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General Science Supplement for Forest Beat Officer · Chapter 9

Universe, Space and Satellites

What to remember

  • The universe contains billions of galaxies; each galaxy has billions of stars. Our galaxy is the Milky Way and our star is the Sun.
  • Planets orbit the Sun under gravity; satellites orbit planets. The speed and height of a satellite decide its orbit and its use.
  • India's space programme is run by ISRO, with launch vehicles (PSLV, GSLV) and satellites for communication, remote sensing and navigation. Sriharikota in Andhra Pradesh is the launch centre.

The universe and stars

The Big Bang theory says the universe began from a hot, dense state about 13.8 billion years ago and has been expanding since. A galaxy is a huge system of stars, gas and dust held by gravity. Galaxy shapes: spiral, elliptical and irregular. The Milky Way is a spiral galaxy. The nearest large galaxy is Andromeda.

A light year is the distance light travels in one year, about 9.46 × 10¹² km. It is a unit of distance, not time. An astronomical unit (AU) is the mean Earth–Sun distance, about 1.5 × 10⁸ km. Light takes about 8 minutes to reach us from the Sun, and the speed of light is about 3 × 10⁸ m/s.

Stars make energy by nuclear fusion: hydrogen nuclei join to form helium. The Sun is a medium-sized yellow star. The star nearest to Earth after the Sun is Proxima Centauri. The Pole Star (Polaris) appears nearly fixed in the northern sky. A constellation is a group of stars that forms a pattern, such as Ursa Major (Great Bear or Saptarishi) and Orion.

Life of a star: a cloud of gas becomes a star; a star like the Sun becomes a red giant and then a white dwarf. A very massive star ends in a supernova and may leave a neutron star or a black hole. A black hole has gravity so strong that even light cannot escape. Chandrasekhar limit (about 1.4 solar masses) is the maximum mass of a white dwarf. It is named after the Indian-born scientist S. Chandrasekhar.

The solar system

The Sun, eight planets, their moons, dwarf planets, asteroids and comets form the solar system.

PlanetKey fact
MercurySmallest planet; closest to the Sun
VenusHottest planet; rotates opposite to most planets; brightest planet in the sky
EarthOnly known planet with life; one moon
MarsRed planet because of iron oxide; two small moons
JupiterLargest planet; Great Red Spot
SaturnProminent rings; least dense
UranusRotates on its side
NeptuneFarthest planet

Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Terrestrial (rocky) planets are the inner four; Jupiter and Saturn are gas giants; Uranus and Neptune are ice giants. In 2006 Pluto was reclassified as a dwarf planet. The asteroid belt lies between Mars and Jupiter. Comets are icy bodies with tails that point away from the Sun. Meteors are streaks of light when small rocky pieces burn in the atmosphere; those that reach the ground are meteorites.

Earth facts: it rotates once in about 24 hours (day and night) and revolves around the Sun in about 365 and a quarter days (year). The tilt of the axis, about 23.5 degrees, causes the seasons. Solstices and equinoxes mark seasonal changes. The Moon takes about 27 days to orbit Earth. A solar eclipse occurs when the Moon lies between the Sun and Earth (new moon). A lunar eclipse occurs when Earth lies between the Sun and Moon (full moon). The Moon is not a source of light; it reflects sunlight.

Gravitation and orbits

Newton's law of gravitation: F = G m₁ m₂ / r². Gravity weakens with distance. Orbital velocity near the Earth's surface is about 7.9 km/s. Escape velocity from Earth is about 11.2 km/s. Escape velocity is √2 times the orbital velocity at the same height. Acceleration due to gravity on Earth is about 9.8 m/s². On the Moon it is about one-sixth of that on Earth. Weight changes with place; mass does not.

Kepler's laws: planets move in ellipses with the Sun at one focus; a line from the planet to the Sun sweeps equal areas in equal times; the square of the orbital period is proportional to the cube of the mean distance (T² ∝ r³).

A satellite stays in orbit because gravity provides the centripetal force that keeps it moving around the Earth. Objects inside a satellite feel weightlessness since they are in free fall.

Satellites and orbits

OrbitHeight (approx.)Use
Low Earth Orbit (LEO)Up to about 2,000 kmRemote sensing, imaging, space stations
Polar / Sun-synchronous orbitAbout 600 to 900 kmEarth observation; passes over the same place at the same local time
Medium Earth OrbitAbout 20,000 kmNavigation systems
Geostationary orbit (GEO)About 36,000 km above the equatorCommunication, weather, broadcasting

A geostationary satellite has a period of 24 hours, orbits in the equatorial plane in the direction of Earth's rotation, and appears fixed over one point. Natural satellite: the Moon. Artificial satellites: communication (INSAT, GSAT), remote sensing (IRS, Cartosat, Resourcesat, RISAT), navigation (NavIC, earlier IRNSS), weather and scientific satellites. India's first satellite was Aryabhata (1975). Space-based navigation systems: GPS (USA), GLONASS (Russia), Galileo (EU), BeiDou (China), NavIC (India, regional).

Uses for forestry: remote sensing satellites give images to map forest cover, detect forest fires, track deforestation and study wildlife habitats. GPS helps in boundary demarcation. Weather satellites help predict cyclones that hit the Andhra Pradesh coast.

India's space programme

  • ISRO (Indian Space Research Organisation) has its headquarters at Bengaluru. Dr Vikram Sarabhai is known as the father of the Indian space programme.
  • Sriharikota (the Satish Dhawan Space Centre) is the main launch site and lies on the east coast in Andhra Pradesh.
  • Launch vehicles: SLV, ASLV, PSLV (Polar Satellite Launch Vehicle, a reliable workhorse for polar orbits), GSLV (Geosynchronous Satellite Launch Vehicle) and LVM3 for heavier payloads. PSLV is known for launching many satellites in a single flight.
  • Missions: Chandrayaan (Moon), Mangalyaan or Mars Orbiter Mission, Aditya-L1 (Sun) and Gaganyaan (planned human spaceflight). Chandrayaan-3 landed near the lunar south pole region. Mangalyaan made India the first country to reach Mars orbit in its first attempt. Check the latest official release for recent missions.
  • Rocket principle: Newton's third law. Hot gases rush out backward and the rocket moves forward. Rockets use solid, liquid or cryogenic fuel. A cryogenic engine uses liquid hydrogen and liquid oxygen at very low temperatures.
  • Yuri Gagarin was the first human in space; Rakesh Sharma was the first Indian in space; Neil Armstrong was the first person on the Moon. Sputnik 1 was the first artificial satellite.

More about the Sun, Moon and Earth

The Sun holds more than 99 percent of the mass of the solar system. Its outer visible layer is the photosphere, its outer atmosphere is the corona (seen during a total solar eclipse), and sunspots are cooler dark patches. The Sun's surface temperature is much lower than its core temperature. Solar flares and storms can disturb satellites and radio signals.

The Moon has no atmosphere and no liquid water on its surface, so its temperature changes sharply between day and night. The Moon always shows the same face to Earth because it rotates once in the same time that it takes to revolve once. Tides on Earth come mainly from the Moon's gravity; the Sun adds to them. Spring tides occur at new and full moon.

Space debris and good use of space

Old satellites and rocket parts form space debris that can damage working satellites. Many satellites help daily life: weather forecasts and cyclone warnings, television, mobile networks, mapping, disaster alerts, and tracking of fishing boats and wildlife.

Space stations orbit in low Earth orbit; the International Space Station is built by several countries together. Space telescopes, such as Hubble, avoid the blur of Earth's atmosphere. Telescope types include optical, radio and X-ray telescopes.

Exam traps

  • 1. A light year measures distance, not time.
  • 2. Venus is the hottest planet, not Mercury; Mercury is closest to the Sun.
  • 3. Jupiter is the largest planet; Mercury is the smallest.
  • 4. Geostationary satellites are about 36,000 km up; remote sensing satellites are much lower.
  • 5. Mass is the same everywhere; weight changes with gravity.
  • 6. A solar eclipse occurs at new moon; a lunar eclipse at full moon.
  • 7. PSLV launches into polar orbits; GSLV is for geosynchronous orbits.
  • 8. Mars is red because of iron oxide, not because of hot temperature.

One-liners

  • 1. The Sun is a star made mostly of hydrogen and helium.
  • 2. Light takes about 8 minutes to reach Earth from the Sun.
  • 3. The Milky Way is a spiral galaxy.
  • 4. Aryabhata was India's first satellite.
  • 5. Sriharikota is in Andhra Pradesh.
  • 6. Escape velocity from Earth is about 11.2 km/s.
  • 7. The asteroid belt lies between Mars and Jupiter.
  • 8. Saturn has the best known ring system.
  • 9. Pluto is a dwarf planet.
  • 10. NavIC is India's regional navigation system.
  • 11. A geostationary period is 24 hours.
  • 12. Weightlessness occurs in free fall.

Worked examples

  • 1. Earth–Sun light time: distance 1.5 × 10¹¹ m, speed 3 × 10⁸ m/s. t = 1.5 × 10¹¹ / 3 × 10⁸ = 500 s, about 8.3 minutes.
  • 2. Weight on the Moon of a 60 kg person (g on Earth 10 m/s², Moon one-sixth): Earth weight 600 N; Moon weight = 100 N.
  • 3. Kepler's third law: if a planet is 4 times farther than Earth, T² ∝ r³ gives T² = 64, so T = 8 Earth years.
  • 4. Gravity at twice the Earth's radius from the centre: g falls to one-fourth, because g ∝ 1/r².

Practice questions

  1. Our galaxy is called the

    1. Sombrero
    2. Andromeda
    3. Triangulum
    4. Milky Way
    Answer

    D. Milky Way

    The Milky Way is a spiral galaxy.

  2. The largest planet of the solar system is

    1. Neptune
    2. Earth
    3. Saturn
    4. Jupiter
    Answer

    D. Jupiter

    Jupiter is the largest.

  3. The hottest planet in the solar system is

    1. Mars
    2. Jupiter
    3. Mercury
    4. Venus
    Answer

    D. Venus

    Venus has a thick CO2 atmosphere with a strong greenhouse effect.

  4. The planet closest to the Sun is

    1. Mars
    2. Venus
    3. Mercury
    4. Earth
    Answer

    C. Mercury

    Mercury is also the smallest planet.

  5. Mars appears red because of

    1. red clouds
    2. sulphur
    3. hot lava
    4. iron oxide on its surface
    Answer

    D. iron oxide on its surface

    Rusty dust gives the red colour.

  6. A light year is a unit of

    1. time
    2. speed
    3. distance
    4. brightness
    Answer

    C. distance

    It is the distance light travels in one year.

  7. India's first satellite was

    1. INSAT-1A
    2. Aryabhata
    3. Rohini
    4. Bhaskara
    Answer

    B. Aryabhata

    Aryabhata was launched in 1975.

  8. India's main satellite launch centre is located at

    1. Thumba
    2. Bengaluru
    3. Sriharikota
    4. Balasore
    Answer

    C. Sriharikota

    Sriharikota is in Andhra Pradesh.

  9. Who is known as the father of the Indian space programme?

    1. Homi Bhabha
    2. Vikram Sarabhai
    3. APJ Abdul Kalam
    4. CV Raman
    Answer

    B. Vikram Sarabhai

    Vikram Sarabhai founded ISRO's early programme.

  10. A geostationary satellite orbits at a height of about

    1. 36,000 km
    2. 2,000 km
    3. 400 km
    4. 600 km
    Answer

    A. 36,000 km

    It has a 24-hour period above the equator.

  11. The time period of a geostationary satellite is

    1. 12 hours
    2. 90 minutes
    3. 365 days
    4. 24 hours
    Answer

    D. 24 hours

    It matches Earth's rotation.

  12. Remote sensing satellites for Earth observation commonly use

    1. geostationary orbits only
    2. interplanetary orbits
    3. no orbit
    4. sun-synchronous polar orbits
    Answer

    D. sun-synchronous polar orbits

    They pass over a place at the same local time.

  13. Which launch vehicle is known as the workhorse of ISRO for polar orbits?

    1. GSLV Mk III only
    2. PSLV
    3. SLV-3
    4. ASLV
    Answer

    B. PSLV

    PSLV is the Polar Satellite Launch Vehicle.

  14. The rocket works on Newton's

    1. first law of motion
    2. law of gravitation only
    3. third law of motion
    4. second law of thermodynamics
    Answer

    C. third law of motion

    Exhaust goes backward, rocket goes forward.

  15. A black hole is a region where

    1. stars are born
    2. light is emitted strongly
    3. only dust exists
    4. gravity is so strong that light cannot escape
    Answer

    D. gravity is so strong that light cannot escape

    This is the standard definition.

  16. A solar eclipse occurs when

    1. the Moon lies between the Sun and Earth
    2. Mars lies between the Sun and Earth
    3. Earth lies between the Sun and Moon
    4. the Sun lies between Earth and the Moon
    Answer

    A. the Moon lies between the Sun and Earth

    It occurs at new moon.

  17. The asteroid belt lies between

    1. Mars and Jupiter
    2. Venus and Earth
    3. Jupiter and Saturn
    4. Earth and Mars
    Answer

    A. Mars and Jupiter

    Standard fact.

  18. Which was reclassified as a dwarf planet in 2006?

    1. Uranus
    2. Pluto
    3. Neptune
    4. Mercury
    Answer

    B. Pluto

    Pluto is a dwarf planet.

  19. Which Indian mission made India the first country to reach Mars orbit in its first attempt?

    1. Gaganyaan
    2. Chandrayaan-1
    3. Mangalyaan
    4. Aditya-L1
    Answer

    C. Mangalyaan

    Mangalyaan is the Mars Orbiter Mission.

  20. The speed of light is about 3 x 10^8 m/s. How long does light take to travel 9 x 10^8 m?

    1. 30 seconds
    2. 9 seconds
    3. 3 seconds
    4. 0.3 seconds
    Answer

    C. 3 seconds

    t = 9 x 10^8 / 3 x 10^8 = 3 s.

  21. Light from a star takes 5 years to reach Earth. Its distance is

    1. 5 AU
    2. 50 light years
    3. 5 seconds
    4. 5 light years
    Answer

    D. 5 light years

    Distance in light years equals travel time in years.

  22. A person weighs 600 N on Earth. On the Moon (gravity one-sixth) the weight is

    1. 600 N
    2. 100 N
    3. 6 N
    4. 3,600 N
    Answer

    B. 100 N

    600 / 6 = 100 N.

  23. The mass of an astronaut is 70 kg on Earth. His mass on the Moon is

    1. 70 kg
    2. zero
    3. 420 kg
    4. about 12 kg
    Answer

    A. 70 kg

    Mass does not change with place.

  24. If a planet is 4 times as far from the Sun as Earth, by Kepler's third law its period in Earth years is

    1. 8
    2. 4
    3. 16
    4. 2
    Answer

    A. 8

    T squared = 4 cubed = 64, so T = 8.

  25. If the distance between two masses is doubled, the gravitational force becomes

    1. half
    2. double
    3. four times
    4. one-fourth
    Answer

    D. one-fourth

    F is inversely proportional to r squared.

  26. If one of the two masses is doubled and the distance is the same, the gravitational force

    1. becomes four times
    2. doubles
    3. remains the same
    4. halves
    Answer

    B. doubles

    F is proportional to m1 x m2.

  27. The escape velocity from Earth is about

    1. 7.9 km/s
    2. 3 km/s
    3. 11.2 km/s
    4. 30 km/s
    Answer

    C. 11.2 km/s

    7.9 km/s is orbital velocity near the surface.

  28. The orbital velocity near the Earth's surface is about 7.9 km/s. The escape velocity is nearly how many times this?

    1. 1.41
    2. 2
    3. 0.7
    4. 4
    Answer

    A. 1.41

    Escape velocity is root 2 times orbital velocity.

  29. If the Earth's radius is R, acceleration due to gravity at a distance of 2R from the centre is about (g = 10 m/s2)

    1. 10 m/s2
    2. 5 m/s2
    3. 20 m/s2
    4. 2.5 m/s2
    Answer

    D. 2.5 m/s2

    g falls with the square of distance: 10 / 4.

  30. The Earth-Sun distance is about 1.5 x 10^11 m. Time for light to travel this is about

    1. 50 seconds
    2. 500 seconds
    3. 8 hours
    4. 5,000 seconds
    Answer

    B. 500 seconds

    1.5 x 10^11 / 3 x 10^8 = 500 s.

  31. Which fuel pair is used in a cryogenic rocket engine?

    1. Petrol and air
    2. Methane and nitrogen
    3. Liquid hydrogen and liquid oxygen
    4. Coal and water
    Answer

    C. Liquid hydrogen and liquid oxygen

    These are very cold liquefied gases.

  32. Astronauts in an orbiting satellite feel weightless because they are

    1. in vacuum
    2. in free fall
    3. very far from Earth
    4. beyond gravity
    Answer

    B. in free fall

    Gravity still acts, but the satellite and astronaut fall together.

  33. The Moon takes about how many days to orbit Earth?

    1. 90
    2. 7
    3. 365
    4. 27
    Answer

    D. 27

    Approximately 27 days.

  34. Which satellite use helps to map forest cover and detect forest fires?

    1. Navigation only
    2. Television broadcast only
    3. Remote sensing
    4. Telephone only
    Answer

    C. Remote sensing

    Remote sensing images show vegetation and fires.

  35. Which is the first artificial satellite in the world?

    1. Sputnik 1
    2. Aryabhata
    3. Apollo 11
    4. Explorer 1
    Answer

    A. Sputnik 1

    Launched by the USSR.

  36. The first Indian in space was

    1. Shubhanshu Shukla
    2. Kalpana Chawla
    3. Rakesh Sharma
    4. Sunita Williams
    Answer

    C. Rakesh Sharma

    Rakesh Sharma flew in 1984.

  37. Consider the statements. 1. A light year is a unit of time. 2. A light year is a unit of distance. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    It is a distance.

  38. Consider the statements. 1. Mercury is the hottest planet. 2. Venus is the hottest planet. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Venus is hotter due to its greenhouse effect.

  39. Consider the statements. 1. PSLV is used for polar orbit launches. 2. GSLV is used for geosynchronous orbits. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  40. Consider the statements. 1. A geostationary satellite orbits above the equator. 2. It appears fixed over one point on Earth. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  41. Consider the statements. 1. Mass of a body changes from place to place. 2. Weight of a body changes from place to place. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Weight depends on g; mass is constant.

  42. Consider the statements. 1. The Sun produces energy by nuclear fusion. 2. The Sun produces energy mainly by burning coal. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Hydrogen fuses into helium.

  43. Consider the statements. 1. A lunar eclipse occurs at full moon. 2. A solar eclipse occurs at full moon. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A solar eclipse occurs at new moon.

  44. Consider the statements. 1. Jupiter is a gas giant. 2. Uranus rotates almost on its side. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

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