My Science School

In 1990, the Galileo space probe began to investigate Earth. It was determining whether it is possible to detect signs of life on a planet when viewed from space. The probe detected that Earth had water on its surface and oxygen in its atmosphere, which told scientists that the planet contained life. As well as this, Earth at night is like a glowing neon signpost, alerting aliens to our whereabouts. The Pioneer space probes had special plates engraved with symbols in case the probes ever encountered intelligent life on their journeys into space.

On December 8, 1990, the spacecraft Galileo swept by the Earth, picking up a gravity assist for its long journey to Jupiter. As it came within 960 kilometers of its home planet Galileo was wide awake, all its instruments active, taking measurements and collecting data. For the first time Earth was viewed and measured just like any other planet in the solar system, from the perspective of a spacecraft fly-by.

What could a spacecraft learn about our planet in such a brief visit? Would it detect the things we consider most significant about it – its rich vegetation, teaming life, and human presence? When in 1993 Carl Sagan, co-founder of The Planetary Society, published an article with colleagues in Science magazine about Galileo's Earth encounter, these were precisely the questions he was trying to address. To their satisfaction, Sagan and his collaborators found that the evidence for the presence of life, and even intelligent life, was plentiful in Galileo's data. No probe passing by Earth would miss the fact that here was a planet worthy of a more sustained investigation.

The signs of Earthly life picked up by Galileo were numerous: the combination of abundant water on the surface with unusual amounts of oxygen, methane and ozone in the atmosphere, radio signals, and more. One particular indicator that caught Sagan’s attention was the strong reflection of the near-infrared color in the Earth’s spectrum. This "red edge," he wrote, pointed to the presence of “a light-harvesting pigment in a photosynthetic system.” It was, simply put, the signature of the green plant life that covers vast swathes of Earth’s surface, and it was strongest in those regions of the Earth that are covered with dense vegetation. A probe like Galileo, flying by an unknown planet, would be sure to detect this unmistakable indicator of rich plant life.

The time will probably come when; Star Trek-like, manmade probes will swoop by distant planets and collect data just as Galileo did for Earth. In the present, however, with our limited means and technology, no such close-range encounters are possible. For the foreseeable future, if life-bearing planets are to be found anywhere in the galaxy, they will have to be detected from distances measured by light years, not kilometers. Under such conditions, will the spectral indicator proposed by Sagan and his colleagues be of any use? Will we, in other words, be able to detect that “red edge” in the spectrum of a distant planet, and deduce from it the presence of plant life?

The question is of enormous interest to contemporary astronomers and planetary scientists. So far, admittedly, no Earth-sized planets have as yet been discovered, not to mention imaged, outside our solar system. But the day is undoubtedly near when the sensitivity of the extrasolar planet search will increase to the point when such planets will be found. A new generation of space missions, including the Space Interferometry Mission (SIM), the Terrestrial Planet Finder (TPF), and Darwin, are designed specifically for that purpose – to detect and observe Earth-like extrasolar planets. Within the coming years scientists will be in possession of spectrum measurements, and perhaps images, of distant planets with a mass and orbit comparable to Earth.

When this data becomes available, scientists will immediately begin mining it for signs of distant life. The most obvious and easy to detect signature of life, explains astronomer Pilar Montanes-Rodriguez of the Big Bear Solar Observatory (BBSO) in California, is the presence of large amounts of gases in a planet’s atmosphere than can coexist together only because life is sustaining them. Oxygen and methane, or oxygen, water, ozone and carbon dioxide, are examples of such combinations.

But this in itself only points to the presence of simple microbial life, such as existed on Earth for billions of years before the emergence of multicellular organisms. Detecting complex life, such as plants, is much more difficult, she explains, and for that scientists will need to rely on subtle indicators such as the “red edge.” How should we look for these signs of complex life on distant planets, and how likely are we to find them? These are the questions that Montanes-Rodriguez and her BBSO colleague, Enric Palle, set out to answer.

As well as receiving signals from outer space, radio telescopes such as Arecibo can also broadcast signals to the entire galaxy and beyond. In 1974, radio waves beamed from the Arecibo telescope carried a message deep into space. The message consisted of 1679 pulses that, when arranged into a grid 23 columns wide and 73 rows tall. The message was aimed at a dense ball of stars called M 13, which is so far away from Earth that it could take up to 50,000 years for a possible reply.

Serendip uses the world's largest radio telescope to scan a fair fraction of the celestial sphere. This means it samples many billions of Milky Way stars and many thousands of background galaxies. No one star gets as deep a scrutiny as Project Phoenix provided, but the number of stars being scanned is immense.

No real-time followup yet. This is a problem for piggyback SETI, partly because weak signals from beyond several hundred light-years should fade in and out of audibility, on a timescale of minutes, due to “interstellar scintillation” caused by the thin gas between the stars. Therefore several repeat observations of each point on the sky will probably be needed to catch a single repeat of a continuous weak signal. And, of course, if the aliens turn their transmitter elsewhere (or off) before a dedicated follow-up is scheduled, the chance to confirm a signal disappears.

Astronomers have detected nearly 2,000 alien planets to date. As that number continues to rise, so too does the prospect of finding intelligent extraterrestrial life. In terms of the search for extraterrestrial intelligence (SETI), it may no longer be a matter of answering the "are we alone" question, some scientists say. Rather, just how crowded is the universe?

And if ET is out there, it may be possible to reach out with direct "radio waving" to potentially habitable exoplanets. This form of cosmic cryptography, called "Active SETI," involves no longer merely listening for a signal but purposefully broadcasting to, and perhaps establishing contact with, other starfolk.

Active SETI sounds like science fiction, but some astronomers are discussing it seriously today. The idea is, as it has been in the past, a controversial, hot-button issue, with some researchers wary of sending signals out to touch base with intelligent aliens.

Radio astronomy is the most effective way to search for alien life. Radio telescopes can be positioned all over the world — as radio waves are not affected by the Earth’s atmosphere, and can pick up signals from across the Universe. Radio telescopes such as Arecibo in Puerto Rico, and the Very Large Array, enable astronomers to view space in all directions for signs of alien intelligence.

Several large searches for extraterrestrial intelligence (SETI) are currently scanning the stars, looking for both radio and laser transmissions from distant civilizations. Either type of signal could be sent across interstellar distances fairly economically, scientists are convinced.

Radio searches have been going on the longest. Most of them follow the same basic strategy: they hunt through the microwave part of the spectrum for any extremely narrowband (single-frequency) signal coming from outside the solar system. According to conventional wisdom, this is the kind of broadcast that has the best chance of being detected across interstellar distances.

Of the entire radio spectrum, the band of frequencies from about 0.5 to 60 gigahertz has the least natural background interference in space. Any alien radio astronomers should realize this too — and perhaps they would build interstellar transmitters accordingly. Our atmosphere generally limits us to frequencies below about 12 gigahertz, but maybe other civilizations would have reason to choose the low end of the frequency range too.

The only kind of transmission that we have much hope of detecting is a "beacon" — a very strong signal that aliens somewhere have deliberately designed to announce "Here we are!" as clearly and loudly as possible to any listeners in the cosmos, such as us. The searches now under way are much too weak to pick up any plausible radio chatter from another civilization's internal traffic — its own broadcasts and point-to-point communications — no matter how advanced the civilization may be. (Indeed, there's every reason to think that internal communications will become less recognizable from a distance as a civilization advances, judging from trends in our own communications technology.)

Considering the huge size of our galaxy, the immense distances between stars, and the immense width of the microwave radio spectrum, it's a daunting task even to search for powerful beacons that are designed to help us out! SETI projects have advanced far in recent years, but we are still looking for needles in very big haystacks that remain almost completely unexplored.

Here is a complete rundown of all the major SETI efforts worldwide, both radio and optical, those have recently been carried out or are currently under way.

The astronomer frank drake pioneered the search for intelligent life elsewhere in the Universe. He claimed that for intelligent life, capable of communicating over inter-stellar distances, to arise on a planet, conditions must be perfect. He came up with an equation to estimate the number of civilizations in the galaxy with the means of communicating with Earth:

It might seem logical to assume that the first confirmed alien life forms will be microscopic bacteria hiding out in damp Martian soils or simple organisms swimming around the hidden seas of Europa. But a leading SETI scientist says it’s more likely we find evidence of extra-terrestrial intelligence before discovering alien bugs.

"There are two horses in the race to find life beyond Earth," Andrew Siemion of the University of California, Berkeley, told the Association for the Advancement of Science conference in Seattle, according to News. "The first is the search for chemical signatures from planets and the second is the search for extra-terrestrial intelligence. Intelligent life has the edge, as it can be detected across the entire galaxy."

Siemion gave a talk entitled “Hunting for Techno signatures” at the conference in the same week he announced a major data release from the Breakthrough Listen initiative, where he is principal investigator. (The data has yet to return any evidence of ETI.) Siemion explained that the search for simpler forms of life that we might intuitively assume are more abundant and therefore easier to find is actually quite limited. The problem is that technology available now and in the near future restricts us to looking at our solar system and nearby stars. "And we can never be sure that methane or similar chemicals which we detect are really produced by living things,” Siemion said. “It just comes down to statistics - basic life may be very common, but we are much less likely to find it.”

Intelligent life, on the other hand, can send evidence of its existence across the cosmos at the speed of light as radio waves, laser pulses or other forms of electromagnetic radiation. SETI scientists increasing refer to looking for these signals that could never be created by nature and other signs of alien technology as the search for technosignatures.

Now, as we race towards 2021 without seeing that detection, x is therefore becoming ever closer to zero. And 1,000 times zero equals zero. But Siemion confirmed that he is actually optimistic and he does believe in the other side of the equation, which indicates that our capacity to detect alien life will be three orders of magnitude greater in the coming decade than in the 2010s. “We’ve seen a dramatic explosion in the number of observatories, the number of scientists... that are working in this field,” he explained.

But if we do find simple life in our solar system before detecting ETI’s signals, it could point to a rather dark conjecture: that life in the universe is plentiful, but it rarely survives long enough to develop the capability to reach beyond its own world.

In May 2002, NASA chose two TPF mission architecture concepts for further study and technology development. Each would use a different means to achieve the same goal—to block the light from a parent star in order to see its much smaller, dimmer planets. The technological challenge of imaging planets near their much brighter star has been likened to finding a firefly near the beam of a distant searchlight. Additional goals of the mission would include the characterization of the surfaces and atmospheres of newfound planets, and looking for the chemical signatures of life.

Purpose: To search for Earth-like planets that might harbor life. Terrestrial Planet Finder will use multiple telescopes working together to take family portraits of stars and their orbiting planets and determine which planets may have the right chemistry to sustain life.

The mission will study all aspects of planets, from their formation as disks of dust and gas around newly forming stars to their subsequent development. It will also look for planets orbiting the nearest stars and study their suitability as homes for any possible life.

One great challenge is how to detect planets against the blinding glare of their parent star, an effort that has been compared to trying to find a firefly in the glare of a searchlight. Terrestrial Planet Finder will reduce the glare of parent stars to see planetary systems up to 50 light-years away. Using spectroscopic instruments on Terrestrial Planet Finder, scientists will measure relative amounts of gases like carbon dioxide, water vapor, ozone and methane. This study will help determine whether a planet is suitable for life--or even whether life already exists there.

NASA has chosen two mission architecture concepts for further study and technology development. The two candidate architectures are: Multiple small telescopes on a fixed structure or on separated spacecraft flying in precision formation would simulate a much larger, very powerful telescope. A technique called nulling would reduce starlight by a factor of one million, enabling the detection of the very dim infrared emission from planets.

Visible Light Coronagraph: A large optical telescope, with a mirror three to four times bigger and at least 10 times more precise than the Hubble Space Telescope, would collect starlight and the very dim light reflected from planets. Its special optics would reduce starlight by a factor of one billion, enabling astronomers to detect the faint planets.

Four hundred years ago, an astronomer named Giordano Bruno was burned at the stake for suggesting the existence of other Earth-like worlds. Today we know that there are potentially billions of extra solar planets in the Milky Way. None found so far resemble Earth. Indeed, many are shockingly different from our world. Although none of the planets investigated so far have shown any signs of life, many astronomers believe that it is only a matter of time before Earth’s twin planet is discovered.

Our solar system is just one specific planetary system—a star with planets orbiting around it. Our planetary system is the only one officially called “solar system,” but astronomers have discovered more than 2,500 other stars with planets orbiting them in our galaxy. That’s just how many we’ve found so far. There are likely to be many more planetary systems out there waiting to be discovered! Our Sun is just one of about 200 billion stars in our galaxy. That gives scientists plenty of places to hunt for exoplanets, or planets outside our solar system. But our capabilities have only recently progressed to the point where astronomers can actually find such planets.

Even our closest neighboring stars are trillions of miles away. And all stars are enormous and extremely bright compared to any planets circling them. That means that picking out a planet near a distant star is like spotting a firefly right next to brilliant lighthouse miles away.

So far, the planets outside our solar system have proven to be fascinating and diverse. One planet, known as HD 40307g, is a “super Earth,” with a mass about eight times that of Earth. The force of gravity there would be much stronger than here at home. You would weigh twice as much there as you do on Earth! Another planet, called Kepler-16b, turns out to orbit two stars. A sunset there would provide a view of two setting stars!

In another planetary system, called TRAPPIST-1, there are not one…not two…but seven Earth-sized planets that could be covered in liquid water. The planets are relatively close together, too. If you were to stand on the surface of a TRAPPIST-1 planet, you might see six other planets on the horizon!

Extra-solar planets are very difficult to see because they are outshone by the light from their parent stars. It can be deter-mined whether or not a star has a planetary system by observing whether or not the star’s- light “wobbles”. As a planet orbits a star, its gravitational pull will cause the star’s light to bend slightly, and thus to change colour. This technique only works for giant planets, however, because an Earth-sized world would have little effect on its parent.

It is not easy to detect another planet so far away from Earth. Unlike stars which are fueled by nuclear reactions, planets only reflect the optical light of their stellar companion. In our solar system, for example, the Sun outshines its planets about one billion times in visible light. Because of the distant planets' faintness near the brightness of the nearby star, astronomers have had to devise clever methods to detect them. Currently, the most successful approach is based on the fact that a nearby planet will cause the star to wobble back and forth just a bit as the planet revolves around it. Astronomers can detect this tiny wobble and then calculate the orbit and mass of the object which is causing it. Even using this technique, however, it is still not easy to detect planets around other stars. Consider this: someone looking at our Sun from 30 light-years away would see it wobbling in a circle whose size would be about as big as a quarter viewed from 10,000 kilometers away!

During the past few years, researchers have detected over a dozen planets orbiting sunlike stars. The first was reported in October 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland. While observing the star 51 Pegasi, they noticed a change in the light from the star - its light repeatedly shifted back and forth between the blue and red ends of the electromagnetic spectrum. The timing of this Doppler shift implied that the star was "wobbling" a little because of a closely orbiting planet. In fact, the planet appeared to be revolving around the star every 4.2 days. Shortly thereafter, a survey of over a hundred other sunlike stars performed by the team of Geoff Marcy and Paul Butler at San Francisco State University and the University of California at Berkeley, turned up six more such planets. Of those, one planet circling the star 16 Cygni B was independently discovered by astronomers William D. Cochran and Artie P. Hatzes of the University of Texas McDonald Observatory. Since 1996, the announcement of the detection of new planets has become fairly routine....but always exciting!

Global positioning satellites beam signals to special receivers on Earth. These receivers, which are not much larger than mobile phones, know the difference between when the satellite signal was sent and when it was received. This allows the receiver to work out the distance between each of the satellites and itself, and there-fore calculate its position.

GPS is accurate and handy to use, so much so that we rely on it more and more every day. It's not often we take the time to learn how it works. The idea of GPS refers to a Global Positioning System; a collection of satellites in orbit above the Earth that transmit location data down to our devices. As hobbyists, we can get GPS modules that will read and interpret this data for us! They're known as GPS receivers, and they are used everywhere, like your phone, tablet, and other electronic devices. GPS receivers will relay a satellite's location data directly to a microcontroller in the form of serial data strings, which we can break down into relevant bite-sized chunks of data about where we are and how we are moving!

Firstly, the signal of time is sent from a GPS satellite at a given point. Subsequently, the time difference between GPS time and the point of time clock which GPS receiver receives the time signal will be calculated to generate the distance from the receiver to the satellite. The same process will be done with three other available satellites. It is possible to calculate the position of the GPS receiver from distance from the GPS receiver to three satellites. However, the position generated by means of this method is not accurate, for there is an error in calculated distance between satellites and a GPS receiver, which arises from a time error on the clock incorporated into a GPS receiver. For a satellite, an atomic clock is incorporated to generate on-the-spot time information, but the time generated by clocks incorporated into GPS receivers is not as precise as the time generated by atomic clocks on satellites. Here, the fourth satellite comes to play its role: the distance from the fourth satellite to the receiver can be used to compute the position in relations to the position data generated by distance between three satellites and the receiver, hence reducing the margin of error in position accuracy.

The Fig 1-3 below illustrates an example of positioning by two dimensions (position acquisition by using two given points). We can compute where we are at by calculating distance from two given points, and the GPS is the system that can be illustrated by multiplying given points and replacing them with GPS satellites on this figure.

GPS, or the Global Positioning System, is designed to aid navigation around the planet. It consists of 24 satellites in six different orbits around Earth. Their position in these orbits means that any receiver, anywhere on Earth, can always receive a signal from four satellites or more. Using data from these signals, a GPS receiver can work out its position, including altitude, to within a few metres.

The Global Positioning System (GPS), originally NAVSTAR GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the relatively weak GPS signals.

The GPS does not require the user to transmit any data, and it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.

The GPS project was started by the U.S. Department of Defense in 1973, with the first prototype spacecraft launched in 1978 and the full constellation of 24 satellites operational in 1993. Originally limited to use by the United States military, civilian use was allowed from the 1980s following an executive order from President Ronald Reagan. Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX). Announcements from Vice President AL Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; this was discontinued in May 2000 by a law signed by President Bill Clinton.

When selective availability was lifted in 2000, GPS had about five-meter (16 ft.) accuracy. The latest stage of accuracy enhancement uses the L5 band and is now fully deployed. GPS receivers released in 2018 that use the L5 band can have much higher accuracy, pinpointing to within 30 centimeters or 11.8 inches.

A satellite cannot do its job properly if it is not stable. A satellite dish must always point towards its location, or signals will be lost in space. In order to keep satellites from flying out of control, some are deliberately designed to spin. In the same way that a spinning top remains stable if it is spinning quickly, a satellite that is spinning will not deviate from its course. Some satellites have small, spinning wheels at various points on their frame. These wheels can be used to realign the satellite if it moves off course.

If you throw a ball into the air, the ball comes right back down. That’s because of gravity—the same force that holds us on Earth and keeps us all from floating away. To get into orbit, satellites first have to launch on a rocket. A rocket can go 25,000 miles per hour! That’s fast enough to overcome the strong pull of gravity and leave Earth’s atmosphere. Once the rocket reaches the right location above Earth, it lets go of the satellite.

The satellite uses the energy it picked up from the rocket to stay in motion. That motion is called momentum. But how does the satellite stay in orbit? Wouldn’t it just fly off in a straight line out into space? Not quite. You see, even when a satellite is thousands of miles away, Earth’s gravity is still tugging on it. That tug toward Earth--combined with the momentum from the rocket… …causes the satellite to follow a circular path around Earth: an orbit. When a satellite is in orbit, it has a perfect balance between its momentum and Earth’s gravity. But finding this balance is sort of tricky.

Gravity is stronger the closer you are to Earth. And satellites that orbit close to Earth must travel at very high speeds to stay in orbit.

For example, the satellite NOAA-20 orbits just a few hundred miles above Earth. It has to travel at 17,000 miles per hour to stay in orbit. On the other hand, NOAA’s GOES-East satellite orbits 22,000 miles above Earth. It only has to travel about 6,700 miles per hour to overcome gravity and stay in orbit. Satellites can stay in an orbit for hundreds of years like this, so we don’t have to worry about them falling down to Earth.

Satellites can help scientists learn a great deal more about the planet than instruments on aircraft and ships can. They use Earth-resources satellites to monitor every part of the world in order to find out information about the planet’s condition. Satellites can detect things such as the amount of water in a field of crops, which will give early warning of a harvest failure. They can also detect large areas of deforestation, showing changes over large periods of time.

ERS (Earth Resources Satellite) are the first two remote sensing satellites launched by ESA (European Space Agency). Their primary mission was to monitor Earth's oceans, ice caps, and coastal regions.

The satellites provided systematic, repetitive global measurements of wind speed and direction, wave height, surface temperature, surface altitude, cloud cover, and atmospheric water vapor level. Data from ERS-1 were shared with NASA under a reciprocal agreement for Seasat and Nimbus 7 data. ERS-2 carries the same suite of instruments as ERS-1 with the addition of the Global Ozone Measuring Equipment (GOME) which measures ozone distribution in the outer atmosphere. Having performed well for nine years – more than three times its planned lifetime – the ERS-1 mission was ended on March 10, 2000, by a failure in the onboard attitude control system.

The length of its operation enabled scientists to track several El Nino episodes through combined observations of surface currents, topography, temperatures, and winds. The measurements of sea surface temperatures, critical to the understanding of climate change by the ERS-1 Along-Track Scanning Radiometer were the most accurate ever made from space. All these important measurements are being continued by ERS-

Meteorology satellites, which orbit in geostationary and polar orbits, can keep a constant watch over the weather systems at work around the planet. They record data, such as cloud formation and movement, pressures, wind speeds and humidities, and send them to Earth, where scientists can use them to predict weather in preparation for weather forecasts. Satellites are also used to detect hurricanes — fierce tropical storms with wind speeds of over 130km/h (80mph). These storms can strike with very little warning, but satellites can detect them before they hit land, warning people of danger in time for them to take cover.

Weather satellites carry instruments called radiometers (not cameras) that scan the Earth to form images. These instruments usually have some sort of small telescope or antenna, a scanning mechanism, and one or more detectors that detect either visible, infrared, or microwave radiation for the purpose of monitoring weather systems around the world.

The measurements these instruments make are in the form of electrical voltages, which are digitized and then transmitted to receiving stations on the ground. The data are then relayed to various weather forecast centers around the world, and are made available over the internet in the form of images. Because weather changes quickly, the time from satellite measurement to image availability can be less than a minute.

Most of the satellites and instruments they carry are designed to operate for 3 to 7 years, although many of them last much longer than that. Weather satellites are put into one of two kinds of orbits around the Earth, each of which has advantages (and disadvantages) for weather monitoring. The first is a "geostationary" orbit, with the satellite at a very high altitude (about 22,500 miles) and orbiting over the equator at the same rate that the Earth turns. This allows the satellite to view the same geographic area continuously, and is used to provide most of the satellite imagery you see on TV or the internet.

For instance, GOES-East and GOES-West provide coverage of much of the Western Hemisphere, from the western coast of Africa to the West Pacific, and the Arctic to the Antarctic. The European Space Agency's Meteosat satellite provides coverage of Europe and Africa. The disadvantages of a geostationary orbit are (1) its very high altitude, which requires elaborate telescopes and precise scanning mechanisisms in order to image the Earth at high resolution (currently, 1 km at best); and (2) only a portion of the Earth can be viewed.

The other orbit type is called near-polar, sun-synchronous (or just "polar"), where the satellite is put into a relatively low altitude orbit (around 500 miles) that carries the satellite near the North Pole and the South Pole approximately every 100 minutes. Unlike the geostationary orbit, the polar orbit allows complete Earth coverage as the Earth turns beneath it.

These orbits are "sun-synchronous", allowing the satellite to measure the same location on the Earth twice each day at the same local time. Of course, the diadvantage of this orbit is that the satellite can image a particular location only every 12 hours, rather than continuously as in the case of the geostationary satellite. To offset this disadvantage, two satellites put into orbits at different sun-synchronous times have allowed up to 6 hourly monitoring.

But because of the lower altitude (500 miles rather than 22,000 miles), the instruments the polar-orbiting satellite carries to image the Earth do not have to be as elaborate in order to achieve the same ground resolution. Also, the lower orbit allows microwave radiometers to be used, which must have relatively large antennas in order to achieve ground resolutions fine enough to be useful. The advantage of microwave radiometers is their ability to measure through clouds to sense precipitation, temperature in different layers of the atmosphere, and surface characteristics like ocean surface winds.

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.