The universe is a very large place in which we huddle in a small corner. It is called the Solar System and is not only a tiny fraction of the known universe, but also a very small part of our galactic environs - the Milky Way galaxy. In short, we are a point in the endless cosmic sea.
Nevertheless, the solar system remains a relatively large place with many secrets (for now). We have only recently begun to closely study the hidden nature of our little world. In terms of exploring the solar system, we barely scratched the surface of this box.
Understanding the Solar System
With few exceptions, before the era of modern astronomy, only a few people or civilizations understood what the solar system was. The vast majority of astronomical systems postulated that the Earth is a stationary object around which all known celestial objects revolve. In addition, it was significantly different from other stellar objects that were considered ethereal or divine in nature.
Although there were some Greek, Arab and Asian astronomers during the ancient and medieval period who believed that the universe was heliocentric (that is, that the earth and other bodies revolve around the sun), it was only when Nicolaus Copernicus developed a mathematical predictive model of the heliocentric system in the 16th century that this the idea was widespread.
Galileo (1564-1642) often showed people how to use a telescope and observe the sky in Piazza San Marco in Venice. Please note, there were no adaptive optics in those days.
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During the 17th century, scientists like Galileo Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics that gradually led to the acceptance that the earth revolves around the sun. The development of theories like gravity has also led to the realization that other planets obey the same physical laws as the Earth.
The widespread adoption of telescopes also led to a revolution in astronomy. After Galileo discovered the moons of Jupiter in 1610, Christian Huygens discovered that Saturn also has moons in 1655. New planets (Uranus and Neptune), comets (Halley's comet) and the asteroid belt were also discovered.
By the 19th century, three observations made by three separate astronomers determined the true nature of the solar system and its place in the universe. The first was done in 1839 by the German astronomer Friedrich Bessel, who successfully measured the apparent shift in the position of a star created by the Earth's motion around the Sun (stellar parallax). This not only confirmed the heliocentric model, but also showed the gigantic distance between the Sun and the stars.
In 1859, Robert Bunsen and Gustav Kirchhoff (German chemist and physicist) used a newly invented spectroscope to determine the spectral signature of the sun. They discovered that the Sun is composed of the same elements that exist on Earth, thereby proving that the earthly firmament and the heavenly firmament are made of the same matter.
Then Angelo Secchi's father - an Italian astronomer and director of the Pontifical Gregorian University - compared the spectral signature of the Sun with the signatures of other stars and found that they were almost identical. This convincingly showed that our sun is composed of the same materials as any other star in the universe.
Further apparent discrepancies in the orbits of the outer planets led the American astronomer Percival Lowell to the conclusion that "Planet X" must lie outside Neptune. After his death, the Lowell Observatory undertook the necessary research that eventually led Clyde Tombaugh to the discovery of Pluto in 1930.
In 1992, astronomers David K. Jevitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered a trans-Neptunian object (TNO) known as (15760) 1992 QB1. It entered a new population known as the Kuiper Belt, which astronomers have been talking about for a long time and which should lie at the edge of the solar system.
Further exploration of the Kuiper Belt at the turn of the century led to additional discoveries. The discovery of Eris and other "plutoids" by Mike Brown, Chad Trujillo, David Rabinowitz and other astronomers has led to a harsh discussion between the International Astronomical Union and some astronomers on the designation of planets, large and small.
The structure and composition of the solar system
At the core of the Solar System is the Sun (a G2 main sequence star), which is surrounded by four terrestrial planets (inner planets), the main asteroid belt, four gas giants (outer planets), a massive field of small bodies extending from 30 AU. e. up to 50 amu. e. from the Sun (Kuiper belt) and a spherical cloud of icy planetesimals, which is believed to have extended to a distance of 100,000 AU. e. from the Sun (Oort cloud).
The sun contains 99.86% of the known mass of the system, and its gravity affects the entire system. Most of the large objects in orbit around the Sun lie near the plane of the Earth's orbit (ecliptic), and most bodies and planets revolve around it in the same direction (counterclockwise when viewed from the North Pole of the Earth). Planets are very close to the ecliptic, while comets and Kuiper belt objects are often at a steep angle to it.
The four largest rotating bodies (gas giants) account for 99% of the remaining mass, with Jupiter and Saturn accounting for more than 90% in total. The rest of the solar system objects (including the four terrestrial planets, dwarf planets, moons, asteroids and comets) together make up less than 0.002% of the total mass of the solar system.
Sun and planets
Sometimes astronomers informally divide this structure into separate regions. The first, the inner solar system, includes four terrestrial planets and the asteroid belt. Behind it lies the outer solar system, which includes four gas giants. Meanwhile, there are also the outermost parts of the solar system, which are considered a separate region containing trans-Neptunian objects, that is, objects beyond Neptune.
Most of the planets of the solar system have their own secondary systems, planetary objects revolve around them - natural satellites (moons). The four giant planets also have planetary rings - thin bands of tiny particles rotating in unison. Most of the largest natural satellites are in synchronous rotation, with one side constantly facing their planet.
The sun, which contains almost all the matter in the solar system, is 98% hydrogen and helium. The terrestrial planets of the inner solar system are composed mainly of silicate rocks, iron and nickel. Behind the asteroid belt, the planets consist mainly of gases (hydrogen, helium) and ices - methane, water, ammonia, hydrogen sulfide and carbon dioxide.
Objects farther from the Sun are composed primarily of materials with lower melting points. Ice matter makes up most of the satellites of the giant planets, as well as Uranus and Neptune (which is why we sometimes call them "ice giants") and numerous objects lying beyond the orbit of Neptune.
Gases and ices are considered to be volatile substances. The boundary of the solar system, beyond which these volatiles condense, known as the "snow line", is at 5 AU. e. from the sun. Objects and planetesimals in the Kuiper belt and Oort clouds are composed mostly of these materials and rock.
The formation and evolution of the solar system
The solar system formed 4.568 billion years ago during the region's gravitational collapse in a giant molecular cloud of hydrogen, helium, and small amounts of heavier elements synthesized by previous generations of stars. When this region, which was to become the solar system, collapsed, the conservation of angular momentum caused it to rotate faster.
The center, where most of the mass had gathered, began to get hotter and hotter than the surrounding disk. As the collapsing nebula rotated faster, it began to align itself into a protoplanetary disk with a hot, dense protostar at its center. The planets were formed by the accretion of this disk, in which dust and gas pulled together and combined to form larger bodies.
Due to the higher boiling point, only metals and silicates can exist in solid form close to the Sun and eventually form the terrestrial planets - Mercury, Venus, Earth and Mars. Since the metallic elements were only a small part of the solar nebula, the terrestrial planets were unable to grow very large.
In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter, where materials were cold enough for the volatile ice components to remain solid (on the snow line).
The ices that formed these planets were more abundant than the metals and silicates that formed the inner terrestrial planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Remaining debris that will never become planets has collected in regions like the asteroid belt, Kuiper belt and Oort cloud.
Over 50 million years, the pressure and density of hydrogen at the center of the protostar became high enough for thermonuclear fusion to begin. Temperature, reaction rate, pressure and density were increased until hydrostatic equilibrium was reached.
At this point, the Sun became a main sequence star. The solar wind from the Sun created the heliosphere and swept away the remaining gas and dust of the protoplanetary disk into interstellar space, ending the planetary formation process.
The solar system will remain much the same as we know it until the hydrogen in the sun's core is completely converted to helium. This will happen in about 5 billion years and will mark the end of the main sequence of the Sun's life. At this time, the core of the Sun will collapse and the energy output will be much greater than it is now.
The outer layers of the Sun will expand about 260 times its current diameter and the Sun will become a red giant. The expansion of the Sun is expected to vaporize Mercury and Venus and render the Earth uninhabitable as the habitable zone leaves Mars orbit. Eventually, the core will become hot enough to start helium fusion, the sun will burn the helium a little more, but then the core will begin to shrink.
At this point, the outer layers of the Sun will go into space, leaving behind a white dwarf - an extremely dense object that will have half the original mass of the Sun, but will be the size of the Earth. The ejected outer layers will form a planetary nebula, returning some of the material that formed the Sun into interstellar space.
Inner solar system
In the inner solar system, we find the "inner planets" - Mercury, Venus, Earth, and Mars - so named because they orbit closer to the Sun. In addition to their proximity, these planets have a number of key differences from other planets in the solar system.
For starters, the inner planets are solid and earthy, composed mostly of silicates and metals, while the outer planets are gas giants. The inner planets are closer together than their outer counterparts. The radius of this entire region is less than the distance between the orbits of Jupiter and Saturn.
Typically, the inner planets are smaller and denser than their counterparts and have fewer moons. The outer planets have dozens of moons and rings of ice and rock.
The inner terrestrial planets are made up mostly of refractory minerals like silicates that form their crust and mantle, and metals - iron and nickel - that lie in the core. Three of the four inner planets (Venus, Earth, and Mars) have sufficiently significant atmospheres to shape the weather. All are dotted with impact craters and have surface tectonics, rift valleys and volcanoes.
Of the inner planets, Mercury is the closest to our Sun and the smallest of the terrestrial planets. Its magnetic field is only 1% of that of the earth, and its very thin atmosphere dictates temperatures of 430 degrees Celsius during the day and -187 at night, as the atmosphere cannot keep warm. It has no satellites and is composed mostly of iron and nickel. Mercury is one of the densest planets in the solar system.
Venus, which is roughly the size of Earth, has a dense toxic atmosphere that traps heat and makes the planet the hottest in the solar system. Its atmosphere is 96% carbon dioxide, along with nitrogen and several other gases. Dense clouds within the Venusian atmosphere are composed of sulfuric acid and other corrosive compounds, with little addition of water. Most of Venus's surface is marked by volcanoes and deep canyons - the largest at over 6,400 kilometers in length.
Earth is the third inner planet and the best studied. Of the four terrestrial planets, Earth is the largest and the only one with liquid water necessary for life. The Earth's atmosphere protects the planet from harmful radiation and helps to retain valuable sunlight and heat under the shell, which is also necessary for the existence of life.
Like other terrestrial planets, Earth has a rocky surface with mountains and canyons and a heavy metal core. The Earth's atmosphere contains water vapor, which helps to moderate daily temperatures. Like Mercury, the Earth has an internal magnetic field. And our Moon, the only satellite, consists of a mixture of various rocks and minerals.
Mars is the fourth and last inner planet, also known as the "Red Planet", thanks to the oxidized, iron-rich materials found on the planet's surface. Mars also has a number of interesting surface properties. The planet has the largest mountain in the solar system (Olympus) with a height of 21,229 meters above the surface and the giant Valles Marineris canyon, 4000 km long and up to 7 km deep.
Most of the surface of Mars is very old and filled with craters, but there are also geologically new zones. The polar caps are located at the Martian poles, which decrease in size during the Martian spring and summer. Mars is less dense than Earth and has a weak magnetic field, which speaks more of a solid core than a liquid one.
The thin atmosphere of Mars has led some astronomers to the idea that liquid water existed on the planet's surface, only evaporated into space. The planet has two small moons - Phobos and Deimos.
Outer solar system
The outer planets (sometimes called Trojan planets, giant planets, or gas giants) are huge planets enveloped in gas, with rings and many satellites. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since ancient times, showing astronomers that the solar system is much larger than they thought.
Jupiter is the largest planet in our solar system, which rotates very quickly (10 Earth hours) relative to its orbit around the Sun (which takes 12 Earth years to pass). Its dense atmosphere is composed of hydrogen and helium, possibly encircling the Earth's core. The planet has dozens of moons, several faint rings and the Great Red Spot, a raging storm that has lasted for 400 years.
Saturn is known for its prominent ring system - seven famous rings with well-defined divisions and spaces between them. How the rings formed is not yet entirely clear. The planet also has dozens of satellites. Its atmosphere is made up mostly of hydrogen and helium, and it rotates quite rapidly (10.7 Earth hours) relative to its time around the Sun (29 Earth years).
Uranium was first discovered by William Herschel in 1781. A planet's day lasts about 17 Earth hours, and one orbit around the Sun takes 84 Earth years. Uranium contains water, methane, ammonia, hydrogen, and helium around a solid core. The planet also has dozens of satellites and a weak ring system. The only vehicle that has visited the planet is Voyager 2 in 1986.
Neptune - a distant planet containing water, ammonia, methane, hydrogen and helium and a possible Earth-sized core - has more than a dozen moons and six rings. The Voyager 2 spacecraft also visited this planet and its system in 1989 while passing through the outer solar system.
Trans-Neptunian region of the solar system
More than a thousand objects have been discovered in the Kuiper belt; it is also assumed that there are about 100,000 objects larger than 100 km in diameter. Given their small size and extreme distance from Earth, the chemical composition of Kuiper Belt objects is difficult to determine.
But spectrographic studies of the region have shown that its members are mostly composed of ice: a mixture of light hydrocarbons (like methane), ammonia and water ice - comets have the same composition. Initial research also confirmed a wide range of colors in Kuiper belt objects, from neutral gray to deep red.
This suggests that their surfaces are composed of a wide variety of compounds, from dirty ice to hydrocarbons. In 1996, Robert Brown obtained spectroscopic data on KBO 1993 SC, which showed that the composition of the object's surface is extremely similar to that of plutons (and of Neptune's moon Triton) in that it has a large amount of methane ice.
Water ice has been found in several Kuiper Belt objects, including 1996 TO66, 38628 Huya, and 2000 Varuna. In 2004, Mike Brown et al. Determined the existence of crystalline water and ammonia hydrate at one of the largest known Kuiper objects of 50,000 Quaoar. Both of these substances were destroyed during the life of the solar system, which means that the surface of Kwavar has recently changed due to tectonic activity or the fall of a meteorite.
Pluto's company in the Kuiper belt is worthy of a mention. Kwavar, Makemake, Haumea, Eris and Ork are all large ice bodies of the Kuiper belt, some of them even have satellites. They are extremely distant, but still within reach.
Oort cloud and distant regions
It is believed that the Oort cloud extends from 2000-5000 AU. e. up to 50,000 a. e. from the Sun, although some extend this range to 200,000 AU. e. This cloud is believed to consist of two regions - the spherical outer Oort cloud (within 20,000 - 50,000 AU) and the disc-shaped inner Oort cloud (2000 - 20,000 AU).
The outer Oort cloud can have trillions of objects greater than 1 km and billions more than 20 km in diameter. Its total mass is unknown, but - assuming Halley's comet is a typical representation of the outer objects of the Oort cloud - it can be roughly delineated at 3 × 10 ^ 25 kilograms, or five Earths.
Based on the analysis of recent comets, the vast majority of objects in the Oort cloud are composed of volatile ice-like substances - water, methane, ethane, carbon monoxide, hydrogen cyanide and ammonia. The appearance of asteroids is believed to be explained by the Oort cloud - there may be 1-2% of asteroids in the population of objects.
The first estimates placed their mass in the 380 Earth masses, but the expanded knowledge of the distribution of comets from long periods lowered these indicators. The mass of the inner Oort cloud has not yet been calculated. The contents of the Kuiper belt and the Oort cloud are called trans-Neptunian objects, since objects in both regions have orbits that are farther from the Sun than Neptune's.
Solar system exploration
Our knowledge of the solar system has expanded dramatically with the advent of robotic robotic spacecraft, satellites and robots. Since the mid-20th century, we have had the so-called "space age", when manned and unmanned spacecraft began to explore the planets, asteroids and comets of the inner and outer solar system.
All the planets of the solar system have been visited to varying degrees by vehicles launched from Earth. During these unmanned missions, people were able to obtain photographs of the planets. Some missions even made it possible to “taste” the soil and atmosphere.
"Sputnik-1"
The first man-made object sent into space was the Soviet Sputnik-1 in 1957, which successfully circled the Earth and collected information about the density of the upper atmosphere and ionosphere. The American probe Explorer 6, launched in 1959, was the first satellite to take pictures of the Earth from space.
Robotic spacecraft have also revealed a lot of meaningful information about the planet's atmospheric, geological and surface features. The first successful probe to fly past another planet was the Soviet probe Luna 1, which was accelerated by the Moon in 1959. The Mariner program led to many successful orbital flybys, with the Mariner 2 probing Venus in 1962, Mariner 4 Mars in 1965 and Mariner 10 Mercury in 1974.
By the 1970s, probes were sent to other planets, beginning with the Pioneer 10 mission to Jupiter in 1973 and the Pioneer 11 mission to Saturn by 1979. Voyager's probes have made a grand tour of other planets since their launch in 1977, both passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then came close to Uranus in 1986 and Neptune in 1989.
Launched on January 19, 2006, the New Horizons probe was the first artificial spacecraft to explore the Kuiper belt. In July 2015, this unmanned mission flew past Pluto. In the coming years, the probe will study a number of objects in the Kuiper belt.
Orbiters, rovers, and lander began deploying to other planets in the solar system by the 1960s. The first was the Soviet satellite Luna-10, sent into lunar orbit in 1966. It was followed by 1971 with the deployment of the Mariner 9 space probe, which orbited Mars, and the Soviet probe Venera 9, which entered Venus orbit in 1975.
The Galileo probe became the first artificial satellite to orbit the outer planet when it reached Jupiter in 1995; it was followed by the Cassini-Huygens mission to Saturn in 2004. Mercury and Vesta were explored in 2011 by the MESSENGER and Dawn probes, respectively, after which Dawn visited the orbit of the dwarf planet Ceres in 2015.
The first probe to land on another body in the solar system was the Soviet Luna 2, which fell on the moon in 1959. Since then, probes have landed or fell on the surface of Venus in 1966 (Venus 3), Mars in 1971 (Mars 3 and Viking 1 in 1976), asteroid Eros 433 in 2001 (NEAR Shoemaker) and Saturn's moon Titan (Huygens) and Comet Tempel 1 (Deep Impact) in 2005.
The Curiosity Rover took this mosaic self-portrait with a MAHLI camera on flat sedimentary rock.
To date, only two worlds in the solar system, the Moon and Mars, have been visited by roving rovers. The first robotic rover to land on another body was the Soviet Lunokhod 1, which landed on the moon in 1970. In 1997, the Sojourner landed on Mars, which traveled 500 meters on the planet's surface, followed by Spirit (2004), Opportunity (2004), Curiosity (2012).
Manned missions to space began in the early 50s, and the two superpowers, the United States and the USSR, who were tied in the space race, had two focal points. The Soviet Union focused on the Vostok program, which included sending manned space capsules into orbit.
The first mission - "Vostok-1" - took place on April 12, 1961, the first man - Yuri Gagarin - went into space. On June 6, 1963, the Soviet Union also sent the first woman into space - Valentina Tereshkova - as part of the Vostok-6 mission.
In the United States, the Mercury project was initiated with the same purpose of putting a capsule with a crew into orbit. On May 5, 1961, astronaut Alan Shepard went into space on the Freedon 7 mission and became the first American in space.
After the Vostok and Mercury programs ended, the focus of attention of both states and space programs turned out to be the development of a spacecraft for two or three people, as well as long-term space flights and extravehicular activities (EVA), that is, the spacewalk in self-contained spacesuits.
As a result, the USSR and the USA began to develop their own programs "Voskhod" and "Gemini". For the USSR, this included developing a capsule for two or three people, while Gemini focused on the development and expertise needed for a possible manned flight to the moon.
This latest effort led to the Apollo 11 mission on July 21, 1969, when astronauts Neil Armstrong and Buzz Aldrin became the first humans to walk on the moon. As part of this program, five more lunar landings were carried out, and the program brought many scientific messages from Earth.
After landing on the moon, the focus of American and Soviet programs began to shift towards the development of space stations and reusable spacecraft. For the Soviets, this resulted in the first manned orbital stations dedicated to space science research and military reconnaissance, known as the Salyut and Almaz space stations.
The first orbital station to accommodate more than one crew was NASA's Skylab, which successfully accommodated three crews from 1973 to 1974. The first real human settlement in space was the Soviet Mir station, which was consistently occupied for ten years, from 1989 to 1999. It was closed in 2001, and its successor, the International Space Station, has maintained a constant human presence in space ever since.
The US space shuttles, which debuted in 1981, have become and remain the only reusable spacecraft that have successfully completed many orbital flights. Five shuttles built (Atlantis, Endeavor, Discovery, Challenger, Columbia and Enterprise) flew a total of 121 missions until the program was closed in 2011.
During its history of functioning, two such devices have died in disasters. These were the disaster of the Challenger, which exploded on takeoff on January 28, 1986, and the Columbia, which collapsed on re-entering the atmosphere on February 1, 2003.
What happened next, you know very well. The peak of the 60s gave way to a short exploration of the solar system and, eventually, decline. Perhaps very soon we will receive a sequel.
All the information received during the missions about geological phenomena or other planets - about mountains and craters, for example - as well as about their weather and meteorological phenomena (clouds, dust storms and ice caps) led to the realization that other planets are experiencing essentially the same phenomena like the Earth. In addition, all this helped scientists to learn more about the history of the solar system and its formation.
As our exploration of the inner and outer solar system continues to gain momentum, our approach to categorizing planets has changed. Our current model of the solar system includes eight planets (four terrestrial, four gas giants), four dwarf planets, and a growing number of trans-Neptunian objects that have yet to be identified.
Given the enormous size and complexity of the solar system, it will take many years to explore it in full detail. Will it be worth it? Certainly.
Ilya Khel