Discovery and exploration of theSolar System is observation, visitation, and increase in knowledge and understanding ofEarth's "cosmic neighborhood".[1] This includes theSun, Earth and theMoon, the major planetsMercury,Venus,Mars,Jupiter,Saturn,Uranus, andNeptune, theirsatellites, as well as smaller bodies includingcomets,asteroids, anddust.[1]
In ancient and medieval times, only objectsvisible to the naked eye—the Sun, the Moon, the fiveclassical planets, andcomets, along with phenomena now known to take place inEarth's atmosphere, likemeteors andaurorae—were known.[dubious –discuss] Ancient astronomers were able to make geometric observations with various instruments. The collection of precise observations in theearly modern period and the invention of thetelescope helped determine theoverall structure of the Solar System. Telescopic observations resulted in the discovery ofmoons andrings aroundplanets, and new planets, comets and theasteroids; 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 aboutouter space and itsdeep-sky objects.
The composition of stars and planets was investigated withspectroscopy. Observations of Solar System bodies with other types of electromagnetic radiation became possible withradio astronomy,infrared astronomy,ultraviolet astronomy,X-ray astronomy, andgamma-ray astronomy.
Roboticspace probes, theApollo program landings of humans on the Moon, andspace telescopes have vastly increased human knowledge about the atmosphere, geology, and electromagnetic properties of other planets, giving rise to the new field ofplanetary science.
The Solar System is one of many planetary systems in the galaxy.[1][2] The planetary system that contains Earth is named the "Solar" System. The word "solar" is derived from the Latin word for Sun,Sol (genitiveSolis). Anything related to the Sun is called "solar": for example,stellar wind from the Sun is calledsolar wind.
The first humans had limited understanding of the celestial bodies that could be seen in the sky. TheSun, however, was of immediate interest, as it generates the day-night cycle. Also,dawn andsunset always appear at roughly the same points of the horizon, which helped to develop thecardinal directions. TheMoon was another body of immediate interest, because of its larger apparent size. Thelunar phases helped them measure time in longer periods than days, and to predict the duration of theseasons.[3]
Prehistoric beliefs about the structure andorigin of the universe were highly diverse, often rooted inreligious cosmology, and many are unrecorded. Many associated theclassical planets (star-like points visible with the naked eye) withdeities, in part due to their puzzlingforward and retrograde motion against the otherwisefixed stars, which gave them their nickname of "wanderer stars", πλάνητες ἀστέρες (planētes asteres) in Ancient Greek, from which today's word "planet" was derived.[4]
Systematic astronomical observations were performed in many areas around the world, and started to inform cosmological knowledge, although they were mostly driven byastrological purposes such asdivination and/oromens. Early historic civilizations inEgypt, theLevant,pre-Socratic Greece,Mesopotamia, andancient China, recorded beliefs in aflat Earth.Vedic texts proposed a number of shapes, including a wheel (flat) and a bag (concave), though they likely promote aspherical Earth, which they refer to asbhugol (orभूगोल in Hindi and Sanskrit), which literally translates to "spherical land".[5] Ancient models were typicallygeocentric, putting the Earth at the center of the universe,[6] 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 inChinese 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 (calledPhosphorus by the Greeks) and the bright planet sometimes seen near the sunset (calledHesperus by the Greeks) were actually the same planet,Venus.[7]
Though unclear if motivated by empirical observations, the concept of aspherical Earth apparently first gained intellectual dominance in thePythagorean school in Ancient Greece in the 5th century BC.[8] Meanwhile, thePythagorean astronomical system proposed the Earth and Sun and acounter-Earth rotate around an unseen "Central Fire". Influenced by Pythagoran thinking andPlato, philosophersEudoxus,Callippus, andAristotle all developed models of the solar system based onconcentric 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 ofclassical elements to explain phenomena such as falling rocks and rising flames; objects in the sky were theorized to be composed of a unique element calledaether.
A later geocentricmodel developed by Ptolemy attached smaller spheres to a smaller number of large spheres to explain the complex motions of the planets, a device known asdeferent and epicycle first developed byApollonius of Perga. Published in theAlmagest, this model ofcelestial spheres surrounding a spherical Earth was reasonably accurate and predictive,[9] 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.[9]
Various astronomers, especially those who had access to more precise[citation needed] observations, were skeptical of the geocentric model and proposed alternatives, including theheliocentric 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 inAncient Greece;Martianus Capella taught in the earlyMiddle Ages that both Mercury and Venus orbit the Sun, while the Moon, the Sun and the other planets orbit the Earth;[10] inAl-Andalus,Arzachel proposed that Mercury orbits the Sun, andheliocentric astronomers worked in the Maragha school in Persia.Kerala-based astronomerNilakantha Somayaji proposed a geoheliocentric system, in which the planets circled the Sun while the Sun, Moon and stars orbited the Earth.
Finally, Polish astronomerNicolaus Copernicus developed in full a system calledCopernican heliocentrism, in which the planets and the Earth orbit the Sun, and the Moon orbits the Earth. This theory was known to Danish astronomerTycho Brahe, but he did not accept it, and proposed his own geoheliocentricTychonic system. Brahe undertook a substantial series of more accurate observations. German natural philosopherJohannes Kepler at first worked to combine the Copernican system withPlatonic solids, in line with his interpretation of Christianity and an ancient musical resonance theory known asMusica 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 inellipses. He formulated and published what are now known asKepler's laws of planetary motion from 1609 to 1619. This became the dominant model among astronomers, though as withcelestial 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 theAge of Discovery already well under way,astronomical thought in America was based on the older Greek theories,[11] but newer western European ideas began to appear in writing by 1659.[12]
The invention of thetelescope 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 polymathGalileo Galilei was an early user and made prolific discoveries, including thephases of Venus, which definitively disproved the arrangement of spheres in the Ptolemaic system.Galileo also discovered that theMoon was cratered, that the Sun was marked withsunspots, and that Jupiter hadfour satellites in orbit around it.[13]Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moonTitan and the shape of therings of Saturn.[14]Giovanni Domenico Cassini later discovered four moremoons of Saturn and theCassini division in Saturn's rings.[15]
Around 1677,Edmond Halley observed atransit of Mercury across the Sun, leading him to realise that observations of thesolar parallax of a planet (more ideally using thetransit of Venus) could be used totrigonometrically determine the distances between Earth,Venus, and the Sun.[16] In 1705, Halley realised that repeated sightings ofa 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,[17] though this had been theorized about comets in the 1st century bySeneca.[18] Around 1704, the term "Solar System" first appeared in English.[19]
English astronomer and mathematicianIsaac Newton, incidentally building on recent scientific inquiries into the speed at which objects fall, was inspired by claims by rivalRobert Hooke of a proof of Kepler's laws. Newton was able to explain the motions of the planets by hypothesizing a force ofgravity acting between all solar system objects in proportion to their mass and aninverse-square law for distance –Newton's law of universal gravitation. Newton's 1687Philosophiæ Naturalis Principia Mathematica explained this along withNewton's laws of motion, for the first time providing a unified explanation for astronomical and terrestrial phenomena. These concepts became the basis ofclassical mechanics, which enabled future advancements in many fields ofphysics.
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 forbinary stars in the constellation ofTaurus 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.[20]
Giuseppe Piazzi discoveredCeres 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 asasteroids.[21]
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 andUrbain Le Verrier's calculations eventually led to the discovery ofNeptune.[22] The excess perihelion precession ofMercury's orbit led Le Verrier to postulate the intra-Mercurian planetVulcan in 1859, but that would turn out not to exist: the excess perihelion precession was finally explained by Einstein'sgeneral relativity, which displaced Newton's theory as the most accurate description of gravity on large scales.
Eventually, new moons were discovered alsoaround Uranus starting in 1787 by Herschel,[23]around Neptune starting in 1846 byWilliam Lassell[24] andaround Mars in 1877 byAsaph Hall.[25]
Further apparent discrepancies in the orbits of the outer planets ledPercival Lowell to conclude that yet another planet, "Planet X", must lie beyond Neptune. After his death, hisLowell Observatory conducted a search that ultimately led toClyde Tombaugh's discovery ofPluto 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 adwarf planet by the IAU.[22]
In 1668Isaac Newton buildshis ownreflecting telescope, the first fully functional of this kind, and a landmark for future developments as it reducesspherical aberration with nochromatic aberration.[26] Today, most powerful telescopes in the world are of that type.
In 1840John W. Draper takes adaguerreotype of the Moon, the first astronomical photograph.[27] Since then,astrophotography is a key tool in the observational studies of the skies.
Spectroscopy is a method that permits to study materials by means of the light they emit,[28]developed around 1835–1860 byCharles Wheatstone,[29]Léon Foucault,[30]Anders Jonas Ångström[31] and others.Robert Bunsen andGustav Kirchhoff further develop thespectroscope, which they used to pioneer the identification of thechemical elements in Earth, and also in the Sun.[32] Around 1862 FatherAngelo Secchi developed theheliospectrograph, enabling him to study both the Sun and the stars, and identifying them as things intrinsically of the same kind.[33] In 1868Jules Janssen andNorman Lockyer discovered a new element in the Sun unknown on Earth,helium, which currently comprises 23.8% of the mass in the solarphotosphere.[34] As of today, spectroscopes are an important tool to know about the chemical composition of the celestial bodies.
By the mid-20th century, new important technologies forremote sensing and observation arose, asradar,radio astronomy andastronautics.
In ancient times, there was a common belief in the so-called "sphere offixed stars", a giant dome-like structure orfirmament centered on Earth which was the boundary of the wholeuniverse, rotating daily around the Earth. SinceHellenistic astronomy and through theMiddle Ages, the estimated radius of such a sphere was becoming increasingly large, up to inconceivable distances. But by the EuropeanRenaissance, it was deemed improbable that such a huge sphere could complete a single revolution of 360° around the Earth in only 24 hours,[35] and this point was one of the arguments ofNicholas Copernicus for discarding the centuries-old geocentric model.
In the 16th century, a number of writers inspired by Copernicus, such asThomas Digges,[36]Giordano Bruno[37] andWilliam Gilbert,[35] argued for an indefinitely extended or even infinite universe, with other stars as distant suns, paving the way to deprecate the Aristotelian sphere of the fixed stars.
WhenGalileo Galilei examined the skies and constellations through atelescope, he concluded that the "fixed stars" which had been studied and mapped were only a tiny portion of the massive universe that lay beyond the reach of the naked eye.[38] He also aimed his telescope at the faint strip of theMilky Way, and he found that it resolves into countless white star-like spots, presumably further stars themselves.[39]
The term "Solar System" had entered the English language by 1704, whenJohn Locke used it to refer to the Sun, planets, and comets as a whole.[40] By then it had been established beyond doubt that planets are other worlds; thus the stars would be other distant suns, and the whole Solar System is actually only a small part of an immensely large universe.
Although it is debatable when the Solar System as such was truly "discovered", three 19th century observations determined its nature and place in the Universe beyond reasonable doubt. First, in 1835–1838,Thomas Henderson[41] andFriedrich Bessel[42] successfully measured twostellar parallaxes, apparent shifts in the position of a nearby star due to Earth's motion around the Sun. This was not only a direct, experimental proof of heliocentrism (James Bradley had already done that in 1729 when he discovered that the cause of theaberration of starlight is the Earth's motion around the Sun),[43] but also accurately revealed, for the first time, the vast distance between the Solar System and the closest stars. Then, in 1859,Robert Bunsen andGustav Kirchhoff, using the newly inventedspectroscope, examined the spectral signature of the Sun and discovered that it was composed of the same elements as existed on Earth, establishing for the first time a physical similarity between Earth and the other bodies visible from Earth.[44] Then, FatherAngelo Secchi compared the spectral signature of the Sun with those of other stars, and found them virtually identical.[33] The realisation that the Sun is a star led to a scientifically updated hypothesis that other stars could have planetary systems of their own, though this was not to be proven for nearly 140 years.
Observational cosmology began with attempts byWilliam Herschel to describe the shape of the then known universe. In 1785, he proposed theMilky Way was a disk, but assumed the Sun was at the center. This heliocentric theory was overturned bygalactocentrism in the 1910s, after more observations byHarlow Shapley placed theGalactic Center relatively far away.
In 1992, the first evidence of aplanetary system other than our own was discovered, orbiting thepulsarPSR B1257+12. Three years later,51 Pegasi b, the firstextrasolar planet around a Sunlike star, was discovered. NASA announced in March 2022 that the number of discovered exoplanets reached 5,000, of several types and sizes.[45]
Also in 1992, astronomersDavid C. Jewitt of theUniversity of Hawaii andJane Luu of theMassachusetts Institute of Technology discoveredAlbion. This object proved to be the first of a new population, which became known as theKuiper belt; an icy analogue to the asteroid belt of which such objects asPluto andCharon were deemed a part, the Kuiper belt objects (KBO).[46][47]
Teams byMike Brown,Chad Trujillo andDavid Rabinowitz discovered thetrans-Neptunian objects (TNO)Quaoar in 2002,[48]Sedna in 2003,[49]Orcus andHaumea in 2004[50][51] andMakemake in 2005,[52] part of the most notable KBOs, some now regarded asdwarf planets. Also in 2005 they announced the discovery ofEris, ascattered disc object initially thought to be larger than Pluto, which would make it the largest object discovered in orbit around the Sun since Neptune.[53]New Horizons' fly-by ofPluto in July 2015 resulted in more-accurate measurements of Pluto, which is slightly larger, though less massive, than Eris.
Radar astronomy is the technique for observing nearbyastronomical objects by reflectingradio waves ormicrowaves off target objects and analyzing their reflections, which provide information about the shapes and surface properties of solid bodies, unavailable by other means. Radar can also accurately measure the position and track the movement of such bodies, specially when they are small, as comets and asteroids, as well as to determine distances between objects in the Solar System. In certain casesradar imaging has produced images with up to 7.5-meter resolution.
TheMoon is comparatively close and was studied by radar soon after the invention of the technique in 1946,[54] mainly precise measurements of its distance and its surface roughness.
Other bodies that have been observed by this means include:
By 2018, there have been radar observations of 138main belt asteroids, 789near-Earth asteroids, and 20 comets, including73P/Schwassmann-Wachmann.[63]
Since the start of theSpace Age, a great deal of exploration has been performed byrobotic spacecraft missions that have been organized and executed by various space agencies.
All planets in the Solar System, plus theirmajor moons along someasteroids andcomets, have now been visited to varying degrees by spacecraft launched from Earth. Through these uncrewed missions, humans have been able to get close-up photographs of all the planets and, in the case oflanders, perform tests of the soils andatmospheres of some.
The first artificial object sent into space was the Soviet satelliteSputnik 1, launched on 4 October 1957, which successfully orbited Earth until 4 January the following year.[65] The American probeExplorer 6, launched in 1959, was the first satellite to image Earth from space.
The first successful probe to fly by another Solar System body wasLuna 1, which sped past the Moon in 1959. Originally meant to impact with the Moon, it instead missed its target and became the first artificial object to orbit the Sun.Mariner 2 was the firstplanetary flyby, passing Venus in 1962. The first successful flyby of Mars was made byMariner 4 in 1965.Mariner 10 first passed Mercury in 1974.
The first probe to explore the outer planets wasPioneer 10, which flew by Jupiter in 1973.Pioneer 11 was the first to visit Saturn, in 1979. TheVoyager probes performed aGrand 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 two Voyager probes are now far beyond Neptune's orbit, and are on course to find and study thetermination shock, heliosheath, andheliopause. According toNASA, both Voyager probes have encountered the termination shock at a distance of approximately 93 AU from the Sun.[66]
The first flyby of a comet occurred in 1985, when theInternational Cometary Explorer (ICE) passed by the cometGiacobini–Zinner,[67] whereas the first flybys of asteroids were conducted by theGalileo space probe, which imaged both951 Gaspra (in 1991) and243 Ida (in 1993) on its way toJupiter.
Launched on January 19, 2006, theNew Horizons probe is the first human-made spacecraft to explore the Kuiper belt. This uncrewed mission flew by Pluto in July 2015. The mission was extended to observe a number of other Kuiper belt objects, including a close flyby of486958 Arrokoth on New Year's Day, 2019.[68]
As of 2011, American scientists are concerned that exploration beyond the Asteroid Belt will hampered by a shortage ofPlutonium-238.[needs update]
In 1966, the Moon became the first Solar System body beyond Earth to be orbited by anartificial satellite (Luna 10), followed by Mars in 1971 (Mariner 9), Venus in 1975 (Venera 9), Jupiter in 1995 (Galileo), the asteroidEros in 2000 (NEAR Shoemaker), Saturn in 2004 (Cassini–Huygens), and Mercury andVesta in 2011 (MESSENGER andDawn respectively).Dawn was orbiting the asteroid–dwarf planetCeres since 2015 and it is still there as of 2023, but it became inactive since 2018. In 2014Rosetta spacecraft becomes the first comet orbiter, aroundChuryumov–Gerasimenko.[69]
The first probe toland on another Solar System body was theSovietLuna 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 untilViking 1 in 1976), the asteroidEros in 2001 (NEAR Shoemaker), Saturn's moonTitan in 2004 (Huygens), the cometsTempel 1 (Deep Impact) in 2005, andChuryumov–Gerasimenko (Philae) in 2014.[70] TheGalileo orbiter also dropped a probe into Jupiter's atmosphere in 1995, this was intended to descend as far as possible into the gas giant before being destroyed by heat and pressure.
As of 2022[update], three bodies in the Solar System, the Moon, Mars andRyugu[71] have been visited bymobile rovers. The first robotic rover to visit another celestial body was the SovietLunokhod 1, which landed on the Moon in 1970. The first to visit another planet wasSojourner, which travelled 500 metres across the surface of Mars in 1997. The first flying probe on in Solar System was theVega balloons in 1985, while first powered flight was undertook byIngenuity in 2020. The only crewed rover to visit another world was NASA'sLunar Roving Vehicle, which traveled with Apollos15,16 and17 between 1971 and 1972.
In 2022, theDARTimpactor crashed intoDimorphos, theminor-planet moon of the asteroidDidymos, with the explicit purpose of intentionally deviate (slightly) the orbit of a Solar System body for the first time ever, which it accomplished.[72]
In some instances, both human and robotic explorers have taken physical samples of the visited bodies and return them back to Earth. Other extraterrestrial materials came to Earth naturally, asmeteorites, or became stuck toartificial satellites; they are specimens which also allows studying Solar System matter.
| # | Spacecraft | Launch year | Mercury | Venus | Mars | Ceres | Jupiter | Saturn | Uranus | Neptune | Pluto | End year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Venera 3 | 1965 | Crash landing | 1966 | ||||||||
| 2 | Pioneer 10 | 1972 | Flyby | 2003 | ||||||||
| 3 | Pioneer 11 | 1973 | Flyby | Flyby | 1995 | |||||||
| 4 | Mariner 10 | 1973 | Flyby | Flyby | 1975 | |||||||
| 5 | Voyager 1 | 1977 | Flyby | Flyby | — | |||||||
| 6 | Voyager 2 | 1977 | Flyby | Flyby | Flyby | Flyby | — | |||||
| 7 | Galileo | 1989 | Flyby | Orbiter | 2003 | |||||||
| 8 | Ulysses | 1990 | Flyby | 2009 | ||||||||
| 9 | Cassini | 1997 | Flyby | Flyby | Orbiter | 2017 | ||||||
| 10 | Mars Odyssey | 2001 | Orbiter | — | ||||||||
| 11 | MER-A /B | 2003 | Rovers | 2010 / 2018 | ||||||||
| 12 | Mars Express | 2003 | Orbiter | — | ||||||||
| 13 | MESSENGER | 2004 | Orbiter | Flyby | 2015 | |||||||
| 14 | MRO | 2005 | Orbiter | — | ||||||||
| 15 | Venus Express | 2005 | Orbiter | 2014 | ||||||||
| 16 | New Horizons | 2006 | Flyby | Flyby | — | |||||||
| 17 | Dawn | 2007 | Orbiter | 2018 | ||||||||
| 18 | Juno | 2011 | Orbiter | — | ||||||||
| 19 | Curiosity (MSL) | 2011 | Rover | — | ||||||||
| 20 | Tianwen-1 | 2020 | Orbiter | — | ||||||||
| 20 | Zhurong | 2020 | Rover | — | ||||||||
| 21 | Perseverance (Mars 2020) | 2020 | Rover | — | ||||||||
| 21 | Ingenuity (Mars 2020) | 2020 | Flying probe | — |
See also the categories for missions tocomets,asteroids,the Moon, andthe Sun.
The first human being to reach space (defined as analtitude of over 100 km) and to orbit Earth wasYuri Gagarin, aSovietcosmonaut who was launched inVostok 1 on April 12, 1961. The first human to walk on the surface of another Solar System body wasNeil Armstrong, who stepped onto theMoon on July 21, 1969 during theApollo 11 mission. From then until 1972, there were five more Moon landings. The United States' reusableSpace Shuttle flew 135 missions between 1981 and 2011. Two of the five shuttles were destroyed in accidents.
The first orbitalspace station to host more than one crew wasNASA'sSkylab, which successfully held three crews from 1973 to 1974. True human settlement in space began with the Soviet space stationMir, which was continuously occupied for close to ten years, from 1989 to 1999. Its successor, theInternational Space Station, has maintained a continuous human presence in space since 2001. In 2004, U.S. PresidentGeorge W. Bush announced theVision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a crewed mission to Mars.
Legend
☄ – orbit or flyby[73]
❏ - Space observatory
Ѫ – successful landing on an object
⚗ – sample return
⚘ – crewed mission[74]
ↂ – permanent inhabited space station[75]
| Country or organisation | LEO | Moon | Mars | Mars moons | SSSBs | Venus | Mercury | Outer Solar System |
|---|---|---|---|---|---|---|---|---|
 United States | ☄❏⚘ↂ | ☄❏Ѫ⚗⚘ | ☄Ѫ | ☄ | ☄Ѫ⚗ | ☄Ѫ | ☄ | ☄ |
 Soviet Union[76] | ☄❏⚘ↂ | ☄Ѫ⚗ | ☄Ѫ | ☄ | ☄ | ☄Ѫ | ||
 People's Republic of China | ☄❏⚘ↂ | ☄❏Ѫ⚗ | ☄Ѫ | ☄ | ||||
 Russia (since 1992) | ☄❏⚘ↂ | |||||||
 Japan | ☄❏ↂ | ☄Ѫ | ☄Ѫ⚗ | ☄ | ||||
 ESA[77] | ☄❏ↂ | ☄ | ☄ | ☄ | ☄Ѫ | ☄ | ☄Ѫ | |
 India | ☄❏ | ☄Ѫ | ☄ | |||||
 Israel | ☄ | ☄ | ||||||
 South Korea | ☄ | ☄ | ||||||
 United Arab Emirates | ☄ | |||||||
 Iran | ☄ | |||||||
 Ukraine (since 1992) | ☄ | |||||||
 North Korea | ☄ | |||||||
 New Zealand | ☄ | |||||||
| Commercial | ☄⚘ | ☄Ѫ |
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| Satellites | |||||||
|---|---|---|---|---|---|---|---|
| Jupiter | Saturn | Uranus | Neptune | Pluto | |||
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| Selected asteroids, by number | Selected comets | ||||||
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| Trans-Neptunian objects (TNO), named and/or with radius above 200 km, ordered by size | |||||||
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See also the radar images at "Near-Earth object".
[...] to him belongs the merit of having [...] enunciated a complete account of its theory, and of thus having firmly established it as a means by which the chemical constituents of celestial bodies can be discovered through the comparison of their spectra with those of the various elements that exist on this earth.
[...][his] theory of the unity of the world and of the identity of the fixed stars and the sun received most profound scientific demonstration and confirmation.
Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium
{{cite book}}:ISBN / Date incompatibility (help)The Puritan Thomas Digges (1546–1595?) was the earliest Englishman to offer a defense of the Copernican theory. ... Accompanying Digges's account is a diagram of the universe portraying the heliocentric system surrounded by the orb of fixed stars, described by Digges as infinitely extended in all dimensions.
