Even though it will be less costly than a fast ship, construction of an interstellar slow boat will be a monumental engineering task. To allow our robot or human population the luxury of a 1,000-year transit time, solar-sail engineering will be stressed to its physical limits.
At their best, current sails could complete this journey in about 7,000 years. It will be necessary to develop very thin, strong and temperature-tolerant sails that are joined to the payload by diamond-strength cables. (Unless, that is, the payload consists of thin-film electronics deposited on the anti-sunward sail face, as suggested by Dr Robert Forward in his "Starwisp" concept.)
The 0.2-AU perihelion distances considered for current-technology extrasolar sails will also not be adequate. To take full advantage of solar radiation pressure, our starship must approach the sun closer than 0.1 AU. Care must be taken that solar radiation pressure does not blow the craft prematurely from the inner solar system during its inbound, pre-perihelion trajectory leg.
Ultimate solar sails will most likely be manufactured in space, rather than launched from Earth. Sail manufacturers might first mine appropriate sail materials—perhaps aluminum, beryllium, scandium, niobium, or tungsten—from asteroids or comets rich in such materials. The nanometer-thin-
film sail could then be manufactured in situ using a process called vapor-phase deposition. To prevent solar-radiation back pressure on the sail during the solar pass, the sail could be partially furled behind a chunk of machined asteroid rock or a multi-sail configuration could be utilized so that the sail would initially be "hove-to" in respect to sunlight.
If we are launching a small robotic starprobe, the unfurled sail dimension would be perhaps 1-10 kilometers. Starships capable of maintaining a human population for centuries will require sails 100-1,000 kilometers or larger in linear dimension. These ships could be small versions of the O'Neill space habitats discussed in Chapter 7.
Carbon could also be mined from comet nuclei for this venture. Space manufacturing facilities would utilize solar energy to create diamond-strength filamentary cables from this material, and the cables would be used to join the sail to the payload.
When the construction project is complete, the probe would be activated or the human population would board the habitat. The ship, perhaps still shielded from the Sun by its chunk of asteroid rock, would depart toward perihelion, perhaps utilizing a Jupiter-flyby and auxiliary solar-electric propulsion for pre-perihelion maneuvers.
If human occupants are on board, they may choose to sleep within solar-radiation shelters during the hours of the close solar pass. Alternatively, like the alien astronauts in Buzz Aldrin's and John Barnes' science-fiction novel Encounter with Tiber (Warner, New York, 1996), they may view the process from their acceleration couches, entranced by the spectacle of a star at close range.
As the ship departs perihelion, the sail is slowly unfurled and ballast is released to maintain moderate acceleration. By the time the ship has passed Jupiter, it may be moving at 1,000 kilometers per second. It is time to furl the sail and prepare for the long interstellar cruise.
One advantage of the photon sail as an interstellar propulsion device is its versatility. After acceleration, sail (and cables) could be wrapped around the habitat to produce extra shielding from cosmic radiation. Since Alpha Centauri A and B are both Sunlike stars, the sail could be unfurled again at journey's end for deceleration.
If humans or their descendents wait until the Sun's giant phase before launching colony ships to nearby stars, the enhanced solar radiant flux will result in shorter trip times. But a magsail or some such interstellar drag brake (see Chapter 20) would be necessary to decelerate from the higher interstellar cruise velocity. Such a technique would also be required if our colony ship is directed toward a subluminous red dwarf star, rather than a near-twin of the Sun.
Traveling between the stars somewhat limits our options for onboard power. A nuclear reactor is possible, as is an electrodynamic tether that would obtain power from the interstellar magnetic field, at the expense of a small reduction in the ship's kinetic energy.
Arriving in the destination solar system, the starship occupants could construct larger habitats from local asteroids or comets. Alternatively, they could land directly on Earthlike worlds or terraform planets like Mars. Some have suggested that human starfarers might become perpetual nomads, picking up roots almost immediately and proceeding toward another stellar destination. Perhaps this process would continue until they reached the home system of another intelligence. One can only speculate on the consequences of such a contact.
If human interstellar explorers encounter intelligent extraterrestrials, we hope that everyone gets along. But since interstellar humans would be interlopers in someone else's home solar system, perhaps our explorers will need a protocol to continue exploring until an uninhabited world is located.
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