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Part B

Directions:

The following paragraphs are given in a wrong order. For Questions 21 ¡ª 25, you are required to reorganize these paragraphs into a coherent article by choosing from the list A ¡ª G to fill in each numbered box. The first and last paragraphs have been placed for you in Boxes. Mark your answers on ANSWER SHEET 1.

£ÛA£Ý Spacecraft have to follow a curved path made up of various orbits usually governed by the Sun¡¯s gravity. And they need to aim at where their target will be, not where it is when they set off, a task requiring precise navigation to ensure that the vehicle doesn¡¯t zoom past its goal and fly for ever into space.

£ÛB£Ý Journeys to the stars would be more formidable, because a new speed limit would come into force ¡ª the speed of light. As Einstein demonstrated, nothing can travel faster than 300,000 km a second. Even if our spaceship could accelerate at 1g until it reached 99 percent of the speed of light, then decelerated at 1g in the final stretch, a trip to our nearest star would take five years and four months.

£ÛC£Ý Without air resistance, an object falling from height accelerates at a rate of 9.8m per second every second. Physicists express that acceleration as 1g ¡ª one times the force of gravity. Space scientists say that a journey in which the craft accelerates at 1g is possibly the limit of human endurance. To guarantee a landing on Mars, the brakes would have to be applied, and the spaceship would need to decelerate at 1g for the second half of the trip. At that rate, the journey would take only fortyª²nine and a half hours.

£ÛD£Ý  Chemical rockets have limits ¡ª 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 seventyª²nine days. American and Russian draft plans for manned missions to Mars estimate that crews will spend two years on a return journey.

£ÛE£Ý It¡¯s reasonable to expect that propulsion systems will improve. But even if we take everything at its best ¡ª boundless energy, a spaceship with ultimate powers of acceleration and the ability to fly in a straight line ¡ª 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.

£ÛF£Ý 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 ¡ª one that takes light 100,000 years to make ¡ª 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.

£ÛG£Ý Without the magical propellants of sci-fi(science-fiction)ª²space travel, we have to rely on chemical rockets to power our spacecraft. Whether their fuel is solid or liquid, the principle is the same: the space vehicle goes off like a firework rocket. Hot exhaust gases thrusting downwards blast the spacecraft beyond the pull of Earth¡¯s gravity and towards its target.

Orders:

G  21.   22.    23.    24.   25.    F

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Part A

Text 1: 1. D 2. D 3. C 4. B 5. B

Part B

 21. D 22. A 23. E 24. C 25. B

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