Going Interstellar (37 page)

From here, the amiable dwarf star about which Guge swings resembles the yolk of a colossal fried egg, more reddish than yellow-orange, with a misty orange corona about it like the egg’s congealed albumin. I’ve made it sound ugly, but Gliese 581 looks edible to me and quickly trips my hunger to reach the planet below.

As for Guge, it gleams beneath us like an old coin.

In our first week on its surface, we have already built a tent camp in one of the stabilized climate zones of the nearside terminator. Across the tall visible arc of that terminator, the planet shows itself marbled by a bluish and slate-gray crust marked by fingerlike snowfields and glacier sheets.

On the ground, our people call their base camp Lhasa and the rugged territory all about it New Tibet. In response to this naming and to the alacrity with which our fellow Kalachakrans adopted it, Minister Trungpa wept openly.

I find I like the man. Indeed, I go down for my first visit to the surface with his blessing. (Simon, my father, already bivouacs there, to investigate ways to grow barley, winter wheat, and other grains in the thin air and cold temperatures.) Kyipa, of course, remains for now on our orbiting strut-ship—in Neddy’s stateroom, which he now shares openly with the child’s grandmother, Karen Bryn Bonfils. Neddy and Karen Bryn dote on my daughter shamefully.

Our descent to Lhasa won’t take long, but, along with many others in this second wave of pioneers, I deliberately drop into a meditative trance. I focus on a photograph that Neddy gave me after the mandala ceremony at the arrival celebration, and I recall his words as he presented it.

“Soon after you became a teenager, Greta, I started to doubt your commitment to the Dharma and your ability to stick.”

“How tactful of you to wait till now to tell me,” I said, smiling.

“But I never lost a deeper layer of faith. Today, I can say that all my unspoken doubt has burned off like a summer meadow mist.” He gave me the worn photo—not a hardened d-cube—that now engages my attention.

In it, a Tibetan boy of eight or nine faces the viewer with a broad smile. He holds before him, also facing the viewer, a baby girl with rosy cheeks and eyes so familiar that I tear up in consternation and joy. The eyes belong to my predecessor’s infant sister, who didn’t live long after the capturing of this image.

The eyes also belong to Kyipa.

I meditate on this conundrum, richly. Soon, after all, the Yak Butter Express will set down in New Tibet.

STARSHIP WITH
24 DROP TANKS

(mid-flight configuration)

Figure 4. The
Kalachakra
is an anti-hydrogen powered starship that sheds its fuel tanks and radiators—required to keep the antihydrogen cold—during flight. (Image courtesy of Geoff Landis.)

SOLAR AND BEAMED
ENERGY SAILS

Les Johnson

Les Johnson’s concern for the human future is readily apparent. He has written widely about the subject, suggesting high-tech methods for preserving the environment while we solve the global energy problem, especially in his collaboration with fellow physicist Gregory Matloff and one-time NASA artist C Bangs to produce
Paradise Regained
( 2010). The same trio gave us a survival handbook to take along when we begin our conquest of space,
Living off the Land in Space
(2007). With Matloff and Giovanni Vulpetti, Johnson suggests in
Solar Sails
(2008) a novel method for expanding through and possibly beyond the solar system. He has also collaborated with physicist Travis Taylor to write Back to the Moon, a novel in which the United States’ return visit, engineered by a future NASA, becomes a desperate rescue effort for a stranded Chinese mission.

When we speculate about traveling to the stars, we tend to think in terms of giant nuclear-powered rocket engines. And, admittedly, if we succeed in making the journey, they may indeed prove to be the key. But not necessarily. In fact, Johnson suggests that softer power may be the ultimate answer. Sailing vessels showed the way for early exploration on the world’s oceans. Their days may not be over.

Johnson is a physicist, the Deputy Manager for the Advanced Concepts Office at the NASA George C. Marshall Space Flight Center in Huntsville, Alabama,. and one of the editors of this book.

***

We can’t feel it,
but the light from the Sun is pushing on us. It’s a small push, less than an ounce per square football field. Whenever we are in sunlight, or any light, we are being pushed. This solar pressure is much smaller than the other forces we experience in our everyday lives. The force of the wind from the room air conditioner vent is far stronger than the force we experience in full sunlight. It is so small that very sensitive instruments are required to measure it. And it can only be measured in a vacuum because the various forces around us will otherwise swamp the effect. But solar pressure is real, it is constant, and it can be used to propel a spacecraft to incredible speeds.

About four hundred years ago, Johannes Kepler observed that the tail of a comet appeared to be created by some sort of cosmic breeze and postulated that this breeze could be used to move ships in space in a manner similar to which the sailing ships of his day were propelled by wind. While Kepler was wrong about the nature of these cosmic winds, he was correct in his observation that something coming from the Sun, which we now know is sunlight itself, can be used to move a spacecraft.

An earthly sail moves a ship by transferring the momentum of the wind to the ship by reflecting it from a sail. The force exerted on the sail pushes the ship, causing it to move. In physics, momentum is defined as the product of mass times velocity. Lots and lots of air molecules, each having mass and some velocity, reflect from a sail and transfer their momentum to it. The ship then begins to move, its momentum coming from the wind.

In 1923, the physicist Arthur Compton observed that photons (particles of light) have momentum even though they have no rest mass. In other words, these massless particles that we call light have momentum even though they would have no mass if we could catch one and slow it down to weigh it. This is yet another weird property of light—but one that will be very useful for taking us to the stars.

Imagine a large, very thin, lightweight and very highly reflective sail deployed in space for the sole purpose of reflecting sunlight. We’ve just imagined a solar sail and they are far from imaginary. Solar sails reflect sunlight, transferring the tiny momentum of each reflected photon to the sail, causing the sail to move. The force is tiny. At the Earth’s distance from the Sun (ninety-three million miles), the force from sunlight is about five pounds per square mile. In other words, we’d have to have a sail area of one square mile to feel five pounds of force. For comparison, just one of the Space Shuttle’s main engines produces about five hundred thousand pounds of thrust. The primary difference is that the shuttle’s engines can only produce this thrust for a very short period of time before running out of fuel while a solar sail can produce thrust as long as it remains in sunlight. And since the distances involved in space travel are so large, the sail will remain in sunlight for a very long time no matter its destination.

In this case, the space shuttle engine is the hare and the solar sail is the tortoise. Chemical rockets will never take us to the stars, but solar sails might. It is important to note that while solar sails may one day take us to Alpha Centauri, they will never get us off the surface of the Earth. To lift from the surface of the Earth, we need a propulsion system that can produce more thrust than the rocket weighs. Chemical rockets are capable of producing these high thrust levels; solar sails cannot.

Before we start building our solar sail-propelled starship, we need to discuss a few more critical issues that will affect our design. First of all, the sail will still be subject to Newton’s Second Law, which states, “a body of mass (m) subject to a force (F) undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass.” In other words, to get a mass to accelerate, we need to apply a force. In order to get the accelerations needed to achieve very high speeds, such as those required for interstellar travel, we need a large force or a small mass, or in this case, we need both.

Newton’s Second Law requires our solar sail design to be very large so the sail can capture as much sunlight as possible in order maximize solar photon thrust. It also requires us to use very lightweight materials so that we can make our ship as low mass as possible. The sail must also be highly reflective so that we can capture as much momentum from each photon as possible.

Is there anything we can do to increase the force acting on the sail from the sunlight? Even though we have the benefit of time, five pounds of thrust per square mile is ridiculously small. We would require a sail almost one hundred thousand square miles in area to equal the thrust produced by one space shuttle engine. Such a sail would have roughly the same surface area as Alabama and Mississippi combined! Surely we can do something to increase our thrust so that we can make a smaller sail.

It turns out that another interesting fact about sunlight allows us to do just this. We can dramatically increase the force acting on the solar sail by flying closer to the Sun thanks to a property of sunlight called The Inverse Square Law. According to this law, if we move an object twice the distance from the light source, it will receive only one quarter of the illumination. Two times the distance (2) means one-fourth (¼) the illumination—two squared is four. If we move out to four times the distance from the Sun, the illumination drops to one sixteenth of the previous amount—four squared is sixteen. Less illumination translates directly into less force. Fortunately, we can use this geometric property to our benefit by moving closer to the Sun. If we reduce the distance to ½ its previous value, we get four (4) times the force. If we reduce it to one fourth, then we get sixteen times the force. And if we get sixteen times the force per square mile, then we can reduce the overall surface area of the sail by the same factor. And when we are talking about sails the size of US states, a factor of sixteen is significant.

This all sounds great, but are solar sails real? Have they been built and tested in space? Has anyone actually used one for sending a spacecraft anywhere? Yes, yes, and yes!

Until the 1970s, Kepler’s vision and Compton’s physics were good science but for space travel they were primarily an intellectual curiosity. With the anticipated return of Halley’s Comet in 1986, NASA commissioned a study of the feasibility of using a solar sail to rendezvous with the comet. The project never got off the ground, but it did get many space scientists and engineers thinking about solar sailing as something real, and the pace of sail technology development accelerated. The first big step was taken by Russia with the launch of their Znamya mirror in 1993. Znamya was a large, lightweight mirror flown in space to test the idea of using reflected sunlight to illuminate large areas on the ground at night. The mirror was made from very lightweight reflective materials and looked, for all practical purposes, like a solar sail.

In the late 1990s, the Europeans entered the picture with the ground-based development of a one hundred foot sail manufactured by the German company DLR (Deutschen Zentrums für Luft- und Raumfahrt). Though the sail never left the laboratory, it inspired NASA to develop a similar capability during the early 2000s that culminated in the testing of two different solar sails in the world’s largest vacuum chamber, which is located at the NASA Glenn Research Center’s Plumbrook Station. The two solar sails were one hundred feet in diameter, made from materials thinner than a human hair, and autonomously deployed under space vacuum conditions to test their space worthiness. Figure 5 shows the sail developed for NASA by L’Garde, Inc. just after a deployment test in the vacuum chamber.