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The Copernican principle states the working assumption that Earth is not special—it is no more than an average planet, orbiting a medium-sized star, in an unremarkable part of an ordinary galaxy. If Earth is not unique, then there is little reason to think that other planets cannot also harbor life. Given the number of stars in the universe—in the order of 1023—this might be a statistical certainty. Over the centuries, many thinkers, such as American Carl Sagan, have pondered the possibility.

Is Earth alone?

In the 16th century, the Italian monk Giordano Bruno proposed that the stars were other suns, each of which could have its own solar system. Life could even populate these other earths. Believing that the universe was infinite, Bruno also insisted that it could have no center. Bruno was tried by the Roman Inquisition for these and other heretical beliefs and burned at the stake in 1600.

Throughout history, various astronomers have claimed to see evidence for life on other planets of the solar system. In the 1890s, American astronomer Percival Lowell claimed to have mapped artificial “canals” on Mars, while the dense clouds of Venus were imagined, by Swedish chemist Svante Arrhenius in 1918, to hide from view a lush surface blooming with life. It is now known that the clouds are acidic, while the surface of Venus is an inhospitable 864°F (462°C). However, these are just two planets out of potentially billions

The universe’s immensity and the apparent universality of its physical laws make it seem likely that microbial life exists elsewhere. Indeed, life may have arisen elsewhere and been transported to Earth. Greek philosopher Anaxagoras first suggested this idea of “panspermia” in the 5th century BCE. Naturalist Charles Darwin briefly turned to this idea while working on his theory of evolution by natural selection, troubled that the accepted figure for Earth’s age did not give enough time for complex organisms to evolve. Earth is now known to be vastly older than was believed in Darwin’s time, so panspermia is not needed to explain the origin of life on the planet.

Recent discoveries show that comets can carry many of the basic chemical components of life, but the exact mechanism by which life on Earth began remains a mystery. Solving that mystery should give a far better idea of how likely life is elsewhere.

“Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.” Albertus Magnus

Computer simulations have shown that it is theoretically possible for simple single-cell life forms to exist inside comets or asteroids and to survive an impact like this with Earth.

Where is everyone?

One day, over lunch in Los Alamos in 1950, Italian scientist Enrico Fermi asked a simple question: “Where are they?” He reasoned that, even if only a small proportion of planets play host to intelligent life, given the unimaginable numbers of stars within the galaxy, one might expect a large number of civilizations to exist on other planets. At least some of them may have chosen to send messages or tried to visit Earth themselves. Earth has been producing electromagnetic signals for 90 years or so, since the dawn of radio and television broadcasting. These modulated radio waves—expanding and extending some 90 light-years in all directions—should be a giveaway of a technologically advanced society to any potential spacefaring intelligence.

In 1959, Giuseppe Cocconi and Philip Morrison suggested a bandwidth to search for alien radio messages. A year later, Frank Drake, at the National Radio Astronomy Observatory in Green Bank, West Virginia, set out to look for them. Drake founded Project Ozma, named for the queen of author L. Frank Baum’s imaginary Land of Oz—a place “difficult to reach and populated by exotic beings.” After a briefly exciting and noisy encounter with some top-secret military radio-jamming equipment, Drake and his team were met with silence. More than 50 years later, the silence has not yet been broken.

Order of the Dolphin

Drake drew together a diverse group of scientists to lay the foundations and protocols for the search for extraterrestrial intelligence (SETI). The group jokingly called itself the Order of the Dolphin, in reference to the work of neuroscientist John Lilly, who pioneered the science of speaking to dolphins. As one of the few people dealing with interspecies communication, Lilly was an important part of the group, which also included a young astronomer Carl Sagan, who was an expert on planetary atmospheres.

In preparation for the Order’s first meeting in 1961, Drake came up with a formula for the number of alien civilizations in the galaxy: N = R × fp × ne × fl × fi × fc × L The total (N) was reached by multiplying the factors necessary for intelligent extraterrestrials to evolve and be discovered. It depends on the rate at which stars suitable for intelligent life form (R*); the fraction of these stars that are orbited by planets (fp); the number of planets in any given planetary system that can support life (ne); the fraction of these planets upon which life actually appears (fl); the proportion
of life-bearing planets that go on to produce intelligent life (fi); the proportion of civilizations that develop technology that betrays detectable signs of their existence (fc); and, finally, the length of time such civilizations survive (L).

With these terms in place, bounding limits could theoretically be placed on each one. In 1961, however, not a single one was known with any confidence. Delegates at the meeting concluded that N was approximately equal to L, and a potential 1,000 to 100 million civilizations might exist in the galaxy. Although values for some of the variables in the Drake equation have been narrowed down over the intervening years, modern estimates of N still vary wildly. Some scientists argue that the figure may be zero.

The Pioneer plaque contains information about the location of Earth within the Milky Way.

“The search for extraterrestrial life is one of those few circumstances where both a success and a failure would be a success by all standards.” Carl Sagan

Message in a bottle

In 1966, Sagan cowrote Intelligent Life in the Universe, perhaps the first comprehensive discussion of planetary science and exobiology. The book was an expanded and revised version of an earlier edition, published in 1962 by the Soviet astronomer and astrophysicist Iosif Shklovsky. Although highly speculative, the book ignited discussion among scientists. It inspired NASA’s Project Cyclops report, an influential document now referred to as the “SETI Bible.”

In 1971, Sagan approached NASA with the idea of sending a message on its Pioneer spacecraft. Sagan and Drake worked on a design that would advertise Earth’s existence to alien civilizations and help them locate Earth in the cosmos. The graphics on the Pioneer plaque establish a unit of measurement using the 21-cm hydrogen emission line. Units defined by reference to Earthbased phenomena, such as meters and seconds, would be meaningless to extraterrestrial scientists. By choosing units from properties of nature, the hope was that they would be understood universally.

All the images on the plaque were scaled in terms of these units. A map of bright and distinctive pulsars points the direction to Earth, and Pioneer’s route is traced on a simple pictogram of the solar system. Images of a man and a woman were drawn by Sagan’s artist wife, Linda Salzmann Sagan.

Pioneer 10 and Pioneer 11, launched in 1972 and 1973, were fitted with Sagan’s plaque, etched on a 6- by 9-in (152 x 199 mm) gold-anodized aluminum plate. Critics warned that it would attract the unwanted attentions of power-hungry (or just hungry) aliens. Feminist groups were unhappy that the man waved in greeting, while the woman’s pose angled her body (they thought) submissively toward the male figure. Salzmann responded that women are smaller, on average; that having both figures waving might be interpreted as it being the natural arm position; and that she merely wished to show how the hips moved. Sagan had initially wanted the man and woman to be holding hands, but decided it might make the Earthlings look like a single creature with two heads.

“We began as wanderers, and we are wanderers still. We have lingered long enough on the shores of the cosmic ocean. We are ready at last to set sail for the stars.” Carl Sagan

The Arecibo message

While the search continued for beacons set up by intelligent beings and likely star systems, Drake and Sagan decided to send planet Earth’s own “we are here” signal. The 3-minute burst of 1,000 kW radio waves was designed to cross distances separating stars. Beamed out from the Arecibo radio dish in Puerto Rico in November 1974, the interstellar message was aimed at the globular cluster M13, a group of about 300,000 stars 25,000 light-years from Earth.

Instead of pictograms, the Arecibo message took the form of densely packed mathematical code, consisting of 1,679 binary digits (chosen because 1,679 is a product of two prime numbers, 73 and 23). The digital message contained the numbers 1 to 10 and information about the identity of the sender—details about DNA, the overall shape and dimensions of a human, and the position of planet Earth.

One of the hopes that accompanied the robotic explorers, as they were dispatched across the solar system from the 1960s onward, was that they might uncover some indication of extraterrestrial life within the solar system itself, even if it were only single-celled organisms. The spacecraft that touched down on planets, such as NASA’s Viking landers on Mars, carried experiments to test for signs of life. To date, no indication of life, either past or present, has been found, although some unexplored corners of the solar system remain candidates for life, such as the deep oceans thought to lie beneath the frozen surface of Jupiter’s moon Europa.

The Arecibo message was broadcast into space a single time in 1974. Coded in binary, its message is arranged in 73 rows of 23 columns.

Continued silence

No reply has ever been received to any of the 10 interstellar radio messages sent since 1962 and no communication has been detected. However, there have been false alarms. The most famous of these came in 1977, when an inexplicably powerful blast of radio signals was recorded coming from the direction of the Chi Sagittarii star system by Jerry Ehman at Ohio State University. He circled the signals on the readout and wrote “Wow!” next to them. The “Wow!” signal was never found again, however, and recent research suggests that it may have come from a hydrogen cloud surrounding a comet.

Given the vast distances between stars, it is still early days, however. The Arecibo message will not reach its target stars for another 25,000 years. Neither the Pioneer plaques nor the gold-plated disks carried by Voyagers 1 and 2—the Voyager Golden Record—are headed toward any particular star system. Unless they are intercepted, they are destined to wander the Milky Way eternally. Sagan, for his part, believed that to find life or to fail was a win-win scenario—either result would show something important about the nature of the universe.

Carl Sagan stands next to a model of the Viking 1 lander. The lander sent back signals from the surface of Mars from 1976 to 1982. Its instruments found no sign of life.

SETI in the modern age

NASA struggled to maintain its SETI funding, and today SETI is privately funded. Since the 1980s, the mantle has been taken up by the SETI Institute, based in Mountain View, California. UCal, Berkeley, through its SETI@home initiative, harnesses a network of volunteer computers to trawl Arecibo Observatory data for patterns that might indicate an unnatural radio source. Meanwhile, in 2016, China announced the completion of the largest ever radio telescope, the Five-hundred-meter Aperture Spherical Telescope (FAST). Among other things, FAST will search for extraterrestrial communications. It will eventually be made available to researchers from around the world.

In recent years, the focus of SETI has moved away from merely listening for messages. Efforts have been directed toward picking up biochemical signs of life or indications of advanced technology. Alien life should leave its signature in evolved planetary atmospheres, volatile molecules, or complex organic chemicals that could only be created by life processes. Highly technological societies may have learned how to harvest the energy of their star. A “Dyson sphere” megastructure completely or partially surrounding a star to capture its energy would affect the star’s observed output. It may also be possible to observe signs of asteroid mining or directly detect extraterrestrial spacecraft.

The SETI Institute’s Allen Telescope Array in California is used daily to search for possible alien communications, as well as for radio astronomy.

A cautious approach

In 2015, the Breakthrough Initiatives program was launched with the backing of Russian billionaire Yuri Milner. In addition to a $1,000,000 prize pool for SETI research and a plan to send a fleet of spacecraft to a nearby star, an open competition was announced to design a digital message to be sent to an extraterrestrial civilization. The Breakthrough Message project aims to accurately and artistically represent humanity and Earth, but pledges not to transmit any message until the risks and reward of contacting advanced civilizations have been debated.

Voyager 1 sent back this image of Earth from beyond the orbit of Pluto. The “pale blue dot” appears in a band of scattered sunlight.

Looking at ourselves

In 1990, Carl Sagan persuaded Voyager 1’s controllers to swivel its camera back toward Earth. From 4 billion miles (6 billion kilometers) away, the craft captured the “Pale Blue Dot” image. Sagan wrote: “Everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives on a mote of dust suspended in a sunbeam.” Sagan stressed the importance of looking at ourselves: “The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.”

SETI represents a series of questions whose answering would tell us about Earth’s place in the universe: whether the Copernican principle is correct, and if so, where else life has evolved. The answers could eventually provide humans with a way to transcend their origins and become a galactic species.

“We are almost certainly not the first intelligent species to undertake the search … Their perseverance will be our greatest asset in our beginning listening phase.” Project Cyclops Report


Carl Sagan is one of the most widely known 20thcentury scientists. His deep, honeyed tones are the instantly recognizable voice of the documentary series Cosmos. Sagan was raised in a workingclass Jewish area of New York and as a boy was an avid reader of science fiction. A talented pupil, he went to the University of Chicago in 1951 on a full scholarship. Sagan received his Ph.D. in 1960, showing that the high surface temperatures of Venus are due to a runaway greenhouse effect. Sagan conducted pioneering research in planetary science and exobiology (the biology of extraterrestrial life), which many in mainstream astronomy viewed with suspicion. In 1985, he wrote the sci-fi book Contact, which was later turned into a movie. With his visionary, positive, and humanist outlook, the Cornell University professor inspired a new generation of astronomers.

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In August 2012, the Mars Science Laboratory Rover, better known as Curiosity, landed on Mars. This 2,000-lb (900-kg) wheeled vehicle, which is still roaming the Martian surface, is a mobile laboratory equipped to conduct geological experiments aimed at figuring out the natural history of the red planet. It is the latest robot explorer to reach Mars, and the largest and most advanced in a long line of rovers sent to explore other worlds.

“Mars has been flown by, orbited, smacked into, radar-examined, and rocketed onto, as well as bounced upon, rolled over, shoveled, drilled into, baked, and even blasted. Still to come: Mars being stepped on.” Buzz Aldrin


The potential of rovers in space was clear as far back as 1971, when Apollo 15 carried a four-wheel Lunar Roving Vehicle to the moon. This agile twoseater widened the scope of lunar exploration for the last three Apollo missions. For instance, during the first moon landing in 1969, Neil Armstrong and Buzz Aldrin spent just two and a half hours moonwalking, and the farthest they moved from their lunar module was 200 ft (60 m). By contrast, however, in the final Apollo moon mission, Apollo 17, in 1972, the crew of two—Eugene Cernan and Harrison Schmitt—spent more than 22 hours outside. In their rover, they covered 22 miles (36 km) in total, with one drive taking the pair 4.7 miles (7.6 km) from their spacecraft. The Lunar Roving Vehicle, or moon buggy, was used to collect rocks. The six Apollo missions returned to Earth with 840 lb (381 kg) of them.

Analysis of these rocks revealed much about the history of the moon. The oldest were about 4.6 billion years old, and their chemical composition clearly showed a common ancestry with rocks on Earth. Tests revealed no evidence of organic compounds, indicating that the moon has always been a dry and lifeless world.

Geologist−astronaut Harrison Schmitt collects samples from the lunar surface during the 1972 Apollo 17 mission. He spent many hours exploring the surface on the moon buggy.

Lunokhod 1

The Soviet lunar program, which began in the early 1960s, relied on unmanned probes to explore the moon. Three of the Soviet Luna probes returned with a total of 11.5 oz (326 g) of rock. Then, in November 1970, the Soviet lander Luna 17 arrived at a large lunar plain called the Sea of Rains (many lunar areas are named after the weather conditions they were once thought to influence on Earth). Luna 17 carried the remote-controlled rover Lunokhod 1 (Lunokhod means “moonwalker”). This was the first wheeled vehicle to traverse an extraterrestrial world, arriving about eight months before the first Apollo buggy. The concept behind it was simple—instead of sending moon rocks to Earth, the rover would do the analysis there.

The Soviet Lunokhod 1 rover, seen here in tests on Earth, was the first rover ever to land on an alien world—its predecessor, Lunokhod 0, was launched in 1969 but never reached orbit

Remote-controlled explorer

The Lunokhod rover was 7½-ft (2.3-m) long and resembled a motorized bathtub. The wheels were independently powered so that they could retain traction on the rough lunar terrain. Lunokhod was equipped with video cameras that sent back TV footage of the moon. An X-ray spectrometer was used to analyze the chemical composition of rocks, and a device called a penetrometer was pushed into the lunar regolith (soil) to measure its density. Lunokhod was powered by batteries that were charged by day using an array of solar panels that folded out from the top of the rover. At night, a source of radioactive polonium inside the machine acted as a heater to keep the machinery working. The rover received commands from controllers on Earth about where to go and when to perform experiments. A human might have done a better job, but rovers could stay in space for months on end, and did not require food and water from Earth.

Lunokhod 1 was designed to work for three months, but lasted almost 11. In January 1973, Lunokhod 2 landed in the Le Monnier Crater on the edge of the Sea of Serenity. By June, Lunokhod 2 had traveled a total of 24 miles (39 km), a record that would stand for more than three decades.

“Over time you could terraform Mars to look like Earth … So it’s a fixer-upper of a planet.” Elon Musk

Martian walker

As Lunokhod 1 was exploring the moon, the Soviet space program was eyeing an even greater prize: a rover on Mars. In December 1971, two Soviet spacecraft, code-named Mars 2 and Mars 3, sent modules to land on the red planet. Mars 2 crashed, but Mars 3 made a successful touchdown—the firstever landing on Mars. However, it lost all communications just 14.5 seconds later, probably due to damage from an intense dust storm. Scientists never found out what happened to Mars 3’s cargo: a Prop-M rover, a tiny 10-lb (4.5-kg) vehicle designed to walk on two ski-shaped feet. It was powered through a 50-ft (15-m) umbilical cord, and once on the surface was designed to take readings of the Martian soil. It is unlikely that the Prop-M ever carried out its mission, but it was programmed to operate without input from Earth. A radio signal between the moon and Earth travels in less than 2 seconds, but a signal to or from Mars takes between 3 and 21 minutes to arrive, varying with the planet’s distance from Earth. For a Martian rover to be a successful explorer, it needed to work autonomously.

Bounce down

In 1976, NASA’s two Viking landers sent back the first pictures of Mars. Following this success, many more rovers were planned, but most of these projects never reached their destination, succumbing to what the press dubbed the “Martian Curse.”

NASA eventually had a success with its 1997 Mars Pathfinder mission. In July of that year, the Pathfinder spacecraft entered the Martian atmosphere. Slowed first by the friction of a heat shield and then by a large parachute, the spacecraft jettisoned its outer shielding, and the lander inside was lowered on a 65-ft (20-m) tether. As it neared the surface, huge protective airbags inflated around the lander, and retrorockets on the spacecraft holding the tether fired to slow the speed of descent. The tether was then cut, and the lander bounced across the Martian surface until it rolled to a stop. Fortunately, once the airbags had deflated, the lander was the right side up. The three upper sides or “petals” of the tetrahedral lander folded outward, revealing the 24-lb (11-kg) rover.

During development, the rover was called MFEX, short for Microrover Flight Experiment. However, it was known to the public as Sojourner, meaning “traveler” and chosen for its link to Sojourner Truth, a 19th-century US abolitionist and rights activist.

“We landed in a nice flat spot. Beautiful, really beautiful.” Adam Steltzner

Rolling on Mars

Sojourner was the first rover to take a tour of the Martian surface. However, the Pathfinder mission was really a test for the innovative landing system and the technology that would power larger rovers in the future. The minuscule vehicle traveled just 300 ft (100 m) during its 83-day mission, and never ventured farther than 40 ft (12 m) from the lander. Now named the Carl Sagan Memorial Station, the lander was used to relay data from the rover back to Earth. Most of the rover’s power came from small solar panels on the top. One of the goals of the mission was to see how these panels stood up to extreme temperatures and what power could be generated in the faint Martian sunlight.

The rover’s activities were run from NASA’s Jet Propulsion Laboratory (JPL) in California, and JPL has remained the lead agency in developing Martian rovers. With the time delays inherent in communicating with Mars, it is not possible to drive a rover in real time, so every leg of a journey must be preprogrammed. To achieve this, cameras on the lander were used to create a virtual model of the surface around Sojourner. Human controllers could view the area in 3-D from any angle before mapping a route for the rover.

During its 83 days of operation, the tiny Sojourner rover explored around 2,691 sq ft (250 sq m) of the planet’s surface and recorded 550 images.

Spirit and Opportunity

Despite its limitations in terms of size and power, Sojourner’s mission was a great success, and NASA pressed ahead with two Mars Exploration Rovers (MERs). In June 2003, MER A, named Spirit, and MER B, Opportunity, were ready for launch. They were about the same size as a Lunokhod rover, but were much lighter, at around 400 lb (180 kg). By the end of January the following year, both were traveling across the Martian deserts, hills, and plains, photographing surface features and chemically analyzing rock samples and atmosphere. They sent back the most glorious vistas of the Martian landscape ever seen, enabling geologists to examine the large-scale structures of the planet.

Spirit and Opportunity had landed using the same airbag-and-tether system as Sojourner. Like Sojourner, both relied on solar panels, but the new rovers were built as self-contained units, able to wander far from their landers. Each vehicle’s six wheels were attached to a rocking mechanism, which made it possible for the rovers to keep at least two wheels on the ground as they crossed rugged terrain. The software offered a degree of autonomy so that the rovers could respond to unpredictable events, such as a sudden dust storm, without needing to wait for instructions from Earth.

“Whatever the reason you’re on Mars is, I’m glad you’re there. And I wish I was with you.” Carl Sagan

Low expectations

Nevertheless, expectations for these rovers were low. JPL expected that they would cover about 2,000 ft (600 m) and last for 90 Martian sols (equivalent to about 90 Earth days). During the Martian winter, however, the team did not know whether the solar-powered rovers would retain adequate power to keep working. Of all the solar system’s rocky planets, the seasons of Mars are the most Earth-like, due to the similar tilts of the planets’ rotational axes. Martin winters are dark and bitterly cold, with surface temperatures falling to as low as –225°F (–143°C) near the polar ice caps.

As predicted, Martian winds blew fine dust onto the solar arrays, cutting their generating power; but the wind also blew the panels clean from time to time. As winter drew nearer, the JPL team searched for suitable locations in which the rovers could safely hibernate. To do this, they used a 3-D viewer built from the images taken from the rover’s stereoscopic cameras. They chose steep slopes that faced the rising sun in order to maximize electricity generation and to top off the batteries. All nonessential equipment was shut down so that power could be diverted to heaters that kept the rovers’ internal temperature above –40°F (–40°C).

An artist’s impression portrays a NASA Mars Exploration Rover. Rovers Opportunity and Spirit were launched a few weeks apart in 2003 and landed in January 2004 at two sites on Mars.

Continuing mission

The hibernation worked, and incredibly, JPL has managed to extend the rover missions from a few days to several years. More than five years into its mission, however, Spirit became bogged down in soft soil; all attempts to free it by remote control from Earth failed, and unable to move to a winter refuge, Spirit finally lost power 10 months later. It had traveled 4.8 miles (7.73 km). Opportunity, meanwhile, has avoided mishap and continues to operate. In 2014, it beat Lunokhod 2’s distance record, and by August 2015 it had completed the marathon distance of 26.4 miles (42.45 km). This was no mean feat on a planet located some hundreds of millions of miles from Earth.

In the “Kimberley” formation on Mars, photographed by Curiosity, strata indicate a flow of water. In the distance is Mount Sharp, named after US geologist Robert P. Sharp in 2012.

Curiosity needed

Spirit and Opportunity were equipped with the latest detectors; including a microscope for imaging mineral structures and a grinding tool for accessing samples from the interiors of rocks.

However, Curiosity, the next rover to arrive on the planet in August 2012, carried instruments that not only studied the geology of Mars but also looked for biosignatures—the organic substances that would indicate whether Mars once harbored life. These included the SAM or Sample Analysis at Mars device, which vaporized samples of ground rock to reveal their chemicals. In addition, the rover monitored radiation levels to see whether the planet would be safe for future human colonization.

Considerably larger than previous rovers, Curiosity was delivered to Mars in an unusual way. During the landing phase of the mission, the radio delay (caused by the sheer distance from Earth) was 14 minutes, and the journey through the atmosphere to the surface would take just seven—all on autopilot (not remotely controlled from Earth). This created “seven minutes of terror”: the engineers on Earth knew that by the time a signal arrived informing them that Curiosity had entered the Martian atmosphere, the rover would already have been on the ground for seven minutes—and would be operational or smashed to pieces.

“The Seven Minutes of Terror has turned into the Seven Minutes of Triumph.” John Grunsfeld

Safe landing

As Curiosity’s landing craft moved through the upper atmosphere, its heat shield glowed with heat, while rockets adjusted the descent speed to reach the Gale Crater, an ancient crater caused by a massive meteorite impact. A parachute slowed the craft to about 200 mph (320 km/h), but this was still too fast for a landing. It continued to slow its descent over a flat region of the crater, avoiding the 20,000-ft (6,000-m) mountain at its center. The craft reached about 60 ft (20 m) above the surface and then had to hover, since going too low would create a dust cloud that might wreck its instruments. The rover was finally delivered to the surface via a rocket-powered hovering platform called a sky crane. The sky crane then had to be detached and blasted clear of the area so that its eventual impact did not upset any future exploration.

Having survived the landing, Curiosity signaled to Earth that it had arrived safely. Curiosity’s power supply is expected to last at least 14 years, and the initial two-year mission has now been extended indefinitely. So far, it has measured radiation levels, revealing that it may be possible for humans to survive on Mars; discovered an ancient stream bed, suggesting a past presence of water and perhaps even life; and found many of the key elements for life, including nitrogen, oxygen, hydrogen, and carbon.


In 2020, the European Space Agency, in collaboration with the Russian space agency, Roscosmos, will launch its first Mars rover, ExoMars (Exobiology on Mars), with the goal of landing on Mars the following year. In addition to looking for signs of alien life, the solar-powered rover will carry a ground-penetrating radar that will look deep into Martian rocks to find groundwater. The ExoMars rover will communicate with Earth via the ExoMars Trace Gas Orbiter, which was launched in 2016. This system will limit data transfer to twice a day. The rover is designed to drive by itself; its control software will build a virtual model of the terrain and navigate through that. The rover software was taught how to drive in Stevenage, England, at a mockup of the Martian surface called the Mars Yard (above).

The ExoMars rover is expected to operate for at least seven months and to travel 2.5 miles (4 km) across the Martian surface. It will be delivered to the surface by a robotic platform that will then remain in place to study the area around the landing site.

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In the early 1960s, the US lagged behind the Soviet Union in the “Space Race.” The Soviets had launched the first satellite in 1957, and on April 16, 1961, Yuri Gagarin became the first human in space. In response, in 1961 US President John F. Kennedy publicly committed to landing a man on the moon before the end of the decade. The project was carefully chosen—landing on the moon was so far beyond the capabilities of either protagonist that the Soviets’ early lead might not seem so significant.

Despite the reservations of many at the time regarding a moon landing’s scientific value, especially given the dangers and technical complexities involved, human spaceflight was now the focus of the US space program. NASA managers felt that with enough funding they could put a man on the moon by 1967. NASA administrator James E. Webb suggested another two years be added as a contingency.

In those six years from 1961 to 1967, NASA tripled its workforce, even though most of the planning, designing, and building of the hardware was undertaken by private industry, research institutes, and universities. NASA claimed that only the construction of the Panama Canal and the Manhattan Project to develop the nuclear bomb rivaled the effort and expense of the Apollo program.

“I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth.” John F. Kennedy

Project Mercury astronaut John Glenn enters the Friendship 7 on February 20, 1962. His mission, lasting just under five hours, was the US’s first manned orbital spaceflight.

Which way to the moon?

At the time of Kennedy’s historic announcement, the US boasted a grand total of 15 minutes of human spaceflight. To move from here to a moon landing, many technological hurdles needed to be overcome. One of the first was the method of getting to the moon. Three options, known as mission architectures, were on the table. The direct ascent (DA) profile, or “all-theway,” required an enormous multistage rocket with enough fuel on board to transport the crew back to Earth. This was initially the favored approach. However, it was also the most expensive, and doubts were raised over the feasibility of building such a monster rocket before the 1969 deadline.

In the Earth-orbit rendezvous (EOR) profile, a moon-bound rocket ship would be assembled in space and dock with modules that had already been placed in orbit. Lifting things into space is the most energy-consuming part of any off-Earth mission, but multiple rocket launches would sidestep the need for a single spaceship. This was the safest option, but it would be slow. The real weight-savings came with the lunar-orbit rendezvous (LOR) profile.

Here, a smaller rocket would put a three-part spaceship on course to the moon. At the moon, a command module would remain in orbit with the fuel for the journey home, while a lightweight two-stage lunar lander would be sent to the surface. This quick and comparatively cheap option carried with it the very real risk of leaving a crew stranded in space should anything go wrong. After much debate and lobbying, influential figures, such as Wernher von Braun, director of NASA’s Marshall Space Flight Center, threw their backing behind LOR, and in 1962, LOR was chosen. This was the first of many leaps of faith for Apollo.

Technological hurdles

On February 20, 1962, John Glenn became the first American to orbit Earth, looping three times around the planet in Friendship 7, as part of the US’s first spaceflight program, Project Mercury, which ran from 1958 to 1963. Three more successful Mercury flights followed, but there was a big difference between operations in low Earth orbit and landing on the moon. An entire new fleet of launch vehicles was required. Unlike Mercury spacecraft, which carried a single astronaut, Apollo missions would need a crew of three. In addition, a more reliable power source was needed and much more experience in space. The world’s first fuel cells were built to provide the power.

Project Gemini, NASA’s second human spaceflight program, provided the skills, with endurance spaceflights, orbital maneuvers, and space walks. Scientists also needed to know more about the moon’s surface. A deep layer of dust could swallow up a spacecraft and prevent it from leaving, clog up the thrusters, or cause the electronics to malfunction.

Unmanned fact-finding missions were mounted in parallel with Apollo, but the first wave of robotic explorers dispatched to the moon was an unmitigated failure. Six Ranger landers failed on launch, missed the moon, or crashed on impact, causing the program to be nicknamed “shoot and hope.” Luckily, the final three Rangers were more successful.

Between 1966 and 1967, five Lunar Orbiter satellites were placed in orbit around the moon. They mapped 99 percent of the surface and helped to identify potential Apollo landing sites. NASA’s seven Surveyor spacecraft also demonstrated the feasibility of a soft landing on the lunar soil.

The Saturn V rocket was developed for the Apollo program. Many private corporations were involved in its production, including Boeing, Chrysler, Lockheed, and Douglas.

“From this day forward, Flight Control will be known by two words: “Tough and Competent.”” Gene Kranz

A gamble and a disaster

At 363 ft (110.5 m), Saturn V—the heavy-lift booster that carried the Apollo astronauts out of Earth’s atmosphere—is still the tallest, heaviest, and most powerful rocket ever built. “Man-rating” the rocket (certifying it to carry a human crew) proved particularly troublesome. The mammoth engines generated vibrations that threatened to break the rocket apart. Knowing that the project was behind schedule, NASA’s associate administrator for manned spaceflight, George Mueller, pioneered a daring “all-up” testing regime. Rather than the cautious stage-by-stage approach favored by von Braun, Mueller had the entire Apollo–Saturn systems tested together.

While striving for perfection, the NASA engineers developed a new engineering concept: that of redundancy. Key or critical components were duplicated in order to increase overall reliability. The Mercury and Gemini projects had taught engineers to expect unforeseen risks. A fully assembled Apollo vehicle had 5.6 million parts, and 1.5 million systems, subsystems, and assemblies. Even with 99.9 percent reliability, the engineers could anticipate 5,600 defects. Nevertheless, over its 17 unmanned and 15 manned flights, the Saturn boosters had shown 100 percent reliability. With two partially successful test flights under its belt, Mueller declared that the next launch would carry astronauts.

Until 1967, progress had been smooth, despite the breakneck pace. Then disaster struck. An electrical short-circuit during a launch rehearsal started a fire that incinerated the Apollo 1 crew inside the Command Module. The toxic smoke and intensity of the fire in a pressurized, pure-oxygen atmosphere killed Virgil “Gus” Grissom, Ed White, and Roger Chaffee in less than five minutes. In the wake of this tragedy, the next five Apollo missions were unmanned tests. Modifications were made, resulting in a safer spacecraft with a new gas-operated hatch, a 60–40 oxygen–nitrogen mix in the cockpit, and fireproof wiring throughout.

The Lunar Orbiter satellites took images of potential landing sites. In 1966, Lunar Orbiter 2 sent back this image of Copernicus Crater, one of the first-ever close-up views of the moon.

“Apollo riding his chariot across the sun was appropriate to the grand scale of the proposed program.” Abe Silverstein

Earth’s place in space

Apollo 8 was the first manned spacecraft to leave Earth’s orbit. On Christmas Eve 1968, Frank Borman, James Lovell, and Bill Anders looped around the far side of the moon and witnessed the astounding sight of Earth rising from behind the moon’s surface. For the first time, humans could see their home from space—a startlingly blue world lost in the immensity of the void. As Anders put it: “We came all this way to explore the moon, and the most important thing is that we discovered the Earth.”

The crew was also the first to pass through the Van Allen radiation belts. This zone of charged particles extends up to 15,000 miles (24,000 km) from Earth, and was initially thought to be a serious barrier to human space travel. As it turned out, it resulted in a dosage of radiation only equivalent to a chest X-ray.

Finally, the program was ready for the last step—to take real steps on the moon itself. On July 21, 1969, an estimated global audience of 500 million tuned in to watch Neil Armstrong land the Lunar Module and step out onto the surface of the moon, closely followed by Buzz Aldrin. It was the culmination of nearly a decade of collaborative effort and effectively ended the Space Race.

There were six more missions to the moon following Apollo 11, including the near-disaster of Apollo 13, whose lunar landing in 1970 was aborted after an oxygen tank exploded on board. The crew was returned safely to Earth on the crippled spacecraft in a real-life drama that played out in front of a worldwide television audience.

In 1968, Apollo 8 broadcast live from moon orbit. Images taken from the spacecraft by astronaut Bill Anders included the iconic Earthrise.

Learning about the moon

Before Apollo, much of what was known about the physical nature of Earth’s only natural satellite was speculation but, with the political goals achieved, here was an opportunity to find out about an alien world firsthand. Each of the six landing missions carried a kit of scientific tools—the Apollo Lunar Surface Experiments Package (ASLEP). Apollo’s instruments tested the internal structure of the moon, detecting seismic vibrations that would indicate a “moonquake.” Other experiments measured the moon’s gravitational and magnetic fields, heat flow from its surface, and the composition and pressure of the lunar atmosphere.

Thanks to Apollo, scientists have compelling evidence from analysis of moon rock that the moon was once a part of Earth. Like Earth, the moon also has internal layers and was most likely molten at some point in its early history. Unlike Earth, however, the moon has no liquid water. Since it has no moving geological plates, its surface is not continually repaved, and so the youngest moon rocks are the same age as Earth’s oldest. The moon is not entirely geologically inactive, however, and occasionally has moonquakes that last for hours.

One Apollo 11 experiment remains active and has been returning data since 1969. Reflectors planted on the lunar surface bounce back laser beams fired from Earth, enabling scientists to calculate the distance to the moon to within an accuracy of a couple of millimeters. This gives precise measurements of the moon’s orbit, and the rate at which it is drifting away from Earth (about 1½ in [3.8 cm] per year).

Neil Armstrong took this famous photograph of Buzz Aldrin on the surface of the moon. Armstrong’s reflection, standing next to the lunar module, can be seen in Aldrin’s visor.

Apollo’s legacy

On December 19, 1972, the sonic boom over the South Pacific, as the Apollo 17 capsule thumped into Earth’s atmosphere, sounded the end of the Apollo program. In total, 12 men had walked on the moon. At the time, it was widely assumed that routine flights to Mars would soon be a reality, but in the intervening 40 years, scientific priorities changed, politicians worried about costs, and human space travel has not ventured farther than Earth’s orbit.

For many, the decision to end manned moon missions was a wasted opportunity, caused by a lack of imagination and leadership. However, the end of the acute Cold War competition that gave rise to the Apollo program heralded a new era of international cooperation for NASA, with Skylab, Mir, and the International Space Station.

Gene Cernan, the last man on the moon, predicted that it could be another 100 years before humankind appreciates the true significance of the Apollo missions. One result could be that it may have made the US smarter—the intake for doctoral degrees at American universities tripled during the 1960s, particularly in the field of physics. Apollo contracts also nurtured nascent industries, such as computing and semiconductors. Several employees of the California-based Fairchild Semiconductors went on to found new companies, including Intel, a technology giant. The Santa Clara area where these firms were based has become today’s Silicon Valley. But perhaps Apollo’s real legacy is the idea of Earth as a fragile oasis of life in space. Photos taken from orbit, such as the “Blue Marble” and “Earthrise”, fed into a growing awareness of planet Earth as a single entity, and the need for careful stewardship.

Apollo 11’s command and service module docked with the lunar module in orbit before heading for the moon. Before touchdown, the service module was jettisoned, and only the command module returned to Earth.

“Houston. Tranquility Base here. The Eagle has landed.” Neil Armstrong

On the final three Apollo missions, astronauts explored the surface of the moon on lunar rovers. The rovers were abandoned and can still be seen where they were left behind


Perhaps the embodiment of the NASA spirit is not the heroic astronauts but the legendary Apollo flight director Gene Kranz. Born in 1933, Kranz was fascinated by space from an early age. He served as a pilot with the US Air Force before leaving to pursue rocket research with the McDonnell Aircraft Corporation and then NASA.

Prominent and colorful, with a brutally close-cut flattop hairstyle, Kranz was unmistakable in Mission Control, dressed in his dapper white “mission” vests made by his wife.

Although he never actually spoke the words “Failure is not an option”— they were written for his character in the movie Apollo 13—they sum up his attitude. Kranz’s address to his Flight Control staff after the Apollo 1 disaster has gone down in history as a masterpiece of motivational speaking. In it, he stated the Kranz Dictum—“tough and competent”—that would guide Mission Control. Kranz was awarded the Presidential Medal of Freedom in 1970 for successfully returning Apollo 13 to Earth.

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Solar system




In the late 1950s, astronomers across the world started to find mysterious, compact sources of radio signals in the sky without any corresponding visible objects. Eventually a source of these radio waves was identified—a faint point of light, which became known as a quasar. In 1963, Dutch astronomer Maarten Schmidt discovered a quasar that was hugely distant (2.5 billion light-years away). The fact that it was so easily detected meant it must be pouring out energy.

Searching for quasars

By the mid 1960s, many radio astronomers were searching for new quasars. One such figure was Antony Hewish, part of a radio astronomy research group at Cambridge University. Hewish had been working on a new technique in radio astronomy based on a phenomenon called interplanetary scintillation (IPS), which is a “twinkling,” or fluctuation, in the intensity of radio emissions from compact radio sources. The twinkling of sources of visible light, such as stars, is caused by disturbances in Earth’s atmosphere that the light has to pass through. The twinkling of radio sources, however, is caused by streams of charged particles emanating from the sun. As radio waves pass through this “solar wind,” they are diffracted, meaning that the waves spread out, making the radio source appear to twinkle.

Hewish hoped that IPS could be used to find quasars. Radio waves coming from a compact source, such as a quasar, twinkle more than radiation from a less compact source, such as a galaxy, and so quasars should twinkle more than other radio sources. Hewish and his team built a large radio telescope designed specifically to detect IPS. It covered an area of nearly 4.5 acres (2 hectares), took two years to construct, and required more than 120 miles (190 km) of cable to carry all the signals.

Members of the Cambridge radio astronomy group built the new telescope themselves. Among them was a Ph.D. student named Jocelyn Bell. When the telescope started operating in July 1967, Bell was made responsible for operating it and analyzing the data, under the supervision of Hewish. Part of her job was to monitor output data from the telescope, made by pen recorders on chart-recorder printouts. Examining about 100 ft (30 meters) of chart paper every day, Bell quickly learned to recognize scintillating sources.

Little Green Man 1

About two months into the project, Bell noticed an unusual pattern of signals, which she described as “scruff.” It looked far too regular and had too high a frequency to be coming from a quasar. Checking back through her records, she found it had appeared in the data before and always came from the same patch of sky. Intrigued, Bell started making more regular chart recordings of the same area of sky. At the end of November 1967, she found the signal again. It was a series of pulses, equally spaced and always 1.33 seconds apart.

Bell showed the signal, dubbed “Little Green Man 1” (LGM-1), to Hewish. His initial reaction was that a pulse occurring every 1.33 seconds was far too fast for something as large as a star, and the signal must be due to human activity. Together, Bell and Hewish ruled out various human-related sources, including radar reflected from the moon, Earth-based radio transmissions, and artificial satellites in peculiar orbits. A second telescope was also found to pick up the pulses, which proved that they could not be due to an equipment fault, and calculations showed that they were coming from well outside the solar system.

Hewish had to revise his opinion that the signals had a human origin. The possibility that they were being sent by extra-terrestrials could not be ruled out. The team measured the duration of each pulse and found it was only 16 milliseconds. This short duration suggested that the source could be no larger than a small planet. But a planet—or an alien civilization living on a planet— was unlikely, since the signal would show slight changes in frequency, called Doppler shifts, as a planet orbited its star.

This image of a pulsar in the Crab nebula, a well-known supernova remnant, was taken in space by the Chandra X-ray Observatory. The white dot at the center is the neutron star.

Publishing dilemma

Hewish, Bell, and their colleagues were unsure how to publish their findings. While it seemed unlikely that the signals were being sent by an alien civilization, no one had any other explanation. Bell returned to her chart analysis, and soon found another “scruff” in a different part of the sky. She discovered it was due to another pulsating signal, this time slightly faster, with pulses every 1.2 seconds. Now she was reassured that the pulses must have some natural explanation—two sets of aliens in different places would surely not be sending signals to Earth at the same time and at nearly the same frequency.

By January 1968, Hewish and Bell had found four pulsing sources in total, which they decided to call “pulsars.” They wrote a paper describing the first source, suggesting that it might be due to pulsed emissions from a theoretical type of superdense collapsed star called a neutron star. Objects of this type had been predicted as long ago as 1934, but up to that time had never been detected.

“My eureka moment was in the dead of night, the early hours of the morning. But when the result poured out of the charts … you realize instantly how significant this is—what it is you’ve really landed on—and it’s great!” Jocelyn Bell Burnell

Explaining the pulses

Three months later, Thomas Gold, an Austrian−American astronomer at Cornell University in the US, published a fuller explanation for the pulsed signals. He agreed that each set of radio signals was coming from a neutron star, but proposed that the star was rapidly spinning. A star like this would not need to be emitting pulsed radiation to account for the pattern of signals observed. Instead, it could be emitting a steady radio signal in a beam that it swept around in circles, just like a beam of light from a lighthouse. When the pulsar’s beam (or perhaps one of its two beams) was pointing at Earth, a signal would be detected, which would show up as the sort of short pulse that Bell had noticed on printouts. When the beam had passed by Earth, the signal would stop until the beam came around again. Challenged about the pulsation rates, which implied extremely rapid spinning, Gold explained that neutron stars could be expected to behave in this way because of the way in which they form—from the collapse of stellar cores in supernova explosions.

A pulsar is a spinning neutron star with an intensely strong magnetic field. It emits beams of radiation from its north and south poles.

Confirming the hypothesis

Initially, Gold’s explanations were not well received by the astronomy community. However, they became widely accepted after the discovery of a pulsar in the Crab nebula, a well-known supernova remnant. Over subsequent years, many more pulsars were found. They are now known to be rapidly rotating neutron stars with intense electromagnetic fields, which emit beams of electromagnetic radiation from their north and south poles. These beams are often, but not always, radio waves and sometimes other forms of radiation, including in some cases visible light. One reason for the excitement regarding the discovery of pulsars was that it increased the likelihood that another theoretical phenomenon—black holes—might also be detected and proven. Like neutron stars, black holes are objects that could result from the gravitational collapse of a stellar core following a supernova explosion.

In 1974, Hewish and Martin Ryle shared a Nobel Prize: “Ryle for his observations and inventions … and Hewish for his decisive role in the discovery of pulsars.” However, Jocelyn Bell Burnell was told that she would not share the award with them because she had still been a student at the time of her work. She graciously accepted that decision.

A spinning neutron star that is emitting radiation beams can be detected as a pulsar on Earth if, as it spins, one or possibly both of its radiation beams recurrently point in Earth’s direction as they are swept around through space. The pulsar will then be detected as a very regular series of signal “blips.”


Jocelyn Bell was born in 1943 in Belfast, Northern Ireland. After earning a physics degree from Glasgow University in 1965, she moved to Cambridge University, where she studied for a Ph.D. There, she joined the team that built a radio telescope to detect quasars. In 1968, Bell became a research fellow at the University of Southampton and changed her last name to Bell Burnell when she married. She has held astronomy and physics-related positions in London, Edinburgh, and at the Open University, where, from 1991 to 2001, she was professor of physics. From 2008 to 2010, she was President of the Institute of Physics. Bell Burnell has received numerous awards for her professional contributions, including the Herschel Medal of the Royal Astronomical Society in 1989. In 2016, she was visiting professor of astrophysics at Oxford University.

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