Extraterrestrial Civilizations - Isaac Asimov [119]
Another way of imagining a spaceship’s gaining acceleration without fuel is to picture a laser beam shining upon a large “sail” surrounding the vessel. The laser beam, based on some convenient body in the Solar system, would be trained continually on the sail and would act as one continuous push serving to steadily accelerate the vessel. The laser beam, to remain in being would, of course, consume the vast quantities of energy that the ship was not consuming. (You can’t beat the system when it comes to energy.) In addition, it would be more and more difficult to remain on target as the ship moved farther and farther from home base. Finally, the laser beam could not be used to decelerate unless someone at the destination point up ahead could supply an obliging beam in the opposite direction.
Still, if all nonfuel methods failed and a speed-of-light vessel had to use fuel, it might perhaps not have to carry that fuel. It might be able to pick it up as it went along. After all, interstellar space is not truly empty, not an utter vacuum. There are occasional atoms of matter present, mostly hydrogen.
In 1960, the American physicist Robert W. Bussard suggested that this hydrogen might be picked up as the spaceship plowed through space. The ship would be a kind of “interstellar ramjet” but since space has much less matter in it than Earth’s atmosphere does, the ship would have to sweep up the matter from a far larger volume of space, compress it, and extract energy through hydrogen fusion.
The ship’s scoop, in order to be effective, would have to be at least 125 kilometers (80 miles) in diameter when it is passing through those volumes of space where there are clouds of dust and gas, and matter is strewn most thickly. In clear interstellar space, the scoop would have to be as much as 1,400 kilometers (870 miles) in diameter, and in intergalactic space, 140,000 kilometers (87,000 miles) across.
Such scoops, if we imagine them built of even the flimsiest materials, would be prohibitively massive. How would the materials in those scoops be carried out into space; or how much time and effort would it take to assemble them out of matter already in space?
Even if the energy problem is somehow beaten by methods we can’t in the least foresee, it remains true that a huge ship traveling very near the speed of light is peculiarly vulnerable. There may be no danger of striking a star, but it may well be that space is fairly full of relatively small bodies from planets down to gravel.
From the viewpoint of the ship, every object in the Universe that happens to be approaching it will be doing so at the speed of light. Such objects will be impossible to avoid, for any conceivable message that heralds their approach (x-rays or anything else) will be traveling only at the speed of light so that the object itself will be hot on the heels of the message. No sooner will a collision warning sound than the collision will take place.
And any massive object colliding with the ship, where the velocity of one relative to the other is that of light, would leave a neat hole in the ship where it entered, where it emerged, and at all intersections in between. The ship might be a Swiss cheese before long.
Even if we discard sizable particles and assume there is nothing but very thin gas in the volume of gas being passed through—that is enough to make trouble.
As the spaceship accelerates and goes faster and faster, the atoms in interstellar space strike harder and harder, and more and more of them do so per second.
From the standpoint of the spaceship, the oncoming particles will be approaching at very near the speed of light and that will make them, to all intents and purposes, cosmic-ray particles.
Under ordinary conditions, cosmic-ray intensity in space is not particularly deadly. Astronauts have remained in space for more than 3 months continuously and have survived handily. Moving through interstellar space at the speed of light, however, with every oncoming particle striking