单项选择题

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Since ancient times, people have dreamed of leaving their home planet and exploring other worlds. In the later half of the 20th century, that dream became reality. The space age began with the launch of the first artificial satellites in 1963. A human first went into space in 1963. Since then, astronauts and cosmonauts have ventured into space for ever greater lengths of time, even living aboard orbiting space stations for months on end. Two dozen people have circled the moon or walked on its surface. At the same time, robotic explorers have journeyed where humans could not go, visiting all but one of the solar system’s major worlds. Unpiloted spacecraft have also visited a host of minor bodies such as moons, comets, and asteroids. These explorations have sparked the advance of new technologies, from rockets to communications equipment to computers. Spacecraft studies have yielded a bounty of scientific discoveries about the solar system, the Milky Way Galaxy, and the universe. And they have given humanity a new perspective on the earth and its neighbors in space.
The first challenge of space exploration was developing rockets powerful enough and reliable enough to boost a satellite into orbit. These boosters needed more than brute force, however; they also needed guidance systems to steer them on the proper flight paths to reach their desired orbits. The next challenge was building the satellites themselves. The satellites needed electronic components that were lightweight, yet durable enough to withstand the acceleration and vibration of launch. Creating these components required the world’s aerospace engineering facilities to adopt new standards of reliability in manufacturing and testing. On Earth, engineers also had to build tracking stations to maintain radio communications with these artificial "moons" as they circled the planet.
Beginning in the early 1920s, humans launched probes to explore other planets. The distances traveled by these robotic space travelers required travel times measured in months or years. These spacecraft had to be especially reliable to continue functioning for a decade or more. They also had to withstand such hazards as the radiation belts surrounding Jupiter, particles orbiting in the rings of Saturn, and greater extremes in temperature than are faced by spacecraft in the closeness of Earth. Despite their great scientific returns, these missions often came with high price tags. Today the world’ s space agencies, such as the United States National Aeronautics and Space Administration (NASA. and the European Space Agency (ESA), strive to conduct robotic missions more cheaply and efficiently.
It was inevitable that humans would follow their unpiloted creations into space. Piloted space flight introduced a whole new set of difficulties, many of them concerned with keeping people alive in the hostile environment of space. In addition to the vacuum of space, which requires any piloted spacecraft to carry its own atmosphere, there are other deadly hazards: solar and cosmic radiation, micrometorites (small bits of rock and dust) that might puncture a spacecraft hull or an astronaut’s pressure suit, and extremes of temperature ranging from frigid darkness to broiling sunlight. It was not enough simply to keep people alive in space -- astronauts needed to have a means of accomplishing useful work while they were there. It was necessary to develop tools and techniques for space navigation, and for conducting scientific observations and experiments. Astronauts would have to be protected when they ventured outside the safety of their pressurized spacecraft to work in the vacuum. Missions and hardware would have to be carefully designed to help insure the safety of space crews in any foreseeable emergency, from liftoff to landing.
The challenges of conducting piloted space flights were great enough for missions that orbited Earth. They became even more daunting for the Apollo missions, which sent astronauts to the moon. The achievement of sending astronauts to the lunar surface and back represents a summit of human space flight.
After the Apollo program, the emphasis in piloted missions shifted to long-duration spaceflight, as pioneered aboard Soviet and U.S. space stations. The development of reusable spacecraft became another goal, giving rise to the U.S. space shuttle fleet. Today efforts focus on keeping people healthy during space missions lasting a year or more w the duration needed to reach nearby planets -- and in lowering the cost of sending satellites into orbit.
In the passage, the author wants to tell us ______.