• orbit
    An orbit is the path of an object around a more massive center of gravity.

    Photograph by Mark Thiessen

    Pluto
    Pluto, a dwarf planet on the edge of our solar system, takes a strange orbit around the sun. Pluto's eccentricity is also much higher than any planet in the solar system, at .249. This is partly why Pluto, an official planet until 2006, was downgraded to a dwarf planet.

    Edge of Orbit
    Voyager II is a spacecraft launched by the United States in 1977. Voyager II passed through the heliosheath, the edge of the sun's gravitational pull, in 2007. Voyager II is nearly outside the Sun's orbit.

    Space Junk
    There are more pieces of space junk orbiting the Earth than useful satellites. Space junk is material from satellites, rockets, or other spacecraft that no longer works.

    Clarke Orbit
    The idea for geostationary orbit was outlined in a 1945 paper by the scientist and science-fiction author Arthur C. Clarke. For this reason, geostationary orbit is sometimes called "Clarke orbit."

    An orbit is a regular, repeating path that one object takes around another object or center of gravity. Orbiting objects, which are called satellites, include planets, moons, asteroids, and manmade devices.

    Objects orbit each other because of gravity. Gravity is the force that exists between any two objects with mass. Every object, from the smallest subatomic particle to the largest star, has mass. The more massive the object, the larger its gravitational pull. Gravitational pull is the amount of force one object exerts on another object.

    The sun is the most massive object in our solar system. All of the other objects in the solar system are subject to the gravitational pull of the sun.

    Many satellites orbit on orbital planes. An orbital plane is a flat, disk-shaped space that connects the center of the object being orbited with the center of orbiting objects. Because all planets in our solar system share a similar orbital plane, planets dont run in to each other.

    All the planets in our solar system line up with each other on the same general orbital plane. However, sometimes orbital paths of other objects in the solar system intersect, and the objects can collide. Comet Tempel-Tuttle, for instance, passes through Earths orbit. The debris from the tail of this comet passes through Earths atmosphere as meteors, or falling stars, at a specific time every year. The debris from the comet's orbit is called the Leonid meteor shower.

    The time it takes for an object to orbit around another object is called its orbital period. Earth completes its orbital period around the sun every 365 days. The further away a planet is from the sun, the longer its orbital period. The planet Neptune, for example, takes almost 165 years to orbit the sun.

    Each orbit has its own eccentricity. Eccentricity is the amount an orbits path differs from a perfect circle. A perfect circle has an eccentricity of zero. Earths eccentricity is .017. Mercury has the largest eccentricity of all the planets in the solar system, at .206.

    Types of Orbits

    Moons orbit planets, while planets orbit the sun. Our entire solar system orbits around the black hole at the center of our galaxy, the Milky Way. There are three major types of orbits: galactocentric orbits, heliocentric orbits, and geocentric orbits. Objects with geocentric orbits have their own types.

    A galactocentric orbit is an orbit that goes around the center of a galaxy. Our solar system follows this type of orbit around the Milky Way.

    A heliocentric orbit is one that goes around the sun. All the planets in our solar system, along with all the asteroids in the Asteroid Belt and all comets, follow this kind of orbit. Each planet's orbit is regular: they follow certain paths and take a certain amount of time to make one complete orbit. The planet Mercury completes its short heliocentric orbit every 88 days. Comet Kohoutek may take 100,000 years to complete its long heliocentric orbit.


    A geocentric orbit is one that goes around the Earth. Our moon follows a geocentric orbit, and so do most manmade satellites. The Moon is Earth's only natural satellite. It takes about 27 days for the moon to complete its orbital period around the Earth. There are three major types of geocentric orbits: low-Earth orbit (LEO), medium-Earth orbit (MEO), and geostationary orbit.

    Low-Earth orbit exists between 160 kilometers (100 miles) and 2,000 kilometers (1,240 miles) above Earth's surface. Most artificial satellites with human crews are in low-Earth orbit. The orbital period for objects in LEO is about 90 minutes.

    Medium-Earth orbit exists between 2,000 kilometers (1,243 miles) and 36,000 kilometers (23,000 miles) above the Earths surface. Satellites in MEO are at greater risk for damage, because they are exposed to powerful radiation from the sun. Satellites in MEO include global positioning system (GPS) and communication satellites. MEO satellites can orbit the Earth in about two hours.

    Satellites in geostationary orbit circle the Earth directly above the Equator. These satellites have geosynchronous orbits, or move at the same rotation of the Earth. Therefore, the orbital period of geosynchronous satellites is 24 hours.

    Geostationary satellites are useful because they appear as a fixed point in the sky. Antennae pointed toward the geostationary satellite will have a clear signal unless objects in the atmosphere (such as storm clouds) between Earth and the satellite interfere. Most weather satellites are geostationary and provide images of Earths atmosphere.

    Satellite Orbits

    Manmade satellites are sent to orbit the Earth to collect information we can only assemble from above the atmosphere. The first satellite, Sputnik, was launched by the Soviet Union in 1957. Today, thousands of satellites orbit the Earth. Weather satellites provide images of weather patterns for meteorologists to study. Communication satellites connect cell phone users and GPS receivers. Military satellites track movement of weapons and troops from different countries.

    Sometimes, manmade satellites have people on them. The most famous manmade satellite is the International Space Station (ISS). Astronauts from all over the world stay on the ISS for months at a time as it orbits the Earth. Astronomers and stargazers can see the ISS and other satellites as they orbit through telescopes and even powerful binoculars.

    Not all artificial satellites orbit the Earth. Some orbit other planets. The Cassini-Huygens mission, for instance, is studying the planet Saturn. The project has a spacecraft, Cassini, in orbit around Saturn.

    Putting satellites into orbit is complex and costly. Few governments can afford large space programs. Manmade satellites from the United States are sent into orbit by the National Aeronautics and Space Administration, or NASA. The European Space Agency (ESA) sends up satellites from countries in the European Union. The Russian Federal Space Agency (Roscosmos), the Japanese Space Agency (JSA), and the Iranian Space Agency (ISA) have all successfully put satellites into orbit.

    Satellites are put into orbit from spaceports, which are carefully constructed for that purpose. The Baikonur Cosmodrome in Kazakhstan and the Kennedy Space Center in the U.S. state of Florida are both well-known spaceports.

  • Term Part of Speech Definition Encyclopedic Entry
    antenna Noun

    structure through which electromagnetic signals are received.

    Arthur C. Clarke Noun

    (1917-2008) British scientist and author.

    asteroid Noun

    irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.

    asteroid belt Noun

    area of the solar system between the orbits of Mars and Jupiter filled with asteroids.

    astronaut Noun

    person who takes part in space flights.

    astronomer Noun

    person who studies space and the universe beyond Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    black hole Noun

    region of space where the gravitational field is so strong even light cannot escape.

    Cassini-Huygens Noun

    mission to study the planet Saturn and its moons.

    comet Noun

    celestial object of matter surrounded by ice and dust that orbits the sun and leaves a tail of debris.

    Comet Kohoutek Noun

    comet that orbits the sun about every 100,000 years; discovered in 1973.

    Comet Tempel-Tuttle Noun

    comet that orbits the sun every 33 years.

    communication satellite Noun

    instrument that orbits the Earth to connect devices such as cell phones, GPS units, and television broadcasts.

    complex Adjective

    complicated.

    construct Verb

    to build or erect.

    debris Noun

    remains of something broken or destroyed, waste, or garbage.

    Earth Noun

    our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

    Encyclopedic Entry: Earth
    eccentricity Noun

    amount an orbit's path differs from a perfect circle.

    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    European Union Noun

    association of European nations promoting free trade, ease of transportation, and cultural and political links.

    exert Verb

    to force or pressure.

    galactocentric orbit Noun

    regular path around the center of a galaxy.

    galaxy Noun

    collection of stars, planets, gases, and other celestial bodies bound together by gravity.

    geostationary orbit Noun

    orbit around the Earth directly above the Equator.

    geosynchronous orbit Noun

    orbit that moves at the same speed as the Earth's rotation.

    Global Positioning System (GPS) Noun

    system of satellites and receiving devices used to determine the location of something on Earth.

    government Noun

    system or order of a nation, state, or other political unit.

    GPS receiver Noun

    device that gets radio signals from satellites in orbit above Earth in order to calculate a precise location.

    gravitational pull Noun

    the physical attraction between two objects.

    gravity Noun

    physical force by which objects attract, or pull toward, each other.

    heliocentric orbit Noun

    regular path around the sun.

    heliosheath Noun

    outer edge of the solar system where the sun loses its gravitational pull.

    intercontinental ballistic missile (ICBM) Noun

    projectile launched into sub-orbit above the Earth and carrying nuclear weapons.

    International Space Station (ISS) Noun

    satellite in low-Earth orbit that houses several astronauts for months at a time.

    intersect Verb

    to cross paths with.

    Leonid meteor shower Noun

    annual event, usually in November, when debris from the tail of Comet Tempel-Tuttle fall as meteors. Also called the Leonids.

    low-Earth orbit Noun

    between 160 kilometers (100 miles) and 2,000 kilometers (1,240 miles) above Earth's surface.

    mass Noun

    unit of measurement (abbreviated m) determined by an object's resistance to change in the speed or direction of motion.

    medium-Earth orbit Noun

    between 2,000 kilometers (1,243 miles) and 36,000 kilometers (22,370 miles) above the Earths surface.

    meteor Noun

    rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star.

    Encyclopedic Entry: meteor
    meteorologist Noun

    person who studies patterns and changes in Earth's atmosphere.

    military satellite Noun

    instrument that orbits the Earth to track the movement of troops, weapons, and information.

    Milky Way Noun

    galaxy in which the Earth and sun are located.

    moon Noun

    natural satellite of a planet.

    Encyclopedic Entry: moon
    Moon Noun

    Earth's only natural satellite.

    NASA Noun

    (acronym for National Aeronautics and Space Administration) U.S. agency responsible for space research and systems.

    Neptune Noun

    eighth planet from the sun in our solar system.

    nuclear weapon Noun

    explosive device that draws power from the splitting and combining of atomic nuclei.

    orbit Noun

    path of one object around a more massive object.

    orbital period Noun

    time it takes for an object to complete an orbit.

    orbital plane Noun

    flat space in which a body orbits.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    Pluto Noun

    dwarf planet in our solar system.

    radiation Noun

    energy, emitted as waves or particles, radiating outward from a source.

    rocket Noun

    device that moves through the atmosphere by release of expanding gas.

    rotation Noun

    object's complete turn around its own axis.

    Encyclopedic Entry: rotation
    satellite Noun

    object that orbits around something else. Satellites can be natural, like moons, or made by people.

    Saturn Noun

    sixth planet from the sun.

    solar system Noun

    the sun and the planets, asteroids, comets, and other bodies that orbit around it.

    Soviet Union Noun

    (1922-1991) large northern Eurasian nation that had a communist government. Also called the Union of Soviet Socialist Republics, or the USSR.

    spacecraft Noun

    vehicle designed for travel outside Earth's atmosphere.

    space junk Noun

    material orbiting Earth that is no longer working or useful.

    spaceport Noun

    facility for launching vehicles or capsules into space.

    Sputnik Noun

    (1957) first artifical satellite, launched by the Soviet Union, from Earth.

    star Noun

    large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

    subatomic particle Noun

    unit that is part of an atom, such as quarks.

    sun Noun

    star at the center of our solar system.

    tail Noun

    stream of gas or dust debris behind a comet.

    telescope Noun

    scientific instrument that uses mirrors to view distant objects.

    troop Noun

    a soldier.

    universe Noun

    all known matter, energy, and space.

    Voyager II Noun

    (1977) satellite launched to study the outer solar system.

    weather satellite Noun

    instrument that orbits the Earth to track weather and patterns in the atmosphere.

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