Going Interstellar (collection)

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.

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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.