Planetary Exploration Missions

James D Burke. Encyclopedia of the Solar System. Editor: Lucy-Ann McFadden, et al., 2nd edition, Elsevier, 2007.

Introduction

Immediately upon launching Sputnik in 1957, it was clear that technical and political conditions would soon permit humans to realize a dream of centuries—exploring the Moon and planets. With large military rockets plus advanced radio techniques and the dawning skills of robotics, it would be possible eventually to send spacecraft throughout the solar system.

At first, however, the effort mostly failed. Driven by Cold War desires to show superiority in both military and civil endeavor, the Soviet and US governments sponsored hectic attempts to penetrate deep space, using strategic-weapon boosters, cobbled-together upper rocket stages and hastily prepared robotic messengers. In time, as the equipment became more reliable and the management more capable, successes came—but in-flight failures have continued for decades to afflict all deep space programs. Lunar and planetary exploration is barely achievable even with the finest skills.

Here, where our purpose is to trace the development of flight missions, we do not dwell on the failures. The accompanying tables list only those missions that yielded some data in accord with their objectives.

In the early years, the Soviet Union garnered all of the main firsts: the first escape from Earth’s gravity, the first man and first woman in orbit, the first lunar impact, the first lunar landing and the first lunar orbit. But the US program came from behind and scored the first data from a planet, Venus, and ultimately the grand prize, the first human exploration of the Moon.

Though Cold War rivalry provided emotional stimulus and government support, both programs were scientific right from the start. The earliest satellites were launched in support of the International Geophysical Year. Every mission carried some instruments to elucidate the character of its target body or region, and this largely continued as more nations and agencies joined the program. As a result, there is now a huge body of data, some of it still unexamined, from flight missions complementing an important archive of ground-based and Earth-orbiting telescopic observations of the Moon, planets, and small bodies in the solar system. In what follows, mission results will be briefly mentioned, with cross references to more extended treatments in other chapters.

Exploration of the Sun’s domain by robots and at the Moon by humans has now placed us in a position to build strong hypotheses about the origin and evolution of the solar system and also to begin the study of other such systems as they are discovered. The missions that made this possible are an unprecedented expression, on a grand international scale, of peaceful human values and achievement.

Program Evolution

Launch Services

Sputnik, orbited on 4 October 1957, galvanized a huge response from the United States. Less than 12 years later, two astronauts walked on the Moon. However, in both the USSR and the USA, it was an existing legacy that enabled launch of the first satellites in 1957 and 1958. Strategic weapons programs had had high priority in both nations for many years. Sputniks, Explorers, and Discoverers were launched on early versions of intermediate-range and intercontinental ballistic missile boosters. With modifications and increasingly powerful upper stages added, these boosters have continued to serve in both programs, up to the present day, for sending spacecraft out into the solar system.

Today, while the Russian Soyuz and American Atlas and Titan carry on as direct descendants of the early ICBMs, they are accompanied by Delta (an IRBM derivative but later vehicles with the same name are wholly new) the air-launched Pegasus, and a whole suite of ex-Soviet vehicles able to launch both smaller and larger robotic spacecraft beyond LEO. The space shuttle was also briefly used as a planetary mission launch vehicle for a period in the late 1980s and early 1990s.

In time, space mission developers in other nations, driven primarily by a desire to have assured, independent access to space but also by a desire for their own organic technology advancement, began to provide their own launch services, at first for low Earth orbit (LEO) missions and later for missions beyond LEO, including geosynchronous (GEO), lunar, interplanetary, and planetary ventures.

In Europe, after some false starts with missile-derived vehicles, the unique Ariane series of rockets, designed exclusively for space, began and has now led to the creation of the powerful Ariane V, capable of sending multiton pay-loads into geosynchronous orbit and beyond.

In Japan, two separate lines of vehicles were developed, one by the Institute of Space and Astronautical Science (ISAS, primarily for science) and one by the National Space Development Agency (NASDA, primarily for applications and technology). ISAS and NASDA are now parts of the Japan Aerospace Exploration Agency (JAXA).

In China, the Long March vehicle series began with Soviet-derived technology but soon diverged into a more indigenous form. In India, launch vehicles were developed for both LEO and GEO applications missions.

The first LEO missions with human crews were launched by Soviet and American ICBM-derived rockets. But when it came time to send humans beyond LEO, far larger vehicles were needed. The Moon Race of the 1960s saw the creation of the giant Saturn V and N-1. Both of them have now passed into history.

Following the end of Apollo and its Soviet lunar competitor (which never flew successfully), both nations fell back to LEO for human missions and both developed partly reusable launch systems intended to service space stations—the American space shuttle and the Soviet/Russian Buran. The shuttle has carried many American human missions into LEO, but Buran flew only once, without crew, and was then mothballed. The ancient and reliable, expendable Soyuz booster continues to deliver crews, equipment, and supplies to the International Space Station (ISS), a successor to the American Skylab and the Soviet and Russian Salyut and MIR stations.

The search for lower cost launch services, regarded as a key to future space development, has led over decades to the spending of resources equaling billions of dollars in studies and aborted vehicle developments, with as yet no promising result. However, work continues on a variety of approaches including air launch, hybrid air-breathing and rocket propulsion, and alternatively just extreme simplification in booster design.

Even without a radical launch cost reduction, a human breakout into the solar system is conceivable through the use of extraterrestrial resources. With energy and especially materials collected off Earth, in a manner that has come to be called in-situ resource utilization (ISRU), great savings are possible in the mass that must be lifted from Earth. However, this technique has yet to be demonstrated at a large enough scale for its true potential and its real comparative costs to be known.

Tracking and Data Acquisition

Without some way of delivering robotic mission results to Earth, it does not matter what else works or does not work. In the time before the invention of radio, space science fiction authors assumed that signaling with light beams would be used. In a way they were right: Optical communications using lasers may yet become the method of choice in certain applications. Meanwhile, however, telemetry, tracking and orbit determination, command, and science in deep space are entirely dependent on radio technique.

For the first satellites, tracking stations were improvised based on previous military communications systems. For missions to the Moon and beyond, however, it was necessary to adapt methods used by strategic defense radar developers and radio astronomers. Huge antennas, supersensitive receivers, transmitters with enormous power output, and advanced data recording and processing all were needed.

From the outset, a difference in philosophy guided Soviet and American deep space engineers. In the then secretive USSR, the initial plan was to have spacecraft turn on their transmitters only when over Soviet territory, thus requiring ground stations in only the eastern hemisphere. (In response to that, an American deep space signals intercept site was built in Eritrea.) In the US, on the other hand, the policy called for continuous contact, meaning that stations would have to be located worldwide, with of course a worldwide ground and space communications system for command, control and data acquisition. That led to the creation of the Deep Space Network (DSN) whose stations today are in California, Spain, and Australia. For Apollo, a dedicated network was built, and it was backed up by the DSN plus a few specially equipped radio astronomy sites.

Meanwhile the Soviet system evolved. At first located only in the Crimea, the Soviet network expanded to include sites in the Far East and in the central USSR, plus a fleet of tracking ships offshore. As additional nations joined in exploring the solar system and the cosmos beyond, many more stations were built for both tracking and radio science. Figure 2 shows three examples of the modern deep space stations that now exist in several countries.

Spacecraft

Sputnik 1 was little more than a ball of batteries plus a beeping transmitter radiating at a frequency that most radio amateurs could tune in. But Sputnik 2 carried the dog Laika. Sputnik 3 was, for its time, a large scientific observatory outfitted to investigate the environment just outside Earth’s atmosphere. The American Explorers, though much smaller, also carried scientific instruments, including the radiation counters that confirmed the existence of trapped charged particles in the Van Allen belts. From that modest beginning, robotic spacecraft in LEO and GEO have evolved into the thousands of diverse science and applications machines that have been sent into orbit. Among these are large, multifunction craft devoted to observing Earth as a planet, such as the European Envisat and the American Terra and Aqua.

Meanwhile, spacecraft designed to explore the solar system beyond Earth underwent a similar evolution. The most important early mission was that of Luna 3 in 1959, ending centuries of speculation by returning the first images of the Moon’s far side. Soon after that, spacecraft design began to elaborate on the features that are essential in interplanetary space: attitude stabilization for pointing cameras and high-gain antennas, capable onboard data handling systems, long-duration power supplies, and long-surviving electronic equipment.

For human spaceflight, the earliest craft were mainly just capsules capable of sustaining life and returning safely to Earth. But as space stations in LEO and flight beyond LEO became program objectives, more functions became the responsibility of human pilots and other crew members. The Apollo and space shuttle designs took full advantage of human capacities, while Soviet missions continued to make more use of teleoperation and onboard automation, as shown by the pilotless flight of Buran and the routine automated dockings of Soyuz and Progress servicing craft with the ISS.

Today, deep space spacecraft design and development is a mature activity as shown by the success of Soviet Venus landers, Apollo Moon missions, the decades-long Pioneer and Voyager missions to Jupiter and beyond, the missions to Halley’s Comet in 1986, Galileo to Jupiter, Cassini/Huygens to Saturn, and the fleet of orbiters, landers, and rovers now exploring Mars. But in-flight failure, as in seven Mars attempts since 1992, is an ever-present threat requiring vigilance and entailing high costs of spacecraft development and operations.

Operations

In even the earliest lunar and planetary missions, it was necessary to keep track of the spacecraft’s trajectory and issue commands for onboard functions both engineering and scientific. Gradually a humans-and-machines art developed, represented today by large rooms full of people and displays backed by buildings full of computers and data systems. Initially centered in main theaters, as missions have become more complex, these facilities have become dispersed, providing work spaces for the many specialized flight management and scientific teams working during a mission. With the Internet and other modern communications available, scientists can now reside at their home institutions and participate in missions in real time.

The latest trend is toward increasing onboard autonomy, which holds the promise of reducing the large staffing needed round the clock to control missions. Some degree of autonomy is needed anyway in deep space, simply because of the round-trip signal times to distant spacecraft, tens of minutes for Mars and Venus, and many hours in the outer solar system.

Operations have become more and more dependent on software whose design and verification now constitute one of the main cost items in each new mission’s budget. With the maturing of the operations art have come numerous stories of remarkable rescues when a distant robot (or, as in Apollo 13, a human crew) got into trouble, but there are also instances where a mistake on Earth sent a mission to oblivion.

Reliability and Quality Assurance

A vital part of the deep space exploration art is the creation of systems having but a small chance of disabling failures, plus an ability to work around failures when they do occur. One reason for the high cost of lunar and planetary missions is the need for multiple levels of checking, testing, reviewing, and documentation at every stage from the manufacturing of thousands of tiny components, through assembly into subsystems and systems for both ground and flight, organization of human teams capable of imagining and analyzing failure scenarios and designing around them, and finally launching and controlling a mission during its years or decades of activity.

These costs are aggravated by the nature of deep space exploration as a work of building very complicated things (hardware, software and human-machine complexes) in ones and twos, as distinct from the repetitive manufacture of highly reliable items such as cars or computers whose teething troubles can be eliminated in early prototype testing. In a sense, every lunar or planetary mission is a first effort.

Management

In the 20th century, as cold and hot warfare became more and more technological, a suite of skills, traditions, and managerial methods grew and created the capability of planning and executing large complicated projects. Many disciplines were involved, ranging from what became known as systems engineering all the way to new ways of organizing academic institutions, industries, and government agencies. The sometimes maligned worldwide military-industrial complex is a product of those developments, and it was the seedbed of the world’s deep space programs.

The great lunar contest of the mid-20th century highlighted some stark differences between American and Soviet management methods and organizations. At the outset, both used existing military hardware and existing military ways of working, but over time the programs evolved along different paths. With their head start the Soviets garnered all the early prizes in robotic lunar exploration, but when planning began for human lunar exploration the Soviet system faltered.

Despite a huge and highly capable engineering and industrial base of talented and motivated people, the Soviet human flight lunar enterprise proved unable to solve problems of interagency rivalry and timely decision making, with the result that Apollo won the day. The USSR cut its losses and canceled its program, and Apollo soon followed because of pressure on the US federal budget and the lack of the political stimulus of Soviet competition. Decades then passed before lunar robotic exploration resumed, and more decades will pass before humans again bestride the Moon.

Sun and Heliosphere

The emphasis in this chapter is on missions to the Moon and planets. However, now that star-planet aggregates are at last being observed as a class of known objects in the cosmos, it is essential for us to include at least a part of the story of missions devoted to our own star as host of a planetary system.

Our tale begins with the International Geophysical Year (IGY). Centuries of ground-based investigations of sunspots and solar and terrestrial magnetism, plus decades of ionospheric and auroral research, had led by the mid-20th century to a drive by scientists for a worldwide campaign of coordinated measurements resembling previous efforts such as international polar years. The new element now was the knowledge that rockets could take instruments beyond Earth’s atmosphere and even into orbit. In both the USSR and the US, satellite experiments were planned and announced in support of this goal, and in 1957 and 1958 it was achieved.

Explorer I found an excess of radiation saturating its detector. Explorer IV showed that this radiation is due to energetic particles trapped in Earth’s magnetic field, the Van Allen belts. Then, in 1962, an instrument aboard Mariner II, en route to Venus, confirmed predictions of a fast outward flow of plasma from the Sun—the solar wind, now known to bathe the entire solar system out to the boundary of the heliosphere, where it meets the oncoming, tenuous interstellar medium. Voyager 1 and 2 are now entering that interaction region, more than 90 AU from the Sun. Over the next 5 to 10 years, they are expected to continue to yield information on phenomena at the outer limits of the Sun’s domain.

Meanwhile, over the past five decades, many spacecraft have journeyed into interplanetary space, investigating the particles and fields environment of the solar system or the Sun itself. Some were errant vehicles from planetary misses. The notable Pioneer series began in 1959 and continued in the late 1960s. The first international solar mission, Helios, a U.S.-German cooperative mission, with interplanetary spacecraft observing the solar wind and radiation, was launched in the mid 1970s (see below).

Now the Sun is continuously observed from space. Currently operating missions include Ulysses, SOHO, and ACE, plus particles-and-fields instruments carried on some new planetary missions. In the aggregate, as described in the chapter on the heliosphere, these investigations have shown a common portrait, with variations, of what happens as the Sun’s streaming plasma, coronal mass ejections and electromagnetic radiations interact with the magnetic fields, ionospheres, and atmospheres and surfaces of solar system bodies. These effects are most dramatic when they result in spectacular comet ion tails, but they are also important in causing magnetic storms and driving the evolution of atmospheres due to dissociation of molecules and ionization and sweeping away of atoms.

Study of these interactions as they are imagined to have happened in the ancient past, for example when our star is thought to have gone through a hugely energetic T Tauri phase, enables not only analyses of early planetary history here but also productive reasoning about what may be observed in other star-planet systems as they are found.

Over the history of spaceflight many space-borne investigations, for example surveys of Earth’s magnetosphere by missions including ISEE-2 and 3, Interbol, Geotail, Wind, Polar, and Cluster have added to knowledge of the Sun via its interactions with the rest of the solar system. Here we do not dwell on those ventures; instead we focus on missions dedicated to investigating the Sun itself as a star, with improved planetary magnetospheric, ionospheric, and atmospheric knowledge being extra benefits. It is appropriate, however, to observe that the long tradition is vigorously continuing with the worldwide International Heliosphere Year (IHY) due to begin in 2007.

Pioneer 6, 7, 8, 9

These missions, making ingenious use of the technology of their time, employed small spinning spacecraft to obtain a rich harvest of data on the solar wind and other interplanetary phenomena over a period beginning in 1965 and continuing for more than 30 years. (See http://samadhi.jpl.nasa.gov/msl/QuickLooks/pioneer6QL.html)

Helios

Two German spacecraft, launched by NASA Titan-Centaurs in 1975 and 1976, explored solar phenomena between Earth’s orbit and as close as 0.29 AU from the Sun. An arrangement of mirrors and radiators enabled the spinning spacecraft to survive the consequent extreme heating. (See http://www.linmpi.mpg.de/english/projekte/helios/

Isee-3

Launched in 1978, the International Sun-Earth Explorer was a small spacecraft maneuvered into a halo orbit around the L1 libration point, 1.5 million km sunward from Earth, where its x-ray and gamma-ray spectrometers enabled the study of both solar flares and cosmic gamma-ray bursts. In 1982 it was maneuvered onto a trajectory toward Comet Giaccobini-Zinner and renamed the International Cometary Explorer. (See SMALL BODIES section below.) (See heasarc.gsfc.nasa.gov/docs/heasarc/missions/isee3.html# instrumentation.)

Solar Maximum Mission

Launched in 1980 by Space Shuttle, SMM carried a suite of instruments investigating the Sun at the height of the sunspot cycle. Ultraviolet, x-ray, gamma ray, and visible light observations combined to give a picture of the Sun’s total radiation and its variations due to flares. The spacecraft failed and was dramatically rescued by a shuttle crew in 1984, whence it continued until atmospheric reentry in 1989. (See umbra.nascom.nasa.gov/smm/.)

Ulysses

Launched in 1990 by the space shuttle with propulsion beyond LEO to send it to Jupiter, ESA’s Ulysses used the giant planet’s gravity to kick its orbit out of the plane of the ecliptic and send the spacecraft back inward, passing over the Sun’s poles to survey a region never before explored. Now the craft goes out to the distance of Jupiter’s orbit and back to the Sun every five years. Its mission is expected to continue until at least 2007. In addition to its huge yield of information about the Sun, solar magnetism, and the solar wind, Ulysses has observed interstellar dust and interstellar helium atoms in interplanetary space. (See helio.estec.esa.nl/ulysses/.)

Yohkoh

Launched in 1991 from Kagoshima, this mission of ISAS, with contributions from the US and UK, was an x-ray and gamma-ray observatory that gave 10 years of nearly continuous imaging of the solar atmosphere. (See solar.physics.montana.edu/sxt/.)

SOHO

The ESA/NASA Solar and Heliospheric Observatory, launched by an American Atlas-Centaur in 1995, orbits about the L1 Lagrangian libration point 1.5 million km sunward from the Earth, where its 14 instruments continuously observe phenomena relevant to understanding the solar interior, the solar atmosphere, and the solar wind. SOHO’s observations are immediately fed to users via the Internet at umbra.nascom.nasa.gov. The mission has already made observations through most of an 11-year solar cycle, and it is expected to continue for several more years. It too survived a massive onboard failure with a dramatic rescue—this time by remote control from Earth. (See: sohowww.estec.esa.nl.)

ACE

The Advanced Composition Explorer, a NASA mission with nine instruments and an international team of 20 investigators, was launched by a Delta II vehicle in 1997. Like SOHO, it orbits in the L1 region where it continuously surveys the isotopic and elemental composition of particles from the solar corona, the interplanetary medium and interstellar space. In 1998, the ACE data system began providing public, real-time observations that can give warning of solar events that cause geomagnetic storms. (See www.srl.caltech.edu/ACE/,)

TRACE

A small Explorer satellite launched in 1998 by the innovative air-launched Pegasus rocket system, TRACE provides nearly continuous solar coronal observations with high spatial and temporal resolution, complementing the data from SOHO. (See umbra.nascom.nasa.gov/trace/.)

Genesis

In an audacious venture using gravity assist at Earth and libration orbiting for two years near L1, the Genesis mission, launched by a Delta II in 2001, in 2004 returned a capsule to Earth bearing actual samples of the solar wind and interplanetary medium embedded in ultraclean collector plates. Due to a failure to signal its parachute to open, the capsule crashed in the Utah desert, but not all was lost: A number of the collector units survived in condition good enough for the recovery of isotopic information and other science data. (See: genesis.lanl.gov.)

RHESSI

Launched by Pegasus in 2002, the Reuven Ramaty High Energy Solar Spectroscopic Imager is a small Explorer spacecraft dedicated to x-ray and gamma-ray observations for exploring the basic physics of particle acceleration and energy release in solar flares. (See hesperia.gsfc.nasa.gov/hessi/index.html.)

Mercury

Mariner 10

Flight to the innermost planet began with Mariner 10, launched on 3 November 1973 by an Atlas-Centaur. It was the first mission to use gravity assist, flying by Venus on 5 February 1974 enroute to Mercury, where it arrived on 29 March. Then using Mercury gravity assist, it flew by again on 21 September 1974 and 16 March 1975, each time passing over the same side of the planet. Mariner 10‘s images showed a scorched, Moon-like cratered surface, while its infrared and ultraviolet spectrometers recorded mineral composition and its magnetic and plasma instruments surveyed Mercury’s surroundings, revealing a weak magnetic field. Precise trajectory analysis confirmed that Mercury has a huge iron core reaching to two thirds of its outer diameter. (See nssdc.gsfc.nasa.gov/nmc/tmp/1973-085A.html.)

Messenger

The Messenger spacecraft, launched by a Delta II on 2 August 2004, will enter obit about Mercury in 2011 after an Earth gravity assist in 2005, Venus gravity assists in 2006 and 2007, then three Mercury assists in 2008 and 2009. The spacecraft carries a suite of instruments to investigate Mercury’s surface and interior composition, its gravity and magnetic fields, its particles and radiation environment and the polar regions where Earth-based radar observations show the possible presence of ices in permanently shadowed craters. (See messenger.jhuapl.edu/.)

Venus

Mariner 2

The first mission to return data from another planet, Mariner 2 in 1962, had amazing escapes from disaster. During ascent its Atlas went into uncontrolled rolling and miraculously stopped in an orientation such that the Agena upper stage could deliver the spacecraft onto a trajectory toward Venus. En route, the spacecraft survived a series of mortal threats, and shortly after flying by Venus it succumbed to overheating. But during the flyby, as described in the Venus chapters of this encyclopedia, it produced proof of the planet’s hellish greenhouse. (See nssdc.gsfc.nasa.gov/nmc/tmp/1962-041A.html.)

Veneras 4 through 16 and Vega

First to enter another atmosphere, Venera 4 in 1967 carried the emblem of the USSR to Venus. It began the Soviets’ most successful interplanetary program. As shown in the table, Venera missions of increasing complexity and scientific yield continued to be launched at nearly every celestial mechanics opportunity until 1983, and then in 1985 the two VEGA spacecraft, en route to Halley’s comet, delivered balloons into the Venus atmosphere. Scientific results of this decades long exploration are described in the Venus chapter. (See www.russianspaceweb.com/spacecraft_planetary_venus/.)

Mariner 5

Launched two days after Venera 4 in 1967, Mariner 5 made flyby observations, including ultraviolet cloud imaging, that revealed the rapid rotation and spiraling equator-to-pole circulation of the Venusian atmosphere. (See nssdc.gsfc.nasa.gov/nmc/tmp/1967-060A.html.)

Mariner 10

During its gravity assist flyby of Venus in 1974 en route to Mercury, Mariner 10 made observations of the Venusian atmosphere and ionosphere, confirming the equator-to-pole circulation and absence of a magnetosphere. (See nssdc.gsfc.nasa.gov/nmc/tmp/1973-085A.html.)

Pioneer Venus

The two Pioneer Venus spacecraft, launched in 1978, had complementary objectives. Pioneer Venus 1 went into orbit with a radar altimeter to survey the surface though the planet’s permanent cloud cover. Pioneer Venus 2 delivered four probes into the atmosphere to measure its character and composition down to the surface. (See nssdc.nasa.gov/planetary/pioneer_venus.html.)

Vega 1 and 2

Two large Soviet spacecraft Vega 1 and 2 flew by Venus in 1985 en route to close encounters with Halley’s comet. Their spherical entry capsules released balloons that were inflated and floated in the Venus atmosphere, returning data for several days. (See www.iki.rssi.ru/IPL/vega.html.)

Magellan

The ubiquitous clouds of Venus forever hide the planet’s surface from outside visual examination. Venera landers in 1975-1981 gave close-up surface panoramas and in 1983 radars on the Venera 15 and 16 orbiters mapped most of the northern hemisphere. Long delayed through years of attempts to gain government approval, Magellan was finally launched in 1989 into a series of orbits enabling it to map the entire planet using synthetic-aperture radar. Once the radar mission was complete, the spacecraft was moved into a lower orbit to map the Venusian gravity field and to test aerobraking techniques. (See www.jpl.nasa.gov/magellan/.)

Galileo Venus Flyby

En route to Jupiter, the Galileo spacecraft performed a gravity assist flyby at Venus in February 1990. Spacecraft observations included infrared imaging of the planet’s cloud layers and even surface features, through infrared “windows” in the atmosphere and clouds. (See galileo.jpl.nasa.gov/facts.cfm.)

Venus Express

By modifying the design to cater for the hot environment near Venus, but otherwise using many proven components and operational techniques, ESA was able to mount a low-cost mission to place in Venus orbit a spacecraft based on the successful Mars Express to be described later below. Launched by a Russian Soyuz-Fregat in 2005, the mission has delivered unique images of Venus’s north polar cloud vortex. (See sci.esa.int/venusexpress/.)

Earth

Among the thousands of spacecraft launched to date, at least hundreds have made some contributions to the study of our Earth as a planet. Here we make no attempt at a catalog of all those ventures. Instead we highlight a few recent and representative missions that illustrate the state of humans’ ongoing endeavor to understand Earth’s interior, its oceans and lands, its atmosphere, its evolution and its fate, including that of its biosphere.

Resurs

Soviet and Russian film-return photo-reconnaissance satellites have operated over many years for Earth observation. Civil uses have been publicized since 1979, with increasingly capable camera systems used for both applications and science.

Corresponding US imagery was mostly kept classified until 1995, when much previously secret overhead reconnaissance information was released for public use in historical and scientific studies. (See www.fas.org/spp/guide/russia/earth/resurs-f.)

Galileo Earth Flybys

En route to Jupiter (see Outer Planets section below) the Galileo spacecraft made gravity assist passes at Earth in 1990 and 1992. Spectrometric observations were made to simulate a search for evidence of life on an unknown planet, and the data did show an out-of-equilibrium, oxygen-rich atmosphere. (See galileo.jpl.nasa.gov/facts.cfm.)

Terra

Launched in 1999, NASA’s Terra spacecraft carries five advanced radiometric and spectrometric instruments observing global phenomena of land, oceans and atmosphere. Measuring Earth’s radiation budget, its carbon cycle and evolution of its climate and biosphere are main mission goals. (See terra.nasa.gov/.)

Topex/Poseidon and Jason-1

Launched in 1992 and 2001 respectively as parts of a collaboration between NASA and the French national space agency CNES, Topex/Poseidon and Jason-1 use radar altimetry and very precise orbit determination to determine ocean topography, aiding studies of currents, winds and climate effects including El Niño. (See topex-www.jpl.nasa.gov.)

Grace

In a collaboration among NASA, the German space agency DLR and other partners, two small satellites, Grace, launched in 2002 use very precise measurements of the distance between them to gain knowledge of the bumps and hollows in Earth’s gravity field, leading to information on the exchanges of mass, momentum and energy between oceans and atmosphere. (See www.csr.utexas.edu/grace/.)

Envisat

ESA’s 8200-kg Earth observing satellite, Envisat, launched in 2002, carries ten large instruments including a synthetic aperture radar, a radar altimeter and a suite of radiometers and spectrometers recording atmospheric, ocean, ice, land and biosphere data, spanning the spectrum from ultraviolet to microwave frequencies. Its polar orbit gives global coverage. (See www.esa.int/export/esaEO/.)

Aqua

NASA’s Aqua satellite, launched in 2002, carries six radiometric and spectrometric instruments surveying Earth’s water cycle, sea and land ice, atmospheric temperature, aerosols and trace gases, and soil moisture, so as to increase understanding of climate and Earth’s radiation balance, with both physical and biological influences. (See science.hq.nasa.gov/missions/satellite_aqua/.)

Aura

Launched in 2004, the Aura satellite’s four instruments complement those of Terra and Aqua by measuring atmospheric chemistry, including the formation and dissipation of polar ozone holes and the distribution of greenhouse gases. (See science.hq.nasa.gov/missions/satellite_aura/.)

Other Recent Earth Observing Missions

In addition to the major efforts noted here, a host of other orbital remote-sensing missions investigating Earth as a planet with its evolving hydrosphere, cryosphere, atmosphere, biosphere, and magnetosphere have been launched in recent years. Summaries are given at the following Web site with links to pages describing each mission in more detail: science.hq.nasa.gov/missions/earth_sun/.

Moon

After centuries of careful naked-eye and telescopic observation from Earth, the Moon has at last become a body to be investigated by robots, visited by human explorers, and perhaps ultimately inhabited by the people of a first outward wave of civilization. At its beginning, scientific lunar exploration was caught up in the great 20th century struggle between the USA and the USSR. With the end of the USSR, the program fell victim to low priority and languished for decades, but now a lively international revival is in progress. Here we list the most important robotic missions of the past, then briefly mention the grand Apollo venture and its failed Soviet competitor, and finally remark on the new missions now established in a widening group of countries.

Luna 1, 2, and 3

The Luna Soviet missions in 1959 yielded the first escape from Earth’s gravity, the first lunar impact, and the first far-side images. (See nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1959-008A.)

Ranger 7, 8, and 9

After two nonlunar tests and three failed attempts to deliver seismometers to the lunar surface, the NASA Ranger missions, launched by Atlas-Agenas in 1964 and 1965, yielded thousands of high-resolution television images of the lunar surface showing that all features are mantled by the impact-generated regolith. (See nssdc.gsfc.nasa.gov/planetary/lunar/ranger.html.)

Zond 3

A Soviet planetary spacecraft, Zond 3, launched on a test flight including a lunar flyby, this mission in 1965 returned improved imagery of parts of the Moon’s far side. (See nssdc.gsfc.nasa.gov/planetary/lunar/lunarussr.html.)

Luna 9 and 13

After many Soviet lunar failures in 1960-1965, Luna 9 and 13 in 1966 achieved history’s first and third successful lunar touchdowns, delivering image panoramas showing fine surface details. (See selena.sai.msu.ru/Home/Spacecrafts/Luna-9/luna-9e.htm.)

Luna 10, 11, 12, and 14

These Soviet missions, Luna, in 1966 and 1968 achieved the first entry into lunar orbit and made some measurements of lunar gravity and geochemistry. (See www.iki.rssi.ru/solar/eng/luna14.htm.)

Lunar Orbiter 1-5

Designed to image landing sites on the Moon in support of Apollo, the first three of the Atlas-Agena-launched Lunar Orbiter NASA photographic missions were so successful that the last two were given the expanded task of mapping the entire Moon. (See www.hq.nasa.gov/office/pao/History/TM-3487/top.htm.)

Surveyor 1, 3, 5, 6, and 7

NASA’s Surveyor 1, launched by Atlas-Centaur, achieved the first lunar soft landing and returned television mosaics of its surroundings. In addition to imagery, the Surveyors in 1966 and 1967 yielded information on the mechanical and chemical properties of the regolith. (See nssdc.gsfc.nasa.gov/planetary/lunar/surveyor.html.)

Zond 5, 6, 7, and 8

The Zond Soviet spacecraft, launched from 1968-1970 by large Proton vehicles, flew on circumlunar trajectories, returning to Earth after passing over the Moon’s far side. They were test flights for a never-completed human lunar flight program. Payloads consisted of environmental instrumentation and biological specimens including tortoises. The later flights demonstrated an ingenious skip re-entry, dipping briefly into the atmosphere over the Indian Ocean and then traveling on to land in central Asia. (See nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1970-088A.)

Apollo 8

When in 1961 US President John F. Kennedy called for starting Apollo, he had asked his advisors to describe a program in which “we can win” in competition with the USSR. Observation of Soviet lunar launch preparations and test flights led to a decision to send a human crew to the Moon as soon as possible. The risky Apollo 8 mission in 1968 was the result. It went into lunar orbit with only the Command and Service Modules (CSM) because the lunar landing module (LM) was not yet available. Thus there was no prospect of saving the mission in “LM Lifeboat” mode as had to be done in Apollo 13 (see below). The Apollo 8 crew broadcast TV images and a Christmas voice message from lunar orbit, took photos, made visual observations, and returned safely to splashdown in the Pacific Ocean. (See www.lpi.usra.edu/expmoon/Apollo8/Apollo8.html.)

Apollo 10

In the final rehearsal for a lunar landing in 1969 (after Apollo 9‘s successful Earth-orbiting test of the LM), the Apollo 10 crew exercised all LM functions in low lunar orbit, rendezvoused with the CSM, and returned to Earth. (See www.lpi.usra.edu/expmoon/Apollo10/Apollo10.html.)

Apollo 11

Apollo 11, the mission that won the greatest peaceful international contest placed, on 20 July 1969, the first human footprints on the Moon. The LM crew gathered rock and soil samples and installed a set of long-lived instruments on the surface. Meanwhile, a photographic survey from the orbiting CSM covered landing sites for future missions. (See-www.hq.nasa.gov/office/pao/History/ap11ann/introduction.htm and nssdc.gsfc.nasa.gov/planetary/lunar.apollo11info.html.)

Apollo 12

An outstanding achievement in 1969 by the Apollo 12 ground and flight crews is shown in Figure 8. Navigating to a landing within 170 meters of Surveyor 3, which had been sitting on the Moon for 31 months, the LM crew walked over to the Surveyor, cut off its camera and soil-sampler claw, and returned them to Earth. The mission also brought back a new harvest of rocks, soils, orbital and surface imagery, and other science data. (See www.lpi.usra.edu/expmoon/Apollo12/Apollo12.html.)

Apollo 13

When the Apollo 13 spacecraft was en route to the Moon in 1970, an oxygen tank in the service module exploded. The dramatic rescue of the mission during the following week is an epic tale of devotion and ingenuity by the ground and flight crews. Moving out of the crippled CSM into the LM, the crew used the LM descent engine to adjust their trajectory to a circumlunar return to Earth. In the midst of the emergency, they even managed to obtain some lunar far-side photography. (See nssdc.gsfc.nasa.gov/planetary/lunar/ap13acc.html.)

Apollo 14

Continuing to expand Apollo‘s science capabilities, the 1971 Apollo 14 mission’s surface exploration included a hand-drawn cart for carrying instruments. (See nssdc.gsfc.nasa.gov/planetary/lunar/apollo14info.html.)

Apollo 15, 16, and 17

During three Apollo missions in 1971 and 1972, human lunar scientific exploration showed its real potential. With augmented geological training of astronauts, plus one crew member a professional geologist, plus a rover to carry the LM crew on extended surface traverses, plus a suite of remote sensing instruments on the CSM, these missions yielded a cornucopia of information that is described in the Moon chapter. (See nssdc.gsfc.nasa.gov/planetary/lunar/apollo17info.html.)

Luna 16, 17, 20, 21, and 24

During the Apollo years the USSR had three lunar programs. The first was the robotic science program that began in 1959 and continued with increasing capabilities until 1976. The second was the Proton-launched circumlunar ZOND (a name meaning sounder) human-precursor tests. The third was the human lunar landing effort based on the giant N-1 vehicle that failed in four launch attempts.

Lunas 16 through 24 were emissaries of the first program. The Proton-launched Luna 16, 20, and 24  drilled into the regolith, encapsulated small soil samples and returned them to Earth. Luna 17 and 21 delivered Lunokhod rovers to the Moon’s surface. (See nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1976-081A.)

Clementine

The mission that revived lunar exploration in 1994 after its decades of stasis, Clementine, had an innovative management and technical plan. Proposed as a test of instrument technologies for the American Strategic Defense Initiative, it was sponsored by the Ballistic Missile Defense Organization and NASA, managed by the Naval Research Laboratory, and launched from the Pacific Missile Range on a Titan II-G.

During two months in lunar orbit, it mapped the entire Moon at many wavelengths and hinted at the presence of theoretically predicted excess volatiles, possibly a signature of cold-trapped water ice near the lunar poles. (See www.cmf.nrl.navy.mil/clementine/.)

Lunar Prospector

Launched in 1998 by an Athena solid-fueled vehicle, the NASA Lunar Prospector continued the trend toward small, highly capable lunar spacecraft and relatively low mission costs. With neutron, gamma-ray, and alpha-particle spectrometers plus measurements of lunar magnetic and gravity fields, the mission yielded data on the Moon’s surface composition and its geochemical and geophysical properties. It added confidence to the Clementine findings of possible polar ices. (See lunar.arc.nasa.gov/project/index.htm.)

Smart-1

ESA’s first lunar mission, Smart-1, was launched in 2003 with a small, highly advanced spacecraft demonstrating solar-electric propulsion, onboard autonomy, and several new instrument technologies. Spiraling slowly outward from Earth and then inward toward the Moon, the craft was captured by the Moon’s gravity late in 2004 and began science operations in lunar orbit in 2005, whence it delivered a fine harvest of imaging and other remote-sensing data until its planned crash into the Moon on 3 September 2006. (See www.esa.int/SPECIALS/SMART-1/.)

New Lunar Missions

Continuing the worldwide revival of interest in the Moon, several robotic lunar orbiting missions are being prepared for launch: Japan’s Lunar-A for seismic penetrators and SELENE for a broad set of remote-sensing objectives; China’s Chang ‘E-1 for remote sensing and surveying for later landing missions, and India’s Chandrayaan-1 for remote sensing. SELENE is an acronym.

In addition, NASA will execute lunar orbital missions both for science and in preparation for a new American space program employing the Moon as a stepping stone toward eventual human exploration of Mars. The first such mission is that of the Lunar Reconnaissance Orbiter, a large, multipurpose remote-sensing spacecraft to be launched in 2008 or 2009. Because its launch vehicle has excess payload capacity, the mission will also carry an experiment called LCROSS for observing a planned lunar crash of the vehicle’s translunar injection stage. The program is intended to continue with robotic landers and rovers exploring the prospects for use of lunar resources, including the polar ice deposits if they do exist.

Mars

With 19th-century telescopic observation showing polar caps and other indications of an atmosphere and changing surface features, Mars became the planet of choice for speculation about other life in the cosmos and about human travel to other worlds.

These pervasive ideas have since driven planetary program priorities with the result that huge resources have been devoted to Martian robotic exploration and to studies of the prospect of human ventures to Mars. But Mars has proved to be a difficult destination: Failure has been an ever present hazard—not only in flight missions but also in the councils where budget decisions are made.

In what follows, we concentrate upon successes, but those must be seen as just the most visible parts of a remarkable, decades-long striving toward a possible breakout of humanity beyond the bounds of Earth.

In addition to the Web page listed for each mission below, a site with a brief story of every publicly acknowledged Mars mission is given at www.planetary.org/learn/missions/marsmissions.html.

Mariner 4

Mars launch opportunities occur about every 26 months. In both the USA and the USSR, the October 1960 window was the favored first chance. The Soviets did launch, with two upper stage vehicle failures. During the 1962 window, the Soviets tried three launches, one of which sent Mars 1 toward the planet. That spacecraft failed en route. In 1964, NASA launched two Atlas-Agenas with one success. Mariner 4 flew by Mars and returned 22 images of the cratered southern highlands, leading to the impression of a Moon-like Mars, proved false by later missions. (See www.jpl.nasa.gov/missions/past/mariner3-4.html.)

Mariner 6 and 7

Two Mars flyby missions, Mariner 6 and 7, launched by Atlas-Centaurs in 1969, demonstrated the rapid advance of deep-space data acquisition technology. Their imaging was greatly improved over that of Mariner 4 in both quality and quantity, and in addition infrared spectrometry gave some first indications of Martian surface compositions. They still covered mainly southern, including polar, ancient landforms, omitting the vast volcanoes and canyons discovered by Mariner 9. (See www.jpl.nasa.gov/missions/past/mariner6-7.)

Mars 2 and 3

During the 1971 Mars window, the USSR and USA each launched two missions. The Soviet Mars 2 and 3 orbiter/landers both arrived successfully into orbit at the planet; Mars 2 returned some orbital science data but its lander crashed. Mars 3, in addition to its orbital operations, delivered its lander with a small tethered mobile platform. But the transmissions from the lander ceased only 20 seconds after touchdown. (See www.earthandspace.org/mars23.htm.)

Mariner 9

The Atlas-Centaur carrying Mariner 8 failed but Mariner 9 became the most rewarding Mars mission up to its time, waiting out a global dust storm in orbit and then sending imagery of most of the Martian surface until its mission ended in 1972, revealing enormous volcanoes, canyons, apparent river channel networks, sapping collapse features and clouds, plus imagery of the two small moons, Phobos and Deimos. (See www.jpl.nasa.gov/missions/past/mariner8-9.)

Mars 4, 5, 6, and 7

At the 1973 opportunity, the Soviets made an all out effort to upstage the American Viking missions planned for 1975. They launched four large spacecraft, all of which arrived in the vicinity of Mars but each of which ultimately failed for a different reason. Mars 4 failed to brake into orbit but did return some flyby data; Mars 5 entered orbit, sent some images and failed after 22 days; Mars 6 released a lander that failed during descent; Mars 7‘s lander missed the planet. (See athena.cornell.edu/mars_facts/past_missions_70s.html.)

Viking 1 and 2

In 1975 two large NASA orbiter/landers, Viking 1 and 2, were launched by powerful Titan-Centaurs. Arriving in June and July 1976, they entered orbit and began surveying for landing sites. The Viking 1 lander set down in Chryse Planitia on 20 July and the Viking 2 lander descended to Utopia Planitia on 3 September on the opposite side of Mars. While the orbiters began imaging the whole planet and making spectrometric remote sensing observations, during descent the landers measured atmospheric composition. Then the landers began to operate a suite of instruments for imaging their surroundings and determining meteorological, geological and biological properties. At first, microbial activity was suspected, but eventually most scientists concluded that no life did or could exist in the soil samples. (See www.jpl.nasa.gov/missions/past_missions.cfm.)

Phobos 1 and 2

After a long pause in Martian exploration, in 1988 two large and complex Soviet spacecraft, Phobos 1 and 2, were launched by Proton vehicles toward the vicinity of Mars. Phobos 1 was lost en route due to a human error in ground control. Phobos 2 arrived and began phasing orbits for a rendezvous with the little moon, where it was to make close-up observations and deposit two small landing packages, one of them a hopping rover. Imagery and some other data of Phobos and Mars were obtained, but the spacecraft failed before the landings could occur. (See www.iki.rssi.ru/IPL/phobos.html.)

Mars Pathfinder and Mars Global Surveyor

The 1996 launch window saw the revival of detailed American exploration of Mars. NASA’s Pathfinder delivered a lander and a small rover, named Sojourner, which explored nearby surroundings in the Ares Vallis outwash plain. The Global Surveyor spacecraft entered an eccentric orbit and was delicately aerobraked down into circular mapping orbit over a period of months, the long period being needed due to structural failure of the attachment of one solar panel. The mission has yielded a continuing stream of imaging and other data, revolutionizing scientists’ knowledge and modeling of Martian geology and atmospheric processes. (See marsweb.jpl.nasa.gov/missions/.)

Failures of the 1990s

The years 1992, 1998, and 1999 saw three US missions fail during arrival at the planet: Mars Observer, Mars Climate Orbiter, and Mars Polar Lander. An elaborate international Russian mission’s launch, Mars-96, failed in 1996—a series of events that led in the USA to a management overhaul and in Russia to the end of Mars exploration for the time being. (See marsweb.jpl.nasa.gov/missions/ and www.iki.rssi.ru/mars96/mars96hp.html.

Mars Odyssey

Mars Odyssey, a NASA orbiter launched in 2001, is instrumented for measurements complementing those of the Global Surveyor. With infrared/visible, gamma-ray, and particle spectrometers, it produces thermal imaging enabling evaluation of surface physical properties, subsurface elemental chemistry, and the planet’s radiation environment. Odyssey‘s findings have greatly stimulated interpretations of many of Mars’s landforms as resulting from the action of subsurface briny water, ice and carbon dioxide. (See marsprogram.jpl.nasa.gov/odyssey/.)

Spirit and Opportunity

In an intense three-year effort, two NASA Mars rover missions, Spirit and Opportunity, were prepared for the 2003 launch opportunity. Both succeeded, and at the time of writing the two rovers are continuing to make astonishing discoveries in Meridiani Planum and Gusev Crater, on opposite sides of the planet, reinforcing the orbiters’ findings of a history dominated by the effects of water. (See marsrovers.jpl.nasa.gov/home/index.html.)

Mars Express

ESA’s Mars Express orbiter, launched on a Russian Soyuz-Fregat vehicle in 2003, delivered the small British Beagle-2 lander, which failed, and has then gone on at the time of writing to yield excellent imaging, plus spectrometric measurements indicating, among other findings, that there is a correlation between regions of enhanced water vapor and methane concentrations in the atmosphere. Mars Express also carries a ground-penetrating radar for detetecting the signatures of subsurface brines and ices. (See sci.esa.int/science-e/www/area/index.cfm?fareaid=9.)

Mars Reconnaissance Orbiter

Launched in 2005 and delivered in 2006 into aerobraking orbit at Mars, Mars Reconnaissance Orbiter is expected to increase by orders of magnitude the quantity and quality of remote-sensing data from Mars, because of its powerful radio system and advanced on-board instruments and system software. Imaging already obtained, while excellent, gives only a small sample of the harvest to come.

Future Mars Missions

The exciting discoveries of the missions listed here and the ongoing debate over the prospect of human missions to Mars have continued to energize an active NASA program. Mars launches are planned for the 2007 and 2009 opportunities. Phoenix is a reflight of the failed Mars Polar Lander, and Mars Science Laboratory is intended to expand on the findings of the Mars rovers, Spirit and Opportunity. (See marsweb.jpl.nasa.gov/missions/.)

Small Bodies

As scientists have come to realize that comets and asteroids contain clues to the ancient history of the solar system—clues largely obliterated by geologic processes in planets and moons—missions to small bodies have increased in importance. Also studies of cratering and meteorite records show that near-Earth asteroids present both a threat and an opportunity. The threat is that of devastating impacts and the opportunity is that of useful resources not found in the Moon. (See www.permanent.com.)

Ice

After completing its solar mission as ISEE-3 (see Sun and Heliosphere sections) the spacecraft was retargeted and renamed International Cometary Explorer. It flew through the tail of Comet Giaccobini-Zinner in 1985, then continued on in heliocentric orbit where it sent low-rate data for the next several years. (See heasarc.gsfc.nasa.gov/docs/heasarc/missions/isee3.html# instrumentation.)

The Halley Armada

As Halley’s comet arrived near the Sun in 1986 on its 76-year orbit, it was met by spacecraft from Japan, Europe, and the USSR. Comet enthusiasts lamented the absence of the USA from this once-in-a-lifetime opportunity. Japan’s Suisei and Sakigaki made distant observations of the ultraviolet coma. ESA’s Giotto passed within 600 km of the nucleus collecting imaging, spectra, and detailed chemical data. The Soviet VEGA 1 and 2 flew by at intermediate distances after their productive en route encounters with Venus. (See www.planetary.org/html/neo/Missions-Research/index.html.)

Galileo En Route Encounters

While en route to Jupiter on its long journey with gravity assists at Venus and Earth, the NASA Galileo spacecraft flew by two asteroids, 951 Gaspra in 1991 and 243 Ida in 1993, and obtained close-up imagery, spectra, and other measurements. A highlight of the Ida encounter was the discovery of the tiny moon Dactyl orbiting Ida. (See galileo.jpl.nasa.gov/facts.htm.)

NEAR-Shoemaker

Launched in 1996, the NASA spacecraft Near-Shoemaker entered orbit about asteroid 433 Eros in 2000, delivered imagery, spectrometric data, and gravitometric data. After one year in orbit it was commanded to a gentle touchdown, which it survived, even though not designed for landing. (See near.jhuapl.edu/.)

Deep Space 1

The NASA craft Deep Space 1 launched in 1998 to demonstrate solar-electric propulsion, autonomous navigation, and other new technologies, encountered asteroid 9969 Braille in 1999, though it only returned a few distant, low-resolution images. Its mission was extended to a close flyby of Comet Borrelly on 22 September 2001 and successfully imaged the nucleus at visible and infrared wavelengths. (See nmp.jpl.nasa.gov/ds1/.)

Stardust

With the goal of collecting samples of cometary dust and returning them to Earth, NASA’s Stardust mission, launched in 1999, flew by asteroid 5535 Annefrank in 2002, and encountered Comet Wild 2 in 2004, returning imaging data. The sample return capsule successfully parachuted to Earth on January 4, 2006, with its precious cargo of thousands of cometary (and interstellar) dust particles. (See star-dust.jpl.nasa.gov/top.html.)

Hayabusa (Muses-C)

An ISAS mission with assistance from NASA, Hayabusa was launched in 2003 and used solar electric propulsion to rendezvous with asteroid 25143 Itokawa in September 2005. It returned multispectral imaging and gravity data and attempted to collect surface samples for return to Earth. The spacecraft is en route to Earth but technical problems have delayed arrival until 2010. (See www.isas.jaxa.jp/e/enterp/missions/hayabusa/index.html.)

Rosetta

Rosetta, an ESA mission launched in March 2004 with an Ariane V, is scheduled to arrive at Comet Churyumov-Gerasimenko in 2014 after three Earth gravity assists and one at Mars. It is also targeted to flyby asteroids 2867 Steins in 2008 and 21 Lutetia in 2010. (See rosetta.esa.int/science-e/www/area/index.cfm?fareaid=13.)

Deep Impact

With the goal of determining the physical and chemical makeup of a cometary nucleus, NASA’s Deep Impact mission, launched in January 2005, successfully delivered a 370-kg projectile to a 10 km/s collision with Comet Tempel 1 on July 4, 2005. Imaging from the impactor and the flyby spacecraft returned the highest resolution pictures of a comet to date and documented the impact event which provided new insights into the nature of cometary nuclei. (See deepimpact.jpl.nasa.gov/mission/facts.html.)

Dawn

Dawn, an ion-propelled spacecraft to be launched in 2007 (after its mission was cancelled and quickly reinstated in 2006) will investigate the surface and interior properties of Vesta and Ceres, the two large asteroids that telescopic observation shows to be quite different from each other. (See dawn.jpl.nasa.gov.)

Outer Planets and Moons

In 1610 when Galileo observed four bright specks moving near Jupiter, he set in motion a quest that culminated in the 20th century with history’s greatest robotic exploration program, giving never-to-be-repeated first close looks at the giant outer planets and their retinue of moons and rings. (See www.solarsystem.nasa.gov./index.cfm.)

Pioneers 10 and 11

Two NASA missions launched the first two of four human artifacts to escape forever from the Sun’s domain. Leaving Earth in 1972, Pioneer 10 flew by Jupiter in 1973 with imaging and magnetospheric measurements. Its signal continued to be detected at Earth until 2003. After launch in 1973, Pioneer 11‘s flyby trajectory was adjusted so that, at its encounter in 1974, Jupiter’s gravity would fling it onward toward Saturn, where it flew by in 1979. Each spacecraft carried a golden plaque illustrating humans and encoded information on where and when in the cosmos the flight had originated. (See www.solarsystem.nasa.gov/missions/profile.cfm?Sort=Target&Target=Saturn&MCode=Pioneer_11.)

Voyagers 1 and 2

Launched in 1977 by Titan-Centaurs and still operating in 2006, the NASA missions Voyagers 1 and 2 are a mighty achievement. Voyager 1 flew by Jupiter in 1979 and Saturn in 1980, whence it is headed toward the heliopause, the boundary between the Sun’s realm and that of interstellar space. Voyager 2 was targeted to a Jupiter flyby and then to Saturn, where Saturn’s gravity would send it on to Uranus and Neptune, taking advantage of a planetary alignment that happens at intervals of 173 years. Voyager 2 passed Uranus in 1986 and Neptune in 1989. The two Voyagers returned a vast harvest of imagery, geochemical and geophysical data on the giant planets and their moons and rings, and magnetospheric information. Each one carried a golden phonograph and video record showing characteristics of our planet, its inhabitants, and human civilization. In 2005 Voyager 1 detected the heliopause, and in 2006 it passed 100 astronomical units from the Sun. (See www.solarsystem.nasa.gov/missions/profile.cfm?Sort=Chron&MCode=Voyager_2&StartYear=2000&EndYear=2009.)

Galileo

NASA’s Galileo mission was launched by the space shuttle plus the Inertial Upper Stage in 1989 after a fraught history of replanning and delays. Galileo entered Jovian orbit in 1995, having made one Venus and two Earth gravity assist flybys en route. During the flybys, some science data were collected, including multispectral observations of the Earth and Moon. Galileo performed the first two asteroid flybys and was in position to image the Comet Shoemaker-Levy 9 impacts on Jupiter. At arrival in the Jovian system the spacecraft delivered a probe into the huge planet’s atmosphere. Despite the failure of the orbiter’s high-gain antenna to deploy, the mission returned a large volume of imaging, spectra and other data on the planet and its moons. In 2003 the craft was commanded to a Jupiter impact, with destruction in Jupiter’s atmosphere to keep it from becoming a contamination risk toward any possible biology in the putative subsurface oceans of Europa and Ganymede. (See www.solarsystem.nasa.gov/missions/profile.cfm?Sort=Target&Target=Jupiter&MCode=Galileo.)

Cassini-Huygens

Launched in 1997 by a Titan-Centaur, NASA Saturn-orbiter spacecraft Cassini-Huygens carried ESA’s Huygens probe designed to enter the dense atmosphere of the huge moon Titan. With a 1998 Venus gravity assist and a 2000 Jupiter flyby with some scientific observations en route, the combination entered Saturn orbit in 2004. The probe descended to Titan in 2005, delivered remarkable images and survived impact on the surface for many hours. Both spacecraft returned unique new observations that will cause active scientific analysis and argument for years to come. (See saturn.jpl.nasa.gov.)

New Horizons

Launched in 2006 at such a high speed that it will pass Jupiter in less than thirteen months from Earth departure, the New Horizons spacecraft is to investigate the surfaces and atmospheres of Pluto and its large moon Charon during a flyby in 2015. After that it is expected to continue functioning for several more years, exploring the mysteries of the Kuiper Belt, that far-out region of the solar system where the first representatives of a likely multitude of small, icy objects have already been discovered.

Conclusion

Thus has ended the first, magnificent phase of investigation throughout the Sun’s domain. Meanwhile, spaceflight in the inner solar system is reinvigorated as robotic missions to the Moon and Mars take on the purpose, in addition to science, of acting as precursors to renewed human exploration and perhaps ultimately settlement of communities off Earth. And discoveries of giant planets orbiting other stars, more than 150 to date, tell us that exploration of star-and-planet systems has a limitless future.