Module 2: Solar System

Written by kasadmin



  • The solar system is located in an outer spiral arm of the vast Milky Way galaxy.
  • The time taken by the solar system to make a rotation around the center of Milky Way galaxy is called a Cosmic Year or Galactic Year. It takes our solar system about 230 million years to complete one orbit around the galactic center.
  • Solar system consists of:
  • The sun, eight planets revolving around it, their satellites and dwarf planets.
  • Interstellar objects such as asteroids, meteoroids and comets.
  • Interplanetary dust particles, electrically charged gases etc.
  • The Sun is the center of solar system. It is also the largest body in solar system, which accounts for more than 99% of the mass of the solar system.

The Sun

  • The Sun is the center of our solar system and makes up 99.8 percent of the mass of the entire solar system.
  • Sun is the major source of energy in the solar system.
  • Sun exerts huge gravitational pull to keep the planets rotate around it in elliptical orbit.
  • Sun is approximately 460 crore years old and it is expected to glow in a similar manner for next 5 billion years or so on.
  • The sun has a surface temperature of about 55000C (99320F) and increases to 20 million 0C (36 million 0F) in the interior.
  • Sun has a diameter of 1,392, 684 km. It is 109 times wider and 3,30,000 times massive than earth.
  • Hydrogen and Helium are the main component of sun. Hydrogen makes up about 92% of all atoms in the sun while Helium makes up about 7.8%.
  • Energy is generated by nuclear fusion at the sun’s core, as Hydrogen converts to Helium.
  • As the gases heats up, atoms break apart into charged particles, turning the gas into plasma. 99% of matter is in plasma state in the sun.
  • Light of sun reach earth in 8 minute 20 seconds (500 seconds). The light of Proxima Centauri, the nearest star to the sun reaches in about 4.3 light years.
  • Sun rotates in the opposite direction to Earth with the sun rotating from west to east instead of east to west like earth


  • Distance from Earth 1,49,59,78,70,700 km
  • Diameter 1,392,684 km
  • Surface Temperature 55000C – 60000C
  • Core Temperature 15 million 0C – 20 million 0C
  • Rotation speed 25.38 days (w.r.t equator), 33 days (w.r.t. poles)
  • Mass 1.99 × 1030 kg
  • Time taken by sunlight 8 minute 20 seconds (500 seconds)

to reach Earth

  • Solar Winds : Solar wind is a stream of fast moving charged particles (protons, electrons and atoms of heavier metals) from the upper layer of sun, corona.
  • Sun spots: Sun spots are dark, cooler spots on the surface of Sunspots have a temperature of about 35000C where the surrounding hotter areas have temperature of 55000C.
  • Aurora: An aurora is a luminous phenomenon visible in Earth’s sky, predominantly seen in high altitudes. It is caused by the collision between air molecules and the charged particles from the sun that is captured by earth’s magnetic field. Aurora in northern hemisphere is called Aurora Borealis or Northern Light and in Southern hemisphere is called Aurora Australias or Southern Light.
  • The brief period between sunrise and full daylight is called dawn, and that between sunset and complete darkness is called twilight.
  • The atmosphere of the sun is composed of three layers: the photosphere, chromosphere and corona.
  • Photosphere is the deepest layer and the visible part of the sun.
  • The layer in the sun above the photosphere is called chromosphere. Its temperature varies from 60000C to about 20,0000C.
  • The corona is the outer most layer of the sun, starting at about 2000 km above the photosphere. The temperature in corona is about 500,0000C.
  • The corona is visible with naked eye only during a total solar eclipse.
  • Coronagraph is a telescope that can see things very close to the sun. It is used to study the sun’s outer atmosphere, corona.
  • Helioseismology is the study of the interior of the sun using observations of waves on the Sun’s surface.
  • Solar and Heliospheric Observatory (SOHO) mission is a joint project of NASA and European Space Agency launched in 1995 to study the internal structure of the sun, its extensive outer atmosphere and the origin of the solar wind.
  • Aditya L1 is the first Indian space mission to study the sun. The satellite will be launched during 2019 – 2020 timeframe by PSLV.

Parker Solar Probe

  • The Parker Solar Probe is a solar mission launched by NASA on August 12, 2018 to study the outer corona of the Sun.
  • The Parker Solar Probe will be the first spacecraft to fly into the low solar corona.
  • The mission was renamed for Eugene Parker, Department of Astronomy and Astrophysics at the University of Chicago.
  • In the 1950s, Parker proposed a number of concepts about how stars—including our Sun—give off energy. He called this cascade of energy the solar wind, and he described an entire complex system of plasmas, magnetic fields, and energetic particles that make up this phenomenon.
  • This is the first NASA mission that has been named for a living individual.
  • Parker Solar Probe now holds the record for closest approach to the Sun by a human-made object. The previous record was set in April 1976 by the Helios 2 spacecraft.
  • The star nearest to the earth – Sun
  • The star nearest to the sun – Proxima Centauri (4.24 light years away)


  • Planets are the celestial bodies which continuously revolve around the sun.
  • The definite elliptical path along with the planets revolve around the sun is called orbit.
  • Movement of planets around its own axis is called rotation and around the sun is called revolution.
  • The word ‘Planet’ came from the Greek word ‘Planetai’, which means wanderers.
  • There are 8 planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
  • Planets are classified into:
    1. Inner planets and outer planets
    2. Terrestrial and Jovian planets
  • Mercury, Venus, Earth and Mars are called Inner Planets because of their proximity to the sun.
  • Inner Planet has very few moons. Earth is the largest of inner planets.
  • Jupiter, Saturn, Uranus and Neptune are much farther from the sun and are called Outer planets. They have more number of moons.
  • Mercury, Venus, Earth and Mars are also called Terrestrial planets because of their structural similarity with the earth.
  • Jupiter, Saturn, Uranus and Neptune are called Jovian planets because of their similarity with Jupiter.

Planets in the Solar System in the decreasing order of size:

1. Jupiter 2. Saturn 3. Uranus 4. Neptune 5. Earth 6. Venus

7. Mars 8. Mercury

Planets in the solar system in the decreasing order of distance from the sun:

1. Neptune 2. Uranus 3. Saturn 4. Jupiter 5. Mars 6. Earth

7. Venus 8. Mercury


  • Period of Revolution : 88 days
  • Period of Rotation : 58 days
  • Mercury is nearest to sun and the smallest planet in the solar system.
  • It is the second hottest and second densest body in the solar system.
  • Mercury is also known as “will-O-the wisp”
  • Mercury is named after Roman God of Messenger.
  • It is the fastest revolving planet with period of revolution (length of a year) 88 earth days.
  • Mercury does not have atmosphere due to low escape velocity.
  • The surface of Mercury is dominated by craters and a younger area of dark plains made from flood of lava.
  • The surface of the planet facing sun receives maximum heat and become hotter; and other side will be extremely cold.
  • Mercury has no moons and rings around it.
  • Mercury has only been visited by two spacecraft—NASA’s Mariner 10 and MESSENGER.
  • First space craft to visit Mercury was Mariner-10 (USA, 1973). Messenger flew past Mercury three times before going into orbit around Mercury in 2011.
  • The third mission to Mercury is BepiColombo, a joint mission between the Japan Aerospace Exploration Agency (JAXA) and the European Space Agency. The two spacecrafts of BepiColombo was launched on 20 October 2018 and is expected to reach Mercury in 2025.


  • Period of Revolution : 224.7 days
  • Period of Rotation : 243 days
  • Venus is the hottest planet in the solar system.
  • Unlike other planets, Venus rotates from East to West. It is also known as Morning Star and Evening Star as it is seen in the East in morning and in the west in the evening.
  • Venus is also known as ‘Earth’s twin’. Its size is nearly same as that of the Earth.
  • Venus is named after the Roman goddess of love and beauty.
  • Venus is the second brightest object in the night sky, because it reflects back 70% of sunlight that falls on it.
  • Venus take more time to complete one rotation than one revolution. It has the slowest rotational speed.(It means that a day in Venus is longer than an year)
  • The atmosphere of Venus is mainly made up of carbon dioxide with clouds of sulfuric acid.
  • Venus has high plateaus, folded mountain belts, numerous volcanoes, and relatively smooth volcanic plains.
  • Maxwell Mount is the highest mountain range in Venus.
  • Venera 1 was the first spacecraft to fly by Venus.
  • The first space craft to land on Venus was Venera-7.
  • The first orbiter which produced detailed maps of Venus using radar was Magellan (USA).
  • The Venus Climate Orbiter mission (PLANET-C), or “Akatsuki”, is studying the atmospheric circulation of Venus.


  • Period of Revolution : 365 days 5 hours 48 minutes 45.51 seconds
  • Period of Rotation : 23 hours 56 minutes 4.09 seconds
  • Earth is known as ‘Blue Planet’ due to presence of water.
  • It is the largest inner planet in the solar system.
  • Shape of earth is called Oblate Spheroid or Geoid.
  • It is the third nearest planet to the sun and ranks fifth in size.
  • Earth is the only planet that supports life due to favourable atmosphere.
  • Our atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface as meteorites.
  • It is the densest planet in the solar system.
  • Moon is the only natural satellite of Earth.
  • The escape velocity of earth is 11.2 km/s.
  • Earth day is observed on 22nd April every year.


  • Period of Revolution : 687 days
  • Period of Rotation : 24 hours 37 minutes 22.63 seconds
  • Mars is also known as ‘Fossil Planet’ and ‘Rusty Planet’.
  • It is also known as ‘Red Planet’ due to the presence of Iron Oxides.
  • It has a thin atmosphere comprising of Nitrogen and Argon.
  • Mars is almost half the size of earth and mass of that of earth.
  • It is visible from the earth for most part of the year.
  • Three huge volcanoes, one more than 28 km high exists at Mars.
  • Mars is named after the Roman god of war.
  • Olympus Mons, the largest volcanic mountain in the solar system is located at Mars.
  • Phobos and Deimos are the two satellites of Mars. They were discovered by Asaph Hall in 1877.
  • Smallest satellite in solar system – Deimos
  • The satellite also known as ‘black moon’ – Phobos
  • First successful space craft launched to observe Mars was Mariner-4 (1964).
  • Mariner 9 was the first spacecraft to orbit around Mars (Nov 14, 1971). It was the first ever space craft orbited around another planet.
  • First space craft to land on the surface of Mars was Viking-1 (USA, launched on 20th August 1975).
  • In 1997, Mars Pathfinder lands and dispatches Sojourner, the first wheeled rover to explore the surface of another planet.
  • Curiosity is a rover designed to explore Gale Crater on Mars. It was launched on November 26, 2011 and landed in 2012.
  • The space probe sent to Mars by NASA on November 2013 to study the upper atmosphere of Mars was MAVEN (Mars Atmosphere and Volatile Evolution).
  • Exo Mars Trace Gas Orbiter is an atmospheric research orbiter built in collaboration between European Space Agency (ESA) and Russia. It was launched on October 19, 2016 to gain a better understanding of Methane and other trace gases in Martian atmosphere.
  • In May 2018, NSA launched InSight, a Mars lander aimed to study the interior of the planet Mars.
  • The rocket that launched InSight also launched a separate NASA technology experiment: two CubeSats called Mars Cube One, or MarCO. Its goal is to test new miniaturized deep space communication equipment. A CubeSat is a type of miniaturized satellite for space research. These two MarCO Cubesats were the first CubeSats to leave earth orbit.


  • Pathfinder is an American Robotic spacecraft launched on 4th December 1996 and landed on the surface of Mars on 4th July 1997.
  • It had a wheeled robotic rover (space exploration vehicle) named Sojourner.
  • Sojourner, the first robot that landed on Martian surface, is also the first rover to operate outside Earth-moon system.
  • The spot where Sojourner was landed is now named Carl Sagan Memorial Station.


  • Curiosity is a robotic rover launched by NASA on 26th November 2011.
  • It landed safely on Mars on 6th August 2012.
  • The main goal was to determine if Mars was ever able to support microbial life.
  • Curiosity landed on Gale Crater. The landing of Curiosity is also known as “Seven Minutes of Terror”.


  • Mangalyaan was India’s first interplanetary mission. The official name of Mangalyaan is Mars Orbiter Mission (MOM).
  • India is the 4th country in the world and first Asian country to launch space craft to Mars.
  • Mangalyaan was launched on 5th November 2013 from Sathish Dhawan Space Centre, Sriharikota
  • MOM was launched aboard PSLV C-25, which was an XL variant of the PSLV
  • It carries a color camera and four scientific instruments to study the Martian surface and atmosphere
  • Mangalyaan reached the Mars orbit on 24th September 2014
  • It is the first Indian spacecraft to escape the Sphere Of Influence of Earth and orbit Sun.
  • It was the first Mars mission in the world to succeed Mars Orbit Insertion in first attempt.
  • MOM completed 1000 Earth days in its orbit on June 19, 2017
  • Mangalyaan is world’s least expensive mission to Mars (450 crore).
  • The launch mass of Mangalyaan was1337 kg.
  • The ISRO chairman at the time of Mangalyaan launching was K. Radhakrishnan


  • Period of Revolution : 12 years
  • Period of Rotation : 9 hours 50 minutes 30 seconds
  • Jupiter is the largest planet in the solar system. It is 11 times bigger than earth.
  • It is the fastest rotating planet and it have shortest day and night in solar system
  • It is too massive to solidify as a planet and composed mostly of gas and liquid. It has no solid surface.
  • Jupiter has strongest gravitational force and magnetic force.
  • Jupiter has the highest escape velocity among other planets (60 km/s)
  • Jupiter rotates once about every 10 hours (a Jovian day), but takes about 12 Earth years to complete one orbit of the Sun (a Jovian year).
  • When viewed from Earth, Jupiter is the second brightest planet in the night sky, after Venus.
  • Jupiter is very windy with wind speed ranges from 192 mph to more than 400 mph.
  • Great Red spot is a giant spinning storm which is more than twice the size of earth is the largest anticyclone in the solar system.
  • Little Red Spot is another storm on planet Jupiter with a wind speed of about 380 mph.
  • Jupiter’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
  • Jupiter has highest number of satellites (67) in solar system. Jupiter and its satellites are together known as ‘Mini Solar System’.
  • On Jul. 17, 2018, scientists announced they had discovered 12 new moons orbiting Jupiter. That raised Jupiter’s total number of moons to 79. The 53 of these moons are confirmed and named; the other 26 are awaiting official confirmation of discovery before they are named.
  • In 1610, Galileo Galilei makes the first detailed observations of Jupiter.
  • Nine spacecraft have visited Jupiter. Seven flew by and two have orbited the gas giant.
  • In 1973 Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter.
  • Galileo spacecraft launched by NASA was the first to orbit the planet Jupiter.
  • Jupiter has a faint ring system that was discovered in 1979 by the Voyager-1 mission
  • Juno is a NASA space probe orbiting the planet Jupiter. It was launched from Cape Canaveral Air Force Station on August 5, 2011 and arrived at Jupiter in 2016. It is conducting an in-depth investigation of the planet’s atmosphere, deep structure and magnetosphere for clues to its origin and evolution.​


  • Largest planet Jupiter
  • Smallest planet Mercury
  • Hottest planet Venus
  • Coldest planet Neptune
  • Fastest rotating planet Jupiter
  • Slowest rotating planet Venus
  • Fastest revolving planet Mercury
  • Slowest revolving planet Neptune
  • Farthest planet to the sun Neptune
  • Closest planet to the sun Mercury
  • Planet with highest density Earth
  • Planet with lowest density Saturn
  • Planet with longest day Venus
  • Planet with shortest day Jupiter
  • Highest escape velocity Jupiter
  • Lowest escape velocity Mercury
  • Planet with highest weight Jupiter
  • Planet with lowest weight Saturn
  • Windiest Planet Neptune


  • Period of Revolution : 30 years
  • Period of Rotation : 10 hours 14 minutes
  • Saturn is the second largest planet in the solar system.
  • It is named after the Roman God of agriculture.
  • Like Jupiter, Saturn does not have a solid surface.
  • It is a gigantic ball made up mostly of lighter gases, in which 63% is Hydrogen
  • Saturn has the least density among other planets. Its density is less than water.
  • Saturn is also known as ‘the jewel of the solar system’.
  • Saturn has the most spectacular ring system which is made up of seven rings with several gaps between them. These rings are probably made up of particles of ice or ice-covered rocks. All four giant planets in our solar system have ring systems.
  • Galileo Galilee was the first person to observe Saturn through a telescope.
  • Pioneer 11 of NASA was the first spacecraft to reach Saturn. Voyager 1, Voyager 2 and Cassini – Huygens are the other missions visited Saturn.
  • Saturn has 53 known moons with an additional nine moons awaiting confirmation of their discovery—that is a total of 62 moons.
  • Titan is the largest satellite of Saturn.
  • Saturn cannot support life as we know it, but some of Saturn’s moons have conditions that might support life.
  • Great white spot (Great White Oval) and Dragon Storm are periodic storms on planet Saturn.
  • Few missions have visited Saturn: Pioneer 11 and Voyagers 1 and 2 flew by; But Cassini orbited Saturn 294 times from 2004 to 2017.
  • Pioneer 11 was the first spacecraft to reach Saturn (1979).


  • Cassini – Huygens Mission is a joint project of NASA, European Space Agency (ESA) and Italian Space Agency (ISA) launched in 1997 to study Saturn and its system.
  • Cassini was the fourth space probe to visit Saturn and the first to enter its orbit in 2004.
  • Huygens was separated from Cassini and landed on Titan, a satellite of Saturn in 2005. It is the first spacecraft to make a soft landing on another planet’s satellite.
  • Cassini ended its mission on September 2017.


  • Period of Revolution : 84 days
  • Period of Rotation : 17 hours 12 minutes
  • Uranus is the third largest planet in the solar system.
  • Uranus rotates at a nearly 90 degree angle from the plane of its orbit. This unique tilt make Uranus appear to orbit the sun like a rolling ball.
  • It is also known as ‘Rolling Planet’ or ‘Lying Planet’
  • It was the first planet found with the help of a telescope. It was discovered by William Herschel in 1781.
  • Uranus is one of two Ice giants (the other is Neptune). Most of the planet’s mass is made up of icy materials – water, methane and ammonia – above a small rocky core.
  • Uranus appears in blue – green colour due to the presence of Methane gas in atmosphere.
  • Uranus is one of two planets that rotate from east to west (Venus is the other one). But Uranus is unique in that it rotates on its side.
  • Uranus has about 27 satellites.
  • Uranus has 13 known rings. The inner rings are narrow and dark and the outer rings are brightly colored.
  • Uranus is named after the primal Greek god of sky.
  • Voyager 2 is the only spacecraft to fly by Uranus. No spacecraft has orbited this distant planet.
  • In 2011, New Horizons passed the orbit of Uranus on its way to Pluto, becoming the first spacecraft to journey beyond Uranus’ orbit since Voyager 2.


  • Period of Revolution : 165 years
  • Period of Rotation : 16 hours 6 minutes
  • Neptune was discovered by Johann Gottfried Galle using the calculations by Urbain Le Verrier and John Couch Adams in 1846.
  • It was the first planet to get its existence predicted by mathematical calculation.
  • The first person to observe Neptune through a telescope was Galileo Galilee.
  • Neptune is named after Roman god of the sea.
  • It has the second largest gravity among other planets (first is Jupiter)
  • Great Dark Spot and Small Dark Spot are large storms found in Neptune.
  • Neptune is the coldest planet having a surface temperature of -2140C.
  • The windiest planet in our solar system is Neptune.
  • Neptune is the planet that take maximum time for revolution (or planet which has longest year).
  • Neptune has 13 moons (and one more awaiting confirmation of discovery), which are named after sea gods and nymphs in Greek mythology. Triton is Neptune’s largest moon
  • The three rings of Neptune are named Liberty, Equality and Fraternity.
  • Neptune’s atmosphere is composed of Hydrogen, Helium and trace amounts of Methane.
  • Voyager 2 is the only spacecraft to have visited Neptune. No spacecraft has orbited this distant planet.


As of now, there are five artificial objects achieved the escape velocity that will allow them to leave the Solar System. These are Voyager-1, Voyager-2, New Horizons, Pioneer 10 and Pioneer 11. Voyager 1 was the first human-made object to leave our Solar System when it crossed into interstellar space in 2012. Voyager also made it to interstellar space in 2018. New Horizons is are still active and will eventually transition to the space between the stars. Pioneers 10 and 11 also have reached escape velocity, but both spacecraft have been inactive for many years.

Voyager -1 is the first space craft to leave solar system to reach interstellar space in 2012. It was launched in September 1977, flew past Jupiter in 1979 and Saturn in 1980, making a special close approach to Saturn’s moon Titan. The probe passed the heliosphere (the area of influence of sun) on 25 August 2012 to enter interstellar space and is still active.

Voyager-2 is the only spacecraft to study all the four outer planets of solar system at close range. It was launched in August 1977, flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Voyager 2 left solar system and entered interstellar space on December 2018, making it second human made object to do so.

New Horizons: The Primary mission of this spacecraft was to study Pluto and a secondary mission to fly by and study one or more other Kuiper belt objects (KBOs). Launched in 2006, the probe flew past Jupiter in 2007 and Pluto on 14 July 2015. It is currently headed towards a Kuiper belt object (486958) 2014 MU69 (nick named Ultima Thule). The flyby will occur in January 2019, which will make it the farthest object in the Solar System ever to be visited by a spacecraft.

Dwarf Planets

  • In 2006, the International Astronomical Union (IAU) created a new classification category for objects in solar system called ‘Dwarf Planets’.
  • A dwarf planet is a celestial body that:
    • Orbits the sun
    • Has enough mass to assume a nearly round shape
    • Has not cleared the neighborhood around its orbit
    • Is not a moon
  • IAU recognised five dwarf planets in our solar system. They are (1) Eris (2) Ceres (3) Pluto (4) Makemake and (5) Haumea.


  • Pluto was discovered by Clyde Tombaugh in 1930.
  • It was long considered the smallest and ninth planet in our solar system.
  • After the discovery of similar planets in Kuiper Belt, Pluto was expelled from planets and reclassified as dwarf planet in 2006.
  • Pluto takes 248 years for revolution and 156 hours for rotation.
  • Pluto has a thin atmosphere of nitrogen, methane and carbon monoxide.
  • The five known satellites of Pluto are Charon, Hydra, Nix, Kerberos and Styx.
  • The largest satellite, Charon, is so big that Pluto and Charon orbit each other like a double planet.
  • NASA launched first mission to Pluto named New Horizons in 2006 which passed close by in July 2015.
  • The name ‘Pluto’ was suggested by a 11 year old girl Venetia Burney of England. It is named after Roman God of underworld.


  • Eris is the second largest dwarf planet in the solar system.
  • It evolves around the sun beyond Pluto.
  • It was discovered by Mike Brown in 2005.
  • Dysnomia is the satellite of Eris.


  • Ceres is the only dwarf planet in the solar system located in the inner solar system (between Mars and Jupiter)
  • Ceres is also an asteroid and the largest object in the asteroid belt. It is now classified as a dwarf Planet.
  • While it is the smallest of the known dwarf planets, it is the largest object in the asteroid belt.
  • Ceres is named for the Roman goddess of corn and harvests.
  • Guiseppe Piazzi discovered the first and the largest asteroid Ceres, orbiting Mars and Jupiter.
  • ‘Dawn’ spacecraft of NASA was launched in 2007 to study about Ceres and Vesta, another object in asteroid belt. When Dawn arrived in 2015, Ceres became the first dwarf planet to receive a visit from a spacecraft.
  • It is the only dwarf planet with no satellites.


  • Makemake is the largest object in Kuiper Belt.
  • It was discovered in 2005 by a team led by Michael E. Brown.
  • The name was adopted from the mythology of Rapa Nui people of Easter Island.


  • Haumea is about one third mass of Pluto and was discovered in 2004.
  • It is the least spherical of all the dwarf planets.
  • It is named after Hawaiian Goddess of childbirth.
  • It is the third brightest object in the Kuiper belt, after the dwarf planets Pluto and Makemake. Haumea has a ring around it.
  • Haumea also has two moons named Hi’iaka and Namaka.


Also known as Kuiper – Edgeworth Belt, it is a disk-shaped region in the outer solar system, extending from the orbit of Neptune to around 50 Astronomical Unit from the sun. This region consists of bits of rock and ice, comets, and dwarf planets. These are also called Kuiper Belt Objects (KBOs). The Kuiper Belt is named after a scientist named Gerard Kuiper. In 1951 he proposed the idea that a belt of icy bodies might have existed beyond Neptune when the solar system formed. The dwarf planets Pluto, Makemake, Eris and Haumea are found in this region. Several dwarf planets within the Kuiper Belt have moons. The first mission to explore the Kuiper Belt is New Horizons. It flew past Pluto in 2015.

In 2018, a dwarf planet nicknamed the Goblin has been discovered well beyond Pluto. It is officially known as 2015 TG387 by the International Astronomical Union. Sedna and 2012 VP113 are the two other dwarf planets discovered recently.


Astronomers are searching for a possible planet that is suspected to be in orbit far beyond Pluto that might explain the strange orbits of several Kuiper Belt Objects.  It is given the nickname Planet 9 or Planet X.


The Oort Cloud is a theorised shell of icy objects that lie beyond the Kuiper Belt, in the outermost reaches of the solar system. It is a predicted, but undiscovered region of space. It is named after astronomer Jan Oort, who first theorised its existence. The Oort Cloud is roughly spherical. Objects in the Oort Cloud are also referred to as Trans-Neptunian objects. This name also applies to objects in the Kuiper Belt.

Astronomers think that long-period comets (which take more than 200 years to orbit the Sun) have their origins in the Oort Cloud. Also the most short-period comets (which take 200 years or less to orbit the sun) have their origins in the Kuiper Belt.


  • Asteroids are small rocky celestial bodies revolving around the sun in elliptical orbit. Most of the asteroids are found between the orbits of Mars and Jupiter.
  • Asteroids do not have rings.
  • The largest asteroid is Ceres, which has a diameter of about 1000 km. It was discovered by Giuseppe Piazzi. It is now classified as Dwarf Planet.
  • Ceres encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.
  • Asteroids that pass close to Earth are called Near-Earth Objects, or NEOs.
  • The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993.
  • Galileo was the spacecraft that visited two asteroids – Gaspra in 1991 and Ida in 1993.
  • Near-Earth Asteroid Rendezvous (NEAR – Shoemaker) mission studied asteroids Mathilde and Eros.
  • The Rosetta mission encountered Steins in 2008 and Lutetia in 2010.
  • Dawn spacecraft of NASA was launched in 2007. It orbited and explored the asteroid Vesta for over one year and then headed towards Ceres, another asteroid and a dwarf planet. Dawn entered the orbit of Ceres on 23 April 2015. It was the first mission to explore a dwarf planet. Dawn is the first spacecraft to orbit two extraterrestrial bodies. It was retired on 1 November 2018.
  • In 2003, Japanese robotic spacecraft Hayabusa landed on near-Earth asteroid Itokawa. On June 3, 2010, Hayabusa successfully returned to Earth with a small amount of asteroid dust now being studied by scientists.
  • The spacecraft Hayabusa2 of Japan landed two robotic rovers, collectively named MINERVA II1 on the surface of the asteroid Rygu on Spetember 21, 2018. MINERVA-II1 is world’s first rover to land on the surface of an asteroid.


  • Natural satellites are celestial bodies that orbit a planet or a minor planet.
  • Natural satellites are sometime referred to as ‘moons’.
  • There are more than 150 known moons in our solar system and several more awaiting confirmation of discovery.
  • Johannes Kepler was the first person to use the term ‘satellite’.
  • They are generally solid bodies and few have atmospheres.
  • Out of 8 planets in solar system, six have their own satellites.
  • Mercury and Venus does not have any satellites. Earth has one satellite and Mars has two. Jovian planets have numerous satellites.
  • Jupiter has most number of satellites (67) Saturn has second highest number of satellites (62)
  • Ganymede is the largest satellite in solar system. It is the satellite of Jupiter.
  • Titan, the satellite of Saturn is the second largest satellite in solar system. It was discovered by Christian Huygens (1655).
  • Titan is the only satellite in the solar system with an atmosphere of its own. Nitrogen is the main component in Titan’s atmosphere.
  • The satellites of Saturn are named after Greek mythical characters.
  • The satellites of Uranus are named after characters created by William Shakespeare and Alexander Pope.
  • The largest satellite of Neptune is Triton. It revolves around Neptune in the opposite direction.
  • Charon is the largest satellite of Pluto.
Jupiter 67 (12 more discovered, making it 79) Ganymede, Io, Europa, Callisto, Himalia Lysithea, Thebe, Elara
Saturn 62 Titan, Prometheus, Pandora, Atlas, Helan, Thethys Phoebe
Uranus 27 Umbriel, Miranda, Ariel, Titania, ophelia,Oberon
Neptune 14 Triton, Nereid, Proteus, Larissa, Despina, Thalassa
Mars 2 Phobos, Deimos
Earth 1 Moon


  • Moon is the only natural satellite of the Earth.
  • It is fifth largest satellite in solar system and has a size of about 27% of the earth
  • Over 59% of Moon’s surface is visible from the earth.
  • The study of moon is called Selenology.
  • The highest point on the surface of moon is Mount Leibnitz (35000 ft), situated on the south pole of the moon
  • The volume of moon is only 2% of volume of earth and the mass of moon is 1.2% of that of earth
  • The escape velocity from the surface of moon is 2.376 km/sec.
  • The period of rotation and revolution of moon are almost same, meaning the same side is always taking the earth.
  • The force due to gravity at the surface of moon is only about one sixth of that on Earth (If an object weigh 60 kg on the surface of earth, it will weigh only 10 kg at the lunar surface.
  • The sky always appears black in moon due to the absence of atmosphere. No sound can be heard on the surface of moon and is unprotected from meteorites and solar winds.
  • Most abundant elements found on lunar surface are Oxygen, Silicon, Iron, Magnesium, Calcium and Titanium.
  • Ilmenite, the ore of titanium is found abundantly on the surface of moon.
  • The light will reach from moon to earth in 1.3 seconds.
  • The moon has a very thin and tenuous (weak) atmosphere, called an exosphere.
  • More than 100 spacecrafts have been launched to explore the moon. It is the only celestial body beyond Earth that has been visited by human beings.
  • The first space craft to reach the surface of moon was Luna – 2 (Russia). It is the first man-made object to land on another celestial body.
  • The first spacecraft to orbit the moon successfully was Luna -10 (the first artificial satellite of moon).
  • Apollo 11 mission of NASA carried three astronauts to moon in 1969. It was launched by Saturn V rocket from Kennedy space station, Florida.
  • Twelve human beings have walked on the surface of the moon.
  • Neil Armstrong was the first astronaut to land on the moon. He was followed by Edwin Aldrin.
  • Neil Armstrong described this as: “one small step for a man, a giant leap for mankind”.
  • The lunar module which they used to land on the moon was called Eagle. They landed on an area called ‘Sea of Tranquility’.
  • Eugene Cernan was the last man to have walked on Moon (1972).


  • Distance from Earth 384400 km
  • Orbital Velocity 3680.5 km/h
  • Density 3.344 g/cm3
  • Diameter 3475 km
  • Mass 7.35 × 1022 kg
  • Circumference 11000 km
  • Maximum distance from earth (Apogee) 406000 km
  • Minimum distance from Earth (Perigee) 364000 km


  • Government of India approved ISRO’s proposal for the first Indian Moon Mission, called Chandrayaan-1 in November 2003.
  • Chandrayaan-1, India’s first mission to Moon, was launched successfully on October 22, 2008 from Sathish Dhavan Space Centre, Sriharikota.
  • The rocket used to launch Chandrayan-1 was PSLV C-11. Vikram Sarabhai Space Centre, Thiruvananthapuram designed and developed PSLV-C11.
  • The spacecraft was orbiting around the Moon at a height of 100 km from the lunar surface
  • The spacecraft carried 11 scientific instruments built in India, USA, UK, Germany, Sweden and Bulgaria.
  • The payload (instrument) in Chandrayaan spacecraft that separated from it and hit on the surface of moon was Moon Impact Probe (MIP). Moon Impact Probe landed on Shackleton crater on the South Pole of Moon .
  • The payload in Chandrayaan spacecraft that detected the presence of water in lunar surface was Moon Mineralogy Mapper.
  • ISRO Chairman at the time of Chandrayaan launching was G. Madhavan Nair.
  • The mission was concluded when the communication with the spacecraft was lost on August 29, 2009
  • ISRO is planning to launch Chandrayaan – 2, the country’s second mission to moon. It is expected to launch before October 2018.
  • The total cost of the Chandrayaan – 2 Mission will be about Rs 800 crore.

Lunar Mare (singular: lunar maria) are large, dark plains on the surface of the moon. These basaltic plains are formed as the result of volcanic eruptions in the ancient period. Sea of Tranquility, Ocean of Storms etc. are some of the lunar mare.


  • The first artificial satellite in the world – Sputnik I (4th October 1957)
  • Sputnik I was launched by – Russia
  • The first animal sent to space – The dog name ‘Leika’
  • The first man in space – Yuri Gagarin (Russia)
  • The spacecraft in which Yuri Gagarin travelled – Vostok – 1
  • Yuri Gagarin went to space on – 12thApril 1961
  • The person who was known as ‘Columbus of the space’ – Yuri Gagarin
  • The first woman in space – Valentina Tereshkova (Russia, 1963)
  • The first American space traveler – Alan Shepard
  • The first American woman in space – Sally K Ride
  • The oldest man in space – John Glenn (77 years) [He died on December 8, 2016)
  • The first person to walk in space – Alexi Leonov (USSR, 1965)
  • The first woman to walk in space – Svetlana Savitskaya
  • The first Indian in space – Rakesh Sharma (1984 – Soyuz T11)(138th space traveler in the world)
  • The space agency of America – NASA
  • The year in which NASA was established – 1958
  • The rocket launching station of NASA – Cape Canaveral
  • The first satellite of America – Explorer – I
  • The first space station in the world – Salyut – I (Russia)
  • The first space shuttle in the world – Colombia (USA)
  • The first woman of Indian origin in space is – Kalpana Chawla (1997)
  • The whole universe is my Native Land” words of – Kalpana Chawla
  • Kalpana Chawla was killed in the space shuttle Columbia disaster on – 1st February 2003
  • The first Indian satellite to study about weather – Metsat (2002)
  • The Metsat was renamed as – Kalpana I
  • The second woman of Indian origin in space is – Sunitha Williams
  • The woman who holds the record of maximum number of space walks (8) – Peggy Whitson [Peggy Whitson breaks Sunitha Williams record of maximum number of space walks (7)]
  • The oldest woman to reach space Peggy Whitson (56 years, NASA)
  • First marathon in space was run by Sunitha Williams
  • Japan’s communication satellite launched by space X þ JCSAT 14
  • The largest space station in the world – International Space Station
  • World’s largest radio telescope – FAST (Five hundred meter Aperture Spherical Telescope), China
  • The robotic spacecraft of NASA to be launched in 2020 to perform in – orbit satellite servicing in low Earth orbit – Restore L. Mission
List of Space Agencies
Names Full Form Nation
NASA National Aeronautics and Space Administration USA
ROSCOSMOS Russian Federal Space Agency Russia
ESA European Space Agency EU
CNES French Space Agency France
JAXA Japan Aerospace Exploration Agency Japan
DLR German Aerospace Centre Germany
ISRO Indian Space Research Organization India
CNSA China National Space Administration China
ASI Italian Space Agency Italy
ISA Iranian Space Agency Iran
UKSA UK Space Agency United Kingdom
SUPARCO Space and Upper Atmosphere Research Commission Pakistan


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