Extraterrestrial Civilizations - Isaac Asimov [105]
It’s just that the astronauts travel at an average speed 1,300 times that of Columbus. Well, increase that speed by another factor of 70 and it will take only three days to reach Venus.
One way to gain the necessary speed is to place a spaceship under seventy times the acceleration of a Moon rocket, using rocket engines with seventy times the capacity for thrust. Even if we build such large engines and are willing to expend so much fuel, it remains true that the human body can only endure so much (and not very much) acceleration. The acceleration required to send the vessel on its way to Venus at a speed that would make the voyage short work would kill the astronauts at once.
The alternative is to use an acceleration no higher than that required to launch a vessel to the Moon, but then to use further acceleration at a bearable level for a prolonged period. In this way, the vessel would go faster and faster till the halfway point was reached. After that the rocket exhaust could be aimed in the other direction and a prolonged and gradual deceleration could reduce the vessel’s speed for the tryst with Venus.
It would take time to accelerate and decelerate, so the voyage would take considerably more than three days. Worse yet, acceleration and deceleration requires the expenditure of energy, and we can make the general rule that to decrease the time required for any voyage means an increase in energy expenditure. (For that matter, if the astronauts move at an average speed 1,300 times that of Columbus, their total energy expenditure is far more than 1,300 times that of Columbus.)
We don’t know of any way of uncoupling time lapse and energy expenditure, and if our understanding of the laws of nature is correct there is no conceivable way. Between the demands of the human body where acceleration is concerned and the demands of the human economy where energy expenditure is concerned, our first manned flights to Venus (if any) are going to take at best four months.
Already men have remained in space for almost that long, but that has been in space stations such as Skylab, in Earth’s immediate neighborhood, with rescue at short notice possible. To spend 120 days in space in cramped quarters, with every moment taking you farther from home, is a psychological hazard indeed.
Worse yet, having arrived in the neighborhood of Venus, there would be no chance of a landing in view of the planet’s almost redhot surface temperature. Any exploration of the surface would have to be carried out by unmanned probes launched by the space vessel, which would itself remain in orbit about Venus and would then launch itself on another four-month journey back to Earth.
Since exploration of Venus’s surface would have to be carried out by an unmanned probe, that probe might as well travel all the way from Earth—as several probes have indeed already done. The benefits achieved by having the probe launched from, and the signals received by, a manned mother ship would scarcely justify the traumatic experience of over eight continuous months in space.
Mercury, the planet nearest the Sun, is farther from us than Venus, being never closer to us than 80 million kilometers (50 million miles) or twice Venus’s closest approach.
Mercury would at least offer a landfall to the long-distance astronauts, for one can visualize them as landing on the night side of Mercury and being able to explore the surface for several weeks before the approach of sunrise makes it absolutely necessary to leave.
The flight to Mercury, however, would carry the astronauts to a distance no farther from the Sun than 65 million kilometers (40 million miles). Solar radiation would be over 4 times as concentrated at that distance as it is in the neighborhood of the Earth. For what might be gained in a manned voyage to Mercury, over an unmanned probe, the price paid in risking the effects of the greater radiation may prove too high.
Since voyages in the direction