Space Chronicles: Facing the Ultimate Frontier (24 page)

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CHAPTER TWENTY-THREE

PROPULSION FOR DEEP SPACE
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L
aunching
a spacecraft is now a routine feat of engineering. Attach the fuel tanks and rocket boosters, ignite the chemical fuels, and away it goes.

But today’s spacecraft quickly runs out of fuel. So, left to itself, it cannot slow down, stop, speed up, or make serious changes in direction. With its trajectory choreographed entirely by the gravity fields of the Sun, the planets, and their moons, the craft can only fly past its destination, like a fast-moving tour bus with no stops on its itinerary—and the riders can only glance at the passing scenery.

If a spacecraft can’t slow down, it can’t land anywhere without crashing, which is not a common objective of aerospace engineers. Lately, however, engineers have been getting clever about fuel-deprived craft. In the case of the Mars rovers, their stupendous speed toward the Red Planet was slowed by aerobraking through the Martian atmosphere. That meant they could land with the help of nothing more than heat shields, parachutes, and airbags.

Today, the biggest challenge in aeronautics is to find a lightweight and efficient means of propulsion, whose punch per pound greatly exceeds that of conventional chemical fuels. With that challenge met, a spacecraft could leave the launchpad with fuel reserves onboard, and scientists could think more about celestial objects as places to visit than as planetary peep shows.

Fortunately, human ingenuity doesn’t often take no for an answer. Legions of engineers are ready to propel us and our robotic surrogates into deep space with a variety of innovative engines. The most efficient among them would tap energy from a nuclear reactor by bringing matter and antimatter into contact with each other, thereby converting all their mass into propulsion energy, just as
Star Trek
’s antimatter engines did. Some physicists even dream of traveling faster than the speed of light by somehow tunneling through warps in the fabric of space and time.
Star Trek
didn’t miss that one either: the warp drives on the starship USS Enterprise were what enabled Captain Kirk and his crew to speed across the galaxy during the TV commercials.

A
cceleration 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 CO
₂
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.

I
n 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 not. 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.

S
ince 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.