What Causes Space Junk?

Debris in orbit can come from many sources:

• Exploding rockets - This leaves behind the most debris in space.
• The slip of an astronaut’s hand - Suppose an astronaut doing repair in space drops a wrench — it’s gone forever. The wrench then goes into orbit, probably at a speed of something like 6 miles per second. If the wrench hits any vehicle carrying a human crew, the results could be disastrous. Larger objects like a space station make a larger target for space junk, and so are at greater risk.
• Jettisoned items - Parts of launch canisters, camera lens caps, etc.

Items initially placed into high orbits stay in space the longest.The European Space Agency tracks more than 7,500 orbiting items with a width of 4 inches (10 cm) or more. Space debris may also be a reason why space shuttles typically orbit with their windows to the rear. This protects the astronauts onboard, at least to some degree.

A special NASA satellite called Long Duration Exposure Facility (LDEF) was put in orbit to study the long-term effects of collisions with space junk. The LDEF was later brought back to Earth via a space shuttle for analysis.

For more information on satellites and related topics, check out the links on the next page.

How Can I See an Overhead Satellite?

This satellite tracking Web site shows how you can see a satellite overhead, thanks to the German Space Operations Center. You will need your coordinates for longitude and latitude, available from the USGS Mapping Information Web site or at the website Topozone.

• Satellite-tracking software is available for predicting orbit passes. Note the exact times.
• Use binoculars on a clear night when there is not a bright moon.
• Ensure that your watch is set to exactly match a known time standard.
• A north-south orbit often indicates a spy satellite!

Satellite Altitudes

Looking up from Earth, satellites are orbiting overhead in various bands of altitude. It’s interesting to think of satellites in terms of how near or far they are from us. Proceeding roughly from the nearest to the farthest, here are the types of satellites whizzing around Earth:80 to 1,200 miles - Asynchronous Orbits

Photo courtesy USGS The island of Manhattan in New York City (Central Park at the top)

Observation satellites, typically orbiting at altitudes from 300 to 600 miles (480 to 970 km), are used for tasks like photography. Observation satellites such as the Landsat 7 perform tasks such as:

• Mapping
• Ice and sand movement
• Locating environmental situations (such as disappearing rainforests)
• Locating mineral deposits
• Finding crop problems

Search-and-rescue satellites act as relay stations to rebroadcast emergency radio-beacon signals from a downed aircraft or ship in trouble.The Space Shuttle is the familiar manned satellite, usually with a fixed duration and number of orbits. Manned missions often have the task of repairing existing expensive satellites or building future space stations.

Photo courtesy NASA The “glass cockpit” on Space Shuttle Atlantis, March 1, 2000

Teledesic, with the financial backing of Bill Gates, promises broadband (high-speed) communications using many planned low Earth orbiting (LEO) satellites.

3,000 to 6,000 miles - Asynchronous Orbits
Science satellites are sometimes in altitudes of 3,000 to 6,000 miles (4,800 to 9,700 km). They send their research data to Earth via radio telemetry signals. Scientific satellite applications include:

• Researching plants and animals
• Earth science, such as monitoring volcanoes
• Tracking wildlife
• Astronomy, using the Infrared Astronomy Satellite
• Physics, by NASA’s future study of microgravity and the current Ulysses Mission studying solar physics

6,000 to 12,000 miles - Asynchronous Orbits
For navigation, the U.S. Department of Defense built the Global Positioning System, or GPS. The GPS uses satellites at altitudes of 6,000 to 12,000 miles to determine the exact location of the receiver. The GPS receiver may be located:

• In a ship at sea
• In another spacecraft
• In an airplane
• In an automobile
• In your pocket

As consumer prices for GPS receivers come down, the familiar paper map may face tough competition. No more getting lost leaving the rental car agency at an unfamiliar airport!

• The U.S. military and the forces of allied nations used more than 9,000 GPS receivers during Operation Desert Storm.
• The National Oceanic and Atmospheric Administration (NOAA) used GPS to measure the exact height of the Washington Monument.

Photo courtesy NASA Taken while the Clementine spacecraft was orbiting the moon

Photo courtesy NASA Advanced Communications Technology Satellite, launched in 1993, used multiple antennas for narrow-beam transmissions.

22,223 Miles - Geostationary Orbits
Weather forecasts visually bombard us each day with images from weather satellites, typically 22,223 miles over the equator. You can directly receive many of the actual satellite images using radio receivers and special personal-computer software. Many countries use weather satellites for their weather forecasting and storm observations.

Data, television, image and some telephone transmissions are routinely received and rebroadcast by communications satellites. Typical satellite telephone links have 550 to 650 milliseconds of round-trip delay that contribute to consumer dissatisfaction with this type of long-distance carrier. It takes the voice communications that long to travel all the way up to the satellite and back to Earth. The round-trip delay forces many to use telephone conversations via satellite only when no other links exist. Currently, voice over the Internet is experiencing a similar delay problem, but in this case due to digital compression and bandwidth limitations rather than distance.

Communications satellites are essentially radio relay stations in space. Satellite dishes get smaller as satellites get more powerful transmitters with focused radio “footprints” and gain-type antennas. Subcarriers on these same satellites carry:

• Press agency news feeds
• Stock market, business and other financial information
• International radio broadcasters moving from short-wave to (or supplementing their short-wave broadcasts with) satellite feeds using microwave uplink feeds
• Global television, such as CNN and the BBC
• Digital radio for CD-quality audio

What Are the Types of Satellite Orbits?

There are three basic kinds of orbits, depending on the satellite’s position relative to Earth’s surface:

• Geostationary orbits (also called geosynchronous or synchronous) are orbits in which the satellite is always positioned over the same spot on Earth. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The “satellite parking strip” area over the equator is becoming congested with several hundred television, weather and communication satellites! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite’s signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.
• The scheduled Space Shuttles use a much lower, asynchronous orbit, which means they pass overhead at different times of the day. Other satellites in asynchronous orbits average about 400 miles (644 km) in altitude.
• In a polar orbit, the satellite generally flies at a low altitude and passes over the planet’s poles on each revolution. The polar orbit remains fixed in space as Earth rotates inside the orbit. As a result, much of Earth passes under a satellite in a polar orbit. Because polar orbits achieve excellent coverage of the planet, they are often used for satellites that do mapping and photography.

How Are Satellite Orbits Predicted?
Special satellite software, available for personal computers, predicts satellite orbits. The software uses Keplerian data to forecast each orbit and shows when a satellite will be overhead. The latest “Keps” are available on the Internet for amateur radio satellites, too.

Satellites use a variety of light-sensitive sensors to determine their position. The satellite transmits its position to the ground station.

What is Inside a Typical Satellite?

Satellites come in all shapes and sizes and play a variety of roles. For example:

• Weather satellites help meteorologists predict the weather or see what’s happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth’s weather, either from fixed geostationary positions or from polar orbits.
• Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder — a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.
• Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
• Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.
• Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.
• Rescue satellites respond to radio distress signals (read this page for details).
• Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.
• Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. Applications may include:
  • Relaying encrypted communications
  • Nuclear monitoring
  • Observing enemy movements
  • Early warning of missile launches
  • Eavesdropping on terrestrial radio links
  • Radar imaging
  • Photography (using what are essentially large telescopes that take pictures of militarily interesting areas)

Despite the significant differences between all of these satellites, they have several things in common. For example:

• All of them have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch.
• All of them have a source of power (usually solar cells) and batteries for storage.Arrays of solar cells provide power to charge rechargeable batteries. Newer designs include the use of fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets (read this page for details). Power systems are constantly monitored, and data on power and all other onboard systems is sent to Earth stations in the form of telemetry signals.
• All of them have an onboard computer to control and monitor the different systems.
• All of them have a radio system and antenna. At the very least, most satellites have a radio transmitter/receiver so that the ground-control crew can request status information from the satellite and monitor its health. Many satellites can be controlled in various ways from the ground to do anything from change the orbit to reprogram the computer system.
• All of them have an attitude control system. The ACS keeps the satellite pointed in the right direction.The Hubble Space Telescope has a very elaborate control system so that the telescope can point at the same position in space for hours or days at a time (despite the fact that the telescope travels at 17,000 mph/27,359 kph!). The system contains gyroscopes, accelerometers, a reaction wheel stabilization system, thrusters and a set of sensors that watch guide stars to determine position.

What is a Satellite Launch Window?

A launch window is a particular period of time in which it will be easier to place the satellite in the orbit necessary to perform its intended function.With the Space Shuttle, an extremely important factor in choosing the launch window is the need to bring down the astronauts safely if something goes wrong. The astronauts must be able to reach a safe landing area where rescue personnel can be standing by. For other types of flights, including interplanetary exploration, the launch window must permit the flight to take the most efficient course to its very distant destination. If weather is bad or a malfunction occurs during a launch window, the flight must be postponed until the next launch window appropriate for the flight. If a satellite were launched at the wrong time of the day in perfect weather, the satellite could end up in an orbit that would not pass over any of its intended users. Timing is everything!

How is a Satellite Launched into an Orbit?

Photo courtesy Arianespace ARIANE 44L (four liquid strap-on boosters) at liftoff from French Guiana, October 1998

All satellites today get into orbit by riding on a rocket or by riding in the cargo bay of the Space Shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it safely into orbit on a regular basis.For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption.

After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments to the rocket’s nozzles to tilt the rocket to the course described in the flight plan. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around Earth is greatest and so rotation is fastest.

How big is the boost from an equatorial launch? To make a rough estimate, we can determine Earth’s circumference by multiplying its diameter by pi (3.1416). The diameter of Earth is approximately 7,926 miles (12,753 km). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 km). To travel around that circumference in 24 hours, a point on Earth’s surface has to move at 1,038 mph (1,669 kph). A launch from Cape Canaveral, Florida, doesn’t get as big a boost from Earth’s rotational speed. The Kennedy Space Center’s Launch Complex 39-A, one of its launch facilities, is located at 28 degrees 36 minutes 29.7014 seconds north latitude. The Earth’s rotational speed there is about 894 mph (1,440 kph). The difference in Earth’s surface speed between the equator and Kennedy Space Center, then, is about 144 mph (229 kph). (Note: The Earth is actually oblate — fatter around the middle — not a perfect sphere. For that reason, our estimate of Earth’s circumference is a little small.)

Considering that rockets can go thousands of miles per hour, you may wonder why a difference of only 144 mph would even matter. The answer is that rockets, together with their fuel and their payloads, are very heavy. For example, the February 11, 2000 lift-off of the Space Shuttle Endeavor with the Shuttle Radar Topography Mission required launching a total weight of 4,520,415 pounds (2,050,447 kg). It takes a huge amount of energy to accelerate such a mass to 144 mph, and therefore a significant amount of fuel. Launching from the equator makes a real difference.

Once the rocket reaches extremely thin air, at about 120 miles (193 km) up, the rocket’s navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.

What is a Satellite?

A satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth’s original, natural satellite, and there are many manmade (artificial) satellites, usually closer to Earth.

• The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee.
• Artificial satellites generally are not mass-produced. Most satellites are custom built to perform their intended functions. Exceptions include the GPS satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).
• Approximately 23,000 items of space junk — objects large enough to track with radar that were inadvertently placed in orbit or have outlived their usefulness — are floating above Earth. The actual number varies depending on which agency is counting. Payloads that go into the wrong orbit, satellites with run-down batteries, and leftover rocket boosters all contribute to the count. This online catalog of satellites has almost 26,000 entries!

Although anything that is in orbit around Earth is technically a satellite, the term “satellite” is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. We commonly hear about weather satellites, communication satellites and scientific satellites.

HistoryThe first known fictional depiction of a satellite being launched into orbit is a short story by Edward Everett Hale, The Brick Moon. The story was serialized in The Atlantic Monthly, starting in 1869.^[1][2] The idea surfaces again in Jules Verne’s The Begum’s Millions (1879).

In 1903 Konstantin Tsiolkovsky (1857–1935) published Исследование мировых пространств реактивными приборами (The Exploration of Cosmic Space by Means of Reaction Devices), which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit around the Earth at 8 km/second, and that a multi-stage rocket fueled by liquid propellants could be used to achieve this. He proposed the use of liquid hydrogen and liquid oxygen, though other combinations can be used. During his lifetime he published over 500 works on space travel and related subjects, including science fiction novels. Among his works are designs for rockets with steering thrusters, multi-stage boosters, space stations, airlocks for exiting a spaceship into the vacuum of space, and closed cycle biological systems to provide food and oxygen for space colonies. He also delved into theories of heavier-than-air flying machines, independently working through many of the same calculations that the Wright brothers were performing at about the same time.

In 1928 Herman Potočnik (1892–1929) published his sole book, Das Problem der Befahrung des Weltraums - der Raketen-Motor (The Problem of Space Travel — The Rocket Motor), a plan for a breakthrough into space and a permanent human presence there. He conceived of a space station in detail and calculated its geostationary orbit. He described the use of orbiting spacecraft for detailed peaceful and military observation of the ground and described how the special conditions of space could be useful for scientific experiments. The book described geostationary satellites (first put forward by Tsiolkovsky) and discussed communication between them and the ground using radio, but fell short of the idea of using satellites for mass broadcasting and as telecommunications relays.

In a 1945 Wireless World article the English science fiction writer Arthur C. Clarke (b. 1917) described in detail the possible use of communications satellites for mass communications.^[3] Clarke examined the logistics of satellite launch, possible orbits and other aspects of the creation of a network of world-circling satellites, pointing to the benefits of high-speed global communications. He also suggested that three geostationary satellites would provide coverage over the entire planet.

[edit] History of artificial satellites

Main article: Timeline of artificial satellites and space probes

See also: Space Race

The first artificial satellite was Sputnik 1, launched by the Soviet Union on 4 October 1957. This triggered the Space Race between the Soviet Union and the United States.

In May, 1946, Project RAND had released the Preliminary Design of an Experimental World-Circling Spaceship, which stated, “A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century.^[4] The United States had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. The United States Air Force’s Project RAND eventually released the above report, but did not believe that the satellite was a potential military weapon; rather, they considered it to be a tool for science, politics, and propaganda. In 1954, the Secretary of Defense stated, “I know of no American satellite program.”

On July 29, 1955, the White House announced that the U.S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On July 31, the Soviets announced that they intended to launch a satellite by the fall of 1957.

Following pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, military interest picked up and in early 1955 the Air Force and Navy were working on Project Orbiter, which involved using a Jupiter C rocket to launch a satellite. The project succeeded, and Explorer 1 became the United States’ first satellite on January 31, 1958.

The largest artificial satellite currently orbiting the Earth is the International Space Station.

[edit] Space Surveillance Network

The United States Space Surveillance Network (SSN) has been tracking space objects since 1957 when the Soviets opened the space age with the launch of Sputnik I. Since then, the SSN has tracked more than 26,000 space objects orbiting Earth. The SSN currently tracks more than 8,000 man-made orbiting objects. The rest have re-entered Earth’s turbulent atmosphere and disintegrated, or survived re-entry and impacted the Earth. The space objects now orbiting Earth range from satellites weighing several tons to pieces of spent rocket bodies weighing only 10 pounds. About seven percent of the space objects are operational satellites (i.e. ~560 satellites), the rest are space debris. USSTRATCOM is primarily interested in the active satellites, but also tracks space debris which upon reentry might otherwise be mistaken for incoming missiles. The SSN tracks space objects that are 10 centimeters in diameter (baseball size) or larger.

[edit] Non-Military Satellite Services

There are three basic categories of non-military satellite services:^[5]

[edit] Fixed Satellite Service

Fixed satellite services handle hundreds of millions of voice, data, and video transmission tasks across all continents between fixed points on the earth’s surface

[edit] Mobile Satellite Systems

Mobile satellite systems help connect remote regions, vehicles, ships and aircraft to other parts of the world and/or other mobile or stationary communications units, in addition to serving as navigation systems

[edit] Scientific Research Satellite (commercial and noncommercial)

Scientific research satellites provide us with meteorological information, land survey data (e.g., remote sensing), and other different scientific research applications such as earth science, marine science, and atmospheric research.

[edit] Types

MILSTAR: A communication satellite

• Anti-satellite weapons, sometimes called “Killer satellites” are satellites designed to destroy “enemy” satellites, other orbital weapons and targets. Some are armed with kinetic rounds, while others use energy and/or particle weapons to destroy satellites, ICBMs, MIRVs. Both the U.S. and the USSR had these satellites.
• Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
• Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation.
• Communications satellites are satellites stationed in space for the purpose of telecommunications. Modern communications satellites typically use geosynchronous orbits, Molniya orbits or Low Earth orbits.
• Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500–200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).
• Navigational satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few meters in real time.
• Reconnaissance satellites are Earth observation satellite or communications satellite deployed for military or intelligence applications. Little is known about the full power of these satellites, as governments who operate them usually keep information pertaining to their reconnaissance satellites classified.
• Earth observation satellites are satellites intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.)
• Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities — instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.
• Tether satellite.
• Weather satellites are primarily used to monitor Earth’s weather and climate.

[edit] Orbit types

Main article: List of orbits

[edit] Centric classifications

• Galacto-centric orbit: An orbit about the center of a galaxy. Earth’s sun follows this type of orbit about the galactic center of the Milky Way.
• Heliocentric orbit: An orbit around the Sun. In our Solar System, all planets, comets, and asteroids are in such orbits, as are many artificial satellites and pieces of space debris. Moons by contrast are not in a heliocentric orbit but rather orbit their parent planet.
• Geocentric orbit: An orbit around the planet Earth, such as the Moon or artificial satellites. Currently there are approximately 2465 artificial satellites orbiting the Earth.
• Areocentric orbit: An orbit around the planet Mars, such as moons or artificial satellites.

[edit] Altitude classifications

• Low Earth Orbit (LEO): Geocentric orbits ranging in altitude from 0–2000 km (0–1240 miles)
• Medium Earth Orbit (MEO): Geocentric orbits ranging in altitude from 2000 km (1240 miles) to just below geosynchronous orbit at 35786 km (22240 miles). Also known as an intermediate circular orbit.
• High Earth Orbit (HEO): Geocentric orbits above the altitude of geosynchronous orbit 35786 km (22240 miles).

[edit] Inclination classifications

• Inclined orbit: An orbit whose inclination in reference to the equatorial plane is not 0.
  • Polar orbit: An orbit 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.

[edit] Eccentricity classifications

• Circular orbit: An orbit that has an eccentricity of 0 and whose path traces a circle.
  • 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.
• Elliptic orbit: An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
  • Geosynchronous transfer orbit: An 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.
  • Geostationary transfer orbit: An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geostationary orbit.
  • Molniya orbit: A highly elliptic orbit with inclination of 63.4° and orbital period of half of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over a designated area of the planet.
  • Tundra orbit: A highly elliptic 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 planet.
• Hyperbolic orbit: An orbit with the eccentricity greater than 1. Such an orbit also has a velocity in excess of the escape velocity and as such, will escape the gravitational pull of the planet and continue to travel infinitely.
• Parabolic orbit: An orbit with the eccentricity equal to 1. Such an orbit also has a velocity equal to the escape velocity and therefore will escape the gravitational pull of the planet and travel until its velocity relative to the planet is 0. If the speed of such an orbit is increased it will become a hyperbolic orbit.
  • Escape orbit (EO): A high-speed parabolic orbit where the object has escape velocity and is moving away from the planet.
  • Capture orbit: A high-speed parabolic orbit where the object has escape velocity and is moving toward the planet.

[edit] Synchronous classifications

• Synchronous orbit: An orbit where the satellite has an orbital period equal to the average rotational period (earth’s is: 23 hours, 56 minutes, 4,091 seconds) of the body being orbited and in the same direction of rotation as that body. To a ground observer such a satellite would trace an analemma (figure in the sky.
• Semi-synchronous orbit (SSO): An orbit with an altitude of approximately 20200 km (12544.2 miles) and an orbital period of approximately 12 hours
• Geosynchronous orbit (GEO): Orbits with an altitude of approximately 35786 km (22240 miles). Such a satellite would trace an analemma (figure 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.
  • Supersynchronous orbit: A disposal / storage orbit above GSO/GEO. Satellites will drift west. Also a synonym for Disposal orbit.
  • 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.
• Areosynchronous orbit: A synchronous orbit around the planet Mars with an orbital period equal in length to Mars’ sidereal day, 24,6229 hours.
• Areostationary orbit (ASO): A circular areosynchronous orbit on the equatorial plane and about 17000 km(10557 miles) above the surface. To an observer on the ground this satellite would appear as a fixed point in the sky.
• Heliosynchronous orbit: An heliocentric orbit about the Sun where the satellite’s orbital period matches the Sun’s period of rotation. These orbits occur at a radius of 24,360 Gm (0,1628 AU) around the Sun, a little less than half of the orbital radius of Mercury.

[edit] Special classifications

• Sun-synchronous orbit: An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planets’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 384403 kilometres (238857 mi), elliptical-inclined orbit.

[edit] Pseudo-orbit classifications

• Horseshoe orbit: An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. 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.
  • Orbital spaceflight: A synonym for exo-orbit.
• Lunar transfer orbit (LTO)
• Prograde orbit: An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.
• Retrograde orbit: An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Apart from those in sun-synchronous orbit, few satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.

Satellites can also orbit Lagrangian points.

[edit] Satellite Modules

The satellite’s functional versatility is imbedded within its technical components and its operations characteristics. Looking at the “anatomy” of a satellite, one discovers two modules.^[6]

[edit] Spacecraft bus or service module

This first module consist of five subsystems:

• The Structural Subsystems

The structural subsystem provides the mechanical base structure, shields the satellite from extreme temperature changes and micro-meteorite damage, and controls the satellite’s spin functions.

• The Telemetry Subsystems

The telemetry subsystem monitors the on-board equipment operations, transmits equipment operation data to the earth control station, and receives the earth control station’s commands to perform equipment operation adjustments.

• The Power Subsystems

The power subsystem consists of solar panels and backup batteries that generate power when the satellite passes into the earth’s shadow.

• The Thermal Control Subsystems

The thermal control subsystem helps protect electronic equipment from extreme temperatures due to intense sunlight or the lack of sun exposure on different sides of the satellite’s body

• The Altitude and Orbit Controlled Control Subsystems

The altitude and orbit controlled subsystem consists of small rocket thrusters that keep the satellite in the correct orbital position and keep antennas positioning in the right directions.

[edit] Communication Payload

The second major module is the communication payload, which is made up of transponders. A transponders is capable of :

• Receiving uplinked radio signals from earth satellite transmission stations (antennas).
• Amplifying received radio signals
• Sorting the input signals and directing the output signals through input/output signal multiplexers to the proper downlink antennas for retransmission to earth satellite receiving stations (antennas).

[edit] Launch-capable countries

Main article: Timeline of first orbital launches by nationality

This list includes countries with an independent capability to place satellites in orbit, including production of the necessary launch vehicle. Note: many more countries have the capability to design and build satellites — which relatively speaking, does not require much economic, scientific and industrial capacity — but are unable to launch them, instead relying on foreign launch services. This list does not consider those numerous countries, but only lists those capable of launching satellites indigenously, and the date this capability was first demonstrated. Does not include consortium satellites or multi-national satellites.

Country	Year of first launch	First satellite	Launches to orbit in 2006[7]

Both North Korea (1998) and Iraq (1989) have claimed orbital launches, but these claims are unconfirmed.

In addition to the above, countries such as South Africa, Spain, Italy, West Germany, Canada, Australia, Argentina, Egypt, and private companies such as OTRAG, have developed their own launchers, but have not had a successful launch.

As of 2007, only seven countries from list above (six ‘major’ — Russia and Ukraine instead of USSR, also USA, Japan, China, India, and one ‘minor’ — Israel) and one regional organization (the European Union, represented by European Space Agency, ESA) have independently launched satellites on their own indigenously developed launch vehicles. (The launch capabilities of the United Kingdom and France now fall under the ESA.)

Also, one international private company (Sea Launch) has launch capability through their purchase of Ukrainian–Russian launchers.

Several other countries, including Brazil, Iran, South Korea, Malaysia, Pakistan, and Turkey, are at various stages of development of their own small-scale launcher capabilities, and seek membership in the club of space powers.

South Korea, with assistance with Russia is building Naro Space Center in Goheung, Jeolla Province. It is schedule to be operating in early 2008, and South Korea is launching KSLV rocket to put the nation’s satellite up into space.

Country	Year of first launch	First satellite	Payloads in orbit in 2008[9]

While Canada was the third country to build a satellite which was launched into Space, it was launched aboard a U.S. rocket from a U.S. spaceport. The same goes for Australia, who launched on-board a donated Redstone rocket. The first Italian-launched was San Marco 1, launched on 15 December 1964 on a U.S. Scout rocket from Wallops Island (VA,USA) with an Italian Launch Team trained by NASA.^[10] Australia’s launch project, in November 1967, involved a donated U.S. missile and U. S. support staff as well as a joint launch facility with the United Kingdom.^[11] Kazakhstan claimed to have launched their satellite independently, but the satellite was built with Russian help.

[edit] Satellite Services

• Satellite Internet
• Satellite phone
• Satellite radio
• Satellite television

A satellite dish is a type of parabolic antenna designed with the specific purpose of transmitting signals to and/or receiving from satellites. A satellite dish is a particular type of microwave antenna. Satellite dishes come in varying sizes and designs, and are most commonly used to receive satellite television. Many of the offset type of satellite dishes are sections of a larger parabolic dish.

The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish’s focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and ‘conducts’ them to a low-noise block downconverter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or K_u-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB. (A new form of omnidirectional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle was announced by the University of Waterloo. [1])

Modern dishes intended for home television use are generally 43 cm (18 in) to 80 cm (31 in) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band satellite dish of up to 3 metres in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites.

Motorised satellite dishes are still popular with enthusiasts. There are three competing standards, which often are all supported by a set-top box, DiSEqC, USALS, and 36v positioners. They can only supply one receiver.

A common misconception is that the LNBF (low-noise block/feedhorn), the device at the front of the dish, receives the signal directly from the atmosphere. See, for instance, this BBC News 24 [2] countdown that shows a “red data stream” being received by the LNBF directly instead of being beamed to the dish, which because of its parabolic shape will collect the signal into a smaller area and deliver it to the LNBF.

In Europe the frequencies used by DBS services are 10.7 - 12.75 GHz on two polarisations H and V. This range is divided into a “low band” with 10.7 - 11.7 GHz, and a “high band” with 11.7 - 12.75 GHz. This results in two frequency bands, each with a bandwidth of about 1 GHz, each with two possible polarizations. In the LNB they become down converted to 950 - 2150 MHz, which is the frequency range allocated for the satellite service on the coaxial cable between LNBF and receiver. Lower frequencies are allocated to cable and terrestrial TV, FM radio, etc. Only one of these frequency bands fits on the coaxial cable, so each of these bands needs a separate cable from the LNBF to a switching matrix or the receiver needs to select one of the 4 possibilities at a time.

In a single receiver residential installation there is a single cable from receiver to LNB and the receiver uses different power supply voltages (14/18V) to select polarization and pilot tones (22 kHz) to instruct the LNB to select one of the two frequency bands. In larger installations each band and polarization is given its own cable, so there are 4 cables from the LNB to a switching matrix, which allows the connection of multiple receivers in a star topology using the same signalling method as in a single receiver installation.

The theoretical gain (directive gain) of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching.

With lower frequencies, C-band for example, dish designers have a wider choice of materials. The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework. At higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations.

The dish is a reflector antenna and almost anything that reflects radio frequencies can be used as a reflector antenna. This has led to dustbin lids, woks and other items being used as “dishes”. Coupled with low noise LNBs and the higher transmission power of DTH satellites, it is easier to get a usable signal on some of these “dishes”.

Another common satellite dish is the VSAT. This provides two way broadband communications for both consumers and private networks for organisations. Today most VSATs operate in Ku band, C band is restricted to less populated regions of the world. There is a move which started in 2005 towards new Ka band satellites operating at higher frequencies, such as Spaceway offering greater performance at lower cost. These antennas vary from 74cm to 120cm in most applications though C-band VSATs may be as large as 2.4m.