Are we alone? There is perhaps no more important single scientific question. People have pondered this issue from the very dawn of sentience, wondering if other, similar, beings inhabited a distant mountain range or the other side of an ocean. The history of humanity is largely one of exploration and expansion, and while at first this was limited to the Earth’s surface, in the last few decades only the power of our interplanetary rockets has kept us from exploring our wider environment. As the science-fiction author Ray Bradbury proclaimed when the Viking landers arrived on Mars in July 1976, “There is life on Mars, and it is us.”

Now we are poised to move far beyond our own planetary system — if not yet physically, then at least intellectually — as we learn how many other habitable worlds exist in our galaxy. The expectation of most astronomers is that Earth-like planets will be commonplace; in other words, Earth-size planets will be found orbiting at distances from their parent suns that would allow liquid water to exist on or near the planet’s surface. This prediction is not simply a conceit concocted by wild-eyed planet hunters, but is a logical outcome of what we have learned in the last decade about extrasolar planetary systems. All of the evidence points to the inescapable conclusion that rocky, terrestrial planets similar to the Earth should be orbiting unseen around many, if not most, of the myriad of stars that we see when we look up at the night sky.

This prediction of the likelihood, or frequency, of Earth-like planets orbiting Sun-like stars is about to be tested rigorously by the Kepler Space Telescope, which is designed specifically to determine the percentage of solar-type stars that harbour Earth-like worlds. Due for launch early this month, NASA’s latest telescope will search for extrasolar Earths using a method known as transit photometry. When an Earth-sized planet with a suitably inclined orbit passes between its star and the line of sight to the Earth, it blocks a tiny fraction — about one 10,000th — of the star’s light. Kepler is capable of measuring these tiny, periodic dimmings, and will monitor more than 100,000 stars for at least three and a half years. In the process it should discover dozens of Earth-like planets. In short, we are about to determine the value of a key factor in any estimate of the prevalence of life in the universe: the frequency of habitable worlds where life could originate and evolve.

Early exoplanetology

Until 1995 the only known planets outside our solar system were two scorched rocks in orbit around a very un-Sun-like pulsar, PSR B1257+12. This shortage of extrasolar planets was not due to a lack of effort in trying to find them. Since 1980 a Canadian research team led by Gordon Walker of the University of British Columbia had been searching a sample of nearly two dozen Sun-like stars for the telltale “Doppler wobble”, or periodic redshift and blueshift in the star’s spectrum, caused by the orbital motion of the host star around the system’s centre of mass. By 1992 the researchers had found no definitive evidence for even a single extrasolar planet comparable in size to Jupiter. They therefore stopped their search and wrote a paper describing the upper limits (of the order of one Jupiter mass) on the masses of any possible planets lurking undetected around the target stars. When this paper was published in the autumn of 1995, the field of extrasolar planetology seemed to have come to a premature dead end.

Two months later, Michel Mayor, an astronomer at the Geneva Observatory in Switzerland, made a startling announcement: he and his colleague Didier Queloz had found the first reproducible evidence of a Jupiter-mass planet orbiting a solar-mass star. Mayor had only begun his planet search in 1994, but his target list contained more than 100 stars — including, crucially, 51 Pegasi, located about 50 light-years away from Earth. The newly discovered planet, named 51 Pegasi b, has a mass at least half that of Jupiter and causes a periodic Doppler shift that the Canadian team could have detected — if 51 Pegasi had been one of their target stars.

With an orbital period of just four days, 51 Pegasi b occupies an orbit 100 times closer to 51 Pegasi than Jupiter is to the Sun, making it the first example of the class of planets known as “hot Jupiters”: gas giants with surface temperatures of about 1500 K, which is closer to those of low-mass stars than of Jupiter (150 K). The existence of 51 Pegasi b was confirmed a week later by the astronomers Geoff Marcy and Paul Butler, then at San Francisco State University and the University of California at Berkeley, respectively, who had been searching for gas giants with Doppler spectroscopy since 1988. After Mayor’s announcement, they combed through their seven years of accumulated data on over 100 stars. They quickly found two more “Doppler planets”, 70 Virginis b and 47 Ursa Majoris b. This time, the gas-giant planets had more sensible orbital periods of about 0.33 and three years, making them warm and cool Jupiters, respectively.

The field of extrasolar planets grew enormously after the Swiss and US teams announced their discoveries in early 1996. To date, these two teams have found most of the more than 300 Doppler planets now known to astronomers. Such planets are surprisingly common, with over 10% of solar-type stars appearing to have gas-giant planets with orbital periods less than about five years, with even more planets orbiting at larger distances. Hot Jupiters like 51 Pegasi b occur around about 1% of solar-type stars: if the Canadian team had searched a sample of 100 stars, as the Swiss did, they might have found the first hot Jupiter.

On the theoretical side, the wave of new exoplanet data has left models of gas-giant planet formation in ferment, with two viable mechanisms that may explain the observed range in mass (roughly Saturn mass to 13 Jupiter masses) and relatively high frequency of gas-giant exoplanets. The first, “disk-instability”, mechanism would allow gas giants to form in even the shortest lived protoplanetary disks. Such disks occur in regions of massive star formation. There is good evidence that the solar system formed in such a commonplace area, implying the ubiquity of similar systems. The other possible mechanism relies on “core accretion”, where a roughly 10-Earth-mass core forms and then slowly accretes sufficient gas from the disk to become a gas giant. If the disk gas disappears faster than the cores form, the result is “failed cores’’ similar to the ice-giant planets Uranus and Neptune.

From hot Jupiters to super-Earths

Short-period planets like the hot Jupiters have a 10% chance of having their orbits inclined such that the planet passes in front of its host star as seen from Earth. By measuring the fractional change in the star’s brightness during these transits, we can calculate a planet’s size relative to the size of the star, which can in turn be estimated based on its stellar classification. Hence, transit measurements offer final proof that the Doppler-planet candidates actually are gas-giant planets similar to Jupiter and Saturn. The 10th hot Jupiter found using Doppler spectroscopy by Butler (by then at the Carnegie Institution) and his colleagues turned out to be the first transiting planet, HD 209458 b.

Transits also provide the opportunity to study a planet’s atmosphere as starlight passes through it during a primary eclipse (which occurs when the planet passes in front of its star), or as the planet’s thermal emission is blocked during a secondary eclipse (planet behind the star). Clever observations with the Hubble and Spitzer space telescopes have allowed astronomers to detect the presence of water, carbon dioxide, methane, sodium and hydrogen in the atmospheres of several known hot Jupiters. Transit surveys are also being used to discover new hot Jupiters, with over 50 such planets having been found so far using this method followed up with Doppler confirmations.

Continued improvements to Doppler spectrometers have allowed the Mayor, Marcy and Butler groups to discover not only Jupiter-type planets but also short-period planets with masses as little as four times that of the Earth. Most of these hot and warm “super- Earths” are known to be accompanied by gas-giant siblings at greater orbital distances. This configuration — reassuringly similar to that of the terrestrial and gas-giant planets in our solar system — suggests that these super-Earths formed closer to their parent stars than the gas giants did. Theoretical models of planet formation firmly predict that these planets must be at the high-mass end of the range of rocky extrasolar planets; hence, rocky planets with even smaller masses must exist as well.

Estimates of the frequency of hot and warm super- Earths based on Doppler surveys imply that perhaps a third of all solar- and lower-mass stars have such planets. Mayor’s team has even found one star, HD 40307, that has three orbiting super-Earths with masses of about four, seven and nine times that of the Earth (see “Super-Earth trio”). Several other multiplanet systems have also been found using Doppler spectroscopy. The 55 Cancri system, for example, contains at least five known planets, with the innermost planet being a hot super-Earth with an orbital period of 2.8 days.

Total eclipse of a star

In addition to the Doppler-wobble and transit methods, a third technique, known as microlensing, has also recently been used to discover planets. When a foreground star happens to pass directly in front of a distant background star, the background star’s light can be bent toward the Earth by the gravity of the foreground star. The result is a gradual brightening and dimming of the background star as the unseen foreground star passes in front of it. If the foreground star has a planet in orbit at a distance similar to the asteroid belt in our solar system, then observations will reveal an additional, sharper brightening and dimming of the background star that can only be caused by a third body. The critical distance is known as the Einstein radius, since Einstein predicted this effect in 1936. However, he assumed that it could never be observed in practice because the chance of having the two stars line up in exactly the right configuration when viewed from the Earth would be small, and the chance of observing it at the right time even smaller.

Acting independently, two teams of astronomers (led by Ian Bond from the Institute of Astronomy in Edinburgh, UK, and Andrzej Udalski of the Warsaw University Observatory in Poland) discovered the first microlensing planet in 2004. The new planet had a mass 2.6 times that of Jupiter, and was designated OGLE2003-BLG-235/MOA2003-BLG-53 b as its discovery involved observations by both Bond’s Microlensing Observations in Astrophysics (MOA) team and Udalski’s Optical Gravitational Lensing Experiment (OGLE) team.

This discovery was made possible by Udalski’s participation in the OGLE search for microlensing events, which involves monitoring hundreds of thousands of stars in our galactic bulge with a modest 1.3 m telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. Einstein could not have foreseen such a dedicated effort to test his 1936 prediction.

Microlensing has detected eight planets to date: five gas giants and three cold super-Earths with masses in the range from about 3 to 13 Earth-masses, including one multiple system containing planets analogous to Jupiter and Saturn. The cold super-Earths discovered using microlensing appear to be similar to the solar system’s ice-giant planets, Uranus and Neptune, because of their similar masses and orbits at large distances where ices will be common. Combined with the Doppler discoveries of abundant gas giants, and hot and warm super-Earths, astronomers have identified extrasolar analogues for each of the three basic classes of solar-system planets.

Seeking (higher) resolution

One of the major goals of planet hunters is to take a spatially resolved image of a planet, so that the planet’s atmosphere can be more easily distinguished from that of the host star. Six claims for the first image of an extrasolar planet were advanced in 2008 alone.

Perhaps the most convincing of the six is the image of three possible planets in orbit around the star HR 8799 by Christian Marois and his colleagues at Canada’s Herzberg Institute of Astrophysics (see “Seeing new worlds” and Physics World January 2009 p5, print edition only). The masses of the planets are estimated to be in the range of 7 to 10 Jupiter masses, and they orbit HR 8799 at distances of about 24, 38, and 68 astronomical units (the mean distance of the Earth from the Sun), distances comparable to those of the ice giants and Kuiper belt in our solar system.

One of the main uncertainties in estimating their masses is in estimating their ages: given a theoretical model of how a newly formed planet cools with time, the age of the star is used as the age of the planet, allowing the planet’s mass to be guessed. Marois has estimated that HR 8799 is about 60 million years old, but if the star were considerably older than that, then the masses of the three orbiting bodies would be greater than about 13 Jupiter masses. This limit is important: once over it, such “planets” can burn deuterium and so are classed as brown dwarf stars rather than planets (see “An exoplanet encyclopaedia”).

Only further observational and theoretical work will decide which, if any, of these six candidate images is indeed that of a planetary system. These ambitious efforts presage the ultimate goal of obtaining direct images of nearby Earth-like planets and studying their atmospheres for evidence of molecules associated with habitable (carbon dioxide, water) and inhabited (oxygen, methane) worlds.

Given the discoveries of the last decade, we cannot argue that the solar system is a fluke of the universe. While the chaotic nature of the planet-formation process rules out finding an exact analogue, it is clear that planetary systems similar to our own must be a common feature of our galactic neighbourhood. Indeed, only last month astronomers using the CoRoT space telescope discovered the smallest sized transiting exoplanet to date: a fiery new world less than twice the radius of the the Earth that orbits its star every 20 hours.

The Kepler mission will determine how frequently Earths occur in our galaxy: do 1% of Sun-like stars have Earth-like companions? 10%? 100%? Given that there are billions of Sun-like stars in our galaxy alone, the number of Earth-like worlds must be similarly immense. Whatever the answer, by the time Kepler finishes its primary mission in 2013, we will know just how crowded the universe really is.

In 1609, when he peered expectantly through his handmade telescope at the mountains of the Moon and the four large satellites of Jupiter, Galileo Galilei could have had no idea that nearly four centuries later these astronomical bodies would be orbited by “artificial satellites” hand-built by like-minded inquisitors of the solar system. That one of these spacecraft would be named after him would probably have been dismissed as idle fantasy. However, the spacecraft known as Galileo, launched in 1989 and de-orbited into Jupiter’s turbulent atmosphere in 2003, is just one of many interplanetary spacecraft dispatched to explore the solar system since the beginning of the space age in 1957.

Although he was spared the levels of particulate and light pollution we suffer today, Galileo must have noticed that the performance of his telescopes was limited by the atmosphere. Heat rising from buildings or the ground itself would have caused a familiar shimmering of the Moon’s image, while the longer atmospheric pathlength of light from stars observed near the horizon would have made them twinkle, just as it does today.

From the late 1940s, when sounding rockets began to be used for research on the upper atmosphere, it was clear to scientists that the Earth’s atmosphere absorbs extraterrestrial radiation at some wavelengths, and that the only way to conduct a complete investigation of the universe is to place instruments beyond it. The concept of the space telescope had already been discussed as early as 1929 by German rocket pioneer Hermann Oberth in his book Wege zur Raumschiffahrt (“Paths to Space Travel”), but it is a paper written in 1946 by the US astronomer Lyman Spitzer that is credited with introducing the advantages of a space-based telescope to the research community. The paper, entitled “Report to Project RAND: Astronomical Advantages of an Extra-Terrestrial Observatory”, pointed out that not only could a space-based telescope detect the infrared and ultraviolet wavelengths absorbed by the atmosphere, but also that its angular resolution would be limited only by diffraction (instead of atmosphere turbulence).

Once the technology had been developed to build and launch satellites, the notion of what is now known as “space astronomy” became a reality. Spitzer’s vision was rewarded by seeing NASA’s Orbiting Solar Observatory (OSO-1), the first astronomical observatory in space, launched in March 1962. The OSO series led to the Orbiting Astronomical Observatory (OAO) in the late 1960s – a telescope with which Spitzer was particularly associated – and eventually to the Hubble Space Telescope in 1990.

Being there

Orbiting observatories have proved crucial to the development of astronomy, astrophysics and cosmology because they provide data on distant stars and galaxies that ground-based observers cannot obtain. But the remote-sensing techniques of space astronomy have important limitations when it comes to the planets and the minor astronomical bodies. Even the Hubble Space Telescope can produce only low-resolution images of the surface of Mars or the cloud tops of Jupiter.

It was clear from the early days of the space age that the best way to conduct planetary astronomy was to transport the observing instruments to the vicinity of planets and satellites. In the 1950s, during the Cold War, politics rather than science was the driving force. After the first heat of the space race had been won by the Soviet Union with its Sputniks, a second heat began: the race to land a spacecraft on the surface of our nearest planetary body, the Moon (see “Sputnik’s legacy”).

Thus the US launched its first Moon probe, Pioneer 1, on 11 October 1958 and the USSR launched Luna 1 on 2 January 1959. The former got little more than quarter of the way to the Moon before succumbing to the Earth’s gravitational pull, while the latter missed its target by some 6000 km. But the fact that both nations were planning “Moon shots” less than a year after Sputnik 1 indicates the depth of their respective desires to be first in the various heats of the space race.

It is interesting to note that spacecraft were sent to both the Moon and Venus even before OSO 1 reached Earth orbit in 1962. The pragmatism of remote sensing was apparently outweighed by the desire to “get there” physically, but mainly, as with most aspects of space exploration at the time, by the political need to score points against the opposition.

From an engineering point of view, however, there are other more fundamental challenges, such as the need to provide accurate guidance and control, tracking and telemetry, thermal and radiation protection, and to launch the object in the first place. Indeed, the early history of space exploration is littered with examples of failed launches, fried electronics and “heat stroke”. For example, NASA’s unmanned lunar programme Ranger began badly when the first two spacecraft fell victim to upper-stage rocket failures. Then, Rangers 3 and 4, having been sterilized by heating to 125 °C for 24 hours (ostensibly to avoid possible contamination of the lunar surface), failed to complete their missions after their central computers and sequencers broke down. Ranger 3 missed its target by 37,000 km and Ranger 4 acquired the dubious honour of being the first spacecraft to crash on the far side of the Moon.

Now that planetary exploration is somewhat more routine, it tends to be pursued in phases – using fly-by, orbital and landing missions – that, by and large, represent increasing accuracy in guidance and control. For the exploration of Jupiter, for example, we saw the Pioneer and Voyager fly-bys of the 1970s followed by the Galileo orbiter of the 1990s; for Saturn, the Voyager fly-bys of the early 1980s and the Cassini orbiter of the present decade. There have so far been no “landing missions” in the Jupiter system, unless one counts the atmospheric entry probe from the Galileo mission that, in 1995, became the first spacecraft to enter Jupiter’s atmosphere, and then Galileo’s later dive into the Jovian cloudscape at the end of its mission. Furthermore, the Huygens probe, developed by the European Space Agency (ESA), made a spectacular landing on Saturn’s largest moon Titan in January 2005 and transmitted a wealth of data that could never have been gathered remotely (including details of atmospheric structure and composition, wind speed, surface chemistry and images of icy boulders) (see “Tuning it to Titan”).

Close-up observation has a number of technical advantages. “Spatial resolution is improved enormously and proximity to the planet also improves the signal-to-noise ratio for spectral measurements, enabling higher spectral resolution,” says Wendell Mendell, head of NASA’s Office for Lunar and Planetary Exploration. A spacecraft in orbit around a planet can observe temporal changes, such as atmospheric storms on the gas giants and volcanic eruptions on their satellites (such as those observed on Io by Voyager in 1979 and Galileo in 1996), mainly because it can conduct long-term global surveys, as opposed to taking snapshots. Also, adds Mendell, “phenomena such as X-ray or gamma-ray emission can often be detected only when close to the object”.

Marcello Coradini, ESA’s co-ordinator of solar system missions, goes as far as to quantify the benefits of space probes. “Between 90% to 95% of what we understand of the planets and their satellites”, he says, “is due to in situ space missions that have allowed us to reveal, both from orbit and on the surface, details, features and geological, geophysical and geochemical processes otherwise impossible to detect and study.”

The lure of Mars

Among the many discoveries made possible by spacecraft, Ian Crawford, a senior lecturer in planetary science at Birkbeck College in London, cites the volcanoes of Io, the methane lakes of Titan and “the probable subsurface ocean of Europa”. But it is one of our nearer planetary neighbours, Mars, that has perhaps excited space scientists the most in recent years. The history of its telescopic exploration is well known – most famously because of Percival Lowell’s mistaken identification of canals on its surface. The first successful attempt at a Mars fly-by mission was NASA’s Mariner 4, which made a close approach of some 9000 km in July 1965. It took just 22 photographs, covering only 1% of the planet’s surface, and their content was immediately disappointing to earthbound canal-spotters and anyone hoping for complex life forms, such as vegetation. Mars was apparently a dead planet.

Mariner 6 and Mariner 7 flew past Mars in July and August 1969, respectively, somewhat closer, at about 3500 km, and taking 200 images each, but the results were much the same. The Martian surface – apart from the polar caps that were visible from the Earth anyway – looked very much like the barren, cratered plains of the Moon. Considering that two men from Earth had landed on one of those plains in the same month that Mariner 6 made its fly-by, it is hardly surprising that its achievements failed to take the world by storm.

Mars did not become truly interesting to space scientists until Mariner 9 entered its orbit in November 1971. It mapped the whole surface of the planet in over 7000 photographs and gathered data on atmospheric composition, density and temperature, which effectively prepared the way for the first landing mission – Viking 1 – in July 1976. As Crawford points out, Mariner 9 revealed the volcanoes, canyons and river valleys of Mars, providing tantalizing “evidence of past habitable conditions on that planet”. Unfortunately, the two Viking missions failed to provide evidence of life on Mars – a failure that contributed to a 20-year hiatus in NASA’s Mars exploration programme that lasted until the mid-1990s. But as Carl Sagan once said, “absence of evidence is not evidence of absence”.

Down among the rocks

Put simply, if any form of life, however simple, can be found on another planet, then the chances that more intelligent life exists elsewhere in the universe are significantly improved. It is thought extremely unlikely that life would have developed independently on two planets in a given solar system and nowhere else.

It is this postulate that has driven most planetary-exploration programmes in recent times, not least the “follow the water” theme initiated by NASA in the late 1990s, following data from the Clementine spacecraft that indicated the presence of ice in a crater near the Moon’s south pole. As for Mars, it was clear from some of the Mariner 9 images that water, or at least some form of liquid, had shaped many of its geomorphological features. It was also clear, however, that these features were a relic of the planet’s past, because liquid water could not exist at the low atmospheric pressures detected on the planet.

Mars’ polar caps were formerly thought to consist mainly of frozen carbon dioxide, but observations from orbiting spacecraft, such as NASA’s Mars Global Surveyor and ESA’s Mars Express, in recent years have shown that the north cap is predominantly water ice and that the south cap also contains water. Since then, however, the emphasis has shifted to finding evidence of water beyond the polar caps. And the only way to do this conclusively is to get down among the rocks, preferably using a rover that can move easily on rocky terrain.

NASA’s first Mars rover was the tiny, 10 kg Sojourner, which was a passenger on the Mars Pathfinder lander (later named Carl Sagan Memorial Station) that was launched in December 1996. It touched down in the Chryse Planitia region of Mars on 4 July 1997. The mission was historic in being the first to deposit a rover on the surface of another planet, but also because it marked the first Mars landing since 1976.

Sojourner operated for a little less than three months, returning some 550 images and analysing rocks and soil in the vicinity of the lander. Among the many scientific highlights of the mission was what NASA called “the possible identification of…conglomerates that formed in running water, during a warmer past in which liquid water was stable”. Indisputable proof of water was, however, elusive.

It was the Mars Exploration Rovers (MERs), Spirit and Opportunity, that really proved the efficacy of the “up close and personal” approach to planetary exploration. Launched in 2003, the rovers survived their airbag-assisted landings in January 2004 and easily completed their primary three-month missions the following April. The fact that they were both still operational as of early 2009 is a testament to the engineering design of the two craft.

John Callas, MER project manager at NASA’s Jet Propulsion Laboratory (JPL), describes the rovers as “our robotic proxies on the surface of Mars”. “As field geologists,” he says, “the two rovers have found clues about the ancient environment, [discovering] extensive evidence of sustained, liquid surface water during the planet’s early history.”

Spirit’s discovery of silica-rich soil in the Gusev crater was described by Albert Yen, a geochemist at JPL, as “some of the best evidence Spirit has found for water”. The reasoning was that acid vapours produced by volcanic activity in the presence of water, or water in a hot-spring environment, could have produced the silica.

However, what NASA scientists felt sufficiently confident to describe as “proof” of the existence of water came only with the Phoenix Mars Lander, which dug a trench in June 2008 to uncover “crumbs” of a bright material thought to be water ice. Had they been composed of carbon dioxide or salt, they would not have vaporized, but four days later the crumbs had disappeared, proving that they contained water. Phoenix principal investigator Peter Smith, of the University of Arizona’s Lunar and Planetary Laboratory, said at the time that “with great pride and a lot of joy” they had “found proof that this hard material really is water ice and not some other substance”.

ESA’s Marcello Coradini sums up the importance of planetary spacecraft. “Without them,” he says, “we would not have discovered ice beneath the surface of Mars, we would not be able to discriminate between carbon dioxide and water layering in the polar caps, and we would never have detected methane in the lower layers of the Martian atmosphere.” These and a wealth of other discoveries can only reinvigorate the search for life on Mars.

But planetary science is still in its infancy and, in common with other disciplines, most missions produce more questions than answers. John Callas repeats a familiar litany: “Did Mars support life? Is there life there today? Can we find evidence of extant or extinct life? And, if Mars was once Earth-like, why did it change?” Future missions will have to answer these questions.

New paradigm

Analysing the history of solar-system exploration in terms of the three phases – fly-by, orbit and landing – shows that all of the planets have been subject to fly-by missions (except “former planet” Pluto, which should receive a visit from NASA’s New Horizons probe in 2015). Venus, Mars, Jupiter and Saturn have been targeted by orbiters, while Mercury is due to join the list in 2011 when NASA’s Messenger spacecraft arrives. So far, only the surface of Venus and Mars (and of course Saturn’s moon Titan) have been explored using landers, a fact that indicates the relative difficulty and expense of this method of research.

That said, the challenge of placing a spacecraft from Earth on the surface of a celestial body – including moons, asteroids and comets – has far-reaching pay-offs in terms of understanding our solar neighbourhood and the Earth itself. As Coradini puts it, “On-surface science will allow us to reveal fine geochemical and mineralogical differences that are at the basis of our understanding of the formation and evolution of the solar system.”

The difference between the planetary astronomy of the past and today’s in situ missions is akin to the difference between aerial archaeology and actually “digging the dirt”. In effect, the development of space technology has allowed astronomy to develop from a science of entirely remote observation to one of immersive experimentation, from sampling Titan’s atmosphere to digging for ice on Mars. The next level of understanding will come from samples returned to the Earth, where sophisticated analyses can be performed in laboratory settings that are impossible to implement on a spacecraft. Returned samples also remain available for the future, when technological advances in scientific instrumentation may allow observations that are not possible at the time of the mission.

Crawford goes even further with his view that the whole paradigm of planetary science changed once space technology made it possible to conduct in situ investigations of other planets. “In a real sense,” he says, “space exploration has removed the planets from ‘astronomy’ by making it possible to apply the expertise and techniques of geology and geophysics (and perhaps even biology) to other planets which formerly could only be applied to the Earth.” This “explosion of new knowledge”, adds Crawford, is epitomized by comparing a modern planetary-science textbook with one written prior to the space age, when the other planets were “little more than indistinct dancing images in telescopes”.

Astronomy has come a long way in the 400 years since Galileo Galilei, and most of the progress has been made in the 50-odd years of the space age. But the indisputable advantages of proximity observation with spacecraft provide a challenge for scientific and budgetary prioritization alike. No nation can afford even a fraction of the missions being proposed by the space-science community, which leaves many disappointed. While the thrill of planetary-science missions will never subside, the discovery of more than 330 extrasolar planets (so far!) may, unintentionally, add to this budgetary dilemma. “Ultimately,” predicts Crawford, “we are going to need space probes to explore these extrasolar planetary systems as well, which will be a real challenge for spacecraft designers.” One wonders what Galileo would have made of that.

Space milestones

2 January 1959
The Soviet Union launches its first lunar spacecraft, Luna 1 (also called Lunik 1 or Mechta, Russian for “dream”). It passes within 6000 km of the Moon two days after launch and becomes the first artificial object to escape Earth’s gravitational field.

11 March 1960
Pioneer 5 becomes the first true “deep space” probe by returning data when 36.2 million kilometres from Earth. It confirms the existence of the interplanetary magnetic field and provides warning of solar storms that could cause electromagnetic interference on Earth.

2 June 1966
Surveyor 1 is the first spacecraft to execute a controlled landing on a celestial body, the Moon. It transmits more than 11,000 photographs of the lunar surface.

20 July 1969
Apollo 11’s lunar module Eagle is the first manned spacecraft to land on the Moon (touchdown: 8.17 p.m. GMT). The first lunar extra-vehicular activity is conducted by Neil Armstrong (“boot down”: 2.56 a.m. GMT on 21 July 1969).

17 November 1970
Luna 17 delivers the first tele-operated rover to the Moon, Lunokhod 1. It and Lunokhod 2, launched three years later, are the only planetary rovers until Sojourner lands on Mars in 1997.

15 December 1970
Venera 7 makes the first radio transmission from the surface of another planet, Venus.

3 December 1973
The first spacecraft to have traversed the asteroid belt, Pioneer 10 flies past Jupiter. It becomes the first spacecraft to escape the solar system (by its former definition) when it passes the orbit of Pluto on 13 June 1983.

20 July 1976
Viking 1 is the first spacecraft to soft-land on Mars and is the first to conduct in situ analyses of the Martian environment.

13 March 1986
The European Space Agency’s Giotto Comet Halley interceptor makes the first close encounter of a comet, approaching to within 600 km.

7 December 1995
A probe from the first Jupiter orbiter Galileo enters the Jovian atmosphere. The craft itself is de-orbited in 2003 as a protective measure.

4 July 1997
Mars Pathfinder delivers the first tele-operated rover, Sojourner, to Mars. It remains operational until late September 1997.

4 January 2005
Launched in October 1997, the Cassini Saturn orbiter drops its Huygens Titan entry probe, which lands on Saturn’s largest moon. Cassini continues its tour of the Saturn system.

19 January 2006
NASA’s New Horizons probe is the first spacecraft launched towards the Pluto/Charon system, which it is expected to reach in July 2015 before continuing on to the Kuiper belt.