What future lies ahead for humans in space? Last year, the 50th anniversary of the first moon landing found a host of private and governmental projects that aim to send astronauts far beyond the near-Earth orbits that have limited human space exploration since 1972. China, which landed the first spacecraft on the lunar far side in 2019, has plans to place astronauts to the moon. India, which crashed a lander on the moon in 2019, dreams of doing likewise. Russia, which doesn’t seem to have much of an ongoing astronaut program, still provides the rockets and launch facilities that provide astronauts with access to the International Space Station. The Trump administration proposes to create a lunar base as a key step in sending astronauts to Mars. Elon Musk and Jeff Bezos have spent large sums on future human space missions. Indeed, Musk has already created a thriving rocket business, which NASA uses to resupply the Space Station, 250 miles above Earth’s surface.

What benefits will flow from these efforts to send humans much farther into space? As children of the 1950s, we were thrilled and inspired by the satellites that began to circle Earth in 1957, the first astronauts—who followed similar paths in the 1960s and made the first spacewalk in 1965—and what turned out to be the culmination of human spaceflight: NASA’s six astronaut explorations of the lunar surface from 1969 through 1972. Beyond any scientific returns, these efforts elevated the human spirit, reaching a peak on July 20, 1969, when Neil Armstrong set humanity’s first footprints on the moon. During the 1960s, we became astronomers with a deep passion to explore the cosmos.

But the past five decades have taught a clear lesson about how best to explore the cosmos. People venturing into space are fragile: They require a continuous supply of oxygen, water, food and shelter. They must endure long intervals of weightlessness. Their physical capabilities remain constant across generations. And their loss, when it occurs, casts a pall over our would-be joy of identifying with their exploration. In contrast, automated spacecraft require only a power supply. They cost far less than humans do, and we know how to improve them every year. And if they fail, we lose only dollars and scientific results.

Since the first moon landing, we have sent several hundred probes throughout the solar system, from innermost Mercury to Arrokoth (previously nicknamed “Ultima Thule”), a planetesimal orbiting far beyond Pluto. Spacecraft have landed on Mercury, Venus and Mars, spent years in orbit around Jupiter and Saturn, and surveyed Uranus and Neptune. And the Japanese Hayabusa2 spacecraft will soon pass by Earth to release a capsule with material from the asteroid Ryugu, one of the oldest members of the solar system. Multiple lunar missions have mapped the moon’s far side, detected the gravitational anomalies that make the “man in the moon” always face Earth and discovered huge amounts of water frozen in the soil at the lunar poles.

Limited to low-Earth orbits, astronauts have basically performed extensive experiments on the hazards and requirements of living in space-bound habitats. By far, the finest achievement of humans in space has been the five servicing missions that allowed astronauts to repair instruments on the Hubble Space Telescope, which orbits Earth at the maximum altitude that the now defunct Space Shuttle could carry it to. Astronomers, who cheered these efforts more than anyone, remained deeply aware of an ironic fact: Hubble suffers from close proximity to our planet, whose reflected and emitted radiation greatly hampers the telescope’s ability to peer clearly and deeply into space. The James Webb Space Telescope, scheduled to supplant the now creaky Hubble next year, will be directed to the much more astronomically favored “L2 point” (for second Lagrange point), a million miles from Earth. Spacecraft at L2 can easily maintain a stable orbit, avoiding the slow drift that gravitational tugs from the sun and moon produce elsewhere. Astronomers have already maintained spacecraft at L2 to observe the cosmos in infrared, ultraviolet and x-radiation, unaffected by interference from our own planet.

Why, then, should we not expect future astronauts, if called upon, to repair one of the numerous space-borne instruments to be sent to L2? An astronaut expedition to repair one of these great observatories at L2 would involve at least as much complexity as a landing on the moon—and possibly a greater expense than the creation and launch of a new and improved observational platform. In fact, the missions to repair the Hubble telescope cost significantly more than replacing it with a newer and better version. But these missions elevated our spirits, whereas writing off the telescope would have been a profound downer—a reminder that public opinion, which would have scorned the latter action while celebrating the former, understandably plays a crucial role in determining what our government chooses to do.

The contrast between astronaut and automated space missions will grow ever stronger as we improve our miniaturization, virtual-reality and artificial-intelligence capabilities. Today a trained geologist on the moon can perform as well as a robotic explorer, but the future of geologic investigation of other worlds lies with highly improved versions of our Mars rovers. These explorers will deploy numerous tools to probe rocks and minerals, using a memory equal—and soon superior—to any human’s. They will traverse the lunar or Martian surface for decades, continuously learning about the topography, seismographic activity and distribution of geologic strata in bulk and in detail. Conceptually similar robots will eventually be able to repair spacecraft at the L2 point, while others could construct complex structures in space, including an array of radio telescopes on the radio-quiet far side of the moon.

The fundamental issue of sending humans into the cosmos asks not how easily astronauts can repair instruments in deep space, how quickly they can land on the moon and construct a base there, or why they should travel to Mars and attempt to create a habitat there. Instead it queries, Why should we do any of this? Four major motivations deserve special attention as answers to this question.

Uplifting the human spirit. Sending humans into space adds glory to our lives. Overcoming the manifold challenges to long-term spaceflight inspires and delights us. Almost everyone naturally responds to heroic accomplishments, and many of us would regard a human landing on Mars as a paramount achievement of our species. But to many scientists, and to some among the public, such potent reactions fail to justify the costs and dangers of these missions.

We should note that several other motivations lie behind the push for astronaut expeditions to our celestial neighbors. These include the desire to outdo our rivals, the belief that space offers an eventual refuge from a debilitated Earth and an eagerness to exploit raw materials in the nearby solar system. Each of these arguments, in our opinion, favor expeditions not with humans but with our ever improved spacecraft and robot explorers—at least until the habitats for the refuge of a chosen population are ready.

National pride. The cold war argument that the Russians could “seize the high ground” by establishing a lunar base never made sense, because any nation seeking to use space to launch weapons would attempt to do so close to Earth, not from a quarter-million miles away. There remains the pride that a nation may feel from sending the first humans to other worlds, as when President Donald Trump exalts a future “when American astronauts will plant our beautiful Stars and Stripes on the surface of Mars,” adding the pride of ownership to the thrill of human achievement.

Human survival. Shortly before his death in 2018, Stephen Hawking stated that “spreading out may be the only thing that saves us from ourselves. I am convinced that humans need to leave Earth.” More recently, Bezos has said that humans need space travel because “we are in the process of destroying this planet.” Among other outcomes, he envisions giant space colonies that would each allow millions of people to live in space.

To their enthusiasts, giant space colonies and human habitats on Mars offer not only sites to develop a better society but also places where we may modify humans themselves, partly for adaptation to the lower gravity on Mars or, in many scenarios, to the artificial gravitational force produced within the enormous rotating wheel of a million-person “posthuman environment,” where genetic engineering could attempt to reduce diseases and prolong human life.

Such future plans appeal to those who see Earth’s future as deeply uncertain or even hopeless. A moment’s thought, however, tends to reveal that (a) the notion that we can learn from our errors on Earth in order to survive in space involves pie-in-the-sky optimism and (b) the billions of people to be left behind deserve greater consideration. If we can’t solve humanity’s problem on our home planet, we seem highly unlikely to be able to do so by establishing ourselves in space.

Raw materials. Although less cited in the wider world, a great incentive for reaching nearby solar system objects springs from an old-fashioned, solid desire: the quest for raw materials for profit. One of the asserted justifications for sending humans back to the moon focuses on their potential for harvesting helium-3, a rare isotope of helium. Unlike those of helium-4, the far more common stable isotope of the element, helium-3 nuclei will fuse readily once they reach a sufficiently high temperature. Because this fusion releases large amounts of energy but no radioactive by-products, helium-3 nuclei could provide an almost ideal nuclear fuel. On Earth, helium-3 nuclei furnish only about one one-millionth of already scarce helium nuclei, but their relative abundance in lunar soil rises 100 times higher. Visionaries propose a future society that runs on helium-3 nuclei from the moon, which contains enough of these nuclei to provide many centuries of the world’s current power consumption.

The asteroids likewise offer a road to wealth. Although most asteroids have a composition that resembles Earth’s, a few of them consist largely of metals such as iron, nickel and cobalt—together with a much smaller amount of silver, gold and platinum. A metal-rich asteroid that is only the size of a house would contain a million pounds of metal, including 100 pounds of platinum, gold and other rare metals. We can imagine future space missions that use the more abundant minerals for the construction of mining colonies but that draw the bulk of their profits from the return of the most valuable metals to Earth.

Do any existing international agreements deal with these issues? In 1967 several countries ratified the United Nations’ Outer Space Treaty, whose full formal title includes “the Moon and Other Celestial Bodies.” The 100-plus nations that ratified this treaty include all those likely to carry out space exploration during the coming decades (with the possible exception of Iran, which signed the treaty but did not ratify it).

The treaty’s key provisions forbid placing weapons of mass destruction on the moon, in orbit or elsewhere in outer space. They also state that celestial bodies are to be used exclusively for peaceful purposes and are “not subject to national appropriation” by any means and that all parties will follow international law in their activities relating to the exploration and use of outer space. One may easily see that space lawyers, who have a growing future ahead of them, can dispute the term “national appropriation,” which hardly seems to rule out operations by private parties that do not add directly to a nation’s wealth. Furthermore, the current geopolitical climate suggests the treaty’s terms might not fully govern the actions of any state or private party.

Let’s examine the arguments in favor of sending humans into space in the in reverse order from which we have posed them.

Raw materials and the transformation of planetary surfaces. In 2015 the U.S. Congress passed legislation, informally known as the SPACE Act, that denies any assertion of authority over cosmic objects but promotes the right of U. S. citizens (which naturally includes corporations) to engage in the “commercial recovery of space resources free from harmful interference ... subject to authorization and continuing supervision by the Federal Government.” The marvelous word “recovery,” common in mineral-extraction circles, tends to hide the obvious impacts of such activities. On a moral basis, do we, as humans, U.S. citizens, or private individuals or corporations, possess the right to alter or even destroy the landscape of other celestial objects? To some, the answer is obvious: Of course we do. And the resources of these worlds belong to those who can first exploit them.

The opposing moral argument begins with the thought that humans ought not to embark on these activities lightly, because whatever we do may not be capable of being repaired. To scientists, the gravest threat from the “recovery” of other worlds’ resources resides in the possibility that human activities can forever cloud our knowledge of the origin and distribution of life in the solar system. Wherever we land, we inevitably leave behind traces of our own forms of life. NASA has worried about this problem ever since the first lunar probes and has taken pains, which the agency knows can never be entirely successful, to avoid the biological contamination of other worlds. Large-scale extraction efforts, however, could never proceed without this contamination. Even the moon or asteroids, hostile to life though they now are, may contain traces of past biological activity.

For colonies on Mars—even “harmless” ones whose purpose is purely exploration—the problem increases steeply: Most experts agree that life probably existed on Mars when water ran freely over its surface and may that it may yet survive in underground pools. The discovery of life on a nearby world should reveal, through comparison of its DNA or equivalent material, whether life in the solar system originated separately or transferred itself from world to world onboard meteoroids or asteroids. If we find Earth-like organisms on Mars, our ability to discriminate will be lost if we cannot tell whether this transfer occurred in recent years or eons before.

Advocates of exploiting other worlds often point to a glorious future on Mars after engineers have “terraformed” the planet to produce more Earth-like conditions. By releasing sufficiently large amounts of carbon dioxide that currently reside in rocks and in Mars’s modest polar caps, along with other gases even better at trapping heat, we could produce a “greenhouse effect” that would raise the planet’s surface temperature and increase its atmospheric pressure to the point that liquid water could once again flow over the Martian surface. Those who oppose reworking an entire planet begin with a hard look at what humans have done to “terraform” Earth.

Human survival. Aside from the immense difficulties of creating sustainable, million-person colonies (required for sufficient long-term genetic variation), any lunar or planetary colonists will bring the same human attributes that have caused problems on Earth. In addition, plans to leave behind eight billion of their fellow humans on a dying planet might produce unrest sufficient to derail the project.

National pride. One need not be a one-worlder to recognize that national competition to explore nearby objects—and, even more so, to exploit space resources—fails to furnish a sustainable basis for rational exploration. By exporting our Earthly competition into space, we increase the likelihood of conflict in both venues.

Uplifting the human spirit. Debating the wisdom of sending humans into space inevitably returns to the immense boost that humanity will receive from tracking astronauts as they explore other worlds. Undeniable though these emotions may be, they hardly settle the issue. What remains is the key question of whether this spiritual uplift so far surpasses any news that may be sent by spacecraft that humans in space must be worth the expense and the danger of contaminating other worlds, along with encouraging the exploitation of these worlds for economic gain.

Approximately 3 percent of astronauts who have begun a journey into space have not survived.” Although our increased understanding of how to launch and return spacecraft safely may reduce this percentage, “space tourism”—a phrase that suggests that ordinary people can enjoy the thrill of travel around the Earth or even farther—conceals the actual risks. Space travelers will long resemble stunt motorcyclist Evel Knievel more than they do Jules Verne’s fictional adventurer Phileas Fogg.

Those who feel that our automated planetary explorers can never come close to the human experience in uplifting our spirits may find a modest rebuttal in our robotic explorers on Mars, which have commanded widespread attention and even some human identification during their years on the red planet. NASA’s Opportunity rover, for example, spent more than 15 years on Mars and traversed complex topography for more than two dozen miles, at a price tag that is almost certainly less than 1 percent of what a comparable human expedition would cost today. In addition, we may reasonably expect that popular culture will expand our identification with our marvelous spacefaring machines.

Private expeditions. This discussion has assumed the existence of a forum to debate the pros and cons of humans in space and reach a (more or less) logical conclusion. What of the superrich who operate free from such constraints? The exploitation of raw materials in space offers a wide range of construction, destruction and confrontation among private parties and corporations. If these parties choose to act, who can stop them? Should we try? Human history shows that no scarcity of volunteers will arrive, including those who would gladly gain fame from being among the first to land, for example, on Mars, without any prospect of a return journey.

What, then, should be done? Should members of the public confront these arguments and attempt to influence governmental decisions? Do we want to regulate space adventuring—and if so, how? Or do we prefer to let the “space frontier” work itself out? Private individuals, though aware of the considerations we have raised, need answer to none but themselves in their spacefaring endeavors. Statements by Musk and Bezos testify to deep beliefs in human activities in space, which they apparently regard as not even requiring justification. Musk has so deeply embraced the belief that only humans in space can fulfill the human desire to explore other worlds that he has tweeted “nuke Mars!” to express his plan to use nuclear weapons in order to release carbon dioxide stored in the Martian soil and polar caps in order to create a greenhouse effect to warm the planet.

Whether the exploits proposed by Musk and Bezos will inspire us to greater efforts on Earth—or cause us to imagine that we can forget about problems on our planet—remains an open question. Because little hope of curtailing these endeavors exists, we may do well to let them advance as the masters of space would wish, believing, as we always must, that humans will soon see the wisest way to proceed.

None of our discussion and suggestions looks beyond the next few decades, a sufficiently long stretch of time to beware the accuracy of our predictive abilities. If human civilization successfully overcomes its current problems and achieves long-term stability, we shall certainly send people to the other worlds in our solar system. If we discover much faster means of propulsion or find a way to prolong human life or invent a means of inducing limitless suspension of life during a multicentury journey, we can send humans to the nearest stars and their planetary systems. If we create human colonies in space, their inhabitants might undergo evolutionary changes that make them more fit for space travel. If, as is likely to happen soon, we can manipulate the human genome as we like, we could manufacture a new set of beings designed for space travel.

None of these “ifs” tells us much about what we should do within the next few years. Nor do they rule out machines as superior to whatever humans we may invent. Designs now exist for spacecraft that include photography and radio communication equipment, but which weigh only one-thirtieth of an ounce. These could be accelerated by lasers to 20 percent of the speed of light and reach the closest stars within a couple of decades. This concept leads to the mind-bending, though hardly impossible, notion that eventually we can send human consciousness, downloaded from individuals or created by artificial intelligence, to nanoexplorers that will range through the vastness of space in the name of humanity. Possibly, they may meet and interact with similar probes from an array of other civilizations.

Meanwhile we would do well to ponder the current advantages that our automated explorers of realms close to Earth maintain over their human counterparts.