Gradual acceleration by low thrust

    The biggest hurdle for spaceflight is the first step: getting off the ground into some stable orbit above the dense atmosphere. Be it by rocket, gun or nuclear power, that step is the hardest, requiring a concentrated expenditure of energy and propellant mass.

    The initial orbit is generally the lowest one possible, the one requiring the smallest thrust. But the final goal could be a higher orbit, the Moon, a planet or comet, or even some target outside the solar system. The thrust needed to reach the final orbit from the initial one does not have to be delivered in a single, powerful blast: gradual acceleration will also work, which opens the door to alternative, more efficient methods of propulsion.

The Solar Sail

Artist's rendering of solar sail.

    One such method is the use of a solar sail, relying on the pressure of sunlight. That pressure (together with the pressure of the solar wind) makes comet tails point away from the Sun--not only when the comet approaches the Sun and its tail trails behind it, but also after it has passed the Sun and moves away, when the tail stretches ahead of the comet.

    The solar sail uses this pressure in much the same way as a sailing ship uses the wind. It would consist of a large sheet of reflective material, such as the mylar film which is used to keep spacecraft from overheating, and a framework of girders to keep it extended and to transmit its pressure to the spacecraft. Various designs have been proposed.

    A solar sail can also be useful for a monitoring spacecraft, placed to warn Earth of approaching "storms" from the Sun at a greater distance from Earth than the Lagrangian L1 point where such spacecraft are usually placed (details here).

In general, solar sails are only expected to be useful where sunlight is sufficiently intense, e. g. inside the orbit of Mars. However, Robert Forward, science fiction author and physicist, has proposed the use of a solar sail in trips to the distant stars.

    In his "Flight of the Dragonfly" a team of brave astronauts sets off in a starship to the water-covered half of Roche, a strange double planet inhabited by intelligent whale-like creatures. The starship is propelled by a huge solar sail, pushed by tight beams of light from lasers which orbit the Sun and are powered by sunlight. As the craft approaches its target, part of the sail is cut free. The laser beams then push it ahead of the starship, and as those beams are reflected back, they hit the remaining solar sail from the opposite direction, slowing down the ship and allowing its passengers to land. Let it never be said that any idea is too far-out for scientists to consider! In a technical appendix to the book, Forward presents technical details of his fantasy.

Further reading:

        Web page on the state in 2004 of solar sail work (using the preceding illustration).

        More about Robert Forward's laser-driven sail in "The Starflight Handbook" (subtitle: "A Pioneer's Guide to Interstellar Travel"), by Eugene F. Mallove and Gregory L. Matloff, John Wiley & Sons, 1989.

        Arthur Clarke wrote in 1963 a short science fiction story "The Wind from the Sun" about an international race in the far future, between spaceships propelled by solar sails. It can be found in a collection of Clarke's stories, also titled "The Wind from the Sun," published in 1972 by Harcourt, Brace and Jovanovich, inc..