique. The spacecraft used its solar panels as "wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...

s" to control its passage through the tenuous upper atmosphere of Mars and lower the apoapsis of its orbit over the course of many months. Unfortunately, a structural failure shortly after launch severely damaged one of the MGS's solar panels and necessitated a higher aerobraking altitude (and hence one third the force) than originally planned, significantly extending the time required to attain the desired orbit. More recently, aerobraking was used by the Mars Odyssey and Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...

 spacecraft, in both cases without incident.

Aerobraking in fiction


In Robert A. Heinlein
Robert A. Heinlein
Robert Anson Heinlein was an American science fiction writer. Often called the "dean of science fiction writers", he was one of the most influential and controversial authors of the genre. He set a standard for science and engineering plausibility and helped to raise the genre's standards of...

's 1948 novel Space Cadet
Space Cadet
Space Cadet is a 1948 science fiction novel by Robert A. Heinlein about Matt Dodson, who joins the Space Patrol to help preserve peace in the Solar System. The story translates the standard military academy story into outer space: a boy from Iowa goes to officer school, sees action and adventure,...

, aerobraking is used to save fuel while slowing the spacecraft Aes Triplex for an unplanned extended mission and landing on Venus, during a transit from the Asteroid Belt to Earth.

In the fourth episode
Darkness (Stargate Universe)
"Darkness" is the fourth episode of military science fiction television series Stargate Universe, and is the first part of a two part story. The episode originally aired on October 16, 2009 on Syfy in the United States, followed by being aired in the United Kingdom and Ireland on October 20...

 of Stargate Universe
Stargate Universe
Stargate Universe is a Canadian-American military science fiction television series and part of MGM's Stargate franchise. It follows the adventures of a present-day, multinational exploration team traveling on the Ancient spaceship Destiny many billions of light years distant from the Milky Way...

, the Ancient
Ancient (Stargate)
The Ancients are a humanoid race in the fictional Stargate universe. They are called "Ancients" in the Milky Way, but are also known as Lanteans or Ancestors in the Pegasus galaxy and as the Alterans in their home galaxy, and they sometimes call themselves Anquietas in their language...

 ship Destiny suffers an almost complete loss of power and must use aerobraking to change course. The episode ends in a cliffhanger with Destiny headed directly toward a star.

The spacecraft Cosmonaut Alexey Lenov in Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...

's novel 2010: Odyssey Two
2010: Odyssey Two
2010: Odyssey Two is a 1982 best-selling science fiction novel by Arthur C. Clarke. It is the sequel to the 1968 novel 2001: A Space Odyssey, but continues the story of Stanley Kubrick's film adaptation with the same title and not Clarke's original novel. The book is a part of Clarke's...

uses aerobraking in the upper layers of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's atmosphere to establish itself at the Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

 of the Jupiter - Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

 system.

In Space Odyssey: Voyage to the Planets
Space Odyssey: Voyage To The Planets
Space Odyssey: Voyage To The Planets is a mockumentary about a manned voyage through the solar system. Space Odyssey premiered in 2004 and was made by the BBC...

 (2004) the crew of the international spacecraft Pegasus perform an aerobraking in Jupiter's upper atmosphere
Atmosphere of Jupiter
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide and water. Although water is...

to slow them down enough to enter Jovian orbit.