How Satellites Work

A satellite is any object that moves in a curved path around a planet. The moon is Earth's original, natural satellite, and there are many man-made (artificial) satellites, usually closer to Earth. The path a satellite follows is an orbit, which sometimes takes the shape of a circle.

To understand why satellites move this way, we must revisit our friend Newton. Newton proposed that a force -- gravity -- exists between any two objects in the universe. If it weren't for this force, a satellite in motion near a planet would continue in motion at the same speed and in the same direction -- a straight line. This straight-line inertial path of a satellite, however, is balanced by a strong gravitational attraction directed toward the center of the planet.

Sometimes, a satellite's orbit looks like an ellipse, a squashed circle that moves around two points known as foci. The same basic laws of motion apply, except that the planet is located at one of the foci. As a result, the net force applied to the satellite isn't uniform all the way around the orbit, and the speed of the satellite changes constantly. It moves fastest when it's closest to the planet -- a point known as perigee -- and slowest when it's farthest from the planet -- a point known as apogee.

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

Newton may have worked through the mental exercise of launching a satellite, but it would take a while before we actually accomplished the feat. One of the early visionaries was sci-fi writer Arthur C. Clarke. In 1945, Clarke suggested that satellites could be placed into orbit so that they moved in the same direction and at the same rate as the spinning Earth. These so-called geostationary satellites, he proposed, could be used for communications.

Many scientists didn't fully embrace Clarke's idea -- until Oct. 4, 1957. That's when the Soviet Union launched Sputnik 1, the first man-made satellite to orbit Earth.Sputnik was a 23-inch (58-centimeter), 184-pound (83-kilogram) metal ball. Although it was a remarkable achievement, Sputnik's contents seem meager by today's standards:

On the outside of Sputnik, four whip antennas transmitted on shortwave frequencies above and below what is today's citizens-band (27 megahertz). Tracking stations on the ground picked up the radio signals and confirmed that the tiny satellite had survived the launch and was successfully tracing a path around our planet. A month later, the Soviets placed a companion craft, Sputnik 2, in orbit. Nestled inside the capsule was a dog by the name of Laika.

In December 1957, desperate to keep up with their Cold War counterparts, American scientists tried to carry a satellite into orbit aboard a Vanguard rocket. Unfortunately, the rocket crashed and burned on the launchpad. Shortly after, on Jan. 31, 1958, the U.S. finally matched the success of the Soviets by using a plan adopted by Wernher von Braun, which called for a U.S. Redstone rocket to propel a satellite -- Explorer 1 -- into Earth's orbit. Explorer 1 carried instrumentation to detect cosmic rays and revealed, in an experiment led by James Van Allen of the University of Iowa, a much lower cosmic ray count than expected. This led to the discovery of two doughnut-shaped zones (eventually named for Van Allen) filled with charged particles trapped by Earth's magnetic field.

Bolstered by these successes, several companies raced to develop and deploy satellites in the 1960s. One of these was Hughes Aircraft and its star engineer Harold Rosen. Rosen led a team that turned Arthur C. Clarke's concept -- a communications satellite positioned in Earth's orbit so it could bounce radio waves from one location to another -- into a feasible design. In 1961, NASA gave Hughes a contract to build the Syncom (synchronous communication) series of satellites. In July 1963, Rosen and his colleagues watched as Syncom 2 soared into space and navigated into a (roughly) geosynchronous orbit. President Kennedy used the new system to have a conversation with the Nigerian prime minister in Africa (you can listen here). This was followed by Syncom 3, which could actually broadcast television.

The age of satellites had begun.

Technically, a satellite is any object that revolves around a planet or smaller celestial body. Astronomers classify moons as natural satellites, and they have, over the years, tallied hundreds of these objects orbiting the planets and dwarf planets in our solar system. For example, they have compiled a list of 67 moons circling Jupiter.

Man-made objects, such as those launched during the Sputnik and Explorer missions, can also be classified as satellites because they, like moons, circle a planet. Unfortunately, the human activity required to get man-made satellites into space has produced an enormous amount of leftover debris. All of these bits and pieces behave just like larger rockets and spacecraft -- they move around their target planet at very high speeds, following circular or elliptical paths. In the strictest interpretation of the definition, each piece of debris qualifies as a satellite. But astronomers generally think of satellites as objects that perform a useful function. Scraps of metal and other detritus hardly count as useful and therefore fall into a different category known as orbital debris.

According to NASA's Orbital Debris program, there are 100 million pieces of orbital debris no larger than 1 centimeter (0.4 inches). There are 500,000 pieces in the 1-10 centimeter (0.4-3.9 inch) range and approximately 21,000 items larger than 10 centimeters. Astronomers sometimes refer to the stuff in the latter category as space junk -- objects large enough to track with radar that were inadvertently placed in orbit and that now pose a threat to other active, properly functioning satellites.

Orbital debris can come from many sources:

A special NASA satellite called the Long Duration Exposure Facility (LDEF) was put in orbit to study the long-term effects of collisions with space junk. The space shuttle Challenger deployed LDEF in April 1984, and the space shuttle Columbia retrieved it in January 1990. Over its nearly six-year mission, the satellite's instruments recorded more than 20,000 impacts, some of which were caused by micrometeorites, others by orbital debris [source: Martin]. NASA scientists continue to analyze the data from LDEF to learn about orbital debris populations and distributions.

Satellites come in a variety of shapes and sizes and perform many different functions, but they all have several things in common.

As you might expect, putting all of these systems together isn’t easy. It can take years. Everything begins with a mission objective. Defining the parameters of the mission enables engineers to specify the instruments needed and how they'll be arranged. Once these specifications (and their budget) are approved, satellite construction can begin. This typically takes place in a clean room, a sterile environment that makes it possible to maintain a constant temperature and humidity and protect the satellite during its development, construction and testing.

Artificial satellites generally aren’t mass-produced; they’re custom-built to perform their intended functions. With that said, some companies have designed their satellites to be modular, making it possible to start with a primary structure that can be customized as needed. For example, Boeing's 601 satellites have two basic modules -- a chassis for carrying the propulsion subsystem, bus electronics and battery packs; and a set of honeycomb shelves to hold arrays of equipment. This modularity enables engineers to assemble purpose-built satellites without starting from scratch. And, of course, some satellites, such as those in GPS and the Iridium system, work together in a coordinated network. Using a repeatable design makes it easier to set up and integrate the various components of the system.

All satellites today get into orbit by riding on a rocket. Many used to hitch a ride 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 into orbit regularly and safely.

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 (see sidebar) 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 kilometers). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 kilometers). 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 Florida's Cape Canaveral doesn't get as big a boost from Earth's rotational speed. The Kennedy Space Center's Launch Complex 39-A 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 Feb. 11, 2000, liftoff of the space shuttle Endeavour required launching a total weight of 4,520,415 pounds (2,050,447 kilograms) [source: NASA]. 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 kilometers) 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.

A rocket must accelerate to at least 25,039 mph (40,320 kph) to completely escape Earth's gravity and fly off into space (for more on escape velocity, visit this article at NASA).

Earth's escape velocity is much greater than what's required to place an Earth satellite in orbit. With satellites, the object is not to escape Earth's gravity, but to balance it. Orbital velocity is the velocity needed to achieve balance between gravity's pull on the satellite and the inertia of the satellite's motion -- the satellite's tendency to keep going. This is approximately 17,000 mph (27,359 kph) at an altitude of 150 miles (242 kilometers). Without gravity, the satellite's inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth. At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line.

The orbital velocity of the satellite depends on its altitude above Earth. The nearer to Earth, the faster the required orbital velocity. At an altitude of 124 miles (200 kilometers), the required orbital velocity is a little more than 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 kilometers) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). That orbital speed and distance permit the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22,223 miles altitude stays in a fixed position relative to a point on Earth's surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called "geostationary." Geostationary orbits are ideal for weather satellites and communications satellites.

In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite like the moon can stay in orbit for centuries.

Down on the ground, satellites can look very similar -- shiny boxes or cylinders adorned with solar-panel wings. But out in space, these gawky machines behave quite differently depending on their flight path, altitude and orientation. As a result, classifying satellites can be tricky business. One approach is to think about how a device orbits its target planet (usually Earth). Recall that there are two basic shapes of an orbit: circular and elliptical. Some satellites start out elliptical and then, with corrective nudges from small onboard rockets, acquire circular paths. Others move permanently in elliptical paths known as Molniya orbits. These objects generally circle from north to south, over Earth's poles, and take about 12 hours to make one complete trip.

Polar-orbiting satellites also pass over the planet's poles on each revolution, although their orbits are far less elliptical. 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. And weather forecasters rely on a worldwide network of polar satellites, which covers the entire globe every 12 hours.

You can also classify satellites based on their height above Earth's surface. Using this scheme, there are three categories [source: Riebeek]:

Finally, it's possible to think about satellites in terms of where they're "looking." Most of the objects sent into space over the last few decades look down at Earth. These satellites have cameras and equipment capable of seeing our world through various wavelengths of light, making it possible to enjoy spectacular visible, ultraviolet and infrared views of our changing planet. A smaller number of satellites turn their "eyes" toward space, where they capture magnificent vistas of stars, planets and galaxies and scan for objects, such as asteroids or comets, that could be heading for a collision course with Earth.

Not so long ago, satellites were exotic, top-secret devices used primarily in a military capacity, for activities such as navigation and espionage. Now they're an essential part of our daily lives. We see and recognize their use in weather reports. We watch television signals transmitted by DIRECTV and the DISH Network. We have GPS receivers in our cars and smartphones to help us find our way to any destination. And we marvel at images captured by the Hubble Space Telescope and by antics of astronauts living on the International Space Station.

Even still, many satellites escape our notice. Let's meet a few of these unsung orbiting heroes.

Landsat satellites have been snapping images of Earth since the early 1970s, offering the longest continuous global record of our planet's surface. Landsat 1, known at the time as the Earth Resources Technology Satellite (ERTS), was launched on July 23, 1972. It carried two primary instruments -- a camera built by RCA and a multispectral scanner, courtesy of Hughes Aircraft Company, capable of recording data in green, red and two infrared bands. The satellite produced such amazing images and was considered so successful that it was followed by a series of companions. NASA launched the most recent addition, Landsat 8, on Feb. 11, 2013. The device contains two Earth-observing sensors, the Operational Land Imager (OLI) and the Thermal Infrared Sensor (TIRS), which collect multispectral images of coastal regions, polar ice, islands and the continents.

Geostationary Operational Environmental Satellites (GOES) circle Earth in geosynchronous orbits, each hovering over a fixed area of the globe. This allows the satellites to keep a watchful eye on the atmosphere and detect changing weather conditions that could lead to tornadoes, hurricanes, flash floods and thunderstorms. Meteorologists use the information to issue watches and warnings for severe weather. They can also use GOES imagery to estimate rainfall amounts and snowfall accumulations, measure the extent of snow cover and track movements of sea and lake ice. Since 1974, 15 GOES satellites have been placed in orbit, but at any one time, it takes two of the devices -- GOES East and GOES West -- to see Earth's weather.

Jason-1 and Jason-2 have played key roles in long-term analysis of Earth's oceans. NASA launched Jason-1 on Dec. 7, 2001, to take over duties provided by the NASA/CNES Topex/Poseidon satellite, which had been circling Earth since 1992. For almost 12 years, Jason-1 mapped sea level, wind speed and wave height for more than 95 percent of Earth's ice-free oceans. The mission revolutionized the study of ocean circulation and provided data to indicate that sea levels across the planet were rising. NASA officially decommissioned Jason-1 on July 3, 2013, but that didn't stop the ocean-gazing operations. In 2008, the space agency launched a successor to Jason-1 from Vandenberg Air Force Base in California. Jason-2 carries high-precision instruments to measure the distance between the satellite and the ocean surface to within a few centimeters. These measurements of ocean topography arm scientists with information about how fast ocean currents are moving and how much heat is stored in the ocean. This data, in turn, provides insights into global climate patterns.

The 66 Iridium communications satellites orbiting 500 miles (800 kilometers) above Earth's surface have created a stir on many fronts. Iridium LLC, the original owner of the satellites, spent $5 billion to build and deploy the machines, then sold them for $25 million in 1999 when the company went bankrupt. Then, in 2009, Iridium 33 collided with a decommissioned Russian satellite over Siberia, creating a large field of space junk and debris that will remain in orbit for years to come. Today, Iridium Communications Inc. owns and operates the satellites, which allow subscribers to use satellite phones to communicate from any point on the globe. Stargazers also enjoy Iridium because the satellites in the "constellation" are easy to spot, especially when their antenna arrays catch sunlight and flare up brightly in the night sky.

The OSCAR (Orbiting Satellite Carrying Amateur Radio) series of satellites facilitate communication between amateur radio stations. They're built and operated by a nonprofit organization of ham radio operators worldwide known as AMSAT. The AMSAT-built satellites "hitch" a rocket launch on a "payload-space-available" basis. For this reason, the devices are usually quite small and have no propulsion systems that could interfere with the rocket's primary payload. AMSAT satellites can often be heard by use of a shortwave receiver or aradio scanner. Ham operators make use of the satellites during natural disasters when terrestrial links and cell phone systems may be down or overloaded.

Space telescopes are satellites that look away from Earth. From their orbits high above our atmosphere, they can view the universe without any distortions or interference. You've undoubtedly seen some of the spectacular images beamed down from the Hubble Space Telescope (HST), which entered its orbit, 308 miles (570 kilometers) above Earth, in 1990. HST 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. In 2018, NASA plans to launch a companion to Hubble -- the James Webb Space Telescope (JWST). JWST will observe infrared light from very distant objects and will do it from a special elliptical orbit, known as L2, located 932,000 (1.5 million kilometers) away from Earth!

In the years following Sputnik and Explorer 1, satellites grew larger and more complicated. Consider TerreStar-1, a commercial satellite designed to provide mobile voice and data communications in North America to smartphone-size handsets. Launched in 2009, TerreStar-1 weighed in at 15,233 pounds (6,910 kilograms). And when it was fully deployed, it unfurled an S-band antenna measuring 60 feet (18 meters) across and massive solar panels giving the final device a wingspan of 106 feet (32 meters) [source: de Selding].

Building such a complex machine requires lots of resources, which is why, historically, only government agencies and corporations with deep pockets have been able to get into the satellite business. Much of the cost is wrapped up in the equipment carried by a satellite -- transponders, computers and cameras. A typical weather satellite carries a price tag of $290 million; a spy satellite might cost an additional $100 million [source: GlobalCom]. Then there's the expense of maintaining and repairing satellites. Companies must pay for satellite bandwidth just like cell phone owners must pay to transmit voice and data. Those bandwidth costs could top $1.5 million a year [source: GlobalCom]!

Another important factor with satellites is the cost of the launch. Launching a single satellite into space can cost anywhere between $10 million and $400 million, depending on the vehicle used. A small launch vehicle such as the Pegasus XL rocket can lift 976 pounds (443 kilograms) into low-Earth orbit for about $13.5 million. That works out to be almost $14,000 per pound. A heavy launch vehicle, on the other hand, costs more to launch but also provides a greater lifting force. For example, the Ariane 5G rocket can lift 39,648 pounds (18,000 kilograms) into low-Earth orbit at a cost of $165 million. That works out to $4,162 per pound, making it more cost-effective on a per-pound basis [source: Futron Corporation]. (Note that all monetary figures are in 2000 U.S. dollars.)

Despite the costs and risks associated with building, launching and operating satellites, some companies have managed to grow their space technology business. Boeing is one of those companies. Its Defense, Space and Security division managed to deliver 10 satellites in 2012 and acquire orders for seven more, contributing to the business unit's nearly $32 billion in revenue [source: The Boeing Company].

When you think of stargazing, you probably picture someone standing behind a telescope, studying the craters on the moon, the spots and streaks of the rusty Martian surface, the rings of Saturn or the galaxies of the Local Group. But many amateur astronomers, even those without expensive optics, take great pleasure in spotting satellites in orbit around Earth. If you want to get in on the action, here are a few tips:

If you want to get more serious, you can try to predict when a specific satellite will be passing overhead. 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 a number of popular satellites. Satellites use a variety of light-sensitive sensors to determine their position. The satellite transmits its position to the ground station.

Of course, no "sat-seeing" expedition would be complete without a glimpse of some special satellites. Tumbling spacecraft are one such treat. These are typically dead satellites that remain in orbit but now spin around one or more axes. As they rotate, their surfaces reflect sunlight, making the objects appear to flash as they move across the sky. Satellites in the Iridium constellation can also provide a similar experience. The so-called Iridium flares occur because each satellite has an unusual six-sided shape that readily bounces light toward Earth-based observers. A single flare can glow with an apparent magnitude much greater than Venus.

The International Space Station (ISS), because of its massive size, also glows as brightly as Venus or Jupiter. But it can be challenging to see because it stays close to the horizon and passes through "spells of visibility" -- times when it's easier to spot than others. Sources like Heavens Above can tell you when and where to look to catch a glimpse of the ISS. You will need your coordinates for longitude and latitude, available from theU.S. Geological Survey, and an accurate measure of the time.

In the more than five decades following the launch of Sputnik, satellites, as well as their budgets, have tended to get bigger. The United States, for example, has spent $200 billion on its military satellite program since its inception and now, despite the investment, has a fleet of aging devices without many replacements waiting in the wings [source: The New Atlantis]. Many experts fear that building and deploying large satellites is simply not sustainable, at least not by taxpayer-funded government agencies. One solution is to turn over satellite programs to private interests, such as SpaceX, Virgin Galactic or other space companies, which often don't suffer the same bureaucratic inefficiencies as NASA, the National Reconnaissance Office and the National Oceanic and Atmospheric Administration.

Another solution involves shrinking the size and complexity of satellites. Scientists at California Polytechnic State University and Stanford University have been working since 1999 on a new type of satellite, called CubeSat, that relies on building blocks as small as 4 inches (10 centimeters) on a side. Each cube receives off-the-shelf components and can be combined with other cubes, usually from different teams, to make a more complex payload. By standardizing the design and spreading the development costs to multiple parties, the costs of the satellite don't escalate as greatly. A single CubeSat spacecraft might cost less than $100,000 to develop, launch and operate [source: Pang].

In April 2013, NASA put this basic principle to the test when it launched three CubeSats built around commercial smartphones. The goal was to place the micro-satellites in orbit for a short time and collect some photographs and system data from the phones. NASA launched the satellites on April 21, and they re-entered Earth's atmosphere six days later. Now the agency is looking at how they can deploy a vast network of CubeSats for a coordinated, long-duration mission.

Big or small, future satellites must be able to communicate efficiently with Earth-based stations. Historically, NASA has relied on radio frequency (RF) communication, but RF is reaching its limit as demand for more capacity increases. To overcome this obstacle, NASA scientists have been developing a two-way communication system based on lasers instead of radio waves. The equipment to run the test hitched a ride on NASA's Lunar Atmosphere and Dust Environment Explorer, which launched in September 2013 and headed for the moon, where it began to orbit and collect information on the lunar atmosphere. On Oct. 18, 2013, researchers made history when they used a pulsed laser beam to transmit data over the 239,000 miles (384,633 kilometers) between the moon and Earth at a record-breaking download rate of 622 megabits per second [source: Buck].

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