Space Chronicles: Facing the Ultimate Frontier (25 page)

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.”

R
isks, 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 stuf
f
? 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
^*

T
he 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 astronauts 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.

I
f 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.)

Centrifugal forces arise as the simple consequence of an object’s tendency to travel in a straight line after being set in motion, and so are not true forces at all. But you can use them in calculations as though they were, as did the brilliant eighteenth-century French mathematician Joseph-Louis Lagrange, who discovered spots in the rotating Earth–Moon system where the gravity of Earth, the gravity of the Moon, and the centrifugal forces of the rotating system all balance. These special locations are known as the points of Lagrange, and there are five of them.

The first point of Lagrange (sensibly called L1) falls slightly closer to Earth than the point of pure gravitational balance. Any object placed at L1 can orbit the Earth–Moon center of gravity with the same monthly period as the Moon’s orbit and will appear to be locked in place along the Earth–Moon line. Although all forces cancel there, L1 is a point of precarious equilibrium. If the object drifts
away from
the Earth–Moon line in any direction, the combined effect of the three forces will return it to its former position. But if the object drifts
along
the Earth–Moon line ever so slightly, it will irreversibly fall toward either Earth or the Moon. It’s like a cart atop a mountain, barely balanced, a hair’s width away from rolling down one side or the other.

The second and third Lagrangian points (L2 and L3) also lie on the Earth–Moon line, but L2 lies far beyond the Moon, while L3 lies far beyond Earth in the opposite direction. Once again, the three forces—Earth’s gravity, the Moon’s gravity, and the centrifugal force of the rotating system—cancel in concert. And once again, an object placed in either spot can orbit the Earth–Moon center of gravity in a lunar month. The gravitational balance points at L2 and L3 are quite broad. So if you find yourself drifting down to Earth or the Moon, a tiny investment in fuel will bring you right back to where you were.

Although L1, L2, and L3 are respectable space places, the award for best Lagrangian points must go to L4 and L5. One of them lives far off to one side of the Earth–Moon centerline, while the other lives far off to the opposite side, and each of them represents one vertex of an equilateral triangle, with Earth and the Moon serving as the other two vertices. At L4 and L5, as with their first three siblings, forces are in equilibrium. But unlike the first three Lagrangian points, which enjoy only unstable equilibrium, the equilibria at L4 and L5 are stable. No matter which direction you lean, no matter which direction you drift, the forces prevent you from leaning farther, as though you were at the bottom of a bowl-shaped crater surrounded by a high, sloped rim. So, for both L4 and L5, if an object is not located exactly where all forces cancel, then its position will oscillate around the point of balance, in paths called librations. (Not to be confused with the particular spots on Earth’s surface where one’s mind oscillates from ingested libations.) These librations are equivalent to the back-and-forth path a ball takes when it rolls down one hill yet doesn’t pick up enough speed to climb the next.

More than just orbital curiosities, L4 and L5 represent special areas where one might decide to establish space colonies. All you need do is ship some raw construction materials to the area (having mined them not only from Earth but perhaps from the Moon or an asteroid); leave them in place, since there’s no risk of their drifting away; and return later with more supplies. Once you’ve collected all your materials in this zero-G environment, you could build an enormous space station—tens of miles across—with very little stress on the materials themselves. By rotating the station, you would induce centrifugal forces that simulate Earth gravity for its hundreds (or thousands) of residents and their farm animals.

In 1975, Keith and Carolyn Henson founded the L5 Society to carry out exactly those plans, although the society is best remembered for its informal association with Princeton physics professor Gerard K. O’Neill, who promoted space habitation through such visionary writings as his 1976 book
The High Frontier: Human Colonies in Space
. The group had a single goal: “to disband the Society in a mass meeting at L5.” Presumably this would be done inside the completed space habitat, during the party celebrating their mission accomplished. In 1987 the L5 Society merged with the National Space Institute to become the National Space Society, which continues today.

T
he idea of locating a large structure at libration points appeared as early as the early 1940s, in a series of sci-fi short stories by George O. Smith collected under the title
Venus Equilateral
. In them the author imagines a relay station at the L4 point of the Venus–Sun system. In 1961 Arthur C. Clarke would reference Lagrangian points in his novel
A Fall of Moondust
. Clarke, of course, was no stranger to special orbits. In 1945 he became the first to calculate, in a four-page memorandum, the altitude above Earth’s surface at which a satellite’s orbital period would exactly match the twenty-four-hour rotation period of Earth. Because a satellite with that orbit “hovers” over Earth’s surface, it can serve as an ideal relay station for radio communications from one part of Earth to another. Today, hundreds of communication satellites do just that, at about 22,000 miles above Earth’s surface.

As George O. Smith knew, there is nothing unique about the balance points in the rotating Earth–Moon system. Another set of five Lagrangian points exists for the rotating Sun–Earth system, as well as for any pair of orbiting bodies anywhere in the universe. For objects in low orbits, such as the Hubble, Earth continuously blocks a significant chunk of its night-sky view. However, a million miles from Earth, in the direction opposite that of the Sun, a telescope at the Sun–Earth L2 will have a twenty-four-hour view of the night sky, because it would see Earth at about the size we see the Moon in Earth’s sky.

The Wilkinson Microwave Anisotropy Probe (WMAP for short), which was launched in 2001, reached the Sun–Earth L2 in a couple of months and is still librating there, having busily taken data on the cosmic microwave background—the omnipresent signature of the Big Bang. And having set aside a mere 10 percent of its total fuel, the WMAP satellite nevertheless has enough fuel to hang around this point of unstable equilibrium for nearly a century, long beyond its useful life as a data-taking space probe. NASA’s next-generation space telescope, the James Webb Space Telescope (successor to the Hubble), is also being designed for the Sun–Earth L2 point. And there’s plenty of room for yet more satellites to come and librate, since the real estate of the Sun–Earth L2 occupies quadrillions of cubic miles.

Another Lagrangian-loving NASA satellite, known as Genesis, librated around the Sun–Earth L1 point. This L1 lies a million miles out between Earth and the Sun. For two and a half years, Genesis faced the Sun and collected pristine solar matter, including atomic and molecular particles from the solar wind—revealing something of the contents of the original solar nebula from which the Sun and planets formed.

Given that L4 and L5 are stable points of equilibrium, one might suppose that space junk would accumulate near them, making it quite hazardous to conduct business there. Lagrange, in fact, had predicted that space debris would be found at L4 and L5 for the gravitationally powerful Sun–Jupiter system. A century later, in 1905, the first members of the Trojan family of asteroids were discovered. We now know that gathered at the L4 and L5 points of the Sun–Jupiter system are thousands of asteroids that follow and lead Jupiter around the Sun, with periods that equal one Jovian year. As though gripped by tractor beams, these asteroids are forever held in place by the gravitational and centrifugal forces of the Sun–Jupiter system. (These asteroids, being stuck in the outer solar system and out of harm’s way, pose no risk to life on Earth or to themselves.) Of course, we would expect space junk to accumulate at L4 and L5 of the Sun–Earth and Earth–Moon systems too. And it does.

A
s an important side benefit, interplanetary trajectories that begin at Lagrangian points require very little fuel to reach other Lagrangian points or even other planets. Unlike a launch from a planet’s surface, where most of your fuel goes to lift you off the ground, a Lagrangian launch would be a low-energy affair and would resemble a ship leaving dry dock, cast into the sea with a minimal investment of fuel. Today, instead of thinking about establishing self-sustaining Lagrangian colonies of people and cows, we can think of Lagrangian points as gateways to the rest of the solar system. From the Sun–Earth Lagrangian points, you are halfway to Mars—not in distance or in time but in the all-important category of fuel consumption.

In one version of our spacefaring future, imagine filling stations at every Lagrangian point in the solar system, where travelers refill their rocket gas tanks en route to visit friends and relatives living on other planets or moons. This mode of travel, however futuristic it sounds, is not without precedent. Were it not for the gas stations scattered liberally across the United States, your automobile would require a colossal tank to drive coast to coast: most of the vehicle’s size and mass would be fuel, guzzled primarily to transport the yet-to-be-consumed fuel for your cross-country trip. We don’t travel that way on Earth. Perhaps the time will come when we no longer travel that way through space.