Imagine a starship—a vessel capable of ferrying human beings from one solar system to another. Would it have wings and a cockpit? Or would it look like an aircraft carrier hauled out into the void and fitted with flame-belching rockets and glowing ion drives?

Science fiction has offered us all sorts of visions of interstellar spacecraft, from avian-inspired Klingon birds of prey to hulking masses such as the Borg cube. In general, sci-fi leans toward sleek designs with lines borrowed from planes or cars, since those are the kinds of looks we've been conditioned to think of as "fast." But if there's no air in space, why make things aerodynamic? Does it matter what a spacecraft looks like?

Yes, it turns out, and it depends upon what kind of space travel you're looking to undertake. The reality of starship design is more complex than anything Hollywood has dreamed up and implanted in our collective unconsciousness.

While a manned interstellar mission isn't exactly on NASA's upcoming schedule, researchers haven't abandoned the topic to science fiction. In fact, the 100 Year Starship initiative—which began as a DARPA-funded contest to lay the foundations for a flight across the stars, gathering physicists, entrepreneurs, and anyone seriously interested in long-distance space travel—just finished its annual symposium this past weekend.

One of the participants of the 100 Year Starship project is Marc G. Millis, founder of the Tau Zero Foundation. The foundation has proposed candidate technologies and designs, including the Icarus unmanned fusion-powered probe, which would accelerate (theoretically, of course) to one-tenth or one-fifth the speed of light. Icarus, as it's currently envisioned, isn't the sleekest space ride. The skyscraper-size behemoth is comprised almost entirely of rows and clusters of spherical fuel tanks. But according to Millis, Icarus isn't a definitive, catch-all prediction of what an interstellar craft might look like. It's simply the design that might make sense to build first.

We asked Millis, who once led NASA's Breakthrough Propulsion Physics Project, to take us through the basics of starship design.

Starships Aren't Spaceplanes

One look at the Icarus design—or its predecessor, the Daedalus—and it's clear what starships don't need: wings. The only real-world spacecraft that bother with wings are ones designed to make regular landings on runways, such as the retired Space Shuttle, the upcoming Lynx (a suborbital two-seater from XCOR) or the Dream Chaser, an in-development orbital craft from Sierra Nevada. And wings aren't even required for landings. Like the Russian Soyuz capsule, SpaceX's Dragon currently splashes down in the ocean (though SpaceX plans to move toward rocket-powered launchpad landings).

In both the near and far-term future, experts such as Millis imagine interstellar vessels won't spend much of their time in an atmosphere. Perhaps the small ships that carry people from surface to starship will remain winged, but truly interstellar vehicles can scrap aerodynamics and all of the design principles that are beholden to reducing wind resistance. A starship doesn't need to be sleek or have a pointy nose—even the stocky Battlestar Galactica is pointlessly aircraft-shaped. If anything, the equivalent Cylon ships in the rebooted TV series are more rational interstellar travelers, with their spindly arms and flagrant disregard for the entire air-centric history of aerospace.

Surviving Sublight

Predicting what the first unmanned starships might look like is relatively simple. In the case of Icarus, for example, the entire structure is devoted to propulsion. It's a colossal rocket, albeit a weird fusion-powered one.

Millis says the first person-carrying starships, however, will be dominated by the technologies that keep those passengers alive. Consider gravity, a necessity on long-distance spaceflights. In prolonged zero-g, the human body erodes, losing bone and muscle density. "With the physics we know, you create gravity with a giant centrifuge, a rotating cabin, basically," Millis says. The spinning disc on the Jupiter-bound Discovery One in 2001: A Space Odyssey illustrated this concept well, but Millis says that to better simulate Earth gravity, the real thing would actually have to be much larger. The smaller the centrifuge, the less consistent the centrifugal force is across a crew member's body—the head, in other words, will feel lighter than the feet. Aside from being disoriented by chronic light-headedness, if the goal is to re-create the way blood circulates under the influence of gravity, consistency is key.

Discovery One, from 2001: A Space Odyssey

Of course, mankind can't survive on gravity alone. A starship designed to keep its occupants alive for years, decades, or even centuries, would require systems unheard of in current spacecraft. Sections for growing crops or livestock, for example, could dwarf more traditional compartments. And spacious recreational facilities, with enough room and resources to support vast interior parks, might be crucial for fighting off the existential crisis of spending an entire lifetime crammed inside a spacecraft. What might seem laughable today, and a colossal waste of mass, could become the most defining feature of a vessel filled not with astronauts, but a wider swath of humanity—including, quite possibly, children born en route. Suddenly, a giant, rotating playground bisecting your vessel isn't such a bad idea.

The look of your starship depends a lot on your method of transportation, too, and all of the proposed methods of interstellar propulsion carry their own problems. Anything that requires the ship to have a massive surface area—such as using a sail propelled by the sun's photons or onboard lasers—would have to contend with intergalactic dust. There isn't much material out there in space, but even tiny particles are a hazard to vessels moving at some significant fraction of the speed of light. Those dust particles could cut through a solar sail; perhaps the crew would have to replace or repair the sail when it comes too perforated.

Ikaros, Japan's solar sail project. Credit: JAXA

Perforated sails might be replaceable, but all fast-moving starships will need to worry about dust. Forget the layouts of Firefly's Serenity or the more recent eponymous vessel from Prometheus, with their swooping birdlike profiles and aircraft-style front-mounted cockpits. The risk of dust impacts probably means turning crew compartments into bunkers, and sticking people and any essential systems behind redundant layers of physical shielding. The result would seem ugly by sci-fi standards closer to the Icarus from 2007's Sunshine (not to be confused with the work-in-progress concept), with its solar shield making it look more like a giant umbrella than a bird of prey.

The more you think about it, the less inherently sexy the starship becomes. Even Star Trek's Enterprise is a star-hopping Ferrari compared to the industrial monstrosities that might actually make the trip survivable, based on our current grasp of physics. The Federation's mastery of time, space, and everything in between allows writers to ignore the dangers of galactic cosmic ray bombardment, using various kinds of force fields to ward off disaster as opposed to the staggeringly thick hulls filled with water that a real starship would probably need. And while impulse drives apparently dump 100 percent of their energy into thrust, a real vessel, beholden to the laws of thermodynamics, would likely be bristling with a dizzying array of panels to radiate the excess heat generated by propulsion systems.

Even if humanity manages to upend physics and learns to directly manipulate gravity, matter, and the properties of time and space, Millis says, "the geometry of the ship will be heavily dependent on those technological breakthroughs." For example, gravity-producing plates might call for wide halls with extremely low ceilings, whereas a field that generates gravity in a long cylinder might lead to a skyscraper-shaped starship. Both concepts, says Millis, "are in the realm of playful speculation," but whatever shapes those miraculous devices take, the end-result is likely to be stranger than fiction.

What Color Is Your Time Machine?

Strangest of all is the faster-than-light (FTL) starship. All his life, Millis has been running the number crazy propulsion theories, including in his 2009 book The Frontiers of Propulsion Science, which he's trying to adapt into a more mainstream version through a Kickstarter project. Since the 1930s, he says sci-fi has remained fixated on concepts like warp drives and hyperspace, when it's the crazier-sounding technologies that might actually realize FTL travel.

Take, for example, the old soap-boat experiment: Put a drop of soap behind a toy boat, and watch it scoot across the surface of the bath. In 1996 physicist Miguel Alcubierre proposed a warp drive that works much the same way—it focuses not on the notional vessel and its own built-in propulsion, but on the distortion of space-time into a ship-propelling wave.

Millis says the only equations that support FTL involve space-time sleight-of-hand, such as wormholes or Alcubierre's warp effect. And a vessel that can cheat its way between the stars isn't exactly a "starship" in the way we think of them. It isn't going fast, so there's no real risk of explosive dust impacts. By its very nature, it isn't traveling for very long durations, so never mind the cavernous hydroponic farms. It might even be a single-stage craft, designed to be towed before and after its bizarre shortcut and with almost no traditional propulsion of its own.

In other words: The vessel look like just about anything, so there's no reason to assume it will look like something that flies. The FTL starship is more of a time machine than a rocket, a device capable of impossibly high-energy physics, none of which involves thermodynamic thrust. If a sub-light interstellar spacecraft has all the sex appeal of a nuclear powerplant, the warp-class version is likely to have the swooping curves of 10 daisy-chained Large Hadron Colliders.

Science Fact?

Millis isn't the kind of physicist who has to be baited into discussing warp drives and wormholes, and considering that his foundation is named after Tau Zero, Poul Anderson's 1970 novel about an interstellar colonization mission gone awry, he doesn't dismiss the role of science fiction in his life's work. "It gives you starting points to picture these capabilities, to imagine and list them all," says Millis. "Then you can distill them down, extract what the workable questions are. That's where you can transition to science investigation."

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He says the closest that Hollywood has come to capturing the nonspaceship weirdness of a notional FTL vehicle is 1997's Contact. If you've seen the movie you know "vehicle" isn't the right term; the character call their FTL device the "machine." It entails both a single-seat spherical pod and the mass of rotating, overlapping rings that the pod (with Jodi Foster inside) drops into.

As for slower-than-light starships, Millis was impressed by the design featured (briefly) in the beginning of James Cameron's Avatar, because of its massive heat radiators, bigger than any ship he's seen on the large or small screen. "If you have excess energy, which is usually in the form of low-grade heat, you need huge radiators to keep the vehicle from destroying itself," he says.

Millis doesn't expect Hollywood to nail the details of space travel. Too much realism could undercut what science fiction does best—inspire new generations of pioneers to tackle problems that can't be solved in their lifetimes. "We need those pioneers. People don't want to start solving a problem till it looks like it can be solved," Millis says. "But if we're dealing with the survival of humanity, do we really want to procrastinate? If it's going to take a couple centuries to figure it out, shouldn't we start now, instead of when the asteroid is spotted, and we have three years to evacuate?"

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