On The Net
Spacecraft of the Mind
by James Patrick Kelly
then and now
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When I was a kid, my dad built us a treehouse. It was about ten feet off the ground and had three rickety walls and half a roof. There was a window with no glass in one of the walls beneath which we drew dials and switches. The window looked back at our house, but when I peered through it, I dreamed of adventures in outer space. My plywood spacecraft was powered by an imagination that I had supercharged with all the science fiction books and shows I could find. I spent many happy hours at its control console.
There were, of course, the real spacecraft back then. The USSR and the US were busy racing to space https://www.history.com/topics/cold-war/space-race. (Spoiler alert: America got to the Moon before Russia, but ultimately didn’t care.) But while the technology was inspiring and the space farers from both countries heroic, the deliberate progress of rocket-based programs stood in stark contrast to that quintessential SF spacecraft of the sixties, the starship Enterprise https://www.entertainment.howstuffworks.com/10-star-trek-technologies.htm. Maybe what NASA needed was a catchy theme song, but by 1972, not only had we abandoned the Moon but Star Trek was snoozing in reruns, seven years from its reawakening as a motion picture franchise https://www.startrek.com/series-and-movies.
A series of groundbreaking robot programs took up the space exploration slack: Pioneer https://www.science.nasa.gov/mission/pioneer-10, Voyager https://www.voyager.jpl.nasa.gov, Viking https://www.science.nasa.gov/mission/viking, Hubble https://www.science.nasa.gov/mission/hubble/overview/about-hubble, and the Mars Rovers https://science.nasa.gov/planetary-science/programs/mars-exploration, to name just five. While these probes made some astonishing discoveries, none of them changed people’s lives. Their lack of immediacy made the beautiful images of our Solar System they sent back seem like postcards from an exotic vacation that someone else was taking. Then there was the Space Shuttle https://www.nasa.gov/space-shuttle, which looked a little like a science fiction spacecraft, but was disappointing, expensive, and dangerous https://www.en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle_program. The reusable shuttle flew 135 missions from 1981 to 2011, an average of just five annual missions instead of the twenty-four a year that the designers had promised.
As the shuttle program wound down, SpaceX https://www.spacex.com was developing its own version of reusable rocketry. The first successful launch of the Falcon 1 https://www.science.howstuffworks.com/elon-musk3.htm came in 2008 after three failures. Elon Musk https://www.forbes.com/profile/elon-musk/?sh=5b20ee167999 and company did not manage a manned mission until the first crewed Dragon https://www.spacex.com/vehicles/dragon spacecraft docked with the International Space Station in 2020. The Dragons are recovered at sea after a splashdown https://www.en.wikipedia.org/wiki/Splashdown, like the capsules of the early American space program, but the latest generation of Falcons land intact on the LZ pads https://www.spacex-guide.weebly.com/landing-zone-1-and-2.html at Cape Canaveral. Both Dragon and Falcon can be reused many times, which has brought the cost of access to space https://www.en.wikipedia.org/wiki/Space_launch_market_competition down dramatically. Consider that fifteen years ago it cost fifty-four thousand dollars to launch a kilogram of payload aboard the shuttle to low earth orbit. Workhorse expendable rockets like Russian Proton https://www.russianspaceweb.com/proton.html or the Chinese Long March https://www.space.com/china-new-long-march-rockets-2024 now reach orbit for about forty-four hundred dollars a kilo. But Falcon 9 has already cut that to twenty-seven hundred dollars, and Falcon Heavy is expected to cost just fifteen hundred dollars per kilo of payload.
Just a couple of years ago, it seemed like we were on the cusp of the Age of Space Cowboys https://www.thespacereview.com/article/4244/1 when billionaire techbros like Musk, Jeff Bezos https://www.forbes.com/profile/jeff-bezos/?sh=7af1f5a71b23, and Richard Branson https://www.en.wikipedia.org/wiki/Richard_Branson would fund their own private space programs. But after several setbacks, stock in Branson’s Virgin Galactic https://www.bloomberg.com/news/newsletters/2024-04-17/virgin-galactic-richard-branson-s-space-tourism-ventureis-fading has dropped below a dollar a share. Bezos’s Blue Origin’s https://www.blueorigin.com reusable New Shepard https://www.blueorigin.com/new-shepard has yet to reach orbit on its own. In fact, its most newsworthy accomplishment to date might have been sending William Shatner to the edge of space https://www.youtube.com/watch?v=zLP5jmZkGwc as part of its space tourism program. And while SpaceX is the primary provider of launch services to NASA and the Pentagon, many have concerns about our erratic TycoonX, who is not only an unpredictable decision maker, but is also stifling other competition in the launch market https://www.nytimes.com/2024/05/28/us/politics/elon-musk-space-launch-competition.html.
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nukes next
Although private and governmental engineers around the world continue to refine chemical rocket technology, it’s clear that we’ll need something better if we are to inhabit our Solar System. Adapting nuclear reactors to power future spacecraft is the first priority. NASA launched its first and only fission reactor https://www.en.wikipedia.org/wiki/SNAP-10A into space back in 1965. Most of NASA’s probes, as well as the Apollo capsules, are powered by Radioisotope thermoelectric generators (RTG) https://www.science.nasa.gov/mission/cassini/radioisotope-thermoelectric-generator, a kind of nuclear battery. The shuttle used fuel cells after its rocket boost. The Russians use RTGs, too, but also fast fission nuclear reactors.
There are several nuclear technologies https://www.foronuclear.org/en/updates/in-depth/nuclear-technology-for-future-space-missions being developed, although they pose radiation risks for crews living on top of reactors. Nuclear thermal propulsion https://www.energy.gov/ne/articles/6-things-you-should-know-about-nuclear-thermal-propulsion provides high thrust and twice the propellant efficiency of chemical rockets. A thermal NTP drive uses a reactor to heat liquid hydrogen into a gas that is forced through a nozzle to produce thrust. Then there is Nuclear Electric Propulsion, which uses a reactor to generate electricity to power an ion thruster https://www.nasa.gov/wp-content/uploads/2015/08/ionpropfact_sheet_ps-01628.pdf. The drive removes electrons from the atoms of a neutral gas like xenon, creating positive ions which are then accelerated by an electric field through the thruster. This thrust is not as powerful as that of a thermal NTP drive, but is more efficient, which makes this technology more suitable for longer, interplanetary missions. It has already been used successfully in several deep space robot missions https://www.en.wikipedia.org/wiki/Ion_thruster. Nuclear pulse propulsion https://www.ans.org/news/article-1294/nuclear-pulse-propulsion-gateway-to-the-stars is conceptually possible but politically fraught, because it uses nuclear explosions for thrust. In the 1950s, a group of scientists including Freeman Dyson https://www.physicsworld.com/a/freeman-dyson-the-visionary-thinker-and-maverick-scientist-who-challenged-authority proposed Project Orion https://medium.com/looking-up/project-orion-5c29e5477a99. A ship that rode the shockwaves of successive atomic explosions promised to be much cheaper and faster than the chemical rockets that NASA was using. The specs were so spectacular that Dyson later recalled the team bragging, “Mars by 1965, Saturn by 1970.” But radiation was again the problem, not only for the astronauts but also for the launch team and any number of innocent bystanders. A catastrophic failure at launch (which some cynical rocket scientists call “a rapid unscheduled disassembly”) could result in radioactive fallout spreading as much as two hundred miles downwind. In any event, the nuclear test-ban treaty signed in August 1963 made Project Orion illegal under international law and the project was canceled.
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later, much later
A less explosive and more elegant spacecraft using solar sails https://www.nasa.gov/general/nasa-next-generation-solar-sail-boom-technology-ready-for-launch after being boosted to orbit might someday carry astronauts to the farthest edges of the Solar System—or even the stars. Solar sailing hearkens back to an ancient technology that served humanity throughout recorded history and propelled it into the Age of Exploration https://www.history.com/news/navigational-tools-ships-age-exploration. Instead of riding the wind, solar sails rely on the radiation pressure exerted by sunlight. A spacecraft is attached to an enormous mirror; when the photons in sunlight reach the mirror they bounce off. Although photons don’t have mass, they do have momentum, and the bounce transfers some of that momentum to the mirrorsail. With continuous sunlight comes a low, but continuous thrust. No other fuel is needed to propel the spacecraft. The key limiting factor in this scheme is the size of the mirrorsail. Small solar sailing demonstration projects have successfully shown proof of concept, most recently in 2022. The Planetary Society’s LightSail 2 https://www.planetary.org/sci-tech/lightsail was tiny, with a core of about 12x4x4 inches—about the size of a Barbie doll—pulled by a sail the size of a boxing ring. Practical solar sails for spacecraft big enough to carry astronauts will have to be fabricated of extremely lightweight material and be many kilometers across. Exactly how big is one of the many questions about solar sailing that have yet to be answered. Of what material can such sails be made? How will they be deployed and held in position? How can they be angled to best catch sunlight? How will they be affected by dust and gas? Despite these design problems, solar sailing is the only technology on the horizon that has the potential to send humans to the stars.
And this, after all, would seem to be the goal. Space aficionados and science fiction writers alike will often argue that if humanity has a future in space, it will have to expand beyond the limits of the Solar System. But just because we can see the stars doesn’t mean we can get to them.
Consider How Long Would It Take to Travel to the Nearest Star? https://www.universetoday.com/15403/how-long-would-it-take-to-travel-to-the-nearest-star/, a sober-minded assessment not only of the distance involved in getting to Proxima Centauri, but also of the potential of many of the spacecraft discussed above. Here’s the bad news on NTP (Nuclear Thermal Propulsion): “But adjusted for a one-way journey to Proxima Centauri, a nuclear rocket would still take centuries to reach a fraction of the speed of light. It would then require several decades of travel time, followed by many more centuries of deceleration before reaching its destination. All told, we’re still talking about one thousand years before it reaches its destination.”
This is why the generation starship https://www.sf-encyclopedia.com/entry/generation_starships is a science fiction sub-genre. We considered the improbability of UFOs in a previous column when Officer Einstein https://www.en.wikipedia.org/wiki/Albert_Einstein warned us about the speed limit of the universe https://www.universetoday.com/38040/speed-of-light-2/: 186,000 miles (300,000 kilometers) per second. Even attempting to accelerate a spacecraft to a significant fraction of the speed of light would require energy generation well beyond our current abilities. So not only humans, but also our hypothetical alien neighbors, will need to take the slow boat to the stars https://www.youtube.com/watch?v=H2f0Wd3zNj0, building closed-ecosystem arks in which a population of astronaut/colonists live their lives traveling to an interstellar destination in the hopes that their offspring or great-offspring or great-great-offspring will find what they are seeking. The design of the life support systems of such a spacecraft will be as technically difficult as the design of its drives, not to mention the ethically troublesome impacts of the journey on its crew. And to the best of our knowledge, aliens have yet to sign on for a generation ship visit to Earth.
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exit
After a lifetime of dreaming and writing about space travel, I’m resigned to the fact that the young treehouse astronaut I once was will never check a trip into orbit off his bucket list. That is, unless some billionaire Space Cowboy decides to gift me a golden ticket on one of their corporate spacecraft. Which I’d eagerly accept—don’t get me wrong! Sure, I’m mindful of the warnings from space medicine researchers about the dangers of spending time in space https://www.nasa.gov/hrp/hazards. But I’d accept a stiff dose of Galactic Cosmic Radiation https://www.nasa.gov/missions/analog-field-testing/why-space-radiation-matters for the chance to look out a real window at the final frontier.
Copyright © 2025 James Patrick Kelly