Acceleration can be gradual and prolonged, or it can come from a brief, spectacular blast. Only a major blast can propel a spacecraft off the ground. You’ve got to have at least as many pounds of thrust as the weight of the craft itself. Otherwise, the thing will just sit there on the pad. After that, if you’re not in a big rush—and if you’re sending cargo rather than crew to the distant reaches of the solar system—there’s no need for spectacular acceleration.
In October 1998 an eight-foot-tall, half-ton spacecraft called Deep Space 1 launched from Cape Canaveral, Florida. During its three-year mission, Deep Space 1 tested a dozen innovative technologies, including a propulsion system equipped with ion thrusters—the kind of system that becomes useful at great distances from the launchpad, where low but sustained acceleration eventually yields very high speeds.
Ion-thruster engines do what conventional spacecraft engines do: they accelerate propellant (in this case, a gas) to very high speeds and channel it out a nozzle. In response, the engine, and thus the rest of the spacecraft, recoils in the opposite direction. You can do this science experiment yourself: While you’re standing on a skateboard, let loose a CO2 fire extinguisher (purchased, of course, for this purpose). The gas will go one way; you and the skateboard will go the other way.
But ion thrusters and ordinary rocket engines part ways in their choice of propellant and their source of the energy that accelerates it. Deep Space 1 used electrically charged (ionized) xenon gas as its propellant, rather than the liquid hydrogen-oxygen combo burned in the space shuttle’s main engine. Ionized gas is easier to manage than explosively flammable chemicals. Plus, xenon happens to be a noble gas, which means it won’t corrode or otherwise interact chemically with anything. For sixteen thousand hours, using less than four ounces of propellant a day, Deep Space 1’s foot-wide, drum-shaped engine accelerated xenon ions across an electric field to speeds of twenty-five miles per second and spewed them from its nozzle. As anticipated, the recoil per pound of fuel was ten times greater than that of conventional rocket engines.
In space as on Earth, however, there is no such thing as a free lunch—not to mention a free launch. Something had to power those ion thrusters on Deep Space 1. Some investment of energy had to first ionize the xenon atoms and then accelerate them. That energy came from electricity, courtesy of the Sun.
For touring the inner solar system, where light from the Sun is strong, the spacecraft of tomorrow can use solar panels—not for the propulsion itself, but for the electric power needed to drive the equipment that manages the propulsion. Deep Space 1, for instance, had folding solar “wings” that, when fully extended, spanned almost forty feet—about five times the height of the spacecraft itself. The arrays on them were a combination of 3,600 solar cells and more than seven hundred cylindrical lenses that focused sunlight on the cells. At peak power, their collective output was more than two thousand watts, enough to operate only a hair dryer or two on Earth but plenty for powering the spacecraft’s ion thrusters.
Other, more familiar spacecraft—such as the deorbited and disintegrated Soviet space station Mir and the sprawling International Space Station (ISS)—have also depended on the Sun for the power to operate their electronics. Orbiting about 250 miles above Earth, the ISS carries more than an acre’s worth of solar panels. For about a third of every ninety-minute orbit, as Earth eclipses the Sun, the station orbits in darkness. So by day, some of the collected solar energy gets channeled into storage batteries for later use during dark hours.
Although neither Deep Space 1 nor the ISS has used the Sun’s rays to propel itself, direct solar propulsion is far from impossible. Consider the solar sail, a gossamer, somewhat kitelike form of space propulsion that, once aloft, will accelerate because of the collective thrust of the Sun’s photons, or particles of light, continually reflecting off the sail’s shiny surfaces. As they bounce, the photons induce the craft to recoil. No fuel. No fuel tanks. No exhaust. No mess. You can’t get greener than that.
Having envisioned the geosynchronous satellite, Sir Arthur C. Clarke went on to envision the solar sail. For his 1964 story “The Wind from the Sun,” he created a character who described how it would work:
Hold your hands out to the sun. What do you feel? Heat, of course. But there’s pressure as well—though you’ve never noticed it, because it’s so tiny. Over the area of your hands, it only comes to about a millionth of an ounce. But out in space, even a pressure as small as that can be important—for it’s acting all the time, hour after hour, day after day. Unlike rocket fuel, it’s free and unlimited. If we want to, we can use it; we can build sails to catch the radiation blowing from the sun.
In the 1990s, a group of US and Russian rocket scientists who preferred to collaborate rather than contribute to mutual assured destruction (aptly known as MAD) began working on solar sails through a privately funded collaboration led by the Planetary Society. The fruit of their labor, Cosmos 1, was an engineless, 220-pound spacecraft shaped like a supersize daisy. This celestial sailboat folded inside an unarmed intercontinental ballistic missile left over from the Soviet Union’s Cold War arsenal and was launched from a Russian submarine. Cosmos 1 had a computer at its center and eight reflective, triangular sail blades made of 0.0002-inch-thick Mylar—much thinner than a cheap trash bag—and reinforced with aluminum. When unfurled in space, each blade would extend fifty feet and could be individually angled to steer and sail the craft. Alas, the rocket engine failed little more than a minute after launch, and the furled sail itself, apparently still attached to the rocket, fell into the Barents Sea.
But engineers don’t stop working just because their early efforts fail. Today not only the Planetary Society but also NASA, the US Air Force, the European Space Agency, universities, corporations, and start-ups are enthusiastically investigating designs and uses of solar sails. Philanthropists have come forth with million-dollar donations. International conferences on solar sailing now take place. And in 2010, space sailors celebrated their community’s first true success: a 650-square-foot, 0.0003-inch-thick sail named IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun), designed and operated by the Japan Aerospace Exploration Agency, JAXA. The sail entered solar orbit on May 21, finished unfurling itself on June 11, and passed Venus on December 8. Meanwhile, the Planetary Society anticipates a launch of its LightSail-1, and NASA is working on a miniature demonstration craft named Nano-Sail-D, which may point the way toward using solar sails as parachutes to tow defunct satellites out of orbit and out of harm’s way.
So let’s look on the sunny side. Having entered space, a lightweight solar sail could, after a couple of years, accelerate to a hundred thousand miles an hour. That’s the remarkable effect of a low but steady acceleration. Such a craft could escape from Earth orbit (where it was lofted by conventional rockets) not by aiming for a destination but by cleverly angling its blades, as does a sailor on a ship, so that it ascends to ever larger orbits around Earth. Eventually its orbit could become the same as that of the Moon, or Mars, or something beyond.
Obviously a solar sail would not be the transportation of choice for anybody in a hurry to receive supplies, but it would certainly be fuel efficient. If you wanted to use it as, say, a low-cost food-delivery van, you could load it up with dried fruit, ready-to-eat breakfast cereals, Twinkies, Cool Whip, and other edible items of extremely high shelf life. And as the craft sailed into sectors where the Sun’s light is feeble, you could help it along with a laser, beamed from Earth, or with a network of lasers stationed across the solar system.
Speaking of regions where the Sun is dim, suppose you wanted to park a space station in the outer solar system—at Jupiter, for instance, where sunlight is only 1/27 as intense as it is here on Earth. If your Jovian space station required the same amount of solar power as the completed International Space Station, your panels would have to cover twenty-seven acres. So you would now be laying solar arrays over an area bigger than twenty football fields. I think no
t. To do complex science in deep space, to enable explorers (or settlers) to spend time there, to operate equipment on the surfaces of distant planets, you must draw energy from sources other than the Sun.
Since the early 1960s, space vehicles have commonly relied on the heat from radioactive plutonium as an electrical power supply. Several of the Apollo missions to the Moon, as well as Pioneer 10 and 11 (now about ten billion miles from Earth and destined for interstellar space), Viking 1 and 2 (to Mars), Voyager 1 and 2 (also destined for interstellar space and, in the case of Voyager 1, farther along than the Pioneers), Ulysses (to the Sun), Cassini (to Saturn), and New Horizons (to Pluto and the Kuiper Belt), among others, have all used plutonium for their radioisotope thermoelectric generators, or RTGs. An RTG is a long-lasting source of nuclear power. Much more efficient, and much more energetic, would be a nuclear reactor that could supply both power and propulsion.
Nuclear power in any form, of course, is anathema to some people. Good reasons for this view are not hard to find. Inadequately shielded plutonium and other radioactive elements pose great danger; uncontrolled nuclear chain reactions pose even greater danger. And it’s easy to draw up a list of proven and potential disasters: the radioactive debris spread across northern Canada in 1978 by the crash of the nuclear-powered Soviet satellite Cosmos 954; the partial meltdown in 1979 at the Three Mile Island nuclear power plant on the Susquehanna River near Harrisburg, Pennsylvania; the explosion at the Chernobyl nuclear power plant in 1986 in what is now Ukraine; the plutonium in old RTGs currently lying in (and occasionally stolen from) remote, decrepit lighthouses in northwestern Russia. The failure of the Fukushima Daiichi nuclear power plant on Japan’s northeast coast, struck by a 9.0 earthquake and then inundated by a horrific tsunami in March 2011, renewed every fear. Citizens’ organizations such as the Global Network Against Weapons and Nuclear Power in Space remember these and other similar events.
But so do the scientists and engineers who worked on NASA’s Project Prometheus.
Rather than deny the risks of nuclear devices, NASA turned its attention to maximizing safeguards. In 2003 the agency charged Project Prometheus with developing a small nuclear reactor that could be safely launched and could power long and ambitious missions to the outer solar system. Such a reactor was to provide onboard power and could drive an electric engine with ion thrusters—the same kind of propulsion tested in Deep Space 1.
To appreciate the advance of technology, consider the power output of the RTGs that drove the experiments on the Vikings and Voyagers. They supplied less than a hundred watts, about what your desk lamp uses. The RTGs on Cassini do a bit better, nearly three hundred watts: about the power required by a small kitchen appliance. The nuclear reactor that should have emerged from Prometheus was slated to yield ten thousand watts of usable power for its scientific instruments, enough to drive a rock concert.
To exploit the Promethean advance, an ambitious scientific mission was proposed: the Jupiter Icy Moons Orbiter, or JIMO. Its destinations were Callisto, Ganymede, and Europa—three of the four moons of Jupiter discovered by Galileo in 1610. (The fourth, Io, is studded with volcanoes and is flaming hot.) The lure of the three frigid Galilean moons was that beneath their thick crust of ice might lie vast reservoirs of liquid water that harbor, or once harbored, life.
Endowed with ample onboard propulsion, JIMO would do a “flyto,” rather than a flyby, of Jupiter eight years after launch. It would pull into orbit and systematically visit one moon at a time, perhaps even deploying landers. Powered by ample onboard electricity, suites of scientific instruments would study the moons and send data back to Earth via high-speed broadband channels. Besides efficiency, a big attraction would be safety, both structural and operational. The spacecraft would be launched with ordinary rockets, and its nuclear reactor would be launched “cold”—not until JIMO had reached escape velocity and was well out of Earth orbit would the reactor be turned on.
Sounded good. But Prometheus/JIMO died after barely having lived, becoming what a committee constituted by the National Research Council’s Space Studies Board and Aeronautics and Space Engineering Board termed, in a 2008 report titled Launching Science, a “cautionary tale.” Formally started in March 2003 as a science program, it was transferred within the year to NASA’s newly established Exploration Systems Mission Directorate. Less than a year and a half later, in the summer of 2005, after spending nearly $464 million (plus tens of millions of dollars simply to fund the preparation of the contractors’ bids), NASA canceled the program. Over the succeeding months, $90 million of its $100 million budget went for closeout costs on the canceled contracts. All that money, and yet no spacecraft and no scientific findings. Prometheus/JIMO thus stands, write the authors of Launching Science, as “an example of the risks associated with pursuing ambitious, expensive space science missions.”
Risks, cancellations, and failures are just part of the game. Engineers expect them, agencies resist them, accountants juggle them. Cosmos 1 may have dropped into the sea, and Prometheus/JIMO may have died in the cradle, but they yielded valuable technical lessons. So, hopeful cosmic travelers have no reason to stop trying or planning or dreaming about how to navigate in deep space. Today’s term of art is “in-space propulsion,” and plenty of people are still avidly pursuing its possibilities, including NASA. More efficient rockets are one approach, and so NASA is developing advanced high-temperature rockets. Better thrusters are another approach, and so NASA now has the NEXT (NASA’s Evolutionary Xenon Thruster) Ion Propulsion System, a few steps up from the system on Deep Space 1. Then there are the aforementioned solar sails. The goals of all of these technologies, individually and/or in combination, are to cut down the travel time to distant celestial bodies, increase the potential range and weight of the scientific payload, and reduce the costs.
Someday there might be wackier ways to explore within and beyond our solar system. The folks at NASA’s now-defunct Breakthrough Propulsion Physics Project, for instance, were dreaming of how to couple gravity and electromagnetism, or tap the zero-point energy states of the quantum vacuum, or harness superluminal quantum phenomena. Their inspiration came from such tales as From the Earth to the Moon, by Jules Verne, and the adventures of Buck Rogers, Flash Gordon, and Star Trek. It’s okay to think about this sort of thing from time to time. But, in my opinion, though it’s possible not to have read enough science fiction in one’s lifetime, it’s also possible to have read too much of it.
My favorite science-fiction engine is the antimatter drive. It’s 100 percent efficient: put a pound of antimatter together with a pound of matter, and they turn into a puff of pure energy, with no by-products. Antimatter is real. Credit the twentieth-century British physicist Paul A. M. Dirac for conceiving of it in 1928, and the American physicist Carl D. Anderson for discovering it five years later.
The science part of antimatter is fine. It’s the science-fiction part that presents a small problem. How do you store the stuff? Behind whose spaceship cabin or under whose bunk bed would the canister of antimatter be kept? And out of what substance would the canister be made? Antimatter and matter annihilate each other on contact, so keeping antimatter around requires portable matterless containers, such as magnetic fields shaped into magnetic bottles. Unlike the fringe propulsion ideas, where engineering chases the bleeding edge of physics, the antimatter problem is ordinary physics chasing the bleeding edge of engineering.
So the quest continues. Meanwhile, next time you’re watching a movie in which a captured spy is being questioned, think about this: The questioners hardly ever ask about agricultural secrets or troop movements. With an eye to the future, they ask about the secret rocket formula, the transportation ticket to the final frontier.
• • • CHAPTER TWENTY-FOUR
BALANCING ACTS*
The first manned spacecraft ever to leave Earth orbit was Apollo 8. This achievement remains one of the most unappreciated firsts of the twentieth century. When that moment arrived, the a
stronauts fired the third and final stage of their mighty Saturn V rocket, and the spacecraft and its three occupants rapidly reached a speed of nearly seven miles per second. As the laws of physics show, just by reaching Earth orbit the astronauts had already acquired half the energy needed to reach the Moon.
After Apollo 8’s third stage fired, engines were no longer necessary except to tune the midcourse trajectory so that the astronauts did not miss the Moon entirely. For most of its nearly quarter-million-mile journey from Earth to the Moon, the spacecraft gradually slowed as Earth’s gravity continued to out-tug the Moon’s gravity. Meanwhile, as the astronauts neared the Moon, its force of gravity grew stronger and stronger. Obviously there had to be a spot en route where the Moon’s and Earth’s opposing forces of gravity balanced precisely. And when the command module drifted across that point in space, its speed increased once again, and it accelerated toward the Moon.
If gravity were the only force to be reckoned with, then that spot would be the only place in the Earth–Moon system where the opposing forces cancel. But Earth and the Moon revolve around a common center of gravity, which lives about a thousand miles beneath Earth’s surface along the length of an imaginary line connecting the center of Earth to the center of the Moon.
When objects move in circles of any size and at any speed, they create a new force that pushes outward, away from the center of rotation. Your body feels this “centrifugal” force when you make a sharp turn in your car or when you survive amusement-park attractions that turn in circles. In a classic example of these nausea-inducing rides, you stand along the edge of a large circular platter, with your back against a perimeter wall. As the ride spins, rotating faster and faster, you feel a stronger and stronger force pinning you against the wall. It’s the sturdy wall that prevents you from being flung through the air. Soon you can’t move. That’s when they drop the floor from below your feet and turn the thing sideways and upside down. When I rode one of these as a kid, the force was so great that I could barely move my fingers: they stuck to the wall along with the rest of me. (If you actually got sick on such a ride and you turned your head sideways, the vomit would fly off at a tangent. Or it might get stuck to the wall. Worse yet, if you didn’t turn your head, it might not make it out of your mouth, owing to the extreme centrifugal forces acting in the opposite direction. Come to think of it, I haven’t seen this particular ride anywhere lately.)
Space Chronicles: Facing the Ultimate Frontier Page 18