Solar System Exploration

Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.

In ancient and medieval times, only objects visible to the naked eye—the Sun, the Moon, the five classical planets, and comets, along with phenomena now known to take place in Earth's atmosphere, like meteors and aurorae—were known. Ancient astronomers were able to make geometric observations with various instruments. The collection of precise observations in the early modern period and the invention of the telescope helped determine the overall structure of the Solar System. Telescopic observations resulted in the discovery of moons and rings around planets, and new planets, comets and the asteroids; the recognition of planets as other worlds, of Earth as another planet, and stars as other suns; the identification of the Solar System as an entity in itself, and the determination of the distances to some nearby stars.

For millennia, what today is known to be the Solar System was regarded as the " whole universe", so the knowledge of both mostly advanced in parallel. A clear distinction was not made until around the mid-17th century. Since then, incremental knowledge has been gained not only about the Solar System, but also about outer space and its deep-sky objects.

The composition of stars and planets was investigated with spectroscopy. Observations of Solar System bodies with other types of electromagnetic radiation became possible with radio astronomy, infrared astronomy, ultraviolet astronomy, X-ray astronomy, and gamma-ray astronomy.

Robotic space probes, the Apollo program landings of humans on the Moon, and space telescopes have vastly increased human knowledge about the atmosphere, geology, and electromagnetic properties of other planets, giving rise to the new field of planetary science.

The Solar System is one of many planetary systems in the galaxy.Solar System Exploration
/ref> The planetary system that contains Earth is named the "Solar" System. The word "solar" is derived from the Latin word for Sun, ''Sol'' (genitive ''Solis''). Anything related to the Sun is called "solar": for example, stellar wind from the Sun is called solar wind.

Pre-telescope

The first humans had limited understanding of the celestial bodies that could be seen in the sky. The Sun, however, was of immediate interest, as it generates the day-night cycle. Also, dawn and sunset always appear at roughly the same points of the horizon, which helped to develop the cardinal directions. The Moon was another body of immediate interest, because of its larger apparent size. The lunar phases helped them measure time in longer periods than days, and to predict the duration of the seasons.

Prehistoric beliefs about the structure and origin of the universe were highly diverse, often rooted in religious cosmology, and many are unrecorded. Many associated the classical planets (star-like points visible with the naked eye) with deities, in part due to their puzzling forward and retrograde motion against the otherwise fixed stars, which gave them their nickname of "wanderer stars", πλάνητες ἀστέρες (''planētes asteres'') in Ancient Greek, from which today's word "planet" was derived.

Systematic astronomical observations were performed in many areas around the world, and started to inform cosmological knowledge, although they were mostly driven by astrological purposes such as divination and/or omens. Early historic civilizations in Egypt, the Levant, pre-Socratic Greece, Mesopotamia, and ancient China, recorded beliefs in a flat Earth. Vedic texts proposed a number of shapes, including a wheel (flat) and a bag (concave), though they likely promote a spherical Earth, which they refer to as ''bhugol'' (or ''भूगोल'' in Hindi and Sanskrit), which literally translates to "spherical land". Ancient models were typically geocentric, putting the Earth at the center of the universe, based solely on the common experience of seeing the skies slowly moving around above our heads, and by feeling the land under our feet to be firmly at rest. Some traditions in Chinese cosmology proposed an outer surface to which planets and the Sun and Moon were attached; another proposed that they were free-floating. All remaining stars were regarded as "fixed" in the background.

One important discovery made at different times in different places is that the bright planet sometimes seen near the sunrise (called Phosphorus by the Greeks) and the bright planet sometimes seen near the sunset (called Hesperus by the Greeks) were actually the same planet, Venus.

Though unclear if motivated by empirical observations, the concept of a spherical Earth apparently first gained intellectual dominance in the Pythagorean school in Ancient Greece in the 5th century BC. Meanwhile, the Pythagorean astronomical system proposed the Earth and Sun and a counter-Earth rotate around an unseen "Central Fire". Influenced by Pythagoran thinking and Plato, philosophers Eudoxus, Callippus, and Aristotle all developed models of the solar system based on concentric spheres. These required more than one sphere per planet in order to account for the complicated curves they traced across the sky. Aristotelian physics used the Earth's place at the center of the universe along with the theory of classical elements to explain phenomena such as falling rocks and rising flames; objects in the sky were theorized to be composed of a unique element called aether.

A later geocentric model developed by Ptolemy attached smaller spheres to a smaller number of large spheres to explain the complex motions of the planets, a device known as deferent and epicycle first developed by Apollonius of Perga. Published in the ''Almagest'', this model of celestial spheres surrounding a spherical Earth was reasonably accurate and predictive,Masip, pp. 16–19 and became dominant among educated people in various cultures, spreading from Ancient Greece to Ancient Rome, Christian Europe, the Islamic world, South Asia, and China via inheritance and copying of texts, conquest, trade, and missionaries. It remained in widespread use until the 16th century.Masip, pp. 16–19

Heliocentric model

Various astronomers, especially those who had access to more precise observations, were skeptical of the geocentric model and proposed alternatives, including the heliocentric theory in which the planets and the Earth orbit the Sun. Many proposals did not spread outside the local culture, or did not become locally dominant. Aristarchus of Samos had speculated about heliocentrism in Ancient Greece; Martianus Capella taught in the early Middle Ages that both Mercury and Venus orbit the Sun, while the Moon, the Sun and the other planets orbit the Earth; in Al-Andalus, Arzachel proposed that Mercury orbits the Sun, and heliocentric astronomers worked in the Maragha school in Persia. Kerala-based astronomer Nilakantha Somayaji proposed a geoheliocentric system, in which the planets circled the Sun while the Sun, Moon and stars orbited the Earth.

Finally, Polish astronomer Nicolaus Copernicus developed in full a system called Copernican heliocentrism, in which the planets and the Earth orbit the Sun, and the Moon orbits the Earth. This theory was known to Danish astronomer Tycho Brahe, but he did not accept it, and proposed his own geoheliocentric Tychonic system. Brahe undertook a substantial series of more accurate observations. German natural philosopher Johannes Kepler at first worked to combine the Copernican system with Platonic solids, in line with his interpretation of Christianity and an ancient musical resonance theory known as ''Musica universalis''. After becoming an assistant of Brahe, Kepler inherited the observations and was directed to mathematically analyze the orbit of Mars. After many failed attempts, he eventually made the groundbreaking discovery that the planets moved around the Sun in ellipses. He formulated and published what are now known as Kepler's laws of planetary motion from 1609 to 1619. This became the dominant model among astronomers, though as with celestial sphere models, the physical mechanism by which this motion occurred was somewhat mysterious and theories abounded.

It took some time for the new theories to spread across the world. For example, with the Age of Discovery already well under way, astronomical thought in America was based on the older Greek theories, but newer western European ideas began to appear in writing by 1659.

Telescopic observations

Early telescopic discoveries

The invention of the telescope revolutionized astronomy, making it possible to see details about the Sun, Moon, and planets not available to the naked eye. It appeared around 1608 in the Netherlands, and was quickly adopted among European enthusiasts and astronomers to study the skies.

Italian polymath Galileo Galilei was an early user and made prolific discoveries, including the phases of Venus, which definitively disproved the arrangement of spheres in the Ptolemaic system.
Galileo also discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Giovanni Domenico Cassini later discovered four more moons of Saturn and the Cassini division in Saturn's rings.

Around 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realise that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun. In 1705, Halley realised that repeated sightings of a comet were 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, though this had been theorized about comets in the 1st century by Seneca. Around 1704, the term "Solar System" first appeared in English.

Newtonian physics

English astronomer and mathematician Isaac Newton, incidentally building on recent scientific inquiries into the speed at which objects fall, was inspired by claims by rival Robert Hooke of a proof of Kepler's laws. Newton was able to explain the motions of the planets by hypothesizing a force of gravity acting between all solar system objects in proportion to their mass and an inverse-square law for distance – Newton's law of universal gravitation. Newton's 1687 ''Philosophiæ Naturalis Principia Mathematica'' explained this along with Newton's laws of motion, for the first time providing a unified explanation for astronomical and terrestrial phenomena. These concepts became the basis of classical mechanics, which enabled future advancements in many fields of physics.

Discovery of additional planets and moons

The telescope made it possible for the first time to detect objects not visible to the naked eye. This took some time to accomplish, due to various logistical considerations such as the low magnification power of early equipment, the small area of the sky covered in any given observation, and the work involved in comparing multiple observations over different nights.

In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. Its orbit revealed that it was a new planet, Uranus, the first ever discovered telescopically.

Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars and Jupiter. It was considered another planet, but after subsequent discoveries of other small worlds in the same region, it and the others were eventually reclassified as asteroids.

By 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. John Adams and Urbain Le Verrier's calculations eventually led to the discovery of Neptune. 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 not to exist: the excess perihelion precession was finally explained by Einstein's general relativity, which displaced Newton's theory as the most accurate description of gravity on large scales.

Eventually, new moons were discovered also around Uranus starting in 1787 by Herschel, around Neptune starting in 1846 by William Lassell

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