Electromagnetic Waves: Radio Message from Mars

Identifying the Hypothesis Statement

Science in space
In the decades following the first Sputnik and Explorer satellites, the ability to put their instruments into outer space gave scientists the opportunity to acquire new information about the natural universe, information that in many cases would have been unobtainable any other way. Space science added a new dimension to the quest for knowledge, complementing and extending what had been gained from centuries of theoretical speculations and ground-based observations.
After Gagarin's 1961 flight, space missions involving human crews carried out a range of significant research, from on-site geologic investigations on the Moon to a wide variety of observations and experiments aboard orbiting spacecraft. In particular, the presence in space of humans as experimenters and, in some cases, as experimental subjects facilitated studies in biomedicine and materials science. Nevertheless, most space science was, and continues to be, performed by robotic spacecraft in Earth orbit or on missions to various bodies in the solar system. In general, such missions are far less expensive than those involving humans and can carry sophisticated automated instruments to gather a wide variety of relevant data.
In addition to the United States and the Soviet Union, several other countries achieved the capability of developing and operating scientific spacecraft and thus carrying out their own space science missions. They include Japan, Canada, and a number of European countries such as the United Kingdom, France, Italy, and Germany, acting alone and through cooperative organizations involving other European countries. Furthermore, many other countries became involved in space activities through the participation of their scientists in specific missions. Bilateral or multilateral cooperation between various countries in carrying out space science missions grew to be the usual way of proceeding.
Scientific research in space can be divided into five general areas: (1) solar and space physics, including study of the magnetic and electromagnetic fields in space and the various energetic particles also present, with particular attention to their interactions with Earth, (2) exploration of the planets, moons, asteroids, comets, meteoroids, and dust in the solar system, (3) study of the origin, evolution, and current state of the varied objects in the universe beyond the solar system, (4) research on nonliving and living materials, including humans, in the very low gravity levels of the space environment, and (5) study of Earth from space.

Solar and space physics

Earth's full north polar auroral oval, in an image taken in ultraviolet light by the U.S. Polar ...
NASA
The first scientific discovery made with instruments orbiting in space was the existence of the Van Allen radiation belts, by Explorer 1 and other spacecraft in 1958. Subsequent space missions investigated Earth's magnetosphere, the surrounding region of space in which the planet's magnetic field exerts a controlling effect (see Earth: The magnetic field and magnetosphere). Of particular and ongoing interest has been the interaction of the flux of charged particles emitted by the Sun, called the solar wind, with the magnetosphere. Early space science investigations showed, for example, that luminous atmospheric displays known as auroras are the result of this interaction, and scientists came to understand that the magnetosphere is an extremely complex phenomenon.
The focus of inquiry in space physics was later extended to understanding the characteristics of the Sun, both as an average star and as the primary source of energy for the rest of the solar system, and to exploring space between the Sun and Earth and other planets (see interplanetary medium). The magnetospheres of other planets, particularly Jupiter with its strong magnetic field, also came under study. Scientists sought a better understanding of the internal dynamics and overall behaviour of the Sun, the underlying causes of variations in solar activity, and the way in which those variations propagate through space and ultimately affect Earth's magnetosphere and upper atmosphere. The concept of space weather was advanced to describe the changing conditions in the Sun-Earth region of the solar system. Variations in space weather can cause geomagnetic storms that interfere with the operation of satellites and even systems on the ground such as power grids.

A spectacular flare on the Sun, photographed in extreme ultraviolet light on December 19, 1973, by ...
NASA
To carry out the investigations required for addressing these scientific questions, the United States, Europe, the Soviet Union, and Japan developed a variety of space missions, often in a coordinated fashion. In the United States, early studies of the Sun were undertaken by a series of Orbiting Solar Observatory satellites (launched 1962-75) and the astronaut crews of the Skylab space station in 1973-74, using that facility's Apollo Telescope Mount. These were followed by the Solar Maximum Mission satellite (launched 1980). ESA developed the Ulysses mission (1990) to explore the Sun's polar regions. Solar-terrestrial interactions were the focus of many of the Explorer series of spacecraft (1958-75) and the Orbiting Geophysical Observatory satellites (1964-69). In the 1980s NASA, ESA, and Japan's Institute of Space and Astronautical Science undertook a cooperative venture to develop a comprehensive series of space missions, named the International Solar-Terrestrial Physics Program, that would be aimed at full investigation of the Sun-Earth connection. This program was responsible for the U.S. Wind (1994) and Polar (1996) spacecraft, the European Solar and Heliospheric Observatory (SOHO; 1995) and Cluster (2000) missions, and the Japanese Geotail satellite (1992).

Solar system exploration

Luna 9, the first spacecraft to soft-land on the Moon. It was launched by the Soviet Union January ...
Novosti Press Agency
From the start of space activity, scientists recognized that spacecraft could gather scientifically valuable data about the various planets, moons, and smaller bodies in the solar system. Both the United States and the U.S.S.R. attempted to send robotic missions to the Moon in the late 1950s. The first four U.S. Pioneer spacecraft, Pioneer 0-3, launched in 1958, were not successful in returning data about the Moon. The fifth mission, Pioneer 4 (1959), was the first U.S. spacecraft to escape Earth's gravitational pull; it flew by the Moon at twice the planned distance but returned some useful data. Three Soviet missions, Luna 1-3, explored the vicinity of the Moon in 1959, confirming that it had no appreciable magnetic field and sending back the first-ever images of its far side. Luna 1 was the first spacecraft to fly past the Moon, beating Pioneer 4 by two months; Luna 2, in making a hard landing on the lunar suface, was the first spacecraft to strike another celestial object. Later Luna spacecraft soft-landed on the Moon, and some gathered soil samples and returned them to Earth.

Viking 2 lander (foreground) on Mars, photographed by one of the spacecraft's own cameras, 1976.
NASA/JPL

Merged pancake domes on the eastern edge of the Alpha Regio highland area of Venus, in an oblique ...
Photo NASA/JPL/Caltech (NASA photo # PIA00246)

Composite image of the nucleus of Comet Halley produced from 68 photographs taken on March ...
Courtesy of H.U. Keller; copyright Max-Planck-Institut fur Aeronomie, Lindau, Ger., 1986
In the 1960s the United States became the first country to send a spacecraft to the vicinity of other planets; Mariner 2 flew by Venus in December 1962, and Mariner 4 flew past Mars in July 1965. Among significant accomplishments of planetary missions in succeeding decades were the U.S. Viking landings on Mars in 1976 and the Soviet Venera explorations of the atmosphere and surface of Venus from the mid-1960s to the mid-1980s. In the years since, the United States has continued an active program of solar system exploration, as did the Soviet Union until its dissolution in 1991. Japan launched missions to the Moon, Mars, and Halley's Comet. Europe's first independent solar system mission, Giotto, also flew by Halley. After the turn of the 21st century, it sent missions to the Moon and Mars and an orbiter-lander to a comet.

U.S. spacecraft Galileo making a flyby of Jupiter's moon Io, in an artist's rendering. At the stage ...
National Aeronautics and Space Administration

U.S. Near Earth Asteroid Rendezvous (NEAR) space probe in orbit around an asteroid, in an artist's ...
NASA

Opposite hemispheres of the asteroid Eros, shown in a pair of mosaics made from images taken by the ...
John Hopkins University/Applied Physics Laboratory/NASA
Early on, scientists planned to conduct solar system exploration in three stages: initial reconnaissance from spacecraft flying by a planet, comet, or asteroid; detailed surveillance from a spacecraft orbiting the object; and on-site research after landing on the object or, in the case of the giant gas planets, by sending a probe into its atmosphere. By the start of the 21st century, all three of those stages had been carried out for the Moon, Venus, Mars, Jupiter, and a near-Earth asteroid. Several Soviet and U.S. robotic spacecraft have landed on Venus and the Moon, and the United States has landed spacecraft on the surface of Mars. A long-term, detailed surveillance of Jupiter and its moons began in 1995 when the U.S. Galileo spacecraft took up orbit around the planet, at the same time releasing a probe into the turbulent Jovian atmosphere. In 2001 the U.S. Near Earth Asteroid Rendezvous (NEAR) spacecraft landed on the asteroid Eros and transmitted information from its surface for more than two weeks. Among the rocky inner planets, only Mercury has remained relatively neglected. In the first half century of space exploration, Mercury was visited just once—the U.S. Mariner 10 probe made three flybys of the planet in 1974-75. In 2004 the U.S. Messenger spacecraft was launched to Mercury for a series of flybys beginning in 2008 and establishment of an orbit around the planet in 2011.

This animation shows the paths of the Voyager 1 and 2 spacecraft, which were designed to explore ...
NASA
As of 2005, the exploration of the other giant gas planets—Saturn, Uranus, and Neptune—remained at the first or second stage. In a series of U.S. missions launched in the 1970s, Pioneer 10 flew by Jupiter, whereas Pioneer 11 and Voyager 1 and 2 flew by both Jupiter and Saturn. Voyager 2 then went on to travel past Uranus and Neptune. The U.S. Cassini spacecraft, launched in 1997, began a long-term surveillance mission in the Saturnian system in 2004; the following year its European-built Huygens probe descended to the surface of Titan, Saturn's largest moon. Thus, every significant body in the solar system except the dwarf planet Pluto and its largest moon, Charon, has been visited at least once by a spacecraft.
These exploratory missions sought information on the origin and evolution of the solar system and on the various objects that it comprises, including chemical composition; surface topography; data on magnetic fields, atmospheres, and volcanic activity; and—particularly for Mars and perhaps eventually for Jupiter's moon Europa and Saturn's moon Titan—evidence of water or other liquids in the present or past and perhaps even of extraterrestrial life in some form.
What has been learned to date confirms that Earth and the rest of the solar system formed at about the same time from the same cloud of gas and dust surrounding the Sun. The four outer giant gas planets are roughly similar in size and chemical composition, but each has a set of moons that differ widely in their characteristics, and in some ways they and their satellites resemble miniature solar systems. The four rocky inner planets had a common origin but followed very different evolutionary paths and today have very different surfaces, atmospheres, and internal activity. Ongoing comparative study of the evolution of Venus, Mars, and Earth could provide important insights into Earth's future and its continued ability to support life.

Gullies along the south-facing wall of the Martian valley Nirgal Vallis, in an image taken July 12, ...
Malin Space Science Systems/JPL/NASA
The question of whether life has ever existed elsewhere in the solar system continues to intrigue both scientists and the general public. The United States sent two Viking spacecraft to land on the surface of Mars in 1976. Each contained three experiments intended to search for traces of organic material that might indicate the presence of past or present life-forms; none of the experiments produced positive results. Twenty years later, a team of scientists studying a meteorite of Martian origin found in Antarctica announced the discovery of possible microscopic fossils resulting from past organic life. Their claim was not universally accepted, but it led to an accelerated program of Martian exploration focused on the search for evidence of the action of liquid water, thought necessary for life to have evolved. A major goal of this program is to return samples of the Martian surface to Earth for laboratory analysis.

An elaborately patterned area of disrupted ice crust on Europa's surface, shown in an image made ...
NASA/JPL
The Galileo mission provided images and other data related to Jupiter's moon Europa that suggest the presence of a liquid water ocean beneath its icy crust. Future missions will seek to confirm the existence of this ocean and search for evidence of organic or biological processes in it.

Exploring the universe
Until the dawn of spaceflight, astronomers were limited in their ability to observe objects beyond the solar system to those portions of the electromagnetic spectrum that can penetrate Earth's atmosphere. These portions include the visible region, parts of the ultraviolet region, and most of the radio-frequency region. The ability to place instruments on a spacecraft operating above the atmosphere (see satellite observatory) opened the possibility of observing the universe in all regions of the spectrum. Even operating in the visible region, a space-based observatory could avoid the problems caused by atmospheric turbulence and airglow.

Two computer-coloured images of the Cat's Eye Nebula (NGC 6543) made from data gathered by ...
NASA; (left) UIUC/Y.Chu et al.; (right) HST
Beginning in the 1960s, a number of countries launched satellites to explore cosmic phenomena in the gamma-ray, X-ray, ultraviolet, visible, and infrared regions. More recently, space-based radio astronomy has been pursued. In the last decades of the 20th century, the United States embarked on the development of a series of long-duration orbital facilities collectively called the Great Observatories. They include the Hubble Space Telescope, launched in 1990 for observations in the visible and ultraviolet regions; the Compton Gamma Ray Observatory, launched in 1991; the Chandra X-Ray Observatory, launched in 1999; and the Spitzer Space Telescope, launched in 2003. Europe and Japan have also been active in space-based astronomy and astrophysics.
The results of these space investigations have made major contributions to an understanding of the origin, evolution, and likely future of the universe, galaxies, stars, and planetary systems. For example, the U.S. Cosmic Background Explorer (COBE) satellite, launched in 1989, mapped the microwave background radiation (see Cosmos: Microwave background radiation) left over from the early universe, providing strong support for the theory that the universe was created in a primordial explosion, known as the big bang (see big-bang model). The striking images of cosmic objects obtained by the Hubble Space Telescope not only have added significantly to scientific knowledge but also have shaped the public's perception of the cosmos, perhaps as significantly as did the astronomer Galileo's observations of the Moon and Jupiter nearly four centuries earlier. Working as complements to ground-based observatories of increasing sensitivity, space-based observatories have helped create a revolution in modern astronomy.

Microgravity research

Conditions of microgravity, or weightlessness, permeate every aspect of daily life aboard the ...
Copyright © 2004 AIMS Multimedia (www.aimsmultimedia.com)
A spacecraft orbiting Earth is essentially in a continual state of free fall. All objects associated with the spacecraft, including any crew and other contents, are accelerating—i.e., falling freely—at the same rate in Earth's gravitational field (see Earth: Basic planetary data). As a result, these objects do not "feel" the presence of Earth's gravity but instead experience a state of weightlessness, or zero gravity. True zero gravity, however, is experienced only at the centre of mass of a freely falling object. With increasing distance from the centre of mass, the influence of gravity increases in both directions perpendicular to the object's flight path. These constant but tiny accelerations make necessary the use of the term microgravity to describe the space environment. (It is possible to create a similar absence of gravity's effects only briefly on Earth or in an aircraft.) Human activity or the operation of equipment in a spacecraft causes vibrations that impart additional accelerations and so raise gravity levels, which can make it difficult to carry out highly sensitive experiments under sufficiently low microgravity conditions. Although spacecraft designers cannot totally eliminate gravitational effects, they hope to reduce them in some parts of the International Space Station to one microgravity—one-millionth of Earth's gravity—by isolating those areas from vibrations and other disturbances as much as possible.
The opportunity to carry out experiments in the absence of gravity has interested scientists from the beginning of activities in orbit. In addition to concern about the effects of the weightlessness on humans sent into space (see above Biomedical, psychological, and sociological aspects), scientists are interested in its effects on the reproductive and developmental cycles of plants and animals other than humans. The overall goal is to use space-based research to add to the general understanding of a wide range of biological processes.
Life-sciences experiments were carried out on the Skylab, Salyut, and Mir space stations and will constitute a significant portion of work aboard the ISS. Such research also was conducted on space shuttle missions, particularly within the Spacelab facility. In addition, the Soviet Union and the United States launched a number of robotic satellites dedicated to life-sciences research. Together these experiments have involved a wide range of nonhuman organisms, from bacteria, plants, and invertebrate animals to fish, birds, frogs, turtles, and mammals such as rats and monkeys. Human crew members also have served as experimental subjects for research on such topics as the functioning of the neurological system and the process of aging. In October 1998, U.S. senator and former Mercury astronaut John H. Glenn, Jr., at age 77 returned to space on a shuttle mission dedicated to life-sciences research, which included studies of similarities between the aging process and the body's response to weightlessness. The hope is that the results of biomedical experiments conducted in microgravity can be used to improve human health and well-being on Earth.

West German physicist-astronaut Ulf Merbold conducting a materials-processing experiment aboard ...
NASA
The microgravity environment also offers unique conditions for experiments designed to explore the behaviour of materials. Among the areas of inquiry are biotechnology, combustion science, fluid physics, fundamental physics, and materials science. Experiments in the microgravity environment on various materials, including metals, alloys, electronic and photonic materials, composites, colloids, glasses and ceramics, and polymers, have resulted in a greater understanding of the role of gravity in similar laboratory and manufacturing processes on Earth. The microgravity environment offers the potential for producing biological materials, including highly ordered protein crystals for crystallographic analysis and even materials resembling human tissues, that are difficult or impossible to make on Earth. Although microgravity research is still largely at the basic level, scientists and engineers hope that additional work—another major focus for the ISS—will lead to practical knowledge of great usefulness to manufacturing processes on Earth.

Observing Earth

Gravity map of Earth's ocean surface, computed from radar-altimetry measurements made from orbit by ...
D.T. Sandwell from Scripps Institution of Oceanography, W.H.F. Smith from National Oceanic and Atmospheric Administration/National Ocean Service/Office of Ocean & Earth Science/Geoscience Lab
Satellites, space stations, and space shuttle missions have provided a new perspective for scientists to collect data about Earth itself. In addition to practical applications (see below Space applications), Earth observation from space has made significant contributions to fundamental knowledge. An early and continuing example is the use of satellites to make various geodetic measurements, which has allowed precise determinations of Earth's shape, internal structure, and rotational motion and the tidal and other periodic motions of the oceans. Fields as diverse as archaeology, seismology, and oceanography likewise have benefited from observations and measurements made from orbit.
Scientists have begun to use observations from space as part of comprehensive efforts in fields such as oceanography and ecology to understand and model the causes, processes, and effects of global climate change, including the influence of human activities. The goal is to obtain comprehensive sets of data over meaningful time spans about key physical, chemical, and biological processes that are shaping the planet's future. This is a coordinated international effort, in which the United States, Europe, and Japan are providing satellites to obtain the needed observations.

Space applications

Basic characteristics of orbits in which a satellite can be placed around Earth, categorized by ...
Encyclopædia Britannica, Inc.
Space visionaries in the early 20th century recognized that putting satellites into orbit could furnish direct and tangible benefits to people on Earth. For example, Arthur C. Clarke in 1945 described a way in which three satellites in orbit about 35,800 km (22,250 miles) above the Equator could relay communications around the globe. In this orbit, called a geostationary orbit, the satellites would have an orbital period equal to Earth's rotational period and thus appear from the ground to be stationary in the sky. (For additional information on satellite orbits, see spaceflight: Earth orbit.) A report for the U.S. Army Air Forces in 1946 by Project RAND (the predecessor of the RAND Corporation) identified the benefits of being able to observe Earth from space, which included distinguishing the impact sites of bombs dropped by U.S. aircraft and improved weather forecasting.
Space development, the practical application of the capabilities of spacecraft and of the data collected from space, has evolved in parallel with space exploration. There are two general categories of space applications. One provides benefits that are considered public goods—i.e., that cannot easily be marketed to individual purchasers—and thus are usually provided by governments, using public funds. Examples of public-good space applications include meteorology; navigation, position location, and timing; and military and national security uses. The other category of applications provides goods or services that can be sold to purchasers at a profit. These applications are the basis for the commercial development of space by the private sector. Examples of existing commercial space applications include various forms of telecommunications via satellites, remote sensing of Earth's surface, and commercial space transportation. Other applications, such as space tourism, space-based power generation, the manufacture of high-value materials in a microgravity environment, and the commercial development of extraterrestrial resources, may appear in the future.
Many space applications have both civilian and military uses, and thus similar systems have been developed by both sectors. How to manage and use these dual-purpose systems effectively is a continuing policy issue.

Meteorology
Meteorologists initially thought that satellites would be used primarily to observe cloud patterns and thus provide warnings of impending storms. They did not expect space observations to be central to improved weather forecasting overall. Nevertheless, as the technology of space-based instrumentation became more sophisticated, satellites were called upon to provide three-dimensional profiles of additional variables in the atmosphere, including temperature, moisture content, and wind speed. These data have become critical to modern weather forecasting.
Meteorological satellites are placed in one of two different kinds of orbit. Satellites in geostationary orbit provide continuous images of cloud patterns over large areas of Earth's surface. From changes in those patterns, meteorologists can deduce wind speeds and locate developing storms. Satellites in lower orbits aligned in a north-south direction, called polar orbits, can obtain more detailed data about changing atmospheric conditions. They also provide repetitive global coverage as Earth rotates beneath their orbit. In the United States, military and civilian agencies each have developed independent polar-orbiting meteorological satellite systems; China, Europe, and the Soviet Union also have deployed their own polar-orbiting satellites. The United States, Europe, the Soviet Union, India, and Japan have orbited geostationary meteorological satellites.
Although the research and development activity needed to produce meteorological satellites has been carried out by various space agencies, control over satellite operation usually has been handed over to organizations with general responsibility for weather forecasting. In the United States the National Oceanographic and Atmospheric Administration (NOAA) operates geostationary and polar-orbiting satellites for short- and long-term forecasting; the Department of Defense (DOD) also has developed similar satellites for military use. Operation of U.S. civilian and military polar satellite programs have been combined under joint NOAA-DOD management. In Europe an intergovernmental organization called Eumetsat was created in 1986 to operate Europe's meteorological satellites and provide their observations to national weather services. Agencies around the world cooperate in the exchange of data from their satellites. Meteorological satellites are an excellent example of both the ability of space systems to provide extremely valuable benefits to humanity and the need for international cooperation to maximize those benefits.

Positioning, navigation, and timing

U.S. Navstar Global Positioning System (GPS) satellite in orbit over Earth, shown in an artist's ...
Courtesy of the Lockheed Martin Corporation
In 1957 scientists tracking the first satellite, Sputnik 1, found that they could plot the satellite's orbit very precisely by analyzing the Doppler shift (see Doppler effect) in the frequency of its transmitted signal with respect to a fixed location on Earth. They understood that if this process could be reversed—i.e., if the orbits of several satellites were precisely known—it would be possible to identify one's location on Earth by using information from those satellites.
This realization, coupled with the need to establish the position of submarines carrying ballistic missiles, led the United States and the Soviet Union each to develop satellite-based navigation systems in the 1960s and early '70s. Those systems, however, did not provide highly accurate information and were unwieldy to use. The two countries then developed second-generation products—the U.S. Navstar Global Positioning System (GPS) and the Soviet Global Navigation Satellite System (GLONASS)—that did much to solve the problems of their predecessors. The original purpose of the systems was the support of military activities, and, at the start of the 21st century, they continued to operate under military control.
GPS requires a minimum of 24 satellites, with four satellites distributed in each of six orbits. Deployment of the full complement of satellites was completed in 1994 and included provision for continual replenishment and updating and the maintenance of several spare satellites in orbit. Each satellite carries four atomic clocks accurate to one nanosecond. Because the satellites' orbits are maintained very precisely by ground controllers and the time signals from each satellite are highly accurate, users with a GPS receiver can determine their distance from each of a minimum of four satellites and, from this information, pinpoint their exact location in three dimensions with an accuracy of approximately 18 metres (59 feet) horizontally and 28 metres (92 feet) vertically. GLONASS, which became operational in 1996, functions on the same general principles as GPS. A fully deployed system would consist of 24 satellites distributed in three orbits. Because of Russia's economic difficulties, however, GLONASS has not been well maintained, and replacement satellite deployment has been slow.
Notwithstanding the military origin of GPS and GLONASS, civilian users have proliferated. They range from wilderness campers, farmers, golfers, and recreational sailors to surveyors, car-rental firms, bus and truck fleets, and the world's airlines. The timing information from GPS satellites is also used by the Internet and other computer networks to manage the flow of information. Users have found ways to increase the accuracy of position location to a few centimetres by combining GPS signals with ground-based enhancements, and affordable GPS receivers make the system widely accessible. The United States regards GPS as a global utility to be offered free of charge to all users, and it has stated its intent to maintain and upgrade the system into the indefinite future. Concern has been expressed, however, that important worldwide civilian activities such as air traffic control should not depend on a system controlled by one country's military forces. In response, Europe began in the late 1990s to develop its own navigation satellite system, called Galileo, to be operated under civilian control.

Military and national security uses of space
Those countries and organizations with armed forces deployed abroad were quick to recognize the great usefulness of space-based systems in military operations. The United States, the Soviet Union, the United Kingdom, the North Atlantic Treaty Organization (NATO), and, to a lesser degree, other European countries and China have deployed increasingly sophisticated space systems—including satellites for communications, meteorology, and positioning and navigation—that are dedicated to military uses. In addition, the United States and the Soviet Union have developed satellites to provide early warning of hostile missile launches. Many of these satellites have been designed to meet unique military requirements, such as the ability to operate in a wartime environment, when an opponent may try to interfere with their functioning.
To date, military space systems have served primarily to enhance the effectiveness of ground-, air-, and sea-based military forces. Commanders rely on satellites to communicate with troops on the front lines, and, in extreme circumstances, national authorities could use them to issue the commands to launch nuclear weapons. Meteorological satellites assist in planning air strikes, and positioning satellites are used to guide weapons to their targets with high accuracy.
Despite the substantial military use of space, no country has deployed a space system capable of attacking a satellite in orbit or of delivering a weapon to a target on Earth. Nevertheless, as more countries acquire military space capabilities and as regional and local conflicts persist around the world, it is not clear whether space will continue to be treated as a weapons-free sanctuary.

Two U.S. Corona reconnaissance satellite images made a year apart—in mid-1961 (top) and ...
National Reconaissance Office
In addition to recognizing the value of space systems in warfare, national leaders in the United States and the Soviet Union realized early on that the ability to gather information about surface-based activities such as weapons development and deployment and troop movements would assist them in planning their own national security activities. As a result, both countries deployed a variety of space systems for collecting intelligence. They include reconnaissance satellites that provide high-resolution images of Earth's surface in close to real time for use in identifying threatening activities, planning military operations, and monitoring arms-control agreements. Other satellites collect electronic signals such as telephone, radio, and Internet messages and other emissions, which can be used to determine the type of activities that are taking place in a particular location. Most national-security space activity is carried out in a highly secret manner. As the value to national security of such satellite systems has become evident, other countries, such as France, China, India, and Israel, have developed similar capabilities, and still others have begun planning their own systems.

Satellite telecommunications

Telstar 1, launched July 10, 1962, relayed the first transatlantic television signals.
NASA
Although some early space experiments explored the use of large orbiting satellites as passive reflectors of signals from point to point on Earth, most work in the late 1950s and early '60s focused on the technology by which a signal sent from the ground would be received by satellite, electronically processed, and relayed to another ground station. American Telephone and Telegraph, recognizing the commercial potential of satellite communications, in 1962 paid NASA to launch its first Telstar satellite. Because that satellite, which operated in a fairly low orbit, was in range of any one receiving antenna for only a few minutes, a large network of such satellites would have been necessary for an operational system. Engineers from the American firm Hughes Aircraft, led by Harold Rosen, developed a design for a satellite that would operate in geostationary orbit. Aided by research support from NASA, the first successful geostationary satellite, Syncom 2, was launched in 1963; it demonstrated the feasibility of the Hughes concept prior to commercial use.
The United States also took the lead in creating the organizational framework for communications satellites. Establishment of the Communications Satellite Corporation (Comsat) was authorized in 1962 to operate American communications satellites, and two years later an international agency, the International Telecommunications Satellite Organization (Intelsat), was formed at the proposal of the United States to develop a global network. Comsat, the original manager of Intelsat, decided to base the Intelsat network on geostationary satellites. The first commercial communications satellite, Intelsat 1, also known as Early Bird, was launched in 1965. Intelsat completed its initial global network with the stationing of a satellite over the Indian Ocean in mid-1969, in time to televise the first Moon landing around the world.
The original use of communications satellites was to relay voice, video, and data from one relatively large antenna to a second, distant one, from which the communication then would be distributed over terrestrial networks. This point-to-point application introduced international communications to many new areas of the world, and in the 1970s it also was employed domestically within a number of countries, especially the United States. As undersea fibre-optic cables improved in carrying capacity and signal quality, they became competitive with communications satellites; the latter responded with comparable technological advances that allowed these space-based systems to meet the challenge. A number of companies in the United States and Europe manufacture communications satellites and vie for customers on a global basis.
Other space-based communications applications have appeared, the most prominent being the broadcast of signals, primarily television programming, directly to small antennas serving individual households. A similar emerging use is the broadcast of audio programming to small antennas in locations ranging from rural villages in the developing world to individual automobiles. International private satellite networks have emerged as rivals to the government-owned Intelsat, which as of 2001 was itself being transformed into a private-sector organization.
Yet another service that has been devised for satellites is communication with and between mobile users. In 1979 the International Maritime Satellite Organization (Inmarsat) was formed to relay messages to ships at sea. Beginning in the late 1990s, with the growth of personal mobile communications such as cellular telephone services, several attempts were made to establish satellite-based systems for this purpose. Typically employing constellations of many satellites in low Earth orbit, they experienced difficulty competing with ground-based cellular systems. At the start of the 21st century, the outlook for their economic viability was not good, but other satellite-based personal communication systems, including some based on geostationary satellites, were under development.
The first commercial space application was satellite communications, and that remained the most successful one. One estimate of revenues associated with the industry for the year 2000 included $16 billion for satellite manufacturing, $18 billion for the associated ground systems, and $40 billion from the users of satellite revenues, for a total of $74 billion. In the same year, there were some 235 commercial geostationary communications satellites around the world, operated by almost 60 different owners.

Remote sensing
Remote sensing is a term applied to the use of satellites to observe various characteristics of Earth's land and water surfaces in order to obtain information valuable in mapping, mineral exploration, land-use planning, resource management, and other activities. Remote sensing is carried out from orbit with multispectral sensors; i.e., observations are made in several discrete regions of the electromagnetic spectrum that include visible light and usually other wavelengths. From multispectral imagery, analysts are able to derive information on such varied areas of interest as crop condition and type, pollution patterns, and sea conditions.

Colour-coded Landsat satellite images of Brazil's Carajás mining area, documenting extensive ...
NASA Landsat Pathfinder/Tropical Rainforest Information Center
Because many applications of remote sensing have a public-good character, a commercial remote-sensing industry has been slow to develop. In addition, the secrecy surrounding intelligence-gathering satellites during the Cold War era set stringent limits on the capabilities that could be offered on a commercial basis. The United States launched the first remote-sensing satellite, NASA's Landsat 1 (originally called Earth Resources Technology Satellite), in 1972. The goals of the Landsat program, which by 1999 had included six successful satellites, were to demonstrate the value of multispectral observation and to prepare the system for transfer to private operators. Despite two decades of attempts at such a transfer, Landsat remained a U.S. government program at the start of the 21st century. In 1986 France launched the first of its SPOT remote-sensing satellites and created a marketing organization, Spot Image, to promote use of its imagery. Both Landsat's and SPOT's multispectral images offered a moderate ground resolution of 10-30 metres (about 33-100 feet). Japan and India also launched multispectral remote-sensing satellites.
In the 1990s, with the end of the Cold War, some of the technology used in reconnaissance satellites was declassified. This allowed several American firms to begin developing high-ground-resolution (less than one metre [3.3 feet]) commercial remote-sensing satellites. The first commercial high-resolution satellite, called Ikonos 1, was launched by the Space Imaging Company in 1999. Among major customers for high-resolution imagery are governments that lack their own reconnaissance satellites. The global availability of imagery previously available only to the leaders of a few countries is troubling to some observers, who express concern that it could lead to increased military activity. Others suggest that this widespread availability will contribute to a more stable world.
Remote sensing from space has yet to develop into a viable commercial business. Nevertheless, as users become more familiar with the benefits of combining space-derived data with other sources of geographic information, the possibility of commercial success could improve.

Commercial space transportation
The prosperity of the communications satellite business was accompanied by a willingness of the private sector to pay substantial sums for the launch of its satellites. As growth in the business slowed at the turn of the 21st century, so did growth in commercial space transportation, because there were few other commercial spacecraft needing access to orbit. Initially, most commercial communications satellites went into space on U.S.-government-operated vehicles. When the space shuttle was declared operational in 1982, it became the sole American launch vehicle providing such services. After the 1986 Challenger accident, however, the shuttle was prohibited from launching commercial payloads. This created an opportunity for the private sector to employ existing expendable launch vehicles such as the Delta, Atlas, and Titan as commercial launchers. In the 1990s, an American commercial space transportation industry emerged. Whereas the Titan was not a commercial success, the other two vehicles found commercial customers for a number of years.
Europe followed a different path to commercial space transport. After deciding in the early 1970s to develop the Ariane launcher, it created under French leadership a marketing organization called Arianespace to seek commercial launch contracts for the vehicle. In the mid-1980s, both the U.S.S.R. and China initiated efforts to attract commercial customers for their launch vehicles. As the industry developed in the 1990s, the United States initiated joint ventures with Russia and Ukraine to market their launchers; Europe also created a similar alliance with Russia. China continued to market its Long March series of launch vehicles for commercial use, and other countries hoped to market their indigenous launchers on a commercial basis.
In the first years of the 21th century, only 20 commercial launches were conducted per year on average, although the launch capacity worldwide was significantly greater.

New commercial applications
Space advocates have identified a number of possible opportunities for the future commercial use of space. For their economic feasibility, many depend on lowering the cost of transportation to space, an objective that has eluded both governments and private entrepreneurs. Access to low Earth orbit continues to cost thousands of dollars per kilogram of payload—a significant barrier to further space development.
The International Space Station originally was expected to be the scene of significant commercially funded research and other activity as its laboratories began to operate. This was projected to include both industry-funded microgravity research in ISS laboratories and less-conventional undertakings such as hosting fare-paying passengers, filming movies on the facility, and allowing commercial endorsements of goods used aboard the station. Commercial success for the ISS was predicted to lead to the development of new, privately financed facilities in low Earth orbit, including research, manufacturing, and residential outposts, and perhaps to privately financed transportation systems for access to those facilities. Because of delays in completing the station—particularly after the grounding of the shuttle fleet following the Columbia accident in 2003—such commercial use did not emerge. It seemed unlikely that there would be significant commercial use of the ISS after its completion in 2010 or later.
Another potential commercial application is the transport of fare-paying passengers into space, known as space tourism. Various surveys have suggested a willingness among many in the general public to spend considerable sums for the opportunity to experience space travel. Although a very limited number of wealthy individuals have purchased trips into Earth orbit at a very high price, large-scale development of the space tourism market will not be possible until less-expensive, highly reliable transportation systems are developed.
One variant of space tourism is to take fare-paying passengers to the edge of space—generally set at 100 km (62 miles) altitude—for brief suborbital flights that offer a few minutes of weightlessness and a broad view of Earth. In 2004, in response to a prize competition initiated in the late 1990s, a privately funded spacecraft, named SpaceShipOne, became the first of its kind to carry human beings on such flights. This achievement could herald the beginning of a commercial suborbital travel business. Nevertheless, the speed reached by SpaceShipOne was just over three times the speed of sound, roughly one-seventh of the speed required to enter a practical low-Earth orbit.
As an alternative to existing sources of energy, suggestions have been made for space-based systems that capture large amounts of solar energy and transmit it in the form of microwaves or laser beams to Earth. Achieving this objective would require the deployment of a number of large structures in space and the development of an environmentally acceptable form of energy transmission to create a cost-effective competitor to Earth-based energy-supply systems.

The Moon's south polar region in a mosaic of images made by the U.S. Clementine spacecraft from ...
NASA/Goddard Space Flight Center
Resources available on the Moon and other bodies of the solar system represent additional potential objectives for commercial development. For example, over billions of years the solar wind has deposited large amounts of the isotope helium-3 in the soil of the lunar surface. Scientists and engineers have suggested that helium-3 could be extracted and transported to Earth, where it is rare, for use in nuclear fusion reactors. In addition, there is evidence to suggest that the Moon's polar regions contain ice, which could supply a manned lunar outpost with drinking water, breathable oxygen, and hydrogen for spacecraft fuel. Significant quantities of potentially valuable resources such as water, carbon, and nitrogen may also exist on some asteroids, and space mining of those resources has been proposed.

Issues for the future
Space exploration and development have been stimulated by a complex mixture of motivations, including scientific inquiry, intense competition between national governments and ideologies, and commercial profit. Underlying them has been a vision of the outward movement of humans from Earth, ultimately leading to permanent settlements in space or on other celestial bodies. In reality, however, as of the start of the 21st century, only 27 people have traveled beyond Earth orbit, all of them Apollo astronauts during the primarily politically inspired race to the Moon. Whether, and under what conditions, human exploration and settlement of the solar system will resume is a major issue for the future.
Scientists will continue to seek answers to leading questions about the physical and biological universe through the deployment of increasingly advanced instruments on orbiting satellites and space probes. The principal space-faring countries appear willing to continue their substantial support for space science. The availability of government funding will set the pace of scientific progress.
The various applications of space capability hold the greatest promise for significant change. If other commercial ventures equal or surpass the success of the satellite communications sector, space could become a major centre of business activity. If governments decide to expand the activity in space of their armed forces, space could become another major military theatre—like the land, sea, and air on Earth—for waging war and deploying weapons. If observing Earth from space becomes crucial for effective planetary management, an assortment of increasingly varied and specialized observation satellites could be launched. Thus, outer space could become a much busier area of human activity in the 21st century than in the first four decades of endeavour there. At some point, it even may become necessary to establish a space traffic-control system analogous to traffic-control systems on Earth.
The development of space as an arena for multiple government and private activities will pose significant policy and legal challenges. The legal framework for space activities is based on the 1967 Outer Space Treaty and four subsequent United Nations treaties implementing its provisions. These agreements were negotiated at a time when governments were the principal players in space and commercial space activities were in their infancy. Whether they form an adequate and appropriate framework for current and future space activities requires review.
The Outer Space Treaty prohibits the deployment of weapons of mass destruction in outer space and on celestial bodies. Other treaties have limited some military activities in space, but there is no general framework regulating the military uses of space. The wisdom of developing space weapons—or, alternatively, of limiting their development and keeping space a weapons-free environment—is an issue for discussion and debate.
To date, the benefits of space exploration and development have accrued mainly to those countries that have financed space activities. The contributions of space to the economic and social development of large regions of Earth have been limited. The Outer Space Treaty identifies space as "the common heritage of mankind." How to ensure that the benefits of this common heritage are more equitably distributed will be a continuing challenge.

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