Pluto, July 2015; photographed by NASA’s New Horizons spacecraft, with enhanced color

Nearly every civilization has recorded celestial events, partly in the hope that they might explain terrestrial ones, but also to place themselves in the grand cosmic scheme. The Nebra Sky Disk, a twelve-inch bronze disk with gold inlay, dated to 1600 BCE and discovered in the Saxony-Anhalt region of Germany, depicts the sun, moon, and what appears to be the star cluster Pleiades. By the second millennium BCE, the Babylonians knew of the inner planets—Mercury and Venus—as well as the outer planets—Mars, Jupiter, and Saturn.

An understanding of the organization of our solar system, however, is relatively recent. It was only in 1543 that Nicolaus Copernicus radically reordered the cosmos with De revolutionibus orbium coelestium, upsetting the geocentric model that had been in place since antiquity. For Copernicus, the universe was finite and bounded, with the sun surrounded by the planets of our solar system and fixed stars. Today we know that our own galaxy is one of perhaps trillions in the universe, and that the universe is not only expanding but that its expansion is accelerating. Moreover, we have many independent lines of evidence suggesting that dark matter and dark energy—two mysterious entities whose nature remains elusive—shape the universe as we know it, and that the matter and energy we can observe make up just over 4 percent of all the contents of the cosmos. Such disorienting revelations have been typical of modern astronomy and cosmology over the last century, as technological advances have increased our ability to gather data, probe greater distances, and reconstruct events that took place billions of years ago. These major refinements in our knowledge have required the continuous redrawing of our cosmic map and extended our vision well beyond our solar system.

Two recent books, Discovering Pluto: Exploration at the Edge of the Solar System by Dale P. Cruikshank and William Sheehan and Chasing New Horizons: Inside the Epic First Mission to Pluto by Alan Stern and David Grinspoon, offer ringside views of the exploration of the outer solar system, from the discovery more than two hundred years ago of planets beyond Saturn to the launch in 2006 of NASA’s New Horizons space mission to study Pluto’s environs. Discovering Pluto begins with the discoveries of Uranus and Neptune, the seventh and eighth planets from the sun. The former was first identified by the English astronomer William Herschel in 1781. Later observation revealed that Uranus, whose orbit takes eighty-four years to complete, did not follow the course astronomers had expected it would take.

First, there was an astrometric anomaly: the measured position in the orbit deviated from the predicted orbit of Uranus as determined by Newton’s law of gravity. By the 1830s, it was established that the orbit of Uranus also seemed to diverge from Johannes Kepler’s second law, which states that planets sweep out equal areas in equal times during their orbits. Uranus was seen to move erratically, as though something else was tugging at it, altering its motion. It occasionally moved faster than predicted and swept out a larger area than it should have, and then moved slower than predicted, sweeping out a smaller area. In the 1840s the astronomer-mathematicians Urbain Le Verrier in France and John Couch Adams in England both began searching for explanations to these anomalies—which turned out to be the existence of Neptune.

Cruikshank, a co-investigator on the New Horizons mission, and Sheehan, a historian of the solar system, describe marvelously the dispute between Adams and Le Verrier over credit for the discovery of Neptune. Though Neptune wasn’t properly identified until 1846, it had been observed much earlier. Michel Lalande, the nephew and pupil of the French astronomer Joseph-Jérôme Lalande, appears to have mistaken Neptune for a star in notes taken on May 8 and 10, 1795, and the Scottish astronomer John Lambert, when he was working at the Munich Observatory in 1845 and 1846, recorded sightings of it but didn’t realize what it was. The earliest known sighting, Cruikshank and Sheehan note, comes from the observation logs of Galileo Galilei. In December 1612 and January 1613, while tracking the motions of Jupiter’s newly discovered satellites, Galileo recorded the presence of a speck nestled close to the planet. He failed to follow up on his finding, but a recent study of his notebooks by the physicist David Jamieson suggests that Galileo noticed that the object did not move rapidly across the sky as a star would, and even seems to have tracked its small drift in position over the course of those months.

Nearly 250 years later, Le Verrier, working at the Paris Observatory, found that the discrepancy between the predicted and observed behavior of Uranus could be attributed to the gravitational attraction of another planet beyond its orbit. He presented the calculated position of this unseen planet to the French Academy of Sciences in Paris on August 31, 1846, barely two days before Adams mailed his own solution to the astronomer royal, George Airy, at the Greenwich Observatory so that his calculations could be checked. Neither Adams nor Le Verrier knew that the other had been researching Uranus’s orbit.


The previous year, Adams had given Airy an earlier calculation of the possible position for the unseen planet between the constellations of Capricorn and Aquarius, but Airy had not paid attention to it until he saw a paper by Le Verrier in the Comptes Rendus of the Paris Academy of Sciences, published in June 1846, that delineated a similar location. Adams corresponded further with Le Verrier (without letting him know of the English campaign), and the search began in earnest under the command of James Challis, director of the Cambridge Observatory, which housed the powerful Northumberland telescope.

As an ambitious and enthusiastic scientist, Le Verrier, who was hungry for the discovery and utterly unaware of the developments in England, transmitted the putative position that he had calculated for the location of the new planet to an old friend of his, Johann Galle at the Berlin Observatory. On September 23, 1846, Neptune was sighted between Aquarius and Capricorn, just as Le Verrier had predicted. After the discovery was reported, Challis realized that he had actually detected Neptune earlier, on August 4 and 12, 1846, but had not recognized it as a planet since he did not have an accurate star chart to cross-check his observations against.

The episode is but one of many that proves science is not a dispassionate, neutral, and objective endeavor but rather one in which the violent clash of ideas and personal ambitions often combines with serendipity to propel new discoveries. The British and French fight over recognition for the discovery of Neptune, which Cruikshank and Sheehan recount in detail, was so fierce that it led to searches in historical astronomical data for evidence of earlier sightings by members of both camps. Although Adams acknowledged that his own failure to persuade British astronomers to promptly follow up on his predictions allowed Le Verrier’s to be confirmed before his own, the two shared credit for the discovery until fairly recently. Fifteen years ago, evidence unearthed by Sheehan and the historians of science Nicholas Kollerstrom and Craig Waff revealed that while Adams did perform some interesting calculations, his were not as precise or as accurate as Le Verrier’s, and, moreover, he had not published his work, while Le Verrier had shared his predictions.1

In 1905 the Harvard astronomer Percival Lowell inferred the presence of a “Planet X” from irregularities in Neptune’s orbit. His findings set off an obsessive pursuit on both sides of the Atlantic for cosmic bodies beyond Neptune—there were two competing groups just at the Harvard College Observatory, one led by Lowell and the other by William Pickering. Yet it was a young amateur astronomer who made the discovery, at the Lowell Observatory in Flagstaff, Arizona. Clyde Tombaugh, a Kansas farm boy who ground his own lenses and built telescopes, had been hired by the observatory’s director, Vesto Slipher, as an assistant to look for Planet X. It was arduous work that involved taking photographs night after night in the unheated dome of the observatory, at an elevation of seven thousand feet, carefully guiding the telescope to the region of the constellation Gemini, one spot where Lowell had calculated that Planet X likely lurked. Even more tedious was the use of a blink comparator to flip back and forth between images taken on subsequent nights, in order to identify objects that had changed position among the stars and could thus be plausible candidates for the planet.

Tombaugh, a self-described perfectionist, was consumed by this task. On February 18, 1930, as he did every day, he viewed hundreds of thousands of stars in a set of photographic plates that he had taken. After lunch, as Tombaugh used the blink comparator, he found an object that had shifted: “a swollen image of Planet X exactly where it should be.” Tombaugh didn’t immediately tell his supervisors but savored the discovery for himself: “for three quarters of an hour, I was the only person in the world who knew exactly the position of Planet X.”

The discovery was reported with great fanfare around the world. Thousands of suggestions for the name poured in by mail to the observatory. “Pluto” was suggested by an eleven-year-old English schoolgirl named Venetia Burney, daughter of the Reverend Charles Fox Burney, Oriel Professor of the Interpretation of Holy Scripture at Oxford. She picked the name for the Roman god who ruled the underworld and possessed the power to render himself invisible. A crater on Pluto is named in her honor.


Pluto is about one four-hundredth the mass of Earth and is composed primarily of methane frost and ice. It takes 248 Earth-years to complete its extremely elliptical orbit, which on average is about forty times farther from the sun than Earth’s, traveling more slowly than any planet in the solar system due to its distance from the Sun. The time between sunrise and sunset is approximately 6.4 Earth-days. Pluto’s moon, Charon, is roughly 10 percent of its mass—our moon is only about one eightieth the mass of Earth. Because of their relative closeness in mass, Pluto and Charon are a binary system, exerting enough gravitational pull on each other that they orbit a point between them.

Although Pluto, with a diameter of about 1,500 miles, is only about half the size of Mercury, it was believed for decades to be larger than anything else in the Kuiper Belt, which is the most densely cluttered zone of the solar system and extends from the orbit of Neptune to fifty-five astronomical units from the sun.2 But more recent observations through larger telescopes have shown that the Kuiper Belt contains over 80,000 objects of similar size to Pluto. This vast new inventory, and the discovery in 2005 by the astronomer Michael Brown and his team at Caltech of a particularly large object, Eris, prompted a closer look at the definition of what constitutes a planet in our solar system. The International Astronomical Union (IAU) decided that the following three criteria must be satisfied: (1) the object must orbit our sun (Pluto passes this); (2) it must be massive and spherical (Pluto passes this one too); and (3) its gravitational field must be strong enough to have cleared the astro-debris in its orbital neighborhood. This decluttering criterion proved to be the fateful one—Pluto was simply not massive enough to destroy the astronomical detritus around it. After a vote by the IAU during its twenty-sixth General Assembly meeting in 2006, Pluto was demoted in status to a “dwarf planet,” a new distinction for objects that meet only the first two criteria, upsetting the view of the solar system that many of us had grown up with and cherished.

NASA sent the first unmanned probes beyond Earth’s orbit in the 1960s, starting with the Mariner probes that journeyed toward Venus, Mars, and Mercury between 1962 and 1973. In 1972 the Pioneer spacecraft was launched, eventually traveling toward Jupiter and Saturn. The Voyager missions, launched in 1977, were originally intended to visit all five known outer planets. They did fly past Jupiter, Saturn, Uranus, and Neptune, but a closer look at Pluto remained elusive.

Engraving of John Couch Adams using a telescope to read Urbain Le Verrier’s announcement of his discovery of Neptune

Pictoral Press/Alamy

John Couch Adams, right, using a telescope to read Urbain Le Verrier’s announcement of his discovery of Neptune, while Le Verrier observes the planet; engraving from the French magazine L’Illustration, November 1846

On Bastille Day, 2015, 3.2 billion miles from Earth, NASA’s New Horizons spacecraft flew past Pluto at an astonishing speed of 32,000 miles per hour, taking the first fleeting but sharp images of the desolate, icy world. Chasing New Horizons reveals the inside story of the mission’s failures and successes. This gripping history comes from the project’s principal investigator, Alan Stern, as well as from other scientists who worked on it, as recounted to the astrobiologist and science writer David Grinspoon.

New Horizons is an interplanetary probe, part of NASA’s New Frontiers program to venture out into the furthest reaches of our solar system. Roughly the size and shape of a grand piano and weighing about a thousand pounds, the spacecraft contains sixteen thrusters, used to correct its trajectory, and carries seven scientific instruments, including imagers, particle spectrometers, a dust sensor, and a radio receiver.

Endeavors to launch probes into space must be fail-proof, as there is only one shot to get it right. The New Horizons mission, whose goal was to fly past and study Pluto and the trans-Neptunian Kuiper Belt object 2014 MU69 (also known as Ultima Thule), was more than two decades in the making. Stern led the mission, working with a talented team of thousands of scientists and engineers, including Cruikshank, specializing in everything from microelectronics to nuclear fission physics. (Like other deep-space interplanetary missions, New Horizons is powered by nuclear energy, specifically from the radioactive decay of plutonium—which, incidentally, was named after Pluto.) According to NASA, the entire project cost approximately $700 million, in line with the median cost for space missions.

Launched in January 2006 by an Atlas V rocket from Cape Canaveral directly into an orbit that would escape the gravitational grip of Earth and the sun, New Horizons is the fastest man-made object ever to leave Earth, at a speed of 36,400 miles per hour. The spacecraft was designed to exploit the gravitational force of Jupiter, in order to gain speed to continue its journey. An enormous amount of new data on that planet’s atmosphere and moons was gathered and beamed back to Earth. After this flyby, the spacecraft trundled along on its voyage in hibernation mode, to conserve power and preserve the fidelity of its instruments, checking in only once a year with mission control until December 2014, about seven months prior to the Pluto flyby, when it was roused from its slumber.

Grinspoon recounts the entire journey, from the launch to a brief but alarming silence a mere ten days before the flyby, when the team lost contact with the probe. NASA’s Deep Space Network, a group of three giant radio dish complexes in Madrid, Canberra, and Goldstone, California, keeps constant contact with long-distance spacecraft. No matter where an object is located in deep space, at least one of these radio stations is pointing in its direction. Yet for a period of ninety minutes, Australia, which at the time lay along the path of contact for New Horizons, heard nothing. It took another couple of days to fully reestablish smooth communication and reset the probe’s systems; in the interim, New Horizons had traveled a million miles on its journey to Pluto, operating in its “safe-mode.”

The first closeup image of Pluto revealed giant plains and a huge heart-shaped region lightly colored by a concentration of carbon monoxide ice, bordered by clusters of hills. The surface appears surprisingly smooth, and the lack of craters from meteor impacts suggests that Pluto is geologically very young. It has a layer of gases—nitrogen, methane, and carbon dioxide—vaporized from its surface ices. This volatile atmosphere appears to be rapidly escaping the planet’s gravitational field. The surface is wrinkled by mountain ranges that rival the Rockies in height but are hundreds of millions of years younger, composed of nitrogen and methane ice, softer substances than the silica compounds of the Earth’s crust. The size of the mountains suggests they are likely undergirded by harder sub-surface water ice.

In January 2019 New Horizons had a close encounter with the asteroid 2014 MU69 in the Kuiper Belt. A “compact binary” of two nearly spherical bodies fused together and measuring twenty-two miles end to end, it is the most distant object ever photographed up close by an earthly spacecraft. It is thought to be nearly as old as our solar system—the gentleness of the impact required to fuse its two lobes suggests a low-speed collision, and such low speeds are characteristic of the period when the planets were still being formed. Such objects in the Kuiper Belt can provide clues about the early conditions of the young solar system. New Horizons has enough fuel to last into the 2030s, making a flyby past another, even further object possible.

In 2016 Michael Brown and his collaborators, the self-described “Pluto killers,” claimed they had strong evidence for a new, more distant planet, now dubbed Planet 9, which is purported to be ten times more massive than Earth. The hunt continues to capture an image of it as it wanders in the outskirts of the solar system, still held in the grip of the sun’s gravity. We may end up with a solar system of nine planets after all.

The discovery by terrestrial observatories and by NASA’s Kepler satellite mission of an abundance of exoplanets—planets that orbit other stars—around nearby stars, some of which may provide the conditions necessary for harboring life, has brought to the fore questions about the uniqueness of Earth. Are there other forms of life out there? If they exist, might they be found in the far reaches of the solar system, or further afield in our galaxy and beyond? Would we even be able to recognize intelligent life if we came across it? Would life forms that evolved on Earth be able to survive elsewhere?

Such questions have compelled us to look more closely at our more immediate surroundings in order to understand the physics of planet formation and the biochemistry of life. For this project, the destination of choice is Mars. At a distance of about 34 million miles at its nearest, it’s tantalizingly close (astronomically speaking): with current technology it would take a spacecraft about three hundred days to get there. The length of a Martian day is roughly the same as a day on Earth, twenty-four hours and thirty-seven minutes, and due to a similar tilt in the planets’ axes, the length of seasons on Mars is also similar. Though Mars is much smaller than Earth, both planets have about the same amount of land, since more than two thirds of Earth’s surface is covered in water.

The proximity of Mars has meant that it has long captured our imagination as a possible cradle for life. In the 1870s the Italian astronomer Giovanni Schiaparelli speculated that the dark regions seen on the Martian surface via telescopic observations appeared to be natural channels through which water could circulate around the planet. But Schiaparelli’s canali was translated as “canals,” artificial irrigation that would suggest the existence of an advanced civilization. Convinced by what he thought was Schiaparelli’s interpretation, Percival Lowell was obsessed with the possibility of intelligent life on Mars.

Though there is no evidence for an elaborate channel system on Mars, evidence of ancient water on the Martian surface continues to emerge. In 2012 NASA’s Curiosity rover discovered clay minerals that had been formed in water that might have been able to support life: slightly salty and neither too acidic nor too alkaline. Last November NASA successfully landed InSight, another probe, on Mars to investigate the planet’s geology by burrowing ten to sixteen feet below the surface. InSight has also captured the first-ever sound recording from another planet—low, rhythmic flapping noises that could be the wind rippling past sand dunes, or even the tremors of a marsquake.3

Back on Earth we are now being offered fantasies of a Mars colony by the billionaire set. In 2016 Elon Musk announced that the colonization of Mars was one of the long-term goals of his company SpaceX. Jeff Bezos, currently the richest man on our planet, recently unveiled a design for pods that he claims could enable us to commute to a permanent base on the moon. President Trump has directed NASA to set up a lunar gateway—a space station in orbit around the moon that would serve as the communications center, scientific lab, short-term habitation module, and a general holding area for rovers and robots en route to landing on the moon’s surface—and a moon base by 2024 as part of the ARTEMIS project, with the eventual goal of sending humans to Mars.

In the recent book Should We Colonize Other Planets? the philosopher Adam Morton concludes that we are maladapted for life anywhere but Earth due to the particular conditions that have driven our evolution on this planet, namely the availability of abundant food, the existence of a stable atmosphere, the salubrious temperature range on our planet, and, crucially, the protection that Earth’s magnetic field and atmosphere provides from harmful ionizing radiation. The results from NASA’s recently released study of the twin astronauts Scott and Mark Kelly alert us to the dangers posed by exposure to radiation. In 2015–2016, Scott spent a year aboard the International Space Station, while Mark remained on Earth. Scott experienced many physiological and epigenetic changes, some of which have since reverted to normal, and some of which appear to be permanent and harmful to his overall health and cognitive function. This includes damage to his DNA and the shortening of his telomeres, which protect chromosomes from decay and affect an organism’s life-span. Before we can travel to Mars, we must figure out how to protect humans from this danger: radiation exposure on the planet’s surface exceeds what Scott Kelly experienced in low-Earth orbit by a factor of about three. If we can’t develop an adequate defense against ionizing radiation, even an Earth devastated by climate change or nuclear fallout would probably still be more hospitable to life than Mars.

Morton believes the end of the human race on Earth is inevitable, since Earth will perish one day, owing either to destruction wrought by humans or external catastrophic events like large asteroid strikes. Reflecting on the nature of human existence, he argues that, while we need to adopt a compassionate stance to deal with the end of the human race here, we should be more actively engaged with securing the future of intelligent life elsewhere in the solar system. He contends that we should focus on shaping a future in which an intelligent species that we evolve into or develop artificially may one day be able to populate the cosmos, and not necessarily on preserving humans as we know them today. Even on our planet, he declares, “the place of humans will be taken sooner or later by other creatures.” Future space explorers may be of a new humanoid species, or humans augmented by artificial intelligence or body parts. In view of this theory, Morton proposes that our best bet for colonizing Mars would be to engineer new life forms specifically designed for the conditions there, perhaps beginning with single-celled radiation-resistant organisms.

As we wait for NASA’s InSight Mission to sift through Martian sands, drilling under the red planet’s terrain for signs of life, there is a certain irony that life on our own planet is increasingly vulnerable to anthropogenic changes. No matter how advanced our technical capacities for space travel, our first call to action must be to preserve Earth. Resources need to be urgently redirected to address the rapidly accelerating pace of climate change and its catastrophic effects, and perhaps it is prudent to invest in the more economical unmanned probes like New Horizons, rather than colonization projects, to satisfy our innate curiosity about the cosmos. Young people like the teenage Swedish climate advocate Greta Thunberg and others of her generation have realized the simple fact that it’s time for the rest of us to recognize the urgency of the problem. As the polar explorer and environmental activist Robert Swan bluntly put it, “The greatest threat to our planet is the belief that someone else will save it.”