Satellite (artificial) facts for kids

Satellite in orbit

A satellite is an object that orbits another object. In space, satellites may be made by man, or they may be natural. The moon is a natural satellite that orbits the Earth. Most man-made satellites also orbit the Earth, but some orbit other planets, such as Saturn, Venus or Mars, or the moon.

Satellites are used for many purposes. There are weather satellites, communications satellites, reconnaissance satellites, astronomy satellites and many other kinds.

Herman Potočnik proposed and Arthur C. Clarke popularized the idea of the communications satellite.

The world's first artificial satellite, the Sputnik 1, was launched by the Soviet Union on October 4, 1957. This surprised the world, and the United States quickly worked to launch their own satellite, starting the space race. Sputnik 2 was launched on November 3, 1957 and carried the first living passenger into orbit, a dog named Laika. The United States launched their first satellite, called Explorer 1 on January 31, 1958. The UK launched its first satellite in 1962.

Since then, thousands of satellites have been launched into orbit around the Earth. Some satellites, notably space stations, have been launched in parts and assembled in orbit.

History

A 1949 issue of Popular Science depicts the idea of an "artificial moon"

Animation depicting the orbits of GPS satellites in medium Earth orbit.

1U CubeSat ESTCube-1, developed mainly by the students from the University of Tartu, carries out a tether deployment experiment in low Earth orbit.

The first published mathematical study of the possibility of an artificial satellite was Newton's cannonball, a thought experiment by Isaac Newton to explain the motion of natural satellites, in his Philosophiæ Naturalis Principia Mathematica (1687). The first fictional depiction of a satellite being launched into orbit was a short story by Edward Everett Hale, "The Brick Moon" (1869). The idea surfaced again in Jules Verne's The Begum's Fortune (1879).

In 1903, Konstantin Tsiolkovsky (1857–1935) published Exploring Space Using Jet Propulsion 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, and that a multi-stage rocket fueled by liquid propellants could achieve this.

In 1928, Herman Potočnik (1892–1929) published his sole book, The Problem of Space Travel – The Rocket Motor. He described the use of orbiting spacecraft for observation of the ground and described how the special conditions of space could be useful for scientific experiments.

In a 1945 Wireless World article, the English science fiction writer Arthur C. Clarke described in detail the possible use of communications satellites for mass communications. He suggested that three geostationary satellites would provide coverage over the entire planet.

In May 1946, the United States Air Force's Project RAND released the Preliminary Design of an Experimental World-Circling Spaceship, which stated that "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century." The United States had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. Project RAND eventually released the report, but considered the satellite to be a tool for science, politics, and propaganda, rather than a potential military weapon.

In 1946, American theoretical astrophysicist Lyman Spitzer proposed an orbiting space telescope.

In February 1954 Project RAND released "Scientific Uses for a Satellite Vehicle", written by R.R. Carhart. This expanded on potential scientific uses for satellite vehicles and was followed in June 1955 with "The Scientific Use of an Artificial Satellite", by H.K. Kallmann and W.W. Kellogg.

In the context of activities planned for the International Geophysical Year (1957–58), the White House announced on 29 July 1955 that the U.S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On 31 July, the Soviets announced that they intended to launch a satellite by the fall of 1957.

The first artificial satellite was Sputnik 1, launched by the Soviet Union on 4 October 1957 under the Sputnik program, with Sergei Korolev as chief designer. Sputnik 1 helped to identify the density of high atmospheric layers through measurement of its orbital change and provided data on radio-signal distribution in the ionosphere. The unanticipated announcement of Sputnik 1's success precipitated the Sputnik crisis in the United States and ignited the so-called Space Race within the Cold War.

Sputnik 2 was launched on 3 November 1957 and carried the first living passenger into orbit, a dog named Laika.

In early 1955, following pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, the Army and Navy were working on Project Orbiter with two competing programs. The army used the Jupiter C rocket, while the civilian/Navy program used the Vanguard rocket to launch a satellite. Explorer 1 became the United States' first artificial satellite on 31 January 1958.

In June 1961, three-and-a-half years after the launch of Sputnik 1, the United States Space Surveillance Network cataloged 115 Earth-orbiting satellites.

Early satellites were constructed to unique designs. With advancements in technology, multiple satellites began to be built on single model platforms called satellite buses. The first standardized satellite bus design was the HS-333 geosynchronous (GEO) communication satellite launched in 1972. Beginning in 1997, FreeFlyer is a commercial off-the-shelf software application for satellite mission analysis, design and operations.

Currently the largest artificial satellite ever is the International Space Station.

Herman Potočnik explored the idea of using orbiting spacecraft for detailed peaceful and military observation of the ground in his 1928 book, The Problem of Space Travel. He described how the special conditions of space could be useful for scientific experiments. The book described geostationary satellites (first put forward by Konstantin 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.

Satellites orbiting now

Artificial satellites come from more than 50 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently working, but thousands of unused satellites and satellite fragments orbit the Earth as space debris. The largest satellite is the International Space Station, which was put together by several different countries (including the organizations of NASA, ESA, JAXA and RKA). It usually has a crew of six astronauts or cosmonauts living on board. It is permanently occupied, but the crew changes. The Hubble Space Telescope has been repaired and updated by astronauts in space several times.

There are also man-made satellites orbiting something other than the Earth. The Mars Reconnaissance Orbiter is one of those orbiting Mars. Cassini-Huygens is orbiting Saturn. Venus Express, run by the ESA, is orbiting Venus. Two GRAIL satellites orbited the moon until December 2012. Several satellites have orbited the Sun for years and one is to be added in 2017.

Man-made satellites have several main uses:

• Scientific Investigation
• Earth observation - including weather forecasting and tracking storms and pollution
• Communications - including satellite television and telephone calls
• Navigation - including the Global Positioning System (GPS)
• Military - including reconnaissance photography and communications (nuclear weapons are not allowed in space)

Orbits

A satellite in a geostationary orbit.

Most of the man-made satellites are in a low Earth orbit (LEO) or a geostationary orbit. To stay in orbit, the satellite's sideways speed must balance the force of gravity. Satellites in low orbit are often less than one thousand kilometers above the ground. Close to the Earth, in LEO, the satellites must move faster to stay in orbit. Low orbits work well for satellites that take pictures of the Earth. Many do jobs that call for high orbital inclination (they swing above and below the equator), so they can communicate, or look at other areas. It is easier to put a satellite in low Earth orbit, but the satellite appears to move when viewed from Earth. This means a satellite dish (a type of antenna) must be always moving in order to send or receive communications with that satellite.

Medium orbit works well for GPS satellites - receivers on Earth use the satellite's changing position and precise time (and a type of antenna that does not have to be pointed) to find where on Earth the receiver is. But constantly changing positions does not work for satellite TV and other types of satellites that send and receive a lot of information. Those need to be in geostationary orbit.

A satellite in a geostationary orbit moves around the Earth as fast as the Earth spins, so from the ground it looks like it is stationary (not moving). To move this way, the satellite must be straight above the equator, and 35,786 kilometers (22,236 miles) above the ground.

End of life

When satellites reach the end of their mission (this normally occurs within 3 or 4 years after launch), satellite operators have the option of de-orbiting the satellite, leaving the satellite in its current orbit or moving the satellite to a graveyard orbit. Historically, due to budgetary constraints at the beginning of satellite missions, satellites were rarely designed to be de-orbited. One example of this practice is the satellite Vanguard 1. Launched in 1958, Vanguard 1, the 4th artificial satellite to be put in Geocentric orbit, was still in orbit as of February 2022^[update], as well as the upper stage of its launch rocket.

Instead of being de-orbited, most satellites in the first six decades of spaceflight were either left in their current orbit or moved to a graveyard orbit. As of 2002, the FCC requires all geostationary satellites to commit to moving to a graveyard orbit at the end of their operational life prior to launch.

In cases of uncontrolled de-orbiting, the major variable is the solar flux, and minor variables are the components and form factor of the satellite itself, as well as gravitational perturbations generated by the Sun and the Moon. The nominal breakup altitude due to aerodynamic forces and temperatures is 78 km, with a range between 72 and 84 km. Solar panels, however, are destroyed before any other component at altitudes between 90 and 95 km.

After the late 2010s, and especially after the advent and operational fielding of large satellite internet constellations—where on-orbit active satellites more than doubled over a period of five years—the companies building the constellations began to propose regular planned deorbiting of the older satellites that reach end of life, as a part of the regulatory process of obtaining a launch license.

Launch-capable countries

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 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. The list does not include the European Space Agency, a multi-national state organization, nor private consortiums.

Applications

Weather

GOES-8, a United States weather satellite.

A weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. These meteorological satellites, however, see more than clouds and cloud systems. City lights, fires, effects of pollution, auroras, sand and dust storms, snow cover, ice mapping, boundaries of ocean currents, energy flows, etc., are other types of environmental information collected using weather satellites.

Weather satellite images helped in monitoring the volcanic ash cloud from Mount St. Helens and activity from other volcanoes such as Mount Etna. Smoke from fires in the western United States such as Colorado and Utah have also been monitored.

Environmental monitoring

Other environmental satellites can assist environmental monitoring by detecting changes in the Earth's vegetation, atmospheric trace gas content, sea state, ocean color, and ice fields. By monitoring vegetation changes over time, droughts can be monitored by comparing the current vegetation state to its long term average. For example, the 2002 oil spill off the northwest coast of Spain was watched carefully by the European ENVISAT, which, though not a weather satellite, flies an instrument (ASAR) which can see changes in the sea surface. Anthropogenic emissions can be monitored by evaluating data of tropospheric NO₂ and SO₂.

These types of satellites are almost always in Sun-synchronous and "frozen" orbits. A sun-synchronous orbit passes over each spot on the ground at the same time of day, so that observations from each pass can be more easily compared, since the sun is in the same spot in each observation. A "frozen" orbit is the closest possible orbit to a circular orbit that is undisturbed by the oblateness of the Earth, gravitational attraction from the sun and moon, solar radiation pressure, and air drag.

Mapping

Terrain can be mapped from space with the use of satellites, such as Radarsat-1 and TerraSAR-X.

Related pages

• Natural satellite