Category Science

A satellite in geostationary orbit takes the same time to orbit the Earth as the Earth does to spin, therefore always remaining over the same point on the planet. This orbit is mainly used for communications satellites. Low-Earth orbits, often used by spy satellites, can be lower than 250km (155 miles) above the planet. Polar-orbit satellites orbit at around 800km (590 miles), while highly-elliptical-orbit satellites have very low altitudes when they are closest to Earth, but pass far beyond the planet when they are at their most distant.

A geosynchronous orbit is a high Earth orbit that allows satellites to match Earth’s rotation. Located at 22,236 miles (35,786 kilometers) above Earth’s equator, this position is a valuable spot for monitoring weather, communications and surveillance. “Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south,” NASA wrote on its Earth Observatory website.

Satellites are designed to orbit Earth in one of three basic orbits defined by their distance from the planet: low Earth orbit, medium Earth orbit or high Earth orbit. The higher a satellite is above Earth (or any other world for that matter), the slower it moves. This is because of the effect of Earth’s gravity; it pulls more strongly at satellites that are closer to its center than satellites that are farther away. 

So a satellite at low Earth orbit — such as the International Space Station, at roughly 250 miles (400 km) — will move over the surface, seeing different regions at different times of day. Those at medium Earth orbit (between about 2,000 and 35,780 km, or 1,242 and 22,232 miles) move more slowly, allowing for more detailed studies of a region. At geosynchronous orbit, however, the orbital period of the satellite matches the orbit of the Earth (roughly 24 hours), and the satellite appears virtually still over one spot; it stays at the same longitude, but its orbit may be tilted, or inclined, a few degrees north or south.

A great many of the satellites sent into space by the USA and Russia are used for military activities. These range from eaves-dropping on important telephone calls to detecting the x-rays and electromagnetic pulses given off by nuclear explosions. Early military satellites were used to take close-up pictures of enemy territory but had to return home to have their film developed. Modern satellites use digital technology to take photographs, so they never run out of film. Amazingly, they can photograph things as small as the headlines on a newspaper.

The military space program is a significant but largely unseen aspect of space operations. Nearly a dozen countries have some kind of military space program, but the U.S. program dwarfs the efforts of all these other countries combined.

Military space operations are divided into five main areas: reconnaissance and surveillance, signals intelligence, communications, navigation, and meteorology. Only the United State and Russia operate spacecraft in all five areas. Several other countries have long used communications satellites for military purposes. In the 1990s, several countries in addition to Russia and the United State began developing reconnaissance satellites.

Reconnaissance and surveillance involve the observation of Earth for various purposes. Dedicated reconnaissance satellites, like the United States’ Improved CRYSTAL and the Russian Terilen, take photographs of targets on the ground and relay them to receiving stations in nearly real time. These satellites, however, cannot take continuous images like a television camera. Instead, they take a black-and-white photograph of a target every few seconds. Because they are in low orbits and are constantly moving, they can photograph a target for only a little over a minute before they move out of range. The best American satellites, which are similar in appearance to the Hubble Space Telescope, can see objects about the size of a softball from hundreds of miles up but they cannot read license plates. The Russians also occasionally use a system that takes photographs on film and then returns the film to Earth for processing. This provides them with higher-quality photos. The United States abandoned this technology in the 1980s after developing superior electronic imaging technology.

Other surveillance satellites, such as the American DSP and Space-Based Infrared System (SBIRS, pronounced “sibirs”) and the Russian Oko (or “eye”), are equipped with infrared telescopes and scan the ground for the heat produced by a missile’s exhaust. They can be used to warn of missile attack and can predict the targets of missiles fired hundreds or thousands of miles away. There are also satellites that look at the ground in different wavelengths to peer through camouflage, try to determine what objects are made of, and analyze smokestack emissions.

Signals intelligence satellites can operate either in low Earth orbit or in extremely high, geosynchronous orbit, where they appear to stay in one spot in the sky. These satellites listen for communications from cellular telephones, walkie-talkies, microwave transmissions, radios, and radar. They relay this information to the ground, where it is processed for various purposes. Contrary to popular myth, these satellites do not collect every conversation around the world. There is far more information being transmitted every day over the Internet than can be collected by evens the best spy agency.

Communications satellites are used for many different tasks, including television broadcasts and telephone calls. A telephone call made from England to the USA would be sent to the nearest Earth station, which would use its giant antenna to beam the call into space in the form of radio waves. The satellite would receive these radio waves and beam them back down to an antenna on the other side of the planet.

A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are about 2,000 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,236 miles (35,785 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it.

The high frequency radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or “bands” certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.

Launched by NASA in 1962, Relay 1 was one of several satellites placed in orbit in the decade after Sputnik to test the possibilities of communications from space. Relay 1 received telephone and television signals from ground stations and then transmitted them to other locations on the Earth’s surface. The satellite relayed signals between North America and Europe and between North and South America, and it also monitored the effects of radiation on its electronics. In conjunction with the Syncom 3 communications satellite, Relay 1 transmitted television coverage of the 1964 Olympics in Japan.

This prototype of Relay 1 is covered with solar cells. The antenna on top is for receiving and transmitting communications signals; those at its base are for telemetry, tracking, and control. In orbit, Relay used spin-stabilization to orient the antennas to communicate with Earth.

Satellites must be launched into orbit with enough speed to prevent Earth’s gravity from pulling them back down to the ground. Imagine throwing a ball horizontally. Gravity pulls the ball back to Earth very quickly. If the ball could be thrown hard enough, however, then it would have enough force to keep on travelling horizontally forever. It would be in orbit. A satellite at an altitude of 200km (120 miles) must be travelling at 7.8km/s (4.8mp/s) to prevent it being pulled back down to Earth.

An artificial satellite is a marvel of technology and engineering. The only thing comparable to the feat in technological terms is the scientific know-how that goes into placing, and keeping, one in orbit around the Earth. Just consider what scientists need to understand in order to make this happen: first, there’s gravity, then a comprehensive knowledge of physics, and of course the nature of orbits themselves. So really, the question of How Satellites Stay in Orbit is a multidisciplinary one that involves a great of technical and academic knowledge.

First, to understand how a satellite orbits the Earth, it is important to understand what orbit entails. Johann Kepler was the first to accurately describe the mathematical shape of the orbits of planets. Whereas the orbits of planets about the Sun and the Moon about the Earth were thought to be perfectly circular, Kepler stumbled onto the concept of elliptical orbits. In order for an object to stay in orbit around the Earth, it must have enough speed to retrace its path. This is as true of a natural satellite as it is of an artificial one. From Kepler’s discovery, scientists were also able to infer that the closer a satellite is to an object, the stronger the force of attraction, hence it must travel faster in order to maintain orbit.

Next comes an understanding of gravity itself. All objects possess a gravitational field, but it is only in the case of particularly large objects (i.e. planets) that this force is felt. In Earth’s case, the gravitational pull is calculated to 9.8 m/s2. However, that is a specific case at the surface of the planet. When calculating objects in orbit about the Earth, the formula v=(GM/R)1/2 applies, where v is velocity of the satellite, G is the gravitational constant, M is the mass of the planet, and R is the distance from the center of the Earth. Relying on this formula, we are able to see that the velocity required for orbit is equal to the square root of the distance from the object to the center of the Earth times the acceleration due to gravity at that distance. So if we wanted to put a satellite in a circular orbit at 500 km above the surface (what scientists would call a Low Earth Orbit LEO), it would need a speed of ((6.67 x 10-11 * 6.0 x 1024)/(6900000))1/2 or 7615.77 m/s. The greater the altitude, the less velocity is needed to maintain the orbit.

So really, a satellites ability to maintain its orbit comes down to a balance between two factors: its velocity (or the speed at which it would travel in a straight line), and the gravitational pull between the satellite and the planet it orbits. The higher the orbit, the less velocity is required. The nearer the orbit, the faster it must move to ensure that it does not fall back to Earth.

Any object in orbit around a celestial body is called a satellite. Earth has had its own natural satellite — the Moon — for billions of years. Since 1957, however, hundreds of artificial satellites have been launched into orbit around Earth, each transmitting a cacophony of radio signals to locations across the planet. Satellites are now vital to modern life and are used in many areas of technology, including communications, entertainment and espionage.

But usually when someone says “satellite,” they are talking about a “man-made” satellite. Man-made satellites are machines made by people. These machines are launched into space and orbit Earth or another body in space. There are thousands of man-made satellites. Some take pictures of our planet. Some take pictures of other planets, the sun and other objects. These pictures help scientists learn about Earth, the solar system and the universe. Other satellites send TV signals and phone calls around the world.

Satellites fly high in the sky, so they can see large areas of Earth at one time. Satellites also have a clear view of space. That’s because they fly above Earth’s clouds and air. Before satellites, TV signals didn’t go very far. TV signals only travel in straight lines. So they would go off into space instead of following Earth’s curve. Sometimes they would be blocked by mountains or tall buildings.

Phone calls to faraway places were also a problem. It costs a lot and it is hard to set up telephone wires over long distances or underwater. With satellites, TV signals and phone calls can be sent up to a satellite. The satellite can then send them back down to different spots on Earth.

There are dozens upon dozens of natural satellites in the solar system, with almost every planet having at least one moon. Saturn, for example, has at least 53 natural satellites, and between 2004 and 2017, it also had an artificial one — the Cassini spacecraft, which explored the ringed planet and its moons.

Artificial satellites, however, did not become a reality until the mid-20th century. The first artificial was Sputnik, a Russian beach-ball-size space probe that lifted off on Oct. 4, 1957. That act shocked much of the western world, as it was believed the Soviets did not have the capability to send satellites into space.

Scientists have sent probes to investigate many kinds of celestial objects. In 1995, the Ulysses probe was launched towards the Sun and took readings of the solar wind and the star’s magnetism. The Giotto probe, launched in 1986, battled its way past flying debris and gas into the heart of Halley’s Comet, taking incredible pictures of its nucleus. Asteroids have also been visited by space probes. The Near Earth Asteroid Rendezvous probe landed on the asteroid Eros in 2001.

The 1985-1986 emergence of Halley’s Comet, the first since the advent of the space age, was explored by a variety of spacecraft. The Vega 1, launched by the USSR together with the Eastern-block alliance, passed 5523 miles from the Comet’s nucleus at 7:20:06 Universal time. It indicated that the Comet was about 300 miles closer to the sun than had been predicted. The Japanese spacecraft, Suisei, was created to map the distribution of neutral hydrogen atoms outside Halley’s visible coma. Its pictures indicated that the Comet’s output of water varied between 25 and 60 tons per second. Five days after the Vega 2’s passage through the Comet, the Giotto (sponsored by the European Space Agency) probe appeared. Giotto’s close approach took place 3.1 minutes after midnight UT on March 14th; the craft had passed 376 miles from its target. Giotto’s data indicated that the nucleus was bigger than expected, and that the Comet was composed primarily of water, CO2 and N2. The Vegas and Giotto found that as the solar wind approaches Halley, it slows gradually and the solar magnetic lines embedded in the wind begin to pile up. Pick-up ions, from the Comet’s halo of neutral hydrogen, were found in this solar wind. Sensors on the Vega spacecraft found a variety of plasma waves propagating inside the bow wave. In order to synthesize all the results, a conference on the exploration of Halley’s Comet will be held this October

Data on the nitrogen-containing compounds that observed spectroscopically in the coma of Comet Halley are summarized, and the elemental abundance of nitrogen in the Comet Halley nucleus is derived. It is found that 90 percent of elemental nitrogen is in the dust fraction of the coma, while in the gas fraction, most of the nitrogen is contained in NH3 and CN. The elemental nitrogen abundance in the ice component of the nucleus was found to be deficient by a factor of about 75, relative to the solar photosphere, indicating that the chemical partitioning of N2 into NH3 and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio N2/NH3 = 0.1, observed in the Comet. It is suggested that the low N2/NH3 ratio in Comet Halley may be explained simply by physical fractionation and/or thermal diffusion.