History of Satellites

One of the most dramatic moments of the twentieth century occurred on October 4, 1957. The Soviet Union sent a small shiny sphere with four long antennas into space. They called it Sputnik I. Sputnik is a Russian word that means “traveling companion.” The satellite traveled so fast that its ballistic flight continued all the way around Earth. A radio transmitter on board sent a simple beeping signal that could be heard everywhere as it passed overhead. It was an artificial moon. Since then, satellites have come to be a vital part of human development. Communications, national defence, space exploration and other areas of human activity are today closely linked with satellites.

Because the Soviet rocket program was so secret, scientists and politicians in the United States were surprised by Sputnik. They feared the Soviet Union might be developing other secret projects. A huge effort to catch up began, and it even included building new facilities in schools so young people could learn more science and mathematics to better prepare for the space age.

The former Soviet Union launched many more Sputniks and other Earth-orbiting satellites, with names like Molniya and Cosmos, over the years.

The United States’ first satellite was Explorer 1, which was sent into orbit on February 1, 1958. In that same year, the U.S. government set up the National Aeronautics and Space Administration (NASA) to explore outer space. Other notable satellites launched by the United States included Vanguard, Echo, Syncom, and Landsat.

The European Space Agency, China, India, and other countries have also launched their own satellites. The many uses of satellites include relaying data, transmitting navigation signals, military surveillance, observing the surface of Earth, following the movement of storms and other weather features, and scientific research.

So many satellites are orbiting Earth that you will probably see at least one, about as bright as an average star, moving among the constellations, if you pay attention during the first hour of darkness on any clear night of the year. In fact, companies that build and operate satellite services were a $100-billion-per-year industry around the globe in 2007.

At the beginning of the space age, the first satellite, Sputnik 1, was an elementary device that contained only a radio transmitter. Sputnik 1, the first satellite launched by humankind, was a 83.6-kg capsule.

It circled Earth every 96 minutes and remained in orbit until early 1958, when it fell back and burned in Earth’s atmosphere.

Sputnik 2 carried the dog Laika, the first living creature to be shot into space and orbit Earth. Eight more Sputnik missions with similar satellites carried out experiments on a variety of animals to test spacecraft life-support systems. They also tested reentry procedures and furnished data on space temperatures, pressures, particles, radiation, and magnetic fields.

Sputnik 3 was the first multipurpose space-science satellite placed in orbit. Launched May 15, 1958, by the Soviet Union, it made and transmitted measurements of the pressure and composition of Earth’s upper atmosphere, the concentration of charged particles, and the influx of primary cosmic rays.

Explorer was the largest series of unmanned U.S. spacecraft, consisting of 55 scientific satellites launched between 1958 and 1975. Explorer 1, launched Jan. 31, 1958, was the first space satellite orbited by the United States. It discovered the innermost of the Van Allen Van Allen radiation belts, two zones of charged particles that surround Earth. Explorer 1’s discovery of the Van Allen belts was the first scientific discovery made by an artificial satellite. Explorer 6 took the first pictures of Earth from orbit.

Other notable craft in the series included Explorer 38 (a.k.a. Radio Astronomy Explorer), which measured galactic radio sources and studied low frequencies in space, and Explorer 53 (a.k.a. Small Astronomy Satellite-C), which was sent out to explore X-ray sources both inside and outside the Milky Way galaxy.

The second artificial satellite placed in orbit around Earth by the U.S., Vanguard, was a series of three unmanned U.S. experimental test satellites. Vanguard 1 consisted of a tiny 1.47-kg (3.25-pound) sphere equipped with two radio transmitters. By monitoring Vanguard’s flight path, scientists found that Earth was almost imperceptibly pear-shaped, in confirmation of earlier theories.

Vanguard 2 carried light-sensitive photocells that were designed to provide information about Earth’s cloud cover, but the tumbling motion of the satellite rendered the data unreadable. Vanguard 3, the last in the series, was launched several months later. It was used to map Earth’s magnetic field.

The first series of unmanned U.S. space probes designed chiefly for interplanetary study was Pioneer. Whereas the first five Pioneers were intended to explore the vicinity of the Moon, all other probes in the series were sent to investigate planetary bodies or to measure various interplanetary particle and magnetic-field effects.

Pioneer 10, which was launched in 1972, flew by Jupiter in December 1973. It was the first space probe to do so, and, in the process, discovered Jupiter’s huge magnetic tail, an extension of the planet’s magnetosphere.

Pioneer 6 was injected into solar orbit to determine space conditions between Earth and Venus. It transmitted much data on the solar wind and solar cosmic rays in addition to measuring the Sun’s corona and the tail of Comet Kohoutek.

Pioneer 11, also called Pioneer Saturn, passed by Jupiter in December 1974 and flew within about 20,900 km (13,000 miles) of Saturn in September 1979. It transmitted data and photographs that enabled scientists on Earth to identify two additional rings around the planet and the presence of radiation belts within its magnetosphere.

Discoverer was a series of 38 unmanned experimental satellites launched by the U.S. Air Force. Although the Discoverer satellites had several apparent applications—such as testing orbital maneuvering and re entry techniques— the program was actually a cover story for Corona, a joint Air Force–Central Intelligence Agency project to develop a military reconnaissance satellite. Discoverer 1, launched in 1959 was equipped with a camera and an ejectable capsule capable of carrying exposed film back to Earth. Like later reconnaissance satellites, it was placed in a low polar orbit.

By orbiting almost directly over the poles, Discoverer was in position to photograph the entire surface of Earth every 24 hours. All other satellites in the series were launched into a similar fixed orbit.

Luna was a series of 24 unmanned Soviet lunar probes launched between 1959 and 1976. Luna 1 was the first spacecraft to escape Earth’s gravity. It failed to impact the Moon as planned and became the first man-made object to go into orbit around the Sun.

Luna 2 was the first spacecraft to strike the Moon, and Luna 3 made the first circumnavigation of the Moon and returned the first photographs of its far side. Luna 9 made the first successful lunar soft landing. Luna 16 was the first unmanned spacecraft to carry lunar soil samples back to Earth.

The Midas program was a series of 12 unmanned U.S. military satellites developed to provide warning against surprise attacks by Soviet ICBMs. Midas (an abbreviation of Missile Defense Alarm System) was the first such warning system in the world. To provide global coverage, the Midas satellites were placed into polar orbits. Because of launch and mechanical failures, the Midas satellites were unable to provide the desired continuous coverage of the Soviet Union. The infrared sensors could not distinguish between missile launches and sunlight reflected off of clouds in the upper atmosphere.

Launched during the early 1960s, the reconnaissance satellites were equipped with infrared sensors capable of detecting the heat of a ballistic missile’s rocket exhaust shortly after firing.

Midas 1 and 2 suffered mechanical failures. The first successful satellite was Midas 3, launched in 1961. The last Midas satellite, Midas 12, was launched in 1966.

The TIROS (Television and Infrared Observation Satellite) Program constituted the first worldwide weather observation system. The first TIROS satellite was launched on April 1, 1960. Equipped with specially designed miniature television cameras, infrared detectors, and videotape recorders, the satellites were able to provide global weather coverage at 24-hour intervals.

The cloud-cover pictures transmitted by the TIROS craft enabled meteorologists to track, forecast, and analyze storms. There were 10 TIROS satellites; the last of which, TIROS 10, was launched in 1965.

The first U.S. navigation satellites were those of the Transit program. Launched by the U.S. Navy from 1960 to 1988, the Transit satellites were developed to provide an accurate, all-weather navigational aid for seagoing vessels (particularly submarines) and aircraft.

The system was so designed that any such craft could pinpoint its position by using a computer specially programmed to translate coded radio signals beamed from the satellites into latitude and longitude.

The Venera probes were a series of unmanned Soviet planetary probes that were sent to Venus between 161 and 1983.

Radio contact was lost with the first probe, Venera 1, before it flew by Venus. Venera 2 ceased operation before it flew to within 24,000 km (15,000 miles) of Venus in February 1966. Venera 3 crash-landed on the surface of Venus, 1966, becoming the first spacecraft to strike another planet.

Venera 4 was an atmospheric probe that descended toward the surface by parachute, analyzed the chemical composition of Venus’s upper atmosphere and provided scientists with the first direct measurements for a model of the planet’s atmospheric makeup.

The Venera 9 and 10 landers sent back the first close-up photographs (in black and white) of the surface of another planet. Veneras 11 and 12 conducted detailed chemical measurements of the Venusian atmosphere on their way to soft landings.

Veneras 15 and 16 were orbiters equipped with the first high-resolution imaging radar systems flown to another planet. They mapped about a quarter of Venus’s surface, primarily around the north pole.

NASA’s earliest attempt at lunar exploration was Project Ranger, a series of nine unmanned probes launched from 1961 to 1965.

The last three probes in the series, Ranger 7, 8, and 9, transmitted more than 17,000 high-resolution photographs of the Moon, including many from as close as 300 metres (1,000 feet) above the lunar surface, before crashing.

Cosmos is a series of unmanned Soviet and then Russian satellites launched from the early 1960s to the present day. As of 2017, there were 2,500 satellites in the series. The first satellite in the series was launched on March 16, 1962. Cosmos satellites have been used for a wide variety of purposes, including scientific research, navigation, and military reconnaissance.

Cosmos 26 and 49, for example, were equipped to measure Earth’s magnetic field. Others were employed to study certain technical aspects of spaceflight as well as physical phenomena in Earth’s upper atmosphere and in deep space.

A number of them, such as Cosmos 597, 600, and 602, were apparently used to collect intelligence information on the Yom Kippur War between the Arab states and Israel in October 1973.

Some Cosmos spacecraft had the ability to intercept satellites launched by other nations. Some other Cosmos satellites have proved much more notable for how their missions ended. Cosmos 954, a Soviet Navy satellite powered by a nuclear reactor, crashed in the Northwest Territories of Canada in 1978, scattering radioactive debris.

Ariel was the first international cooperative Earth satellite. It was launched in 1962, as a joint project of agencies of the United States and the United Kingdom.

Design, construction, telemetry, and launching was handled in the United States by NASA. The United Kingdom was responsible for designing the equipment and the experiments to measure electron density and temperature and composition of positive ions, intensity of solar radiation in ultraviolet Lyman-alpha line, and cosmic rays.

The probes in the U.S. Mariner series were sent to the vicinities of Venus, Mars, and Mercury between 1962 and 1973.

Mariner 3 was supposed to fly by Mars, but contact was lost shortly after liftoff. Mariners 4, 6 and 7, and 9 obtained striking photographs of the Martian surface and made significant analyses of the atmosphere of that planet. Mariner was intended to study Mars with Mariner 9, but its upper stage malfunctioned shortly after launch.

Mariner 1 was intended to fly by Venus, but it was destroyed shortly after liftoff when it veered off course. Mariners 2 and 5 passed Venus within 35,000 and 4,000 km (22,000 and 2,500 miles), respectively, and made measurements of temperature and atmospheric density.

Mariner 10, which flew by Venus once and Mercury three times, came within 330 km (200 miles) of the latter planet on its third pass. It transmitted back to Earth the first close-up pictures of Mercury’s surface, as well as analyses of its atmosphere and magnetic field.

The 12 U.S. Velas were reconnaissance satellites developed to detect radiation from nuclear explosions in Earth’s atmosphere. Launched from 1963 to 1970, the Vela satellites were supposed to make certain that no countries violated the 1963 international treaty banning the testing of nuclear weapons on the ground or in the atmosphere. Although their primary function was military reconnaissance, the Velas made several significant astronomical discoveries, including the discovery of gamma-ray bursts.

Each Vela spacecraft carried radiation detectors sensitive to X-ray and gamma ray emissions. The satellites were always launched in pairs to an orbit of more than 60,000 miles (96,000 km) above Earth.

The first twin craft were orbited on Oct. 17, 1963. By 1967, an advanced version of the satellite had been developed. The new model was equipped with more sophisticated detection instruments and was designed to continually point toward Earth, unlike the earlier version, which viewed the heavens as well.

Zond was a series of eight unmanned Soviet lunar and interplanetary probes. Zond 1 and Zond 2 were aimed at Venus and Mars, respectively, but failed to send back data on the planets. Zond 3 transmitted close-up photographs of 7,800,000 square km (3,000,000 square miles) of the lunar surface, including the hidden side, before going into solar orbit. The remaining flights in the Zond program were tests of Soyuz spacecraft modified for flights around the Moon.

Zond 4 was placed into an orbit away from the Moon that carried it 330,000 km (205,000 miles) from Earth. When a landing in the Soviet Union became impossible, Zond 4 was ordered to explode in Earth’s atmosphere.

Zond 5 became the first spacecraft to orbit the Moon and return to a splashdown on Earth. Zond 6, 7, and 8 also made circumlunar flights. They carried biological specimens and transmitted photography of the Moon’s surface.

The seven Surveyor U.S. space probes were sent to the Moon between 1966 and 1968 to photograph and study the lunar surface.

Surveyor 1, carrying a scanning television camera and special sensors, landed on the Moon and transmitted 11,150 photographs as well as information about environmental conditions on the Moon.

Surveyor 5 measured the proportions of chemical elements in lunar soil and studied other surface properties. It returned 18,000 photographs.

Surveyor 2 crashed on the Moon. Surveyor 3 included additional equipment such as a surface-sampling device and two small mirrors to expand the camera vision.

After taking photographs of one area of the Moon’s surface, Surveyor 6 was lifted, moved 2.4 metres (8 feet), and repositioned to continue photographing another area. This marked the first liftoff from an extraterrestrial body. Altogether, 27,000 photographs were obtained.

Surveyor 7 was the only probe in the series that was soft landed in the highland region of the Moon. Data transmitted by the craft revealed that the chemical composition and landscape of this region was quite different from those of sites at lower elevations. This craft obtained 21,000 photographs.

Ōsumi was the first Earth satellite orbited by Japan. It was launched on Feb. 11, 1970, from Kagoshima Space Center on Kyushu and was named for the peninsula on which the centre is located.

Ōsumi consisted of the fourth stage of the U.S.-built Lambda-4S launch rocket that was used to place it into an elliptic orbit 400 km (250 miles) above Earth. It was equipped with several sounding devices and weighed 18 kg (40 pounds). Its purpose was to practice using a rocket to put a satellite into orbit. Ōsumi was destroyed upon reentry into Earth’s atmosphere in 2003.

The first Earth satellite orbited by the People’s Republic of China was China 1 (also known as Chicom 1 or PRC 1). It was launched on April 24, 1970, from the rocket facility at Shuang Cheng Tsu, and it made China the fifth nation to place a satellite into Earth orbit.

Little is known about China 1. It weighed approximately 173 kg (381 pounds) and carried a radio transmitter that broadcast a patriotic anthem.

Helios was a series of two unmanned solar probes developed by West Germany in cooperation with NASA. Helios 1 and Helios 2 were launched by NASA from the John F. Kennedy Space Center in Cape Canaveral, Florida. Both traveled closer to the sun than any other spacecraft.

Helios 1 passed within 45,000,000 km (28,000,000 miles) and Helios 2 within 43,400,000 km. Equipped with special heat-dispersal systems, the probes were able to withstand extremely high temperatures, which reached an estimated 700°F (370°C).

Both returned useful data about the sun’s magnetic field, the solar wind, the relative strength of cosmic rays, and measurements of meteoroid loss from the solar system.

The first Earth satellite built by India was Aryabhata. It was named for a prominent Indian astronomer and mathematician of the 5th century CE. The satellite was assembled at Peenya, near Bangalore, but was launched from within the Soviet Union by a Russian-made rocket.

Aryabhata weighed 360 kg (794 pounds) and was instrumented to explore conditions in Earth’s ionosphere, measure neutrons and gamma rays from the sun, and perform investigations in X-ray astronomy. The scientific instruments had to be switched off during the fifth day in orbit because of a failure in the satellite’s electrical power system. Useful information, nevertheless, was collected during the five days of operation.

The two U.S. Viking spacecraft were launched by NASA for extended study of the planet Mars. The Viking project was the first planetary exploration mission to transmit pictures from the Martian surface.Viking 1 and Viking 2, which lifted off on Aug. 20 and Sept. 9, 1975, respectively, each comprised an instrumented orbiter and lander.

After completing nearly year long journeys, the two spacecraft entered orbit around Mars and spent about a month surveying landing sites. They then released their landers, which touched down on fl at lowland sites in the northern hemisphere about 6,500 km (4,000 miles) apart. Viking 1 landed in Chryse Planitia. Viking 2 landed in Utopia Planitia seven weeks later.

The Viking orbiters mapped and analyzed large expanses of the Martian surface, observed weather patterns, photographed the planet’s two tiny moons—Deimos and Phobos—and relayed signals from the two landers to Earth. The landers measured various properties of the atmosphere and soil of Mars and made colour images of its yellow-brown rocky surface and dusty pinkish sky.

The satellites of the 1980s were built upon the knowledge gained in previous decades. The U.S.-British-Netherlands satellite Infrared Astronomical Satellite (IRAS) was the first space observatory to map the entire sky at infrared wavelengths.

After a series of brief studies by infrared instruments carried on sounding rockets had detected about 4,000 celestial sources of infrared radiation, the United States, the United Kingdom, and The Netherlands built IRAS to map the sky at infrared wavelengths It was launched on a Delta rocket from Vandenberg Air Force Base in California into a polar orbit at an altitude of 900 km.

Its telescope was cooled by superfluid helium. This was necessary because if the telescope were not cooled down, its own thermal radiation at infrared wavelengths would swamp the much fainter radiation from astronomical objects. IRAS discovered many previously unknown galaxies that emit most of their energy in the infrared portion of the electromagnetic spectrum (these are known as ultraluminous infrared galaxies), apparently owing to a massive burst of star formation during the merger of two galaxies.

The European space probe Giotto came within 596 km (370 miles) of the nucleus of Halley’s Comet in 1986. Giotto was the first solar system exploration mission carried out by the ESA. Its objective was to image and analyze the nucleus of Halley’s Comet and to study other characteristics of the comet during its next periodic swing through the inner solar system.

Giotto was named after the 14th-century Italian painter Giotto di Bondone, whose 1305–06 fresco The Adoration of the Magi includes a realistic depiction of a comet as the Star of Bethlehem in the Nativity scene. This image is believed to have been inspired by the artist’s observation of the passage of Halley’s Comet in 1301. Data from the Soviet Vega spacecraft, which also investigated Halley’s Comet, enabled Giotto’s controllers to home in on the comet’s nucleus. In its approach to the nucleus, Giotto returned a wealth of scientifically valuable data, including vivid images. It determined that the comet was 80 percent water, with a dust-covered, uneven surface that was darker than coal. It was also discovered that the comet was composed of primitive material dating from the formation of the solar system.

Hipparcos (or the High Precision Parallax Collecting Satellite) was launched by the ESA in 1989. Over the next four years, it measured the distances to more than 100,000 stars by direct triangulation using observations of parallax from either side of Earth’s orbit around the sun. Hipparcos was named after the ancient Greek astronomer Hipparchus, who drew up an accurate star catalog in the 2nd century BCE.

Hipparcos got off to a rocky start when a rocket engine failed to insert the satellite into a circular geostationary orbit, leaving the satellite in a highly elliptical orbit that passed in and out of Earth’s radiation belts. It was nevertheless able to operate, and the computer analysis was modified to take the non circularity of the orbit into account. Observations from Hipparcos showed that the Andromeda Galaxy—the dominant member of the Local Group, to which the Milky Way Galaxy belongs—is actually 24 percent farther away from the Milky Way than had been previously believed.

Galileo was a U.S. spacecraft launched to Jupiter for extended orbital study of the planet, its magnetic field, and its moons. Galileo was a follow-up to the much briefer fly by visits of Pioneers 10 and 11 and Voyagers 1 and 2. Galileo was placed into Earth orbit on Oct. 18, 1989, by the space shuttle Atlantis. It then was boosted into a roundabout trajectory toward Jupiter.

Galileo flew a series of orbits that produced close encounters with Jupiter’s four largest moons—in order of distance from the planet, Io, Europa, Ganymede, and Callisto. Despite the fouling of its high gain main antenna early in the mission Galileo yielded revealing close-up portraits of selected features on the moons and dramatic images of Jupiter’s cloud layers, auroras, and storm systems, including the long-lived Great Red Spot.

The U.S. satellite Cosmic Background Explorer (COBE) was placed in Earth orbit in 1989 to map the “smoothness” of the cosmic background radiation field and, by extension, to confirm the validity of the Big Bang theory of the origin of the universe. In 1964 Arno Penzias and Robert Wilson—working together at Bell Laboratories in New Jersey to calibrate a large microwave antenna prior to using it to monitor radio-frequency emissions from space—discovered the presence of microwave radiation that seemed to permeate the cosmos uniformly. Now known as the cosmic background radiation, this uniform field provided spectacular support for the Big Bang model.

Penzias and Wilson shared a Nobel Prize for Physics in 1978 for their discovery, but, in order to test the theory of the early history of the universe, cosmologists needed to know whether the radiation field was isotropic (the same in every direction) or anisotropic (having spatial variation) The COBE satellite was launched by NASA on a Delta rocket to make these fundamental observations. COBE’s Far Infrared Absolute Spectrophotometer (FIRAS) was able to measure the spectrum of the radiation field 100 times more accurately than had previously been possible using balloon-borne detectors in Earth’s atmosphere. In doing so, it confirmed that the spectrum of the radiation precisely matched what had been predicted by the theory.

The Differential Microwave Radiometer (DMR) produced an all-sky survey that showed “wrinkles” indicating that the field was isotropic to 1 part in 100,000. Although this may seem minor, the fact that the Big Bang gave rise to a universe that was slightly denser in some places than in others would have stimulated gravitational separation and, ultimately, the formation of galaxies. In 2006, John Mather, COBE project scientist and FIRAS team leader, and George Smoot, DMR principal investigator, won the Nobel Prize for Physics for the FIRAS and DMR results.

Large observatories in space were a highlight of the 1990s. The Great Observatories was a semi formal grouping of four U.S. satellite observatories that had separate origins: the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope. The grouping came about because the four would provide unprecedented spatial and temporal coverage across much of the electromagnetic spectrum from gamma rays (Compton) through X-rays (Chandra) and visible light (Hubble) to the infrared (Spitzer).

The Great Observatories concept was developed in the mid-1980s by American engineer Charles Pellerin—then Director of Astrophysics at NASA—as a way of providing an umbrella for four large, expensive astrophysics missions that otherwise might be viewed as funding competitors. The idea was that, by spanning the electromagnetic spectrum, the four would offer a comprehensive view of the universe that would help unify previous diverse perceptions.

The joint European-U.S. space probe Ulysses was the first spacecraft to fly over the poles of the sun and return data on the solar wind, the sun’s magnetic field, and other activity in the sun’s atmosphere at high solar latitudes. Understanding such solar activity is important, not only because the sun is an average star that is available for close scrutiny, but also because its activity has important consequences for Earth and its inhabitants. Clementine was a joint project of the Department of Defense’s Strategic Defense Initiative and NASA. The ingenious mission design used the Moon as a “target” for testing various sensors and spacecraft components intended for ballistic-missile-defense applications. In the process, it returned a vast amount of scientific data.

Dependence is increasing on space-based systems that can be affected by what has come to be called “space weather,” which is largely driven by solar phenomena. Among Ulysses’s discoveries was that the solar wind speed did not increase continuously toward the poles, but rather at high latitudes leveled off at 750 km (450 miles) per second. The elemental composition of the solar wind was found to differ between fast and slow solar wind streams. In the polar regions, the cosmic ray flux was not enhanced as much as was expected because the sun’s magnetic waves, themselves discovered by Ulysses, scattered the cosmic rays.

The U.S. spacecraft Clementine orbited and observed all regions of the Moon over a two-month period in 1994 for purposes of scientific research and in-space testing of equipment developed primarily for national defense. It carried out geologic mapping in greater detail than any previous lunar mission. Some of its data hinted at the possibility that water exists as ice in craters at the Moon’s south pole.

The Infrared Space Observatory (ISO) was a satellite of the ESA that observed astronomical sources of infrared radiation from 1995 to 1998. ISO was launched by an Ariane 4 rocket and was placed into a highly elliptical 24-hour orbit. This enabled it to spend most of its time both far from terrestrial thermal interference and in communication with the control centre at Villafranca, Spain. ISO’s program included both solar-system and deep sky objects.

The satellite was able to see through the dust that prevents optical astronomers from viewing the centre of the Milky Way Galaxy and found a large number of red giant stars expelling vast quantities of dust. It made significant observations of protoplanetary disks of dust and gas around young stars, suggesting that individual planets can form over periods as brief as 20 million years.

The satellite also found a large number of brown dwarfs—objects in interstellar space that are too small to become stars but too massive to be considered planets. In its “deep field” survey, ISO found that stars were being formed at a rate several times greater than that inferred from optical observations of the relatively dust-free regions of starburst galaxies in the early universe.

The U.S. Mars Global Surveyor spacecraft launched to the planet Mars to carry out long-term study from orbit of the entire surface, the atmosphere, and aspects of the interior. High-resolution images returned from the spacecraft indicated that liquid water may have existed on or near the planet’s surface in geologically recent times and may still exist in protected areas.

After a 10-month journey, Mars Global Surveyor took up a highly elliptical orbit above Mars on Sept. 12, 1997. It employed a technique known as aerobraking—using the drag of the Martian upper atmosphere on the spacecraft to slow it down gradually— to achieve a final 400-km (250- mile) circular polar orbit in which it circled Mars 12 times a day. This orbital configuration allowed the spacecraft to collect data from the entire Martian surface once about every seven days as Mars rotated beneath it.

In its first three years of operation, Mars Global Surveyor returned more data about Mars than all prior Mars missions combined. Close-up images of erosional features on cliffs and crater walls that resembled fresh-appearing gullies suggested the possibility of recent water seepage from levels near the surface. In addition, the mission yielded new information about the global magnetic field and interior of early Mars, allowed real time observation of the changing weather over the Martian seasonal cycle, and revealed that Mars’s moon Phobos is covered with a dust layer at least 1 metre (about 3 feet) thick—caused by millions of years of meteoroid impacts.

The U.S.-European space mission to Saturn, Cassini-Huygens, was launched in 1997. The mission consisted of NASA’s Cassini orbiter, which was the first space probe to orbit Saturn, and the ESA’s Huygens probe, which landed on Titan, Saturn’s largest moon.

Cassini-Huygens was one of the largest interplanetary spacecraft. The Cassini orbiter weighs 2,125 kg (4,685 pounds) and is 6.7 metres (22 feet) long and 4 metres (13 feet) wide. The instruments onboard Cassini include radar to map the cloud-covered surface of Titan and a magnetometer to study Saturn’s magnetic field. The disk shaped Huygens probe was mounted on the side of Cassini.

Cassini was named for French astronomer Gian Domenico Cassini, who discovered four of Saturn’s moons and the Cassini division, a large gap in Saturn’s rings.