I am older than the space age, although not by much. I was only a toddler when Sputnik 1 was launched in 1957, was too young to take any notice of the news about Yuri Gagarin's path-breaking orbital flight four years later. But I have a very clear memory of seeing, on the black-and-white television set the teacher brought into the classroom of Selsley Primary School for the occasion, images of the Moon's surface transmitted by the robot craft Surveyor 1 after its soft landing on the Ocean of Storms. That was on June 2, 1966. A little over three years later I was woken by my mother in the middle of a summer night to see Neil Armstrong make his historic first step. I remember the images captured by Pioneer 10 as it sped through Jupiter's system, Viking 1's first glimpse of Mars's rock-strewn surface, the softly tinted picture of a frozen beach sent from Titan by the little Huygens lander. And a few days ago I watched on the same screen that I'm now typing this, via NASA TV's internet channel, the presentation of the first high resolution images of Pluto and its largest moon, Charon, captured by the New Horizons spacecraft. Less than sixty years after Sputnik the first era of solar system exploration is over.
What a long, strange trip it's been. In my lifetime, the reach of the human race has extended beyond Earth's orbit to every one of the nine planets of the old, classical definition of the solar system. When I was born, those planets and their moons were still largely unknown: blank spaces that science fiction writers populated with monsters and princesses, fey aliens, shape-changing aliens, wise old aliens, ravening hordes of insensately hostile aliens, doughty space miners searching for fortunes, and settlements of emigrant families as American as apple pie. As late as the 1950s astronomers believed that the dark patches at the Martian equator that expanded in the summer and shrank during winter were some kind of seasonal plant growth. But imagination is only the first draft of reality. Planet by planet, moon by moon, our robot explorers have rewritten our ideas about the worlds of the solar system. Some lovely dreams have been shattered. The American Mariner 2 spacecraft and the Russian Venera landers showed that beneath the planet-girdling clouds of Venus were not dripping wet jungles and vast seas of soda water, but lava deserts baking beneath a crushing acidic atmosphere hot enough to melt lead. Instead of Martian canals, crystal cities, and lichen forests, Mariner 4 found a desolation of dust and impact craters. But there have also been revelations and unexpected wonders: the biggest volcano in the solar system on Mars; active sulphur volcanoes on Jupiter's moon Io, and an ocean beneath the icy surface of Europa; jets of water ice blasting from the south pole of Saturn's little moon Enceladus; Titan's methane and ethane lakes and dendritic river systems and huge dunes built from grains of frozen petroleum; Uranus's jigsaw moon, Miranda; nitrogen geysers on Neptune's moon, Triton. And now we have the first detailed images of the surface of Pluto and its moon Charon, and they are as strange and unexpected as the landscapes of the comet 67P/Churyumov-Gerasimenko and the planetoid Ceres, also first mapped this year.
Pluto was discovered by Clyde Tombaugh in 1930. Working in the Lowell Observatory near Flagstaff, Arizona, the young astronomer was actually looking for something else: Planet X, which Percival Lowell believed to account for discrepancies in the predicted orbits of Uranus and Neptune. Pluto was no more than a moving dot of light when it was discovered. Its eccentric orbit is an elongated ellipse that's inclined with respect to the plane in which Earth and the other seven planets orbit. Aphelion, the furthest point in its orbit from the Sun, is more than seven billion kilometers, or almost 49 astronomical units (where 1 AU is the distance of the Earth from the Sun), while at its closest approach to the Sun Pluto crosses the orbit of Neptune. Until this year, this month, the week I'm writing this, it was still largely terra incognita. The furthest outpost of the old classic solar system.
Recently demoted to a dwarf planet, Pluto is about two-thirds the diameter of Earth's moon. And because its largest moon, Charon, is about half its size they are really a binary dwarf planet, orbiting a common centre of mass (and like Uranus, their axes of rotation are tilted so that they appear to be rotating on their sides). The Hubble space telescope has provided no more than tantalising glimpses of shades of dark and light on its surface; there are indications of a tenuous atmosphere; in addition to Charon, it possesses four tiny moons, all discovered within the past ten years. And until New Horizons arrived, that was about all we knew about what was once the ninth planet. It was still possible to imagine that it might be a hideout for space pirates (Stanley G. Weinbaum's "The Red Peri"), an outpost for aliens (H. P. Lovecraft's "The Whisperer in Darkness"; E. E. "Doc" Smith's First Lensman; Robert Heinlein's Have Spacesuit—Will Travel), or populated by strange lifeforms (superfluid ameobae in Larry Niven's "Wait It Out"; snowflake creatures that spin cobwebs from Charon to Pluto in Stephen Baxter's "Goose Summer"). Even as late as 2006, it wasn't incredible to speculate that aliens might use Pluto as a laboratory for experiments in cryogenic lifeforms (Gregory Benford's The Sunborn).
But Pluto is a blank space no longer. Although latter-day conspiracy theorists will no doubt search New Horizons's images for hints of alien construction sites and monuments, the last outpost of the classic solar system has yielded to the mapmakers.
When we first encounter the unfamiliar, we look for recognisable points of reference. In images of the surface of the comet 67P/CG taken by the Rosetta comet chaser, we see cliffs and boulders, fans of scree, and even what look like sand dunes. Earlier this year, after orbiting and mapping the large asteroid Vesta, the Dawn spacecraft reached the dwarf planet Ceres. The largest body in the asteroid belt, massive enough to have been pulled into a sphere by its own gravity, with a surface battered by impact craters big and small, craters overlapping craters, craters within craters, Ceres looks somewhat like one of the small frozen moons of Saturn. Dione, say. Or Rhea. But appearances are deceptive, comparisons are useful only up to a point, and every world and moon we've encountered harbours something unexpected. The dark surface of Ceres is punctuated by strange bright spots that may be freshly exposed patches of water ice, although how they were exposed—impacts, landslides, vents, geysers—is as yet unknown. There's also a pyramidal mountain five kilometres high sitting all alone on a cratered plain—some kind of volcano, perhaps, or the central peak of an impact crater whose rim has been buried or otherwise erased, or the result of tectonic movement. As for comet 67P/CG, its gravity is so vanishingly weak that if you stepped off a hundred metre high ridge it would take you twenty minutes to fall to the bottom; if you jumped, you'd either go into orbit or escape the comet entirely. So while from some angles its landscapes look familiar, it is really an Escher world, with boulders perched on the faces of cliffs at right angles to each other and gravity varying over the two conjoined lobes of its body. And because it is a comet, an icy body warming and growing active as it approaches the sun, its surface is dotted with huge sink holes that blast foggy jets of sublimed water vapour and dust into space.
As on Ceres and 67P/CG, so on Pluto and Charon. As New Horizons sped towards it, the close-coupled binary of Pluto and Charon at first resembled Earth and the Moon. Then, when the first colour images revealed Pluto's ochre tint, it looked as if Mars had somehow captured the Moon. But while Mars's colouration is due to iron oxides, Pluto's is caused by complex hydrocarbons, tholins, formed by the interaction of the sun's ultraviolet light with methane. And the first images sent by New Horizons after its flyby showed that the dwarf planet and its moon are unique, complex worlds that may still be geologically active, with features not found elsewhere. "Baffling in a very interesting and wonderful way," as one of the New Horizons team put it.
A high resolution image of a small section of Pluto's huge bright heart-shaped basin, provisionally named Tombaugh Regio, revealed a range of sharp-peaked mountains of as tall as the Rockies and almost certainly built from water ice, which is as hard as rock in Pluto's frigid temperatures. There's also pitted terrain, strange pillowy terrain cut by what may be a fault line, and a smooth plain of bright ice, Sputnik Planum, that's fractured into large polygons, with dark material accumulated in some joints and rounded hills bordering others, and streaks of what might be wind-blown material or deposits from as yet undiscovered geysers. And there are no impact craters anywhere, suggesting that this is a relatively young landscape, formed or resurfaced by some recent geologic process (a second high-resolution image does show some scattered craters on darker terrain adjacent to Sputnik Planum). Charon's surface also seems relatively youthful. There are some impact craters, but there are also smooth plains, and canyons deeper than Earth's Grand Canyon, and what looks like a mountain circled by a moat, as if some gigantic spaceship were slowly sinking into the icy surface.
Perhaps the interiors of these tiny worlds are warmed and kept active by radioactivity, or perhaps the residual heat of their formation was stored in subsurface seas of water and ammonia which release energy as they slowly freeze out and also, because ice is less dense than water, cause the surface to expand and crack. There is much we don't yet know, and much data yet to be downloaded from New Horizons as it speeds away from Pluto after its brief encounter. The distance between Pluto and the Earth is presently about 32 AU: radio signals take four and a half hours to travel between the spacecraft and Earth, and the rate of data transmission is far less than that of an old dial-up modem. It will take more than a year to download everything New Horizons has gathered, and even more time to understand it. But these first glimpses are enough to tell us that Pluto and Charon are dynamic, diverse worlds. They are no longer distant unknowable points of light, no longer blank spaces that can be populated by unfettered imaginations. They are places.
It's still possible, of course, to write stories set in the old unexplored solar system. George R. R. Martin and Gardner Dozois have edited two anthologies, Old Mars and Old Venus, of original stories that do just that. But such tales are deliberate exercises in nostalgia (which, unfortunately, is threatening to become the default mode of science fiction). What about stories that use the known of the here and now as their departure point into the future? A pleasing variety of authors (M. J. Locke [Up Against It], Kim Stanley Robinson  and Alastair Reynolds [On the Steel Breeze], to name but three) have responded to the mapping of the solar system's planets and moons by using the various and varied landscapes for what they are, not for what they might have been. Populating them not with monsters or even princesses, but with people. Speculating about who might live there and why, and how living there might change them. Others have shown a renewed interest in interstellar voyages of discovery. For the past twenty years astronomers have been cataloguing exoplanets around other stars. To date, more than two thousand have been discovered. We have glimpsed one or two by their reflected light, have even seen hints of weather systems, but know most only by their effect on the rotation of their stars, can make only educated guesses as to their surface temperatures and the composition of their atmospheres, know nothing at all about their landscapes. All are blank maps, worlds unknown that can still be shaped by fine frenzies of imagination, and given local habitation and a name.
But we don't have to voyage to the stars to find new worlds. Pluto is at the inner edge of the Kuiper belt, where millions of icy bodies trace regular orbits around the Sun. There are also scattered disc objects with eccentric orbits that cross the Kuiper belt and range out as far as a hundred astronomical units from the Sun—the largest, Eris, is a planetoid only a little smaller than Pluto. Only a few Kuiper belt objects (KBOs) are similar in size to Pluto or Charon, but power-law distributions suggest that there are several thousand with an average diameter of around a hundred kilometres, and millions more much smaller. The differences between Pluto and Charon, and Triton and Phoebe, moons of Neptune and Saturn widely believed to be captured wanderers from the Kuiper belt, suggests that KBOs are wildly various. Not only that, but the total surface area of KBOs exceeds that of the moons and terrestrial planets in the inner solar system (the gas and ice giants, strictly speaking, have no surfaces). A million worldlets to be explored. Some, like Eris, Haumea and Orcas, with moons of their own. And because they are made of water ice and frozen organics, KBOs could provide the construction material, once thawed, for a million garden habitats, a million statelets of every political hue.
There's also much to explore in the solar system within Pluto's orbit. Most of the asteroids in the asteroid belt, the trojan asteroids that share an orbit with Jupiter, the Centaurs which orbit between Neptune and Jupiter, the small outer moons of Saturn and the parts of Uranus's moons that were facing away from Voyager 2 when the spacecraft passed the planet. . . . Even the ocean floors of Earth are still largely unexplored. In the same week that New Horizons flashed past Pluto, scientists tracking lobster larvae discovered five large unknown undersea volcanoes off the coast of Australia. New Horizons's flyby of Pluto and its moons is not the end of the exploration of the solar system. But it is, perhaps, the end of the beginning.