Orbital mechanics

The shape of a satellite's orbit depends on its function. The further from the Earth a satellite is, the longer it takes to complete an orbit - not only because it has further to travel, but also because objects in orbit naturally move more slowly as they get further away from the object they are orbiting. One particularly useful orbit is the geostationary orbit first identified by Arthur C. Clarke (see p.246) and used by comsats and other craft that need to stay over a single spot on the Earth's surface. In geostationary orbit, a satellite sits precisely 35,786km (22,227 miles) above the equator, orbiting the Earth in 23 hours 56 minutes - the same time the planet itself takes to rotate. The satellite therefore stays over a single point on the equator, providing a fixed location in the Earth's skies.

However, most satellites fly much closer to our planet, in low Earth orbits (LEOs) a few hundred kilometres up, circling the world several times every day. This kind of altitude is close enough in for Earth-observing satellites to pick up fine details but far enough out to avoid drag from the upper

BETWEEN ORBITS

Many satellites have their own integral rocket stages, called kick motors, to push them into their final orbit. Intelsat 603, launched into LEO by a Titan rocket in 1990, became stranded after its kick motor failed. It took a rescue mission from the Space Shuttle (see p.207) to finally get it to geostationary orbit.

BETWEEN ORBITS

Many satellites have their own integral rocket stages, called kick motors, to push them into their final orbit. Intelsat 603, launched into LEO by a Titan rocket in 1990, became stranded after its kick motor failed. It took a rescue mission from the Space Shuttle (see p.207) to finally get it to geostationary orbit.

TECHNOLOGY

DELTA LAUNCHERS

TECHNOLOGY

DELTA LAUNCHERS

atmosphere. For astronomy satellites, it is also high enough to avoid the atmospheric absorption of various types of radiation (see pp.250-57). Only space stations and large spacecraft typically orbit lower than this, skimming the upper atmosphere so that their orbits are unstable unless repeatedly boosted.

Very few orbits are circular - most are ellipses, dipping closer to the Earth on one side and rising higher above it on the other. Often the difference is minor and has no operational effect, but some projects (such as the Soviet Molniya satellites) use highly elliptical orbits and deliberately take advantage of the satellite's slower speed when further from the Earth. In Molniya's case, the shape of the satellites' orbits ensures that they move very slowly across Russian skies, acting as easily tracked communications platforms in northern regions, where equatorial comsats may not be visible.

Differing inclinations

Another important factor in a satellite's orbit is its inclination, or tilt relative to the equator. Although there are fuel advantages to launching satellites into orbits over the equator (see p.249), such orbits are useless for applications such as remote sensing, since the spacecraft will pass repeatedly over the same narrow strip of land and ocean. Instead, many types of satellite use inclined orbits that take them over high latitudes. Although the satellite only covers a narrow strip of land during each pass, the combined effect of repeated orbits and the Earth's daily rotation can gradually build up coverage of much of our planet's surface. The most extreme inclinations, known as polar orbits, allow a satellite to study the entire planet - but, just as some launch sites make it easier to reach equatorial and geostationary orbits, so only certain sites are suitable for polar launches.

The Delta rocket originated as a three-stage vehicle built for NASA by the Douglas Aircraft Company (now part of Boeing), using elements of the Thor missile and the Vanguard rocket. Deltas rapidly became a mainstay of the US satellite launch programme, but after five decades of constant improvement the current variants bear little resemblance to the original. The present Delta II is a three-stage rocket assisted by nine solid-fuelled boosters around its base, while the heavy-lift Delta IV is a modular system that may use two boosters (as seen here) or even multiple rocket cores to lift heavy loads.

10 July 1962

Telstar, the world's first fully functional comsat, is launched.

14 February 1963

Syncom 1 is the first attempt to launch a geostationary comsat.

19 August 1964

Syncom 3 becomes the world's first truly geostationary comsat.

20 August 1964

INTELSAT is established to set up a global comsat network.

23 April 1965

The Soviet Union launches the first of its Molniya comsats.

30 May 1974

NASA's Applications Technology Satellite (ATS) 6 is launched. As the first satellite to offer direct broadcast to small receivers, it paves the way for satellite TV.

22 February 1978

Construction of the US Global Positioning System begins with the launch of NAVSTAR 1.

28 December 2005

Construction of Europe's Galileo Positioning System begins with the launch of a test satellite. The system is due to be operational by 2010.

LAUNCHING GALILEO

This artist's impression shows the release of the first Galileo technology test satellite from its Fregat upper stage in December 2005. The antenna on top of the satellite is able to broodcast GPS signals across the whole of the Earth below it.

SATELLITE PREPARATIONS

A Galileo GPS satellite is loaded into its protective shroud prior to launch on a Soyuz rocket with a Fregat upper stage. The satellite's manoeuvring motors are wrapped in gold foil.

Comsats and GPS

LAUNCHING GALILEO

This artist's impression shows the release of the first Galileo technology test satellite from its Fregat upper stage in December 2005. The antenna on top of the satellite is able to broodcast GPS signals across the whole of the Earth below it.

Orbiting communications relays and the Global Positioning System have had a huge impact on everyday life and spearheaded the commercialization of space.

Even before the dawn of the Space Age, it was clear that satellites had the potential to revolutionize communications, and the military were among the first to see the advantages. Telephone lines had limited capacity and were vulnerable to physical damage. Radio signals were fast but travelled in straight lines and so had limited range due to the curvature of the Earth (though there were attempts to bounce signals off the ionosphere, a layer of the upper atmosphere that reflects radio waves, with unpredictable results). In contrast, a satellite high in the sky could be used as a reflector, allowing signals beamed from one part of the world to be received anywhere else that the satellite was visible above the horizon. NASA's Echo project (see p.51) tested the idea with a simple reflector in orbit, while Telstar (see panel, opposite) was the first satellite with the capability to receive, amplify, and re-transmit signals.

Geostationary satellites

The ideal orbit for a comsat is the geostationary one noted by Arthur C. Clarke in 1945 (see panel, below, and p.244). From this point, a satellite maintains a steady position in the sky - it never rises or sets, and a receiver or transmitter aimed at it will not require adjustment. NASA pioneered the use of this orbit with its experimental series of Syncom satellites. Syncom 1 was lost before it could begin operations in February 1963, while Syncom 2, launched five months later, just missed the geostationary sweet

SATELLITE PREPARATIONS

A Galileo GPS satellite is loaded into its protective shroud prior to launch on a Soyuz rocket with a Fregat upper stage. The satellite's manoeuvring motors are wrapped in gold foil.

spot but was close enough to prove the principle. The first truly geostationary communications satellite was Syncom 3, launched in August 1964. This mission had an immediate public impact, as the satellite was used to beam live pictures from the 1964 Tokyo Olympics in Japan to American broadcasters.

The benefits of comsats were so clear that even as the technology was being proved President Kennedy was calling for the establishment of an international organization to set up a global communications network. Nine months after Kennedy's death, the International Telecommunications Satellite Organization, or INTELSAT, came into being in August 1964 with 11 initial member states. It launched its first satellite, Early Bird, in April 1965, and over four decades, its membership grew to more than 100 nations, before it was privatized in 2001. Its satellite technology, meanwhile, advanced from the relatively primitive Early Bird to the 5,500kg (12,1001b) Intelsat 10. The success of INTELSAT opened the way for commercial companies to follow in its footsteps, and the organization also formed a template for similar efforts to develop regional comsat networks and

ARTHUR C. CLARKE

After serving in the RAF during the Second World War (working on the development of radar), Arthur C. (b.1917) became an active member of the British Interplanetary Society. From the 1950s, he found fame as a science-fiction writer and shrewd prophet of technology - his most famous book is probably 2001: A Space Odyssey. Clarke suggested the use of geostationary satellites as signal relays in a 1945 article for Wireless World magazine, and although he was not the first to note the usefulness of geostationary orbits, his (independent) proposal was the first to be widely noticed. However, Clarke did fail to predict the rise of microelectronics, believing at the time that his relays would have to be manned.

HISTORY FOCUS

other satellite applications. One well-known example is the Iridium network, a series of 66 satellites that offers direct satellite communications for anyone with a suitable telephone. Iridium is widely used by the US military as well as commercial customers.

HISTORY FOCUS

other satellite applications. One well-known example is the Iridium network, a series of 66 satellites that offers direct satellite communications for anyone with a suitable telephone. Iridium is widely used by the US military as well as commercial customers.

After various experiments with orbiting reflectors and taped transmissions from orbit, Telstar became the world's first true communications satellite when it was launched by a Delta rocket on 10 July 1962. Built at Bell Telephone Laboratories in the United States, the satellite was part of a joint programme developed by Britain, France, and the US. Telstar had the ability to receive signals from the ground, amplify them (using power from the solar cells on its spherical surface), and retransmit them through the horn antennae around its equator. It went into service as soon as it reached orbit, successfully transmitting television signals, phone calls, and even faxes across the Atlantic.

distance to each satellite is calculated by receiver

TRIANGULATION FOR GPS

The details of GPS are complex, but the principle is simple - by calculating the time taken for signals from a number of different satellites to reach it, the receiver works out its distance from each one. It then uses a detailed model of the satellite orbits to calculate its location on Earth - a process called triongulation.

TRIANGULATION FOR GPS

The details of GPS are complex, but the principle is simple - by calculating the time taken for signals from a number of different satellites to reach it, the receiver works out its distance from each one. It then uses a detailed model of the satellite orbits to calculate its location on Earth - a process called triongulation.

Navigation from space

Another important spin-off from initial military interest has been the Global Positioning System (GPS). In the 1970s and 1980s, the United States established a satellite network called NAVSTAR that would allow anyone using a suitable receiver to calculate their location anywhere on Earth to within a few metres. In 1983, President Reagan announced it would be made available for civilian use, with some restrictions. Today, applications range from handheld devices to in-car navigation systems and emergency rescue beacons. Since the primary use of the NAVSTAR system was military, it is little wonder that the Soviet Union chose to develop its own version, called GLONASS, in the 1980s. Concerns about overdependence on a US military system also led the European Union to devise its own rival civilian system, called Galileo, which should be complete by 2010.

distance to each satellite is calculated by receiver

COMMERCIAL SPY

lkonos-2 is the world's first commercially operated reconnaissance satellite. Its operator GeoEye uses it to provide high-resolution mult ¡spectral and true-colour (panchromatic) images such as this one of San Francisco (right).

TSUNAMI WATCH

In December 2004, a massive Tsunami laid waste to coastlines around the Indian Ocean. Land sat 7 captured an image (above) of the waves breaking at Devi Point on the eastern shoreline of India.

HURRICANE WARNING

From high obove the Earth, GOES satellites send back images to assist in weather forecasting, climote science, and storm preparation. In September 2004, GOES-12 monitored Hurricane Jeanne obove the Gulf of Mexico.

GOES-L READY FOR FLIGHT

Weather satellites of the long-running Geostationary Operational Environmental Satellite (GOES) series (above) are built by NASA for NOAA, the Notional Oceanic ond Atmospheric Administration.

Watchers from above

Early satellites looked down at Earth to monitor the weather or collect military intelligence, but remote-sensing orbiters developed since the 1970s have probably done most to change the way we look at our world.

The first spy satellites were launched in the earliest days of the Space Race in the late-1950s. American Discoverer and Corona vehicles, and various Soviet Cosmos satellites, carried automatic cameras onboard, usually attached to downward-pointing telescopes. Exposed film from these cameras then had to be physically returned to Earth for processing.

Alternatives were pioneered by the US Satellite and Mssile Observation System (SAM0S) programme, and the Soviet Luna 3 spaceprobe. In 1959 Luna 3 developed its film onboard, then scanned the images and sent them back electronically (see p.53). The SAMOS satellites of the 1960s used television cameras, recording pictures over foreign territory and sending them back to Earth over the US. But for most intelligence purposes, only the highest resolutions will do - and since electronic cameras could not hope to match film until the 1980s, re-entry satellites remained in use by the military for some time.

Other applications for satellites viewing the Earth did not generally require such high-quality images and so relied much sooner on data transmitted from orbit. One obvious use for such satellites was for meteorology - the first experimental NASA weather satellite was TIROS (Television and Infrared Observation Satellite) 1, launched in April 1960. Early weather satellites remained close to the Earth in polar orbits, photographing one strip of the planet at a time - the first geostationary weather satellite, able to keep an entire hemisphere of Earth in constant view, was not launched until 1974.

THE BIG PICTURE

An early display of remote sensing's potential was this giant, cloud-free photographic map of the contiguous United States, assembled in 1972 from 595 satellite images by the US Department of Agriculture. ERTS 1's orbit allowed it to take images from a constant altitude of 912km (560 miles) in the same lighting conditions.

The Landsats are among the most successful of remote-sensing satellites, but many others have been developed. ESA and the Soviet Union launched their own equivalents, but as scientists have found new ways to probe our planet's properties, many more specialized missions have been launched.

Today's satellites monitor every aspect of the Earth, from air temperature to ocean circulation, and from wave height to wind speed. Satellites equipped with spectrometers can analyze radiation emitted or reflected from the ground below, revealing everything from crop usage to buried water and mineral resources. Synthetic aperture radar (SAR) can gather detailed information about the landscape (see p.267) and microwave radars can even penetrate the topsoil to reveal underground features.

THE BIG PICTURE

An early display of remote sensing's potential was this giant, cloud-free photographic map of the contiguous United States, assembled in 1972 from 595 satellite images by the US Department of Agriculture. ERTS 1's orbit allowed it to take images from a constant altitude of 912km (560 miles) in the same lighting conditions.

1 April 1960

NASA launches TIROS 1, its first weather satellite.

12 August 1960

The film capsule from US spy satellite Discoverer 13 is successfully recovered.

28 July 1962

The Soviet Union launches its first successful Zenit spy satellite under the codename Cosmos 7.

3 March 1969

Apollo 9 carries an experimental multi-spectral imager into orbit.

June 1971

The first civilian Soviet space station, Salyut 1, studies the potential of multispectral imaging.

23 July 1972

ERTS 1, the first remote-sensing satellite, is launched.

17 May 1974

NASA launches SMS 1 (Synchronous Meteorological Satellite), the first geostationary weather satellite.

25 June 1974

The first successful military space station, Salyut 3, carries a large reconnaissance camera onboard.

28 June 1978

NASA's Seasat carries the first synthetic aperture radar into orbit.

Earth remote sensing

The full potential of satellites for Earth observation became clear only when early astronauts reported seeing surprising detail from high altitudes. The Earth-orbiting Apollo 9 mission carried experiments to test ways of exploiting the view from orbit, including the first use of multispectral imaging (see panel, right). Soviet cosmonauts carried out similar experiments onboard the Salyut space stations, as did the astronauts aboard Skylab.

NASA launched its first satellite dedicated to these new remote-sensing techniques in 1972. Satellites in the Earth Resources Technology Satellite (ERTS) series was renamed Landsat after the launch of Landsat 2 in 1975, and the programme continues today.

TECHNOLOGY

MULTISPECTRAL IMAGING

The technique of multispectral imaging is simple but powerful - a camera fitted with a series of filters takes images of the same area in specific wavelengths (colours) of visible light and sometimes in infrared or ultraviolet light. When the images are compared or combined, features and properties that are normally invisible can be seen and analyzed - as in this map of the Malaspina Glacier in Alaska, which in visible light is simply lost in the snowy landscape. Often researchers will want to revisit the same area at intervals in order to reveal changes in the landscape. In this case, it is important that all the images are lit from the same angle, so remote-sensing satellites frequently occupy "Sun-synchronous" polar orbits: the satellite maintains its orientation to the Sun at all times (in other words, its orbit progresses slightly each day, circling the Earth once every year, while the Earth rotates daily beneath it).

28 June 1978

NASA's Seasat carries the first synthetic aperture radar into orbit.

8 April 1966

NASA launches OAO-1, but it fails in orbit.

4 July 1968

NASA launches RAE-1, which deploys a cross-shaped 450m (1,500ft) antenna in orbit.

7 December 1968

The Orbiting Astronomical Observatory 2 is launched and begins ultraviolet observations.

8 April 1966

NASA launches OAO-1, but it fails in orbit.

4 July 1968

NASA launches RAE-1, which deploys a cross-shaped 450m (1,500ft) antenna in orbit.

Astronomy in orbit

The arrival of the Space Age provided huge new opportunities for astronomers - finally they could send instruments above the Earth's atmosphere for a clear view of the Universe.

solar panel

OAO-1 SATELLITE

The Orbiting Astronomical Observatories were a series of ultraviolet telescopes launched from 1966 onwards.

| HISTORY FOCUS

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OBbtKVAIOKItb ON IHt MOON

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Before the microelectronics boom of the 1970s, it seemed as though many satellite instruments, including those for astronomy, would be impossible to automate. As a result, astronomy looked like being one of the main roles of any future moonbase. Operating under a long day-night cycle and in a thin atmosphere, a telescope on the Moon could see clearly into deep space - with protection from the surface glare, it could even operate when the Sun was above the horizon. A lunar observatory would also make it possible to construct larger telescopes than any that could be put in orbit. Today, advances in electronics and ingenious design have overcome many limitations of orbiting telescopes, but radio astronomers in particular still dream of building a telescope^ on the lunar far side.

IRAS SATELLITE

Launched in 1983, the Infrared Astronomical Satellite (IRAS, above) was the first orbital telescope to study the sky in the infrared (right). In order to pick up faint heat rodiation from the sky, it was heavily insuloted and cooled with liquid helium.

Astronomers have always been frustrated by the Earth's atmosphere - even on a clear night, turbulent air currents can distort and blur telescope images. In the 19th and 20th centuries, their frustration grew as they learned that visible light was just a small part of a spectrum of electromagnetic waves ranging from very long radio waves to ultra-short, high-energy gamma rays - and that the atmosphere did a thorough job of blocking out nearly all of them.

So when captured V-2 rockets became available at the end of the Second World War, astronomers were eager to utilize them to take a look at the Universe from beyond the atmosphere. Rocket-borne detectors soon revealed that space was full of exotic radiations - ultraviolet radiation from the Sun was discovered in 1946, and solar X-rays in 1949. Radio waves from the Milky Way (the plane of our galaxy) had previously been discovered from the ground in 1932 by American engineer Karl Jansky.

Early satellites added to these discoveries, sometimes unexpectedly. Long-wavelength radio waves (thought to originate in clouds of cool dust and gas) were revealed in the early 1960s by satellites that were actually built to study the Earth's ionosphere. The first X-ray source beyond the Solar System (now suspected to be a black hole) was identified in 1962. Dedicated observatories soon followed, though the earliest, such as the Orbiting thermal insulation around telescope

7 December 1968

The Orbiting Astronomical Observatory 2 is launched and begins ultraviolet observations.

July 1969

Gamma-ray bursts are first observed by US Vela satellites.

12 December 1970

SAS-1 (Uhuru), the first X-ray detector in space, is launched.

15 November 1972

NASA launches SAS-2, the first dedicated gamma-ray observatory.

12 November 1978

The Einstein satellite (HEA0-2) becomes the first imaging X-ray telescope in orbit.

25 January 1983

IRAS, the first infrared space telescope, is launched and operates for ten months.

solar panel

Before the microelectronics boom of the 1970s, it seemed as though many satellite instruments, including those for astronomy, would be impossible to automate. As a result, astronomy looked like being one of the main roles of any future moonbase. Operating under a long day-night cycle and in a thin atmosphere, a telescope on the Moon could see clearly into deep space - with protection from the surface glare, it could even operate when the Sun was above the horizon. A lunar observatory would also make it possible to construct larger telescopes than any that could be put in orbit. Today, advances in electronics and ingenious design have overcome many limitations of orbiting telescopes, but radio astronomers in particular still dream of building a telescope^ on the lunar far side.

Solar Observatory (OSO) series, were intended to reveal more about the Sun rather than target more distant objects. The first successful satellites designed to look further afield were NASA's Radio Astronomy Explorer (RAE) missions, launched from 1968 onwards. The RAEs recorded radio waves from the Sun, Jupiter, and various sources elsewhere in our galaxy.

The first X-ray astronomy satellite was NASA's Small Astronomy Satellite (SAS) 1, also known as Uhuru, launched in 1970. This revealed X-ray sources scattered across the sky and led to a variety of later missions.

OAO-1 SATELLITE

The Orbiting Astronomical Observatories were a series of ultraviolet telescopes launched from 1966 onwards.

CLOSE TO THE SUN

In addition to astronomy satellites for looking into deep space, there are many spacecraft that direct similar instruments towards the Sun. Launched in 1995, SOHO, the Solar and Heliospheric Observatory (above), is a joint project of NASA and ESA. It observes the Sun in visible and ultroviolet light, revealing turbulent activity such as prominences (main picture) and enormous outbursts known as coronal moss ejections (below).

Another new field of astronomy opened when US Vela spy satellites, designed to look for evidence of nuclear testing on Earth, detected bursts of gamma rays from deep space. Their existence was confirmed in 1972 by SAS-2, which also found gamma-ray sources in the remnants of exploded stars. ESA's Cos-B gamma-ray satellite of 1975 was followed by several French experiments carried aboard Soviet spacecraft and space stations.

Beyond the blue, within the red

The first successful ultraviolet satellite was Orbiting Astronomical Observatory (0A0) 2. Launched in 1968, it studied the ultraviolet properties of thousands of stars, as well as objects such as comets and galaxies. Other ultraviolet observatories soon followed.

solar array

45cm ( 18in) reflector telescope solar array

ULTRAVIOLET OBSERVATORY

The highly productive International Ultraviolet Explorer (IUE), a joint venture between NASA, ESA, and the United Kingdom, operated for 18 years and was the first satellite that astronomers could operate from the ground in real time.

The last major area of the electromagnetic spectrum to be explored from space was the infrared. The nature of this heat radiation creates unique challenges - since the telescope itself is an infrared source, it must be cooled to very low temperatures to avoid swamping the weak infrared light from stars and other objects. The challenges were first overcome with the Infrared Astronomical Satellite (IRAS), a joint British, American, and Dutch mission launched in 1983.

ULTRAVIOLET OBSERVATORY

The highly productive International Ultraviolet Explorer (IUE), a joint venture between NASA, ESA, and the United Kingdom, operated for 18 years and was the first satellite that astronomers could operate from the ground in real time.

instrument casing solar array

45cm ( 18in) reflector telescope solar array

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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