Astronomy Picture of the Day 3-08-03

Explanation: Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, the illustration above represents one concept explored by NASA centers to develop an interstellar probe pushed along by sunlight reflected from an ultrathin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets -- without requiring any fuel! If launched in 2010 such a probe could overtake Voyager 1, the most distant spacecraft bound for interstellar space, in 2018 going as far in eight years as the Voyager will have journeyed in 41 years.

A prominent researcher in the area of space propulsion was Dr. Robert Forward. He advocated both tethers and sails for accelerating spacecraft; those continuing his work are presently focusing on a "gray sail" system made of carbon. The sail would absorb the light, getting a push from it, and reradiate it as infrared energy. With the sail oriented properly to the source, this would generate a significant amount of thrust in the desired direction.

A mission to interstellar space could be accomplished with a combination sail. An aluminum coating - just 70 atoms thick - would serve as a traditional reflective solar sail to boost the spacecraft out of Earth orbit, then cancel its solar orbital velocity so it plunges on a near-miss trajectory towards the sun.

As it passes just 3 solar diameters from the sun's visible surface, the aluminum would evaporate, exposing the carbon structure underneath. The carbon would absorb sunlight and heat to 2,000 K (almost 3,600 deg. F). Radiating infrared light would accelerate the craft at 14 times Earth's gravity (the Space Shuttle reaches a maximum of 3 G during launch).

Dr. Forward (1932-2002) was not only a theoretical physicist, but a science-fiction author also. I have several of his books. He wrote "hard" sci-fi; no magic or spacewarps or replicators needed. His stories were fascinating enough without them. And his aliens - absolutely the MOST imaginative ones I've ever read about. They remain the only ones I've ever DREAMED about.

Flight of the Dragonfly will introduce you to the flouwen, jellylike intelligent blobs in a cold world's ammonia-water oceans. There is a whole series of Rocheworld stories; they are intriguing.

Dragon's Egg is my favorite. It, along with its sequel Starquake details the lives of cheela, who exist on a neutron star. They're the size of sesame seeds but contain the same amount of mass we do. They are nucleonic creatures, of course, and their days (rotation of the neutron star) are 6 per second. Sound interesting? I guarantee that it is. And human-cheela contact is perhaps the best feature. Figure out how you could get near a neutron star? Forward did.

If you like sci-fi but prefer your science facts and physical laws undamaged (some theories are bandied about, like magnetic monopoles), read his books. The dialogue is not always smooth, but hey, not every sci-fi writer is an Asimov.

Books at Amazon by Robert L. Forward

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