A Traveler's Guide to the Stars by Les Johnson

Author:Les Johnson
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2022-08-17T00:00:00+00:00

By accelerating along the existing velocity vector (in the orbital direction the ship is already going), the orbital energy of the sail craft is increased, thereby spiraling it away from the sun. If you tip the sail to get thrust in a direction opposite to the velocity vector, the orbital energy decreases, allowing the sun’s gravity to pull it inward, spiraling toward the sun. Since the force of sunlight increases when you get closer to the sun and weaker as you get farther away, the net acceleration on the sail craft increases as it falls toward the sun, making it easier to keep moving closer. Conversely, as the sail craft sails away from the sun, the force from sunlight gets weaker, reducing the acceleration until it gets so low as to be negligible. By tipping the sail upward or downward, you can accelerate out of the orbital plane and allow the ship to get above or below the ecliptic plane.‡

It is important to note that the force of sunlight not only goes up as you decrease the distance between your sail craft and the sun, but it also goes up nonlinearly according to the inverse square law, or as 1/r2, where r is the radial distance from the sun. Simply put, if you double the distance from the sun from that of Earth’s orbital distance to two times that distance, the force is not halved (as intuition might lead you to think) but is rather decreased by a factor of four (½2 = ¼, therefore providing only ¼ the amount of thrust as was experienced by the sail near Earth). Conversely, if you decrease the distance to ½ that of Earth, the force on the sail doesn’t double; it goes up by a factor of four! If you decrease the distance to ⅓ that of Earth, then the resulting forces are nine times (32) larger. It is this force increase that makes sails attractive for interstellar travel. A large, extremely lightweight solar sail deployed close to the sun would experience the force required to rapidly escape from the solar system and travel to another star.

Professor Gregory Matloff, a pioneer in the field of solar sailing, calculated that a sail made from one-atomic-layer and very strong material, with diameters measured in kilometers, could attain velocities of 0.003 c or better, arriving at Alpha Centauri in 1,400 years.2 When he did his initial calculations, building a sail that large required materials not yet known, and they could have been called unobtainium. However, in 2004, graphene (there it is again) was discovered, which has the properties that would be required to build such a large sail. Unfortunately, as of today, making graphene on the scale required to build even the simplest solar sails is beyond current technological capabilities. But it is physically possible.

How far away are sails such as those required to enable an interstellar voyage in a reasonable amount of time? To answer that question, consider the recent spate of successful solar sail demonstration missions: NanoSail-D,3 LightSail 1 and 2,4 and IKAROS,5 all flown in the 2010s.

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