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An orbiting cannon ball showing various sub-orbital and orbital possibilities.

Various earth orbits drawn to scale; the inner cyan (light blue) ring represents low Earth orbit (LEO).

Roughly half an orbit of the ISS

A low Earth orbit (LEO) is generally defined as an orbit below an altitude of 2,000 km. Given the rapid orbital decay of objects below approximately 200 km, the commonly accepted definition for LEO is between 160–2,000 km (100–1,240 miles) above the Earth's surface.^[1][2] With the exception of the lunar flights of the Apollo program, all human spaceflights have taken place in LEO (or were suborbital). The altitude record for a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 km. All manned space stations to date, as well as the majority of artificial satellites, have been in LEO.

Orbital characteristics[link]

Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere (approximately 80–500 km up) or exosphere (approximately 500 km and up), depending on orbit height. LEO is an orbit around Earth between the atmosphere and below the inner Van Allen radiation belt. The altitude is usually not less than 300 km because that would be impractical due to the larger atmospheric drag.

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times and have the lowest delta-v requirement of any orbit. Orbits with a high inclination angle are usually called polar orbits.

Higher orbits include medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation.

Human use[link]

The International Space Station is in a LEO that varies from 320 km (199 mi) to 400 km (249 mi) above the Earth's surface.^[3]

While a majority of artificial satellites are placed in LEO, making one complete revolution around the Earth in about 90 minutes, many communication satellites require geostationary orbits, and move at the same angular velocity as the Earth. Since it requires less energy to place a satellite into a LEO and the LEO satellite needs less powerful amplifiers for successful transmission, LEO is still used for many communication applications. Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage. Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. ENVISAT is one example of an Earth observation satellite that makes use of this particular type of LEO.

Although the Earth's pull due to gravity in LEO is not much less than on the surface of the Earth, people and objects in orbit experience weightlessness due to the effects of freefall.

The speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude. The delta-v needed to achieve low earth orbit starts around 9.4km/s. Atmospheric and gravity drag associated with launch typically adds 1.5–2.0 km/s to the delta-v launch vehicle required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h). The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sails, a proposed propulsion system for interplanetary travel.

Space debris[link]

The LEO environment is becoming congested with space debris. This has caused growing concern in recent years, since collisions at orbital velocities can be damaging or dangerous, and can produce more space debris in the process (Kessler Syndrome). The Joint Space Operations Center, part of United States Strategic Command (formerly the United States Space Command), currently tracks more than 8,500 objects larger than 10 cm in LEO,^[4] however a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters,^[5] which are too small to be visible from Earth.^[6]

See also[link]

	Spaceflight portal
	Astronomy portal
	Solar System portal
	Space portal

• List of orbits
• Escape velocity
• Medium Earth orbit (MEO)
• High Earth orbit (HEO)
• Highly Elliptical Orbit (HEO)
• Specific orbital energy examples
• International Space Station
• Atmospheric reentry
• Satellite phone
• Suborbital spaceflight

References[link]

Broken link. Use: http://www.iadc-online.org/index.cgi?item=docs_pub instead, then choose "Space Debris Mitigation Guidelines".

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations. (April 2009)

A geocentric orbit involves any object orbiting the Earth, such as the Moon or artificial satellites. Currently there are approximately 2,465 artificial satellites orbiting the Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. Over 16,291 previously launched objects have decayed into the Earth's atmosphere.

List of terms and concepts[link]

Altitude

as used here, the height of an object above the average surface of the Earth's oceans.

Analemma

a term in astronomy used to describe the plot of the positions of the Sun on the celestial sphere throughout one year. Closely resembles a figure-eight.

Apogee

is the farthest point that a satellite or celestial body can go from Earth, at which the orbital velocity will be at its minimum.

Eccentricity

a measure of how much an orbit deviates from a perfect circle. Eccentricity is strictly defined for all circular and elliptical orbits, and parabolic and hyperbolic trajectories.

Equatorial plane

as used here, an imaginary plane extending from the equator on the Earth to the celestial sphere.

Escape velocity

as used here, the minimum velocity an object without propulsion needs to have to move away indefinitely from the Earth. An object at this velocity will enter a parabolic trajectory; above this velocity it will enter a hyperbolic trajectory.

Impulse

the integral of a force over the time during which it acts. Measured in (N·sec or lb * sec).

Inclination

the angle between a reference plane and another plane or axis. In the sense discussed here the reference plane is the Earth's equatorial plane.

Orbital characteristics

the six parameters of the Keplerian elements needed to specify that orbit uniquely.

Orbital period

as defined here, time it takes a satellite to make one full orbit about the Earth.

Perigee

is the nearest approach point of a satellite or celestial body from Earth, at which the orbital velocity will be at its maximum.

Sidereal day

the time it takes for a celestial object to rotate 360°. For the Earth this is: 23 hours, 56 minutes, 4.091 seconds.

Solar time

as used here, the local time as measured by a sundial.

Velocity

an object's speed in a particular direction. Since velocity is defined as a vector, both speed and direction are required to define it.

Geocentric orbit types[link]

The following is a list of different geocentric orbit classifications.

Altitude classifications[link]

Low (cyan) and Medium (yellow) Earth orbit regions to scale. The black dashed line is the geosynchronous orbit. The green dashed line is the 20,230 km orbit used for GPS satellites.

Low Earth Orbit (LEO) - Geocentric orbits ranging in altitude from 160 kilometeres (100 statue miles) to 2,000 kilometres (1,200 mi) above mean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8,000 metres per second (26,000 ft/s).

Medium Earth Orbit (MEO) - Geocentric orbits with altitudes at apogee ranging between 2,000 kilometres (1,200 mi) and that of the geosynchronous orbit at 35,786 kilometres (22,236 mi).

Geosynchronous Orbit (GEO) - Geocentric circular orbit with an altitude of 35,786 kilometres (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3,000 metres per second (9,800 ft/s).

High Earth Orbit (HEO) - Geocentric orbits with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is the highly elliptical orbit, where altitude at perigee is less than 2,000 kilometres (1,200 mi).^[1]

Inclination classifications[link]

Inclined orbit - An orbit whose inclination in reference to the equatorial plane is not 0.

Polar orbit - A satellite that passes above or nearly above both poles of the planet on each revolution. Therefore it has an inclination of (or very close to) 90 degrees.

Polar sun synchronous orbit - A nearly polar orbit that passes the equator at the same local time on every pass. Useful for image-taking satellites because shadows will be the same on every pass.

Eccentricity classifications[link]

Circular orbit - An orbit that has an eccentricity of 0 and whose path traces a circle.

Elliptic orbit - An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.

Hohmann transfer orbit - An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.

Geosynchronous transfer orbit - A geocentric-elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.

Highly elliptical orbit (HEO) - Geocentric orbit with apogee above 35,786 km and low perigee (about 1,000 km) that result in long dwell times near apogee.

Molniya orbit - A highly elliptical orbit with inclination of 63.4° and orbital period of ½ of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over a designated area of the Earth.

Tundra orbit - A highly elliptical orbit with inclination of 63.4° and orbital period of one sidereal day (roughly 24 hours). Such a satellite spends most of its time over a designated area of the Earth.

Hyperbolic trajectory - An "orbit" with eccentricity greater than 1. The object's velocity at perigee reaches some value in excess of the escape velocity, therefore it will escape the gravitational pull of the Earth and continue to travel infinitely with a velocity (relative to Earth) decelerating to some finite value, known as the hyperbolic excess velocity.

Escape Trajectory - This trajectory must be used to launch an interplanetary probe away from Earth, because the excess over escape velocity is what changes its heliocentric orbit from that of Earth.

Capture Trajectory - This is the mirror image of the escape trajectory; an object traveling with sufficient speed, not aimed directly at Earth, will move toward it and accelerate. In the absence of a decelerating engine impulse to put it into orbit, it will follow the escape trajectory after periapsis.

Parabolic trajectory - An "orbit" with eccentricity exactly equal to 1. The object's velocity at perigee equals the escape velocity, therefore it will escape the gravitational pull of the Earth and continue to travel with a velocity (relative to Earth) decelerating to 0. A spacecraft launched from Earth with this velocity would travel some distance away from it, but follow it around the Sun in the same heliocentric orbit. It is possible, but not likely that an object approaching Earth could follow a parabolic capture trajectory, but speed and direction would have to be precise.

Directional classifications[link]

Prograde orbit - an orbit in which the projection of the object onto the equatorial plane revolves about the Earth in the same direction as the rotation of the Earth.

Retrograde orbit - an orbit in which the projection of the object onto the equatorial plane revolves about the Earth in the direction opposite that of the rotation of the Earth.

Geosynchronous classifications[link]

Semi-synchronous orbit (SSO) - An orbit with an altitude of approximately 20,200 km (12,600 mi) and an orbital period of approximately 12 hours

Geosynchronous orbit (GEO) - Orbits with an altitude of approximately 35,786 km (22,236 mi). Such a satellite would trace an analemma (figure 8) in the sky.

Geostationary orbit (GSO): A geosynchronous orbit with an inclination of zero. To an observer on the ground this satellite would appear as a fixed point in the sky.

Clarke orbit - Another name for a geostationary orbit. Named after the writer Arthur C. Clarke.

Earth orbital libration points: The libration points for objects orbiting Earth are at 105 degrees west and 75 degrees east. More than 160 satellites are gathered at these two points.^[2]

Supersynchronous orbit - A disposal / storage orbit above GSO/GEO. Satellites will drift west.

Subsynchronous orbit - A drift orbit close to but below GSO/GEO. Satellites will drift east.

Graveyard orbit - An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation.

Disposal orbit - A synonym for graveyard orbit.

Junk orbit - A synonym for graveyard orbit.

Special classifications[link]

Sun-synchronous orbit - An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planet's surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites.

Moon orbit - The orbital characteristics of Earth's Moon. Average altitude of 384,403 kilometres (238,857 mi), elliptical–inclined orbit.

Non-geocentric classifications[link]

Horseshoe orbit - An orbit that appears to a ground observer to be orbiting a planet but is actually in co-orbit with it. See asteroids 3753 (Cruithne) and 2002 AA₂₉.

Exo-orbit - A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.

Sub-orbital spaceflight - A synonym for Exo-orbit.

[edit] Earth orbits

See also[link]

Spaceflight portal

• Earth's orbit
• List of orbits
• Astrodynamics
• Celestial sphere
• Heliocentric orbit
• Areosynchronous satellite
• Areostationary satellite
• Escape velocity
• Satellite
• Space station

References[link]

• "Satellite Situation Report". NASA Goddard Space Flight Center. 2000-02-01. Archived from the original on 2006-08-23. http://web.archive.org/web/20060823011134/http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/ssr.html. Retrieved 2006-09-10. 
• http://www.freemars.org/jeff/speed/index.htm
• http://www.tech-faq.com/medium-earth-orbit.shtml
• http://www.hq.nasa.gov/office/pao/History/conghand/traject.htm
• http://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html
• http://www.astro.uwo.ca/~wiegert/3753/3753.html
• http://www.astro.uwo.ca/~wiegert/AA29/AA29.html

Chris Austin Hadfield
CSA Astronaut
Nationality	Canadian
Status	Active
Born	(1959-08-29) 29 August 1959 (age 52) Sarnia, Ontario
Other occupation	Test pilot
Rank	Colonel, RCAF
Time in space	20d 02h 02m
Selection	1992 CSA Group
Total EVAs	2
Total EVA time	14 hours 50 minutes
Missions	STS-74, STS-100
Mission insignia

Chris Austin Hadfield, O.Ont, MSC, CD (born 29 August 1959) is a Canadian astronaut from the Canadian Space Agency (CSA) who was the first Canadian to walk in space. Hadfield has flown two space shuttle missions, STS-74 in 1995 and STS-100 in 2001. He has served as CAPCOM for both Space Shuttle and International Space Station (ISS) expeditions. He is currently training for a long duration stay on board the ISS, which will include command of Expedition 35 in 2012–13. He will be the first Canadian to command the ISS.^[1] He is enthusiastic about the prospects for a manned mission to Mars, and when asked if he would consider a one-way journey to Mars to be the first to visit, he said "I would be honoured to be given the opportunity."^[2]

Personal life[link]

Chris Hadfield was born in Sarnia, Ontario. His parents are Roger and Eleanor Hadfield who currently reside in Milton, Ontario. Hadfield was raised on a corn farm in southern Ontario and became interested in flying at a young age—from his own words in the interview to STRF.RU, he came to the idea of being astronaut when he was nine on the day of Apollo moon landing, which he had seen on TV then. Hadfield is married to his high-school girlfriend Helene Hadfield, and they have three adult children together.

Education[link]

Chris graduated as an Ontario Scholar from Milton District High School in 1977. After graduating from high school he went on to earn a bachelor degree in mechanical engineering at the Royal Military College of Canada in Kingston, Ontario in 1982. Hadfield conducted post-graduate research on aviation systems at the University of Tennessee in 1993.

Special honours and affiliations[link]

Chris Hadfield is the recipient of numerous awards and special honours. These include the 1988 Liethen-Tittle Award (top pilot graduate of the USAF Test Pilot School) and U.S. Navy Test Pilot of the Year (1991). He received an honorary Doctorate of Engineering from the Royal Military College (1996). Hadfield is a member of the Order of Ontario (1996). He also received an honorary Doctorate of Laws from Trent University (1999). Chris has also received Vanier Award (2001), Meritorious Service Cross (2001), NASA Exceptional Service Medal (2002) and the Queen's Golden Jubilee Medal (2003). He was inducted into Canada's Aviation Hall of Fame (2005) and Commemorated on Royal Canadian Mint silver and gold coins for his spacewalk to install Canadarm2 on the International Space Station (2001).

As an Air Cadet, he earned a glider pilot scholarship at age 15 and a powered pilot scholarship at age 16.

His affiliations include the Royal Military College Club, Society of Experimental Test Pilots, Canadian Aeronautics and Space Institute, honorary patron of Lambton College, trustee of Lakefield College School, Board member of the International Space School Foundation, executive with the Association of Space Explorers.

Sarnia Airport was renamed in his honour in 1997.^[3]

Military experience[link]

Hadfield joined the Canadian Forces in May 1978. He spent two years at Royal Roads Military College, in Victoria, British Columbia followed by two years at the Royal Military College of Canada in Kingston, Ontario where he received a bachelor's degree in Mechanical Engineering (with honors) in 1982. Hadfield underwent basic flight training for the Royal Canadian Air Force at CFB Portage La Prairie in Portage La Prairie, Manitoba for which he was named top pilot in 1980. In 1983, he took honors as the overall top graduate from Basic Jet Training at CFB Moose Jaw in Moose Jaw, Saskatchewan and in 1984–1985, he trained as a fighter pilot at CFB Cold Lake in Cold Lake, Alberta on CF-5s and CF-18s.

For the next three years Hadfield flew CF-18s for the North American Aerospace Defence Command (NORAD) with 425 Squadron, during which time he flew the first CF-18 intercept of a Soviet Tupolev Tu 95 "Bear" aircraft. He attended the United States Air Force (USAF) Test Pilot School at Edwards Air Force Base, in California, and upon graduation, served as an exchange officer with the U.S. Navy at Strike Test Directorate at the Patuxent River Naval Air Station. His accomplishments from 1989 to 1992 included testing the F/A-18 and A-7 aircraft; performing research work with NASA on pitch control margin simulation and flight; completing the first military flight of F/A-18 enhanced performance engines; piloting the first flight test of the National Aerospace Plane external burning hydrogen propulsion engine; developing a new handling qualities rating scale for high angle-of-attack test; and participating in the F/A-18 out-of-control recovery test program. In total, Hadfield has flown over 70 different types of aircraft.

NASA experience[link]

Chris Hadfield in 2012

Chris Hadfield was selected to become one of four new Canadian astronauts from a field of 5,330 applicants in June 1992. Three of those 4 (Dafydd Williams, Julie Payette and Hadfield) have flown in space. Hadfield was assigned by the Canadian Space Agency (CSA) to the NASA Johnson Space Center in Houston, Texas in August of the same year, where he addressed technical and safety issues for Shuttle Operations Development, contributed to the development of the glass shuttle cockpit, and supported shuttle launches at the Kennedy Space Center, in Florida. In addition, Hadfield was NASA's Chief CAPCOM, the voice of mission control to astronauts in orbit, for 25 space shuttle missions. From 1996 to 2000, he represented CSA astronauts and coordinated their activities as the Chief Astronaut for the CSA.^[4]

Chris Hadfield was the Director of Operations for NASA at the Yuri Gagarin Cosmonauts Training Center (GCTC) in Star City, Russia from 2001 until 2003. Some of his duties included coordination and direction of all International Space Station crew activities in Russia, oversight of training and crew support staff, as well as policy negotiation with the Russian Space Program and other International Partners. He also trained and became fully qualified to be a flight engineer cosmonaut in the Soyuz TMA spacecraft, and to perform spacewalks in the Russian Orlan spacesuit.

Hadfield is a civilian CSA astronaut, having retired as a Colonel from the Canadian Air Force in 2003 after 25 years of military service. He was Chief of Robotics for the NASA Astronaut Office at the Johnson Space Center in Houston, Texas from 2003–2006 and was Chief of International Space Station Operations from 2006–2008. In 2008 and 2009, Hadfield trained as a back-up to Robert Thirsk on the Expedition 21 mission.^[5] In May 2010, Hadfield served as the commander of the NEEMO 14 mission aboard the Aquarius underwater laboratory, living and working underwater for fourteen days.^[6][7] NASA has announced that Hadfield will become the first Canadian commander of the International Space Station, leading Expedition 35, when he blasts off in December 2012.^[8]

Space flights[link]

STS-74[link]

Chris Hadfield served as Mission Specialist 1 on STS-74 in November 1995. It was NASA's second space shuttle mission to rendezvous and dock with the Russian Space Station Mir. During the flight, the crew of Space Shuttle Atlantis attached a five-tonne docking module to Mir and transferred over 1,000 kg of food, water, and scientific supplies to the cosmonauts. Hadfield flew as the first Canadian to operate the Canadarm in orbit, and the only Canadian ever to board Mir.^[4]

STS-100[link]

Hadfield spacewalking during the STS-100 mission

In April 2001 Hadfield served as Mission Specialist 1 on STS-100, International Space Station (ISS) assembly Flight 6A. The crew of Space Shuttle Endeavour delivered and installed Canadarm2, the new Canadian-built robotic arm, as well as the Italian-made resupply module Raffaello. During the 11-day flight, Hadfield performed two spacewalks, which made him the first Canadian to ever leave a spacecraft and float freely in space. In total, Hadfield spent 14 hours, 50 minutes outside, traveling 10 times around the world during his spacewalk.^[9]

See also[link]

Listen to this article (info/dl)

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• Canadian space program

References[link]

 This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.

Books[link]

• H16511 Dr. Richard Arthur Preston "Canada's RMC – A History of Royal Military College" Second Edition 1982
• H1877 R. Guy C. Smith (editor) "As You Were! Ex-Cadets Remember". In 2 Volumes. Volume I: 1876–1918. Volume II: 1919–1984. RMC. Kingston, Ontario. The R.M.C. Club of Canada. 1984

External links[link]

Wikimedia Commons has media related to: Chris Hadfield

• NASA astronaut biography
• CSA astronaut biography
• Chris Hadfield on Twitter
• Spacefacts biography of Chris Hadfield
• Video of Chris Hadfield on The Agenda with Steve Paikin, "Are We Bound for Space?" panel discussion with Lawrence Krauss, Donna Shirley, Chris McKay, Karl Schroeder and Robert D. Richards