When it comes to exploration, robots can outperform astronauts at a far lower cost and without risk of human life.  Why, then, do so many people conceive of space exploration as the domain of humans rather than robotic explorers? Martin Rees and Donald Goldsmith explore why robots are the future of space exploration.

How much do we need humans in space? How much do we want them there? Astronauts embody the triumph of human imagination and engineering. Their efforts shed light on the possibilities and problems posed by travel beyond our nurturing Earth. Their presence on the moon or on other solar-system objects can imply that the countries or entities that sent them there possess ownership rights. Astronauts promote an understanding of the cosmos and inspire young people toward careers in science.

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When it comes to exploration, however, our robots can outperform astronauts at a far lower cost and without risk of human life. This assertion, once a prediction for the future, has become reality today, and robot explorers will continue to become ever more capable, while human bodies will not. Fifty years ago, when the first geologist to reach the moon suddenly recognized strange orange soil (the likely remnant of previously unsuspected volcanic activity) no one claimed that an automated explorer could have accomplished this feat. Today we have placed a semiautonomous rover on Mars, one of a continuing suite of orbiters and landers, with cameras and other instruments that probe the Martian soil, capable of finding paths around obstacles as no previous rover could. The first helicopter flight achieved in another body’s atmosphere marks the opening of new methods of exploration.  NASA has plans to search for signs of ancient or even present-day life on Mars by bringing carefully selected samples of Martian soil back to Earth. Our robot explorers have visited all the sun’s planets (including that former planet Pluto), as well as two comets and an asteroid, securing immense amounts of data about them and their moons, most notably Jupiter’s Europa and Saturn’s Enceladus, where oceans that lie beneath an icy crust may harbor strange forms of life. Future missions from the United States, the European Space Agency, China, Japan, India, and Russia will only increase our robot emissaries’ abilities and the scientific importance of their discoveries. Each of these missions has cost far less than a single voyage that would send not robots but humans, which in any case remains an impossibility, for the next few decades, for any destination save the moon and Mars.

Why, then, do so many people conceive of space exploration as the domain of human rather than robotic explorers? Here we may cite six chief factors, often interrelated:

Tradition: From Marco Polo to Columbus, from Ernest Shackleton to Yuri Gagarin and Neil Armstrong, we conceive of exploration as requiring the direct engagement of humans.

Engagement: We naturally relate to humans far more than to machines, though we may identify, to some degree, with the latter, from the Little Engine that Could to the rovers on Mars to the James Webb Space Telescope.

Adventure: The difficulties and dangers of exploration bring a dramatic tension that has always appealed to us. If Columbus had merely sailed the Atlantic to visit friendly nations in the Americas, his voyages would hardly have captured as much attention from European powers.

Inspiration: Children now easily imagine going into space, and from these dreams, great interest in science may arise. Along with adults, they receive continual stimulation from movies and television programs that feature humans who travel through space almost instantaneously (while in real life, a journey to Mars requires six months) and meet extraterrestrial beings who almost always have humanoid characteristics (not least because actors in costumes cost less than computer-generated aliens).

Ownership: Just as Spain and Portugal vied to control the New World until the Pope drew a line of demarcation, modern nations seem ready to assert claims to portions of the moon, most notably over the “Peaks of Eternal Light,” mountains near the lunar south pole where the sun’s rays shine forever. This competition includes the creation of large-scale lunar colonies as arguments for ownership, or to mine the moon for material to create enormous numbers of free-orbiting space colonies, an important part of Jeff Bezos’s future plans (the moon’s low gravity strongly favors our satellite over our planet for such purposes).

Wealth: Despite the immense distances to be traversed, entrepreneurs dream of obtaining rare and useful deposits, from a rare isotope of helium (for nuclear fusion) to the rare-Earth elements, available from only a few terrestrial locations (primarily in China), that have become essential for products ranging from cell phone to electric cars to fighter aircraft. Intriguingly, except for helium-3 buried in lunar soil, certain metal-rich asteroids with orbits that bring them comparatively close to Earth offer the most promising objects for such mining.

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The best argument against long-term plans to “terraform” Mars by creating a more Earthlike environment remains the sad results of our “terraforming” of Earth

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The first four of these six motivating factors spring from deeply embedded attitudes, relatively insusceptible to logic. Number three, Adventure, could be satisfied to a large degree by private space missions, all the more appropriate so long as the rate of catastrophe per launch stays above one percent. The last two, however, spring directly from the long history of conquest and exploitation of Earth’s resources, whose long and effective history has profoundly altered our planet. (The best argument against long-term plans to “terraform” Mars by creating a more Earthlike environment remains the sad results of our “terraforming” of Earth.) Whether or not one approves of them, both ownership claims and mineral extraction can be successfully prosecuted with machines. This also applies to scientific activities. For example, astronomers would dearly love to have a giant radio telescope on the far side of the moon, which would screen out terrestrial radio interference marvelously well. In the near future robots could build this telescope more efficiently and much more cheaply than humans.

This discussion has barely touched on astronauts in low-Earth orbit, their only sphere of activity since Apollo 17 left the moon in 1972. In this realm, astronauts’ greatest achievement by far came with their five repair missions to the Hubble Space Telescope, which saved the giant instrument from uselessness and extended its life by decades by providing upgraded cameras and other systems.  Each of these missions cost about a billion dollars in today’s money (Hubble’s total operational cost comes to $16 billion). The cost of a replacement telescope to replace the Hubble would likewise have been about a billion dollars; in fact, the director of the Space Telescope Science Institute said that the cost of the five repair missions would have paid for seven replacement telescopes. Astronauts could reach the Hubble only because the Space Shuttle, which launched it, could go no farther from Earth, which produces all sorts of interfering radiation and light. Today, astrophysicists have managed to send all of their new spaceborne observatories to distances to a region of space four times farther than the moon, where the James Webb Space Telescope now prepares to study a host of cosmic objects. 

Since 1988, multinational cooperation on the International Space Station, 250 miles above the Earth’s surface, has proven successful in achieving almost all of the tasks that NASA and its collaborators have set for the rotating teams of astronauts from 16 different countries. A closer look at these tasks, however, demonstrates the weakness of justifications for humans in even the most readily available realms of space.

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In 2020, NASA revealed its list of “20 Breakthroughs from 20 Years of Science aboard the International Space Station.” Seventeen of these dealt with processes that robots can perform, such as launching small satellites, the detection of cosmic particles, employing microgravity conditions for drug development and the study of flames, and spaceborne 3-D printing. The remaining three dealt with muscle atrophy and bone loss, growing food, or identifying microbes in space—important for humans in that environment, though hardly a rationale for sending them into space. How deeply will the arguments made here affect anyone who reads them? Opinions that have formed without considerations of logic are unlikely to be changed by appeals that rely on rational argument. Large numbers of us identify with the emotions that led Donald Trump to promise to put the first woman on the moon and to assure that the first person on Mars would be American. Others without such nationalistic impulses will insist that a key aspect of our destiny resides in sending humans into space. We have no good answer to these emotions. Nor can we successfully predict what may happen beyond a time horizon—let us say two decades—about which we can make reasonable forecasts. We can only urge our readers to think things over, to distinguish between scientific exploration and the other motivations for going into space, and to form—what else? —their own conclusions.

Martin Rees' book The End of Astronauts is published by Harvard University Press.