For many thousands of years, people, with a few notable exceptions, did not believe the Solar System existed. The Earth was believed not only to be stationary at the centre of the universe, but to be categorically different from the divine or ethereal objects that moved through the sky. While Nicolaus Copernicus and his predecessors, such as the Indian mathematician-astronomer Aryabhata and the Greek philosopher Aristarchus of Samos, had speculated on a heliocentric reordering of the cosmos, it was the conceptual advances of the 17th century, led by Galileo Galilei, Johannes Kepler, and Isaac Newton, which led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same physical laws that governed the Earth, and therefore could be material worlds in their own right, with such earthly phenomena as craters, weather, geology, seasons and ice caps.
The five closest planets to Earth – Mercury, Venus, Mars, Jupiter, and Saturn – are amongst the brightest objects in the night sky and were called "πλανήτης" (planētēs, meaning "wanderer") by the Ancient Greeks. They were known to move across the fixed stars; this is the origin of the word "planet". Uranus is also visible without optical aid at its brightest, but it is at the very limit of naked-eye detectability and therefore evaded discovery until 1781.
Telescopic observations
Main article: Timeline of solar system astronomy
A replica of Isaac Newton's telescope
The first exploration of the Solar System was conducted by telescope, when astronomers first began to map those objects too faint to be seen with the naked eye.
Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. He discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it.[89] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn.[90] Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings, and the Great Red Spot of Jupiter.[91]
Edmond Halley realised in 1705 that repeated sightings of a comet were in fact recording the same object, returning regularly once every 75-76 years. This was the first evidence that anything other than the planets orbited the Sun.[92]
In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered.[93]
Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars and Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual reclassification as asteroids.[94]
By 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. Urbain Le Verrier's calculations eventually led to the discovery of Neptune.[95] The excess perihelion precession of Mercury's orbit led Le Verrier to postulate the intra-Mercurian planet Vulcan in 1859 —but that would turn out to be a red herring.
Further apparent discrepancies in the orbits of the outer planets led Percival Lowell to conclude that yet another planet, "Planet X," must still be out there. After his death, his Lowell Observatory conducted a search which ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and its discovery was therefore coincidental. Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was reclassified in 2006 as a dwarf planet by the IAU.[95]
In 1992, astronomers David Jewitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered (15760) 1992 QB1. This object proved to be the first of a new population, which came to be known as the Kuiper belt; an icy analogue to the asteroid belt of which such objects as Pluto and Charon were deemed a part.[96][97]
Mike Brown, Chad Trujillo and David Rabinowitz announced the discovery of Eris in 2005, a scattered disc object larger than Pluto and the largest object discovered in orbit round the Sun since Neptune.[98]
Observations by spacecraft
Main article: Timeline of Solar System exploration
Artist's conception of Pioneer 10, which passed the orbit of Pluto in 1983. The last transmission was received in January 2003, sent from approximately 82 AU away. The 35-year-old space probe is now receding at over 27,000mph from the Sun.[99]
Since the start of the Space Age, a great deal of exploration has been performed by robotic spacecraft missions that have been organized and executed by various space agencies.
All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. Through these unmanned missions, humans have been able to get close-up photographs of all of the planets and, in the case of landers, perform tests of the soils and atmospheres of some.
The first manmade object sent into space was the Soviet satellite Sputnik 1, launced in 1957, which successfully orbited the Earth for over a year. The American probe Explorer 6, launched in 1959, was the first satellite to image the Earth from space.
The first successful probe to fly by another Solar System body was Luna 1, which sped past the Moon in 1959. Originally meant to impact with the Moon, it instead missed its target and became the first manmade object to orbit the Sun. Mariner 2 was the first probe to fly by another planet, Venus, in 1962. The first successful flyby of Mars was made by Mariner 4 in 1964. Mercury was first encountered by Mariner 10 in 1974.
A photo of Earth (circled) taken by Voyager 1, 6 billion km (4 billion miles) away. The streaks of light are diffraction spikes radiating from the Sun (off frame to the left).
The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets 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 Neptune's orbit, and are on course to find and study the termination shock, heliosheath, and heliopause. According to NASA, both Voyager probes have encountered the termination shock at a distance of approximately 93 AU from the Sun.[69][100]
No Kuiper belt object has yet been visited by a spacecraft. Launched on January 19, 2006, the New Horizons probe is currently en route to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.[101]
In 1966, the Moon became the first Solar System body beyond Earth to be orbited by an artificial satellite (Luna 10), followed by Mars in 1971 (Mariner 9), Venus in 1975 (Venera 9), Jupiter in 1995 (Galileo, which also made the first asteroid flyby, 951 Gaspra, in 1991), the asteroid 433 Eros in 2000 (NEAR Shoemaker), and Saturn in 2004 (Cassini–Huygens). The MESSENGER probe is currently en route to commence the first orbit of Mercury in 2011, while the Dawn spacecraft is currently set to orbit the asteroid Vesta in 2011 and the dwarf planet Ceres in 2015.
The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on or impacting the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3, although a fully successful landing didn't occur until Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn's moon Titan in 2005 (Huygens). The Galileo orbiter also dropped a probe into Jupiter's atmosphere in 1995; since Jupiter has no physical surface, it was destroyed by increasing temperature and pressure as it descended.
Manned exploration
Manned exploration of the Solar System is currently confined to Earth's immediate environs. The first human being to reach space (defined as an altitude of over 100 km) and to orbit the Earth was Yuri Gagarin, a Soviet cosmonaut who was launched in Vostok 1 on April 12, 1961. The first man to walk on the surface of another Solar System body was Neil Armstrong, who stepped onto the Moon on July 21, 1969 during the Apollo 11 mission. The United States' Space Shuttle is the only reusable spacecraft to successfully make multiple orbital flights. The first orbital space station to host more than one crew was NASA's Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then. In 2004, SpaceShipOne became the first privately funded vehicle to reach space on a suborbital flight. That same year, President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars
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