Space MissionsClass 10 Science Part 1 Chapter 10 Notes – Maharashtra Board

Summary: Space Missions

When you check the weather on your phone, watch a cricket match live or use Google Maps on a bike ride, a satellite high above the Earth is helping you. Space missions have turned the world into a global village: they carry phone, TV and internet signals, warn us about cyclones, help find minerals and keep watch on our borders.

Space missions are of two kinds. Some place artificial satellites in orbit around the Earth for useful work. Others send spacecraft far away to study the Moon, planets, asteroids and comets. Satellites are grouped by their job — weather, communication, broadcast, navigation, military and Earth observation — and India has its own series like INSAT, GSAT, IRNSS and IRS.

To stay in orbit, a satellite must move sideways with a particular speed called the critical velocity, v_c = √(GM/(R+h)). The higher the orbit, the slower the satellite moves and the longer it takes to go round. At about 35,780 km height it takes about 24 hours, so it seems to stand still over one place — a geosynchronous satellite. Orbits are called low, medium or high depending on their height.

Launch vehicles work on Newton's third law: hot gases rush out backwards and push the rocket forwards. Multi-stage rockets like ISRO's PSLV drop empty stages to become lighter. To leave Earth completely, a spacecraft needs at least 11.2 km/s, the escape velocity. With this, India sent Chandrayaan-1, which found water on the Moon, and Mangalyaan, which reached Mars on its first try. Today, growing space debris is a new problem that scientists must solve.

Key points to remember

• Space missions are of two types: placing artificial satellites in Earth orbit, and sending spacecraft to study other objects in space.
• The Moon is Earth's only natural satellite; a man-made object revolving around a planet in a fixed orbit is an artificial satellite.
• Sputnik (1957, Soviet Union) was the first artificial satellite; Yuri Gagarin was the first human in space (1961).
• Satellites run on solar energy using photovoltaic panels fitted like wings.
• Critical velocity v_c = √(GM/(R+h)); it does not depend on the satellite's mass and decreases as height increases.
• High Earth orbit: height ≥ 35,780 km; a satellite here above the equator takes about 24 h per revolution and appears stationary (geosynchronous).
• Medium Earth orbit: 2000 km to 35,780 km; includes polar orbits and GPS satellites (about 20,200 km).
• Low Earth orbit: 180 km to 2000 km; period about 90 minutes; ISS and Hubble telescope are in LEO.
• Launch vehicles work on Newton's third law of motion; multi-stage design reduces weight step by step.
• PSLV has four stages: solid, liquid, solid, liquid, plus strap-on solid boosters.
• Escape velocity v_esc = √(2GM/R) = 11.2 km/s for the Earth.
• Chandrayaan-1 (2008) discovered water on the Moon's surface; Mangalyaan (launched Nov 2013) reached Mars orbit in Sept 2014.
• Vikram Sarabhai is the father of the Indian space programme; India's first satellite was Aryabhata.
• Space debris — dead satellites, rocket parts and collision fragments — threatens working satellites and must be managed.

Space Missions – Notes

1. Space missions and why we need them

• Since the late 20th century, progress in space technology has made space travel possible. More than a thousand artificial satellites now orbit the Earth.
• Two kinds of missions: (1) satellites in orbit around Earth for research and daily services; (2) spacecraft sent into outer space to study the solar system and beyond.
• Communication satellites let us talk to anyone in the world within a second and bring internet, TV and phone signals to our homes.
• Weather images of monsoon clouds that you see in newspapers come from satellites; they also give advance warnings of cyclones and floods.
• Satellites help defence (watching enemy activity), and help locate mineral and fossil fuel reserves.
• Famous space travellers: Yuri Gagarin (USSR, first human in space, 1961), Neil Armstrong (USA, first on the Moon, 1969), Rakesh Sharma (first Indian in space, 1984), and Kalpana Chawla and Sunita Williams of Indian origin with NASA.

2. Artificial satellites and their types

• A natural satellite is an astronomical object orbiting a planet; the Moon is Earth's only one. A man-made object orbiting a planet in a fixed path is an artificial satellite.
• Sputnik, launched by the Soviet Union in 1957, was the first artificial satellite.
• Satellites use solar photovoltaic panels for power and carry instruments to receive signals from and send signals to the Earth.
• Weather satellites — study and forecast weather (INSAT, GSAT; launched by GSLV).
• Communication satellites — link places across the world using specific waves; broadcast satellites — telecast TV programmes (INSAT, GSAT; GSLV).
• Navigational satellites — give the exact latitude and longitude of any place (IRNSS; launched by PSLV).
• Military satellites collect security information. Earth observation satellites study forests, deserts, oceans and polar ice, manage natural resources and guide relief during floods and earthquakes (IRS; PSLV).
• Student power: COEP Pune students built 'Swayam', a satellite of about 1 kg, launched by ISRO in 2016 to about 515 km height for point-to-point messaging.

3. Orbits of artificial satellites and critical velocity

• The satellite's job decides its orbit: height above Earth, shape (circular or elliptical) and whether it is above the equator or tilted.
• A launcher carries the satellite to the required height, then gives it a sideways (tangential) speed called the critical velocity, v_c.
• For an orbit of radius r = R + h, the centripetal force mv_c²/(R+h) is supplied by gravity GMm/(R+h)². Equating them gives v_c = √(GM/(R+h)).
• The satellite's mass m cancels, so v_c does not depend on the satellite's mass — a 1 kg Swayam and a 2000 kg INSAT at the same height move at the same speed.
• As h increases, (R+h) increases, so v_c decreases and the time period T = 2π(R+h)/v_c increases.
• Values used: G = 6.67 × 10^-11 N m²/kg², M = 6 × 10²⁴ kg, R = 6.4 × 10⁶ m = 6400 km.

4. Types of orbits: high, medium and low

• High Earth Orbit (HEO): height ≥ 35,780 km. At this height one revolution takes about 24 hours — the same as Earth's rotation.
• If such a satellite moves above the equator in the same direction as Earth spins, it seems fixed over one spot, like two cars moving side by side at the same speed. These are geosynchronous (geostationary) satellites, used for weather watching and for TV, radio and telephone signals.
• Medium Earth Orbit (MEO): 2000 km to 35,780 km. Geostationary satellites stay over the equator and cannot see the poles well, so elliptical polar orbits in MEO are used to study polar regions; their period is 2 to 24 hours.
• Global positioning (navigation) satellites move in circular MEO orbits about 20,200 km high.
• Low Earth Orbit (LEO): 180 km to 2000 km. Used for scientific experiments and atmospheric studies; a revolution takes about 90 minutes. The International Space Station and Hubble Space Telescope are in LEO.
• Rule to remember: lower orbit → faster satellite → shorter period.

5. Satellite launch vehicles

• A launch vehicle places a satellite in its orbit. It works on Newton's third law: burning fuel produces hot, expanding gas that is pushed out through rear nozzles; the reaction force (thrust) pushes the rocket upward.
• Everyday examples: a Diwali rocket shoots up as gas escapes from its tail; a blown-up balloon released with its mouth open flies the opposite way.
• The satellite's weight and its orbit decide the design and fuel of the launcher. Fuel is the largest part of the launcher's weight.
• Multi-stage rockets: when one stage's fuel is used up, its empty tank and engine are dropped into the sea or an unpopulated area and the next stage fires. The vehicle becomes lighter and goes faster.
• ISRO's PSLV has four stages — 1st solid fuel, 2nd liquid fuel, 3rd solid fuel, 4th liquid fuel — with strap-on solid boosters that give the initial thrust. The satellite sits at the top.
• Ordinary launchers can be used only once, so they are costly. The USA built the space shuttle, which returns to Earth and can be reused (except its fuel tank).
• India's launchers (PSLV, GSLV) can put satellites up to about 2500 kg into all types of orbits.

6. Escape velocity and missions away from the Earth

• To go to the Moon or planets, a spacecraft must break free of Earth's gravity. Its starting velocity must exceed the escape velocity.
• Escape velocity v_esc = √(2GM/R). For Earth it is √(2 × 6.67 × 10^-11 × 6 × 10²⁴ / 6.4 × 10⁶) ≈ 11.18 × 10³ m/s ≈ 11.2 km/s.
• Escape velocity depends only on the planet's mass and radius, not on the spacecraft's mass. The Moon, with much smaller mass, has a lower escape velocity than the Earth.
• Note: v_esc = √2 × (orbital speed just above the surface), so escaping needs about 1.4 times the speed of a very low orbit.
• Light takes only about 1 s to come from the Moon, but spacecraft are much slower — the fastest trip to the Moon so far took 8 hours 36 minutes.
• Such missions help us understand how the solar system was formed and how it changed.

7. Moon, Mars and other planetary missions

• The Moon, our nearest neighbour, was the first target. USA, Soviet Union, Europe, China, Japan and India have sent Moon missions.
• Soviet Luna series: Luna 2 (1959) was the first to reach the Moon; till 1975, 15 crafts studied its chemistry, gravity, density and radiation; the last four landed and brought back rock samples. All were unmanned.
• USA's Moon missions (1962–1972) included manned flights; Neil Armstrong stepped on the Moon in July 1969.
• Chandrayaan-1 (ISRO, 2008) orbited the Moon and sent data for about a year. Its biggest discovery: water on the Moon's surface — India was the first to find it.
• Mars is the next nearest object. About half of all Mars missions by various nations failed, but ISRO's low-cost Mangalyaan (launched November 2013) entered Mars orbit in September 2014 and studied its surface and atmosphere.
• Other missions have orbited, landed on, or flown past other planets, and some have collected dust and rocks from asteroids and brought them back to Earth.
• Indian astronauts: Rakesh Sharma stayed 8 days in space (1984, joint Indo-USSR mission). Kalpana Chawla spent 336 hours in space but died when the Columbia shuttle broke up on 1 February 2003. Sunita Williams stayed 192 days in space and worked 29 hours outside the space station.

8. India and space technology

• Vikram Sarabhai is the father of the Indian space programme. He helped found the Physical Research Laboratory (PRL), Ahmedabad, chaired the Indian National Committee for Space Research (1962) and set up the first launch centre at Thumba (1963). His efforts led to ISRO and India's first satellite, Aryabhata.
• INSAT and GSAT series serve telecommunication, TV broadcasting and weather services — they make TV, phone and internet available across India.
• EDUSAT is a satellite in this series used especially for education.
• The IRS series monitors natural resources and helps in disaster management; the IRNSS series gives precise location (latitude and longitude).
• Launch centres: Thumba (Thiruvananthapuram), Sriharikota (Andhra Pradesh), Chandipur (Odisha).
• Research centres: Vikram Sarabhai Space Centre (Thiruvananthapuram), Satish Dhawan Space Centre (Sriharikota), Space Applications Centre (Ahmedabad).

9. Space debris and its management

• Besides working satellites, many useless objects also circle the Earth: dead satellites, rocket parts dropped during launch, and pieces from collisions. This is space debris.
• A 2016 estimate counted about 2 crore pieces longer than 1 cm orbiting the Earth.
• Debris moves very fast, so even a small piece can damage or destroy a satellite or spacecraft it hits.
• Debris keeps increasing, and soon launching new spacecraft safely may become difficult.
• Scientists are studying and testing ways to remove or control debris so future missions stay safe.

Formulas at a glance

Key terms

Space Missions – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Space Missions – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Space Missions

⏱ About 4 hours over 3 days

1. Day 1 — Satellites and their uses
   Read the need for space missions and the types of satellites. Make a 3-column table: type of satellite, function, Indian series and launcher (INSAT/GSAT–GSLV, IRNSS–PSLV, IRS–PSLV). Learn the first-in-space facts and dates.
2. Day 2 — Orbits and the numericals
   Derive v_c = √(GM/(R+h)) from centripetal force = gravity twice on paper. Learn the height ranges of LEO, MEO and HEO. Solve the solved examples here, then try your own heights (500 km, 2000 km, 20,200 km). Practise escape velocity, including ratio problems where mass and radius change.
3. Day 3 — Launchers, missions and revision
   Draw and label the PSLV diagram. Write why multi-stage rockets are used and how Newton's third law applies. Revise Chandrayaan-1, Mangalyaan, Vikram Sarabhai and space debris. Finish with our flashcards and 15-question quiz.

✍️ Diagrams to practise

• Orbit of an artificial satellite showing R, h, r and the tangential critical velocity v_c
• Labelled structure of PSLV (four stages, fuel type of each, strap-on boosters, satellite position)
• Different orbits of satellites around the Earth (LEO, MEO, HEO)
• Communication through a satellite: Earth station → satellite → mobile tower → phone