《毕金献阅读理解》电子书节选之三_大学英语阅读

《毕金献阅读理解》电子书节选之三

　　[D] Chemical rockets have limits practical and economic. Take speed: Voyager 2, the fastest space probe yet launched, is traveling at 18.5km a second. That sounds impressively fast. But for space travel it s a snail s pace. Mars, which at times is the planet closest to us, is on average about 78 million km distant. Even if a spaceship could travel to Mars at 18km a second in a straight line, the journey would take seventy?nine days. American and Russian draft plans for manned missions to Mars estimate that crews will spend two years on a return journey.

　　[E] It s reasonable to expect that propulsion systems will improve. But even if we take everything at its best boundless energy, a spaceship with ultimate powers of acceleration and the ability to fly in a straight line nobody knows the limits of human endurance in space. To travel faster requires a faster breakout from the constraints of Earth s gravity. A spacecraft s rapid lift-off creates within the vehicle an artificial gravity that presses its occupants fiercely downwards. High speed over a long journey would make limbs feel useless, and possibly damage the heart.

　　[F] Five years-plus is how we on Earth would time the journey. But, strangely, the astronauts would find the trip much faster. As Einstein predicted in his theory of relativity, the spaceship s clocks would slow down compared with those on Earth. A voyage across our whole galaxy one that takes light 100,000 years to make might happen while the astronauts had their morning coffee. Those left on Earth would age at the normal rate. When the astronauts returned from the stars after a five-year trip, by their reckoning, they would land in a world that had aged by several million years.