(B)
In the geocentric perspective from which humans viewed the solar system, its nature and structure were long misperceived. The apparent motions of solar system objects as viewed from a moving Earth were believed to be their actual motions about a stationary Earth. In addition, many solar system objects and phenomena are not directly sensible by humans without technical aids. Thus, both conceptual and technological advances were required in order for the solar system to be correctly understood.
The first and most fundamental of these advances was the Copernican Revolution, which adopted a heliocentric model for the motions of the planets. Indeed, the term “solar system” itself derives from this perspective. But the most important consequences of this new perception came not from the central position of the Sun, but from the orbital position of the Earth, which suggested that the Earth was itself a planet. This was the first indication of the true nature of the planets. Also, the lack of perceptible stellar parallax despite the earth’s orbital motion indicated the extreme remoteness of the fixed stars, which prompted the speculation that they could be objects similar to the Sun, perhaps with planets of their own.
Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another planet or moon was the Soviet Union’s Luna 2 probe, which impacted on the moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, and Saturn’s moon Titan in 2005. Spacecrafts have also made close approaches to other planets: Mariner 10 passed Mercury in 1973, while the Voyager probes performed a grand tour of the solar system following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Pluto’s orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years.
Through these unmanned missions, we have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soil and atmosphere. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the moon has prompted speculation that mankind may return to the moon in the next decade or so. The long-mooted manned mission to Mars does not currently look like coming to fruition in the near future.
What is the main idea of the passage?
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