照片来源 Dan Winters ；星云图由 Ash Thorp 提供
太空，毫无疑问，比烟波浩淼的大海更加凶险；逃离地球引力，跟划离海岸边相比，需要付出更多的努力。不过，那些船是他们那个时代的尖端技术。 航海者们精心策划着他们这昂贵的、危险的旅程， 他们中的许多人在试图超越地平线时，付出了生命的代价。为什么还要继续这样做?
Humanity was born on Earth. Are we going to stay here? I suspect—I hope—the answer is no. —Ann Leckie
Ann Leckie is the Hugo- and Nebula-award-winning author ofAncillary Justice.
Gravity’s a Drag
Getting off Earth is a little like getting divorced: You want to do it quickly, with as little baggage as possible. But powerful forces conspire against you—specifically, gravity. If an object on Earth’s surface wants to fly free, it needs to shoot up and out at speeds exceeding 25,000 mph.
That takes serious oomph—read: dollars. It cost nearly $200 million just to launch the Mars Curiosity rover, about a tenth of the mission’s budget, and any crewed mission would be weighed down by the stuff needed to sustain life. Composite materials like exotic-metal alloys and fibered sheets could reduce the weight; combine that with more efficient, more powerful fuel mixtures and you get a bigger bang for your booster.
But the ultimate money saver will be reusability. “As the number of flights increases, economies of scale kick in,” says Les Johnson, a technical assistant at NASA’s Advanced Concepts Office. “That’s the key to getting the cost to drop dramatically.” SpaceX’s Falcon 9, for example, was designed to relaunch time and again. The more you go to space, the cheaper it gets. —Nick Stockton
Our Ships Are Way Too Slow
Hurtling through space is easy. It’s a vacuum, after all; nothing to slow you down. But getting started? That’s a bear. The larger an object’s mass, the more force it takes to move it—and rockets are kind of massive. Chemical propellants are great for an initial push, but your precious kerosene will burn up in a matter of minutes. After that, expect to reach the moons of Jupiter in, oh, five to seven years. That’s a heck of a lot of in-flight movies. Propulsion needs a radical new method. Here’s a look at what rocket scientists now have, or are working on, or wish they had. —Nick Stockton
problem: space junk
It’s a Minefield Up There
Congratulations! You’ve successfully launched a rocket into orbit. But before you break into outer space, a rogue bit of broke-ass satellite comes from out of nowhere and caps your second-stage fuel tank. No more rocket.
This is the problem of space debris, and it’s very real. The US Space Surveillance Network has eyes on 17,000 objects—each at least the size of a softball—hurtling around Earth at speeds of more than 17,500 mph; if you count pieces under 10 centimeters, it’s closer to 500,000 objects. Launch adapters, lens covers, even a fleck of paint can punch a crater in critical systems.
Whipple shields—layers of metal and Kevlar—can protect against the bitsy pieces, but nothing can save you from a whole satellite. Some 4,000 orbit Earth, most dead in the air. Mission control avoids dangerous paths, but tracking isn’t perfect.
Pulling the sats out of orbit isn’t realistic—it would take a whole mission to capture just one. So starting now, all satellites will have to fall out of orbit on their own. They’ll jettison extra fuel, then use rocket boosters or solar sails to angle down and burn up on reentry. Put decommissioning programs in 90 percent of new launches or you’ll get the Kessler syndrome: One collision leads to more collisions until there’s so much crap up there, no one can fly at all. That might be a century hence—or a lot sooner if space war breaks out. If someone (like China?) starts blowing up enemy satellites, “it would be a disaster,” says Holger Krag, head of the Space Debris Office at the European Space Agency. Essential to the future of space travel: world peace. —Jason Kehe
There’s No GPS for Space
The Deep Space Network, a collection of antenna arrays in California, Australia, and Spain, is the only navigation tool for space. Everything from student-project satellites to the New Horizons probe meandering through the Kuiper Belt depends on it to stay oriented. An ultraprecise atomic clock on Earth times how long it takes for a signal to get from the network to a spacecraft and back, and navigators use that to determine the craft’s position.
But as more and more missions take flight, the network is getting congested. The switchboard is often busy. So in the near term, NASA is working to lighten the load. Atomic clocks on the crafts themselves will cut transmission time in half, allowing distance calculations with a single downlink. And higher-bandwidth lasers will handle big data packages, like photos or video messages.
The farther rockets go from Earth, however, the less reliable this method becomes. Sure, radio waves travel at light speed, but transmissions to deep space still take hours. And the stars can tell you where to go, but they’re too distant to tell you where you are. For future missions, deep-space navigation expert Joseph Guinn wants to design an autonomous system that would collect images of targets and nearby objects and use their relative location to triangulate a spaceship’s coordinates—no ground control required. “It’ll be like GPS on Earth,” Guinn says. “You put a GPS receiver on your car and problem solved.” He calls it a deep-space positioning system—DPS for short. —Katie M. Palmer
Space Turns You Into a Bag of Cancer
Outside the safe cocoon of Earth’s atmosphere and magnetic field, subatomic particles zip around at close to the speed of light. This is space radiation, and it’s deadly. Aside from cancer, it can also cause cataracts and possibly Alzheimer’s.
When these particles knock into the atoms of aluminum that make up a spacecraft hull, their nuclei blow up, emitting yet more superfast particles called secondary radiation. “You’re actually making the problem worse,” says Nasser Barghouty, a physicist at NASA’s Marshall Space Flight Center.
A better solution? One word: plastics. They’re light and strong, and they’re full of hydrogen atoms, whose small nuclei don’t produce much secondary radiation. NASA is testing plastics that can mitigate radiation in spaceships or space suits.
Or how about this word: magnets. Scientists on the Space Radiation Superconducting Shield project are working on a magnesium diboride superconductor that would deflect charged particles away from a ship. It works at –263 degrees Celsius, which is balmy for superconductors, but it helps that space is already so damn cold. —Sarah Zhang
problem: food and water
Mars Has No Supermarkets
Lettuce got to be a hero last August. That’s when astronauts on the ISS ate a few leaves they’d grown in space for the first time. But large-scale gardening in zero g is tricky. Water wants to float around in bubbles instead of trickling through soil, so engineers have devised ceramic tubes that wick it down to the plants’ roots. “It’s like a Chia pet,” says Raymond Wheeler, a botanist at Kennedy Space Center. Also, existing vehicles are cramped. Some veggies are already pretty space-efficient (ha!), but scientists are working on a genetically modified dwarf plum tree that’s just 2 feet tall. Proteins, fats, and carbs could come from a more diverse harvest—like potatoes and peanuts.
All that’s for naught, though, if you run out of water. (On the ISS, the pee-and-water recycling system needs periodic fixing, and interplanetary crews won’t be able to rely on a resupply of new parts.) GMOs could help here too. Michael Flynn, an engineer at NASA Ames Research Center, is working on a water filter made of genetically modified bacteria. He likens it to how your small intestine recycles what you drink. “Basically you are a water recycling system,” he says. “with a useful life of 75 or 80 years.” This filter would continually replenish itself, just like your innards do. —Sarah Zhang
problem: bone and muscle wasting
Zero Gravity Will Transform You into Mush
Weightlessness wrecks the body: It makes certain immune cells unable to do their jobs, and red blood cells explode. It gives you kidney stones and makes your heart lazy. Astronauts on the ISS exercise to combat muscle wasting and bone loss, but they still lose bone mass in space, and those zero-g spin cycles don’t help the other problems. Artificial gravity would fix all that.
In his lab at MIT, former astronaut Laurence Young is testing a human centrifuge: Victims lie on their side on a platform and pedal a stationary wheel as the whole contraption spins around. The resulting force tugs their feet—just like gravity, but awkward.
Young’s machine is too cramped to use for more than an hour or two a day, though, so for 24/7 gravity, the whole spacecraft will have to become a centrifuge. A spinning spaceship could be shaped like a dumbbell, with two chambers connected by a truss. As it gets easier to send more mass into space, designers could become more ambitious—but they don’t have to reinvent the wheel. Remember the station in 2001: A Space Odyssey? The design has been around since 1903. —Sarah Zhang
problem: mental health
Interplanetary Voyages Are a Direct Flight to Space Madness
When physicians treat stroke or heart attack, they sometimes bring the patient’s temperature way down, slowing their metabolism to reduce the damage from lack of oxygen. It’s a trick that might work for astronauts too. Which is good, because to sign up for interplanetary travel is to sign up for a year (at least) of living in a cramped spacecraft with bad food and zero privacy—a recipe for space madness. That’s why John Bradford says we should sleep through it. President of the engineering firm SpaceWorks and coauthor of a report for NASA on long missions, Bradford says cold storage would be a twofer: It cuts down on the amount of food, water, and air a crew would need and keeps them sane. “If we’re going to become a multiplanet species,” he says, “we’ll need a capability like human stasis.” Sleep tight, voyagers. —Sarah Zhang
Crashing Is Not an Option
Planet, ho! You’ve been in space for months. Years, maybe. Now a formerly distant world is finally filling up your viewport. All you have to do is land. But you’re careening through frictionless space at, oh, call it 200,000 mph (assuming you’ve cracked fusion). Oh yeah, and there’s the planet’s gravity to worry about. If you don’t want your touchdown to be remembered as one small leap for a human and one giant splat for humankind, follow these simple steps. —Nick Stockton
You Can’t Take a Mountain of Aluminum Ore With You
When space caravans embark from Earth, they’ll leave full of supplies. But you can’t take everything with you. Seeds, oxygen generators, maybe a few machines for building infrastructure. But settlers will have to harvest or make everything else.
Luckily, space is far from barren. “Every planet has every chemical element in it,” says Ian Crawford, a planetary scientist at Birbeck, University of London, though concentrations differ. The moon has lots of aluminum. Mars has silica and iron oxide. Nearby asteroids are a great source of carbon and platinum ores—and water, once pioneers figure out how to mine the stuff. If blasters and drillers are too heavy to ship, they’ll have to extract those riches with gentler techniques: melting, magnets, or metal-digesting microbes. And NASA is looking into a process that can 3-D-print whole buildings—no need to import special equipment.
In the end, a destination’s resources will shape settlements, which makes surveying the drop zone critical. Just think of the moon’s far side. “It’s been pummeled by asteroids for billions of years,” says Anita Gale, a space shuttle engineer. “Whole new materials could be out there.” Before humanity books a one-way ticket to Kepler-438b, it’ll have to study up. —Chelsea Leu
We Can’t Do Everything By Ourselves
Dogs helped humans colonize Earth, but they’d survive on Mars about as well as we would. To spread out on a new world, we’ll need a new best friend: a robot.
See, settling takes a lot of grunt work, and robots can dig all day without having to eat or breathe. Theoretically, at least. Current prototypes— bulky, bipedal bots that mimic human physiognomy—can barely walk on Earth. So automatons will have to be everything we aren’t—like, say, a lightweight tracked bot with backhoe claws for arms. That’s the shape of one NASA machine designed to dig for ice on Mars: Its two appendages spin in opposite directions, keeping it from flipping over as it works.
Still, humans have a big leg up when it comes to fingers. If a job requires dexterity and precision, you want people doing it—provided they have the right duds. Today’s space suit is designed for weightlessness, not hiking on exoplanets. NASA’s prototype Z-2 model has flexible joints and a helmet that gives a clear view of whatever delicate wiring needs fixing. When the job’s done, just hop on an autonomous transporter to get home. Attaboy, Rover. —Matt Simon
problem: space is big
Warp Drives Don’t Exist … Yet
The fastest thing humans have ever built is a probe called Helios 2. It’s dead now, but if sound traveled in space, you’d hear it screaming as it whips around the sun at speeds of more than 157,000 miles per hour. That’s almost 100 times faster than a bullet, but even at that velocity it would take some 19,000 years to reach Earth’s first stellar neighbor, Alpha Centauri. It’d be a multigenerational ship, and nobody dreams of going to space because it’s a nice place to die of old age.
To beat the clock, you need power—and lots of it. Maybe you could mine Jupiter for enough helium-3 to fuel nuclear fusion—after you’ve figured out fusion engines. Matter-antimatter annihilation is more scalable, but smashing those pugilistic particles together is dangerous. “You’d never want to do that on Earth,” says Les Johnson, technical assistant for NASA’s Advanced Concepts Office, which works on crazy starship ideas. “You do that in deep space, so if you have an accident, you don’t destroy a continent.” Too intense? How about solar power? All you’d need is a sail the size of Texas.
Far more elegant would be hacking the universe’s source code—with physics. The theoretical Alcubierre drive would compress space in front of your craft and expand space behind it so the stuff in between—where your ship is—effectively moves faster than light. Tweaking the Alcubierre equations gets you a Krasnikov tube, an interstellar subway that shortens your return trip.
All aboard? Not quite. Humanity will need a few more Einsteins working at places like the Large Hadron Collider to untangle all the theoretical knots. “It’s entirely possible that we’ll make some discovery that changes everything,” Johnson says. “But you can’t count on that breakthrough to save the day.” If you want eureka moments, you need to budget for them. That means more cash for NASA— and the particle physicists. Until then, Earth’s space ambitions will look a lot like Helios 2: stuck in a futile race around the same old star. —Nick Stockton
problem: THERE’S ONLY ONE EARTH
Let’s Not Boldly Go—Let’s Boldly Stay
A couple decades back, sci-fi author Kim Stanley Robinson sketched out a future utopia on Mars built by scientists from an overpopulated, overextended Earth. His Mars trilogy made a forceful case for colonization of the solar system. But, really, other than science, why should we go to space?
The need to explore is built into our souls, goes one argument—the pioneer spirit and manifest destiny. But scientists don’t talk about pioneers anymore. “You did hear that frontier language 20, 30 years ago,” says Heidi Hammel, who helps set exploration priorities at NASA. But since the New Horizons probe passed by Pluto last July, “we’ve explored every type of environment in the solar system at least once,” she says. Humans could still go dig in the dirt to study distant geology—but when robots can do it, well, maybe not.
As for manifest destiny? Historians know better. Western expansion was a vicious land grab, and the great explorers were mostly in it for resources or treasure. Human wanderlust expresses itself only in the service of political or economic will.
Of course, Earth’s impending destruction could provide some incentive. Deplete the planet’s resources and asteroid-belt mining suddenly seems reasonable. Change the climate and space provides room for humanity (and everything else).
But that’s a dangerous line of thinking. “It creates a moral hazard,” Robinson says. “People think if we fuck up here on Earth we can always go to Mars or the stars. It’s pernicious.” His latest book, Aurora, again makes a forceful case about settlement beyond the solar system: You probably can’t. As far as anyone knows, Earth is the only habitable place in the universe. If we’re going to leave this planet, let’s go because we want to—not because we have to. —Adam Rogers
This article appears in the March 2016 issue.
Illustrations by 520 Design; Nebula by Ash Thorp