Will Humans Travel to the Stars? Challenges and Motivations

Starbound: Interstellar Travel and the Limits of the Possible. By Ed Regis. Cambridge University Press, Cambridge, U.K., March 2025. 300 pages, $29.95.

Between December 21, 1968, and December 14, 1972, 24 men traveled from Earth to the Moon; of those, 12 actually set foot on the Moon for a total of about seven man-days. Since then, no human has ventured more than roughly 250 miles from the Earth’s surface. Five manmade space probes have attained escape velocity from the Sun’s gravitational field: Pioneer 10 and 11, launched in 1972 and 1973; Voyager 1 and 2, launched in 1977; and New Horizons, launched in 2006. We are very far from sending an astronaut to the stars and have made little direct progress toward that goal over the last 50 years. Nevertheless, some scientists and futurists continue to argue that it is both feasible and important for humans to do so; as such, they labor zealously to accomplish just that.

Starbound: Interstellar Travel and the Limits of the Possible, by philosopher and science writer Ed Regis, reviews and evaluates these arguments and plans. The title of the book is deliberately ambiguous; is humanity destined to reach the stars or forever bound to a single star? Regis claims not to have a decisive answer:

It is no part of the purpose of this book to argue that human interstellar travel is impossible (although it might well be) … It is also no part of the purpose of the book to argue that interstellar travel is easy, or that it is going to happen anytime soon.

In fact, however, Starbound argues strongly that there is no reason to believe that human interstellar travel is possible. It asserts that achieving such a feat would require overcoming many difficulties which may well be insuperable; that all of the plans thus far are hopelessly inadequate or completely farfetched; and that there is no justification for the claim that doing so would be worthwhile, even if possible. In my mind, Regis makes a convincing case.

The first and most obvious difficulty is the method of propulsion. To reach a star that is 10 light-years away within a time span of 1,000 years, one must travel at 1/100th the speed of light — that is, 3,000 kilometers per second (for context, the fastest spacecraft on record is the Parker Solar Probe, which reached speeds of 192 kilometers per second). Proposed methods range from complete unknowns (e.g., controlled fusion or solar sails) and those of dubious feasibility (e.g., propulsion with immensely powerful lasers) to those that are very likely infeasible (e.g., propulsion by nuclear bombs or the use of antimatter) to pure fantasy (e.g., warping spacetime). If a spacefarer does not want to wait a millennium, then the issue of propulsion becomes correspondingly greater and further problems appear. For instance, if a spacecraft is traveling at a significant fraction of the speed of light, then hitting even a small particle will create a sizable explosion.

The second major setback is human survival on a lengthy trip through outer space. Astronauts who are in space for long stints tend to develop both temporary and lasting health problems. Weightlessness and exposure to cosmic rays are particularly unhealthy over an extended period of time. While solutions or workarounds for these issues might exist, there is no guarantee. Moreover, if 30 generations are to spend their entire lives on a millennium trip, then the population cannot be small and the spacecraft would have to be large and extremely complex to carry all necessary supplies. Size exacerbates the propulsion problem, while complexity increases the chances of a catastrophic breakdown.

Some of these complications could be ameliorated if passengers were placed into a state of suspended animation, wherein they do not age and have minimal metabolic requirements. But there is no evidence that doing so is possible. Another option would be to modify the human body via genetic engineering, but we again do not know with any certainty what kinds of alterations would be useful or even workable; furthermore, the ethical issues in such deliberate teratology are formidable.

The third problem is the destination. At the end of the long voyage, the travelers would have to arrive at a hospitable exoplanet. And though astronomers have detected almost 6,000 exoplanets over the last two decades, very few of them would be comfortable for human habitation. Regis writes:

Teegarden’s Star b and TOI-700 d are the two top candidates for habitability in The Habitable Worlds Catalog: both tidally locked to dim red dwarf stars, and neither of which is known to have a breathable atmosphere. Other members of the catalogue’s top 20 candidate exoplanets lie at distances ranging from 11 to 1,193 light-years from Earth. Without exception, all of the 20 orbit M-type red dwarf stars, the smallest and coolest stars which would be unlikely to support human vision in the form of visible light.

A useful point of comparison is Antarctica. It would be thrilling if we found an exoplanet that seemed as hospitable as Antarctica. It has the right atmosphere, the right gravity, the right kind of sunlight (during the summer), plenty of (frozen) water, the proper magnetic field to shelter inhabitants from cosmic rays, and advanced native species; plus, it is only a two-hour flight from southern Chile. Antarctica is certainly cold and dark during its six-month winter, but that is survivable; people have been overwintering there since 1899. Still, the population of the entire continent is presently only about 5,000 during the summer and 1,000 during the winter.

Enthusiasts for human interplanetary travel tend to have a can-do attitude, noting that technology has achieved many things that were once thought to be impossible and proudly citing the fact that Apollo 11 landed on the Moon a mere seven years after President John F. Kennedy announced the goal. However, there are plenty of things that human technology simply cannot accomplish. While we may be able to cleverly build a variety of tools, there are limitations to our prowess.

Finally, what is the point? What would human interstellar travel accomplish that would be worth the enormous expense? Regis sorts through the justifications, finding none of them convincing. While we do expect the Sun to turn into a red giant in roughly four billion years, possibly destroying the Earth and certainly making it uninhabitable, why should we concern ourselves with that now? How likely is it that humans will even exist in their current form by then? Four billion years is a long time, even in geological terms; four billion years ago, Earth hosted either no life at all or only the most primitive forms. In all likelihood, the last human will have perished eons before that (though a long time from now). While this might seem like a sad thought, the last human will be no better off dying on some barely livable exoplanet, let alone mid-voyage on a starship.

An even less cogent justification is that exploration is in the nature of our deep inner soul, part of our DNA, and so on. It isn’t. Long-distance migration is presumably built into the DNA of Arctic terns, monarch butterflies, humpback whales, and so forth. But many people have led happy, productive lives without ever journeying more than 100 miles from their birthplace, let alone venturing into the unknown.

Historically, migrations have mostly been carried out by people who were hoping for conquest, wealth, or better lives for themselves and their children, and who were willing to suffer hardships and risk danger to attain their goals. But it is difficult to imagine why anyone would willingly volunteer to endure the discomfort, tedium, and risk of an interstellar space voyage so their descendants—30 generations in the future—can survive on some barely habitable exoplanet.

Regrettably, Regis dilutes his many strong arguments with weak ones. He proffers the failure of the Roanoke Colony as a warning, but in light of the overwhelming success of Europeans’ migration to the Americas, proponents of space travel are unlikely to find the so-called “Lost Colony” discouraging. Similarly, as scientists have now identified nearly 6,000 exoplanets, there is no reason for Regis to spend two pages describing Peter van de Kamp’s announced discovery of an exoplanet in 1963 that turned out to be an error. Regis also demonstrates at length that Christopher Mason’s argument that the human species has a moral obligation to ensure its continued existence is not an Aristotelian syllogism, but this demonstration in no way establishes Mason’s argument as invalid. Regis’ often intemperate language further detracts from his claims.

It seems to me that the obsession with this impossible dream of spacefaring does two kinds of actual harm. First, it distracts from the many urgent problems that face us here on Earth. Powerful and influential people argue that we should focus on colonizing space—first within the solar system and then outside it—rather than addressing climate change, which is a frivolous and irresponsible suggestion. Even in the realm of astrophysics, there are much more urgent concerns than the Sun’s eventual evolution into a red giant. The recurrence of a solar flare on the scale of the Carrington Event of 1859 would do untold damage to our electronic infrastructure, but we have no capacity for advance detection, no method to shield ourselves, and no contingency plans for dealing with the aftermath [1].

Second, the fixation on human space travel sidetracks attention from the amazing accomplishments in unmanned space exploration. Voyager 2 is now in the interstellar medium—three times as far away as Pluto—and still sending us information! Rovers are traveling around the surface of Mars, analyzing soil samples and launching drones through the Martian atmosphere. The James Webb Space Telescope sits in stationary orbit at the L2 Lagrange point, where it probes the utmost depths of the universe and delivers a steady stream of eerily beautiful images of the extraordinary entities that populate the heavens. Personally, I find all of these triumphs more exciting than the thought that an astronaut may set foot on Mars in the near-ish future, and much more inspiring than the fantasy that—in the far distant future—some spaceship with human passengers might land on a planet that is orbiting another star.

References
[1] Graham, D.A. (2015, September 9). When the lights go out. The Atlantic. Retrieved from https://www.theatlantic.com/technology/archive/2015/09/how-safe-is-the-us-electrical-grid-really/402640.