Space Missions Class 10 Notes, Formulas, Questions | SSC

This chapter describes how space missions have changed communication, weather forecasting, navigation and our understanding of the solar system. It explains artificial satellites, their classification by function, and how the height of an orbit decides the critical velocity and period. It also covers satellite launch vehicles, escape velocity, missions to the Moon and Mars, India's achievements in space technology and the growing problem of space debris.

Summary

Space missions are of two kinds: those that place artificial satellites in orbit around the earth for research and practical uses, and those that send spacecraft into outer space to study objects in the solar system and beyond. Yuri Gagarin of the USSR was the first person to travel into space (1961), Neil Armstrong of the USA was the first to step on the Moon (1969), and Rakesh Sharma of India orbited the earth in 1984 in a Russian spacecraft. Kalpana Chawla and Sunita Williams, of Indian origin, took part in NASA missions.

Space missions have made the world a global village: instant communication, internet, television, advance warning of natural calamities, surveillance during war and exploration of minerals and fossil fuels all depend on them. A natural satellite is an astronomical object revolving around a planet; the Moon is the earth's only natural satellite. A man-made object revolving around the earth or another planet in a fixed orbit is an artificial satellite. The first one, Sputnik, was launched by the Soviet Union in 1957; today more than a thousand satellites orbit the earth. They run on solar energy from photovoltaic panels and carry instruments to receive and transmit signals.

By function, satellites are weather satellites, communication satellites, broadcast satellites (INSAT and GSAT series, launched by GSLV), navigational satellites (IRNSS, launched by PSLV), military satellites and earth observation satellites (IRS series, launched by PSLV), which study forests, deserts, oceans, polar ice and natural resources and help during floods and earthquakes.

To put a satellite into orbit, a launcher takes it to the required height and gives it a specific tangential velocity called the critical velocity. The earth's gravitational force provides the centripetal force, giving vc = square root of GM/(R + h). The critical velocity does not depend on the mass of the satellite and decreases as the height of the orbit increases.

Orbits are classified by height. High earth orbits are 35780 km or more above the surface; a satellite at 35780 km above the equator takes about 24 hours per revolution, the same as the earth's rotation, so it appears stationary and is called geosynchronous (geostationary). Such satellites watch one region continuously and are used for meteorology and telephone, television and radio signals. Medium earth orbits lie between 2000 km and 35780 km; elliptical polar orbits in this range take 2 to 24 hours per revolution and are used to study polar regions, and navigation (global positioning) satellites revolve at about 20,200 km. Low earth orbits lie between 180 km and 2000 km, a revolution takes about 90 minutes, and they are used for scientific experiments and atmospheric studies; the International Space Station and the Hubble telescope are in low earth orbit.

Satellite launch vehicles work on Newton's third law of motion: hot gases from burning fuel are expelled forcefully through nozzles at the rear and the reaction thrust pushes the vehicle up. Since fuel makes up most of the weight, multistage vehicles are used; when a stage's fuel is exhausted, its empty tank and engine are detached, so the lighter vehicle can move faster. The PSLV of ISRO is an example. Launch vehicles can be used only once, so the USA developed the space shuttle, which returns to earth (except the fuel tank) and can be reused.

To go away from the earth, a spacecraft must have an initial velocity greater than the escape velocity, given by vesc = square root of 2GM/R, which is about 11.2 km/s for the earth. The first missions were to the Moon: the Soviet Luna series (Luna 2 in 1959) and American missions from 1962 to 1972, some of which were manned. ISRO's Chandrayaan-1 (2008) orbited the Moon for about a year and discovered water on the lunar surface; India was the first to discover this. ISRO's low-cost Mangalyaan was launched in November 2013 and entered orbit around Mars in September 2014, studying its surface and atmosphere. Other missions have studied planets, asteroids and comets, some even bringing back dust and stones.

Vikram Sarabhai is regarded as the father of the Indian space programme; he founded the Physical Research Laboratory, chaired the Indian National Committee for Space Research (1962), helped establish the Thumba launch centre (1963) and ISRO, and his efforts led to India's first satellite, Aryabhatta. India's launchers, chiefly PSLV and GSLV, can place satellites of up to 2500 kg in all types of orbits. INSAT and GSAT serve telecommunication, TV and weather; EDUSAT serves education; IRS monitors natural resources and disasters; IRNSS gives precise location. Space debris, including dead satellites, discarded launcher parts and fragments from collisions (about 2 crore pieces longer than 1 cm in 2016), can damage working satellites, so its management is essential.

Mind map

Deriving the critical velocity of a satellite

Important definitions

Formulas

Key points to remember

• Sputnik (Soviet Union, 1957) was the first artificial satellite.
• Critical velocity does not depend on the satellite's mass and decreases as the orbit height increases.
• A satellite at about 35780 km above the equator takes about 24 hours per revolution and appears stationary.
• Geostationary satellites are above the equator, so they are not useful for studying polar regions; polar orbits are used instead.
• Low earth orbit satellites take about 90 minutes per revolution; ISS and Hubble are in LEO.
• Launch vehicles work on Newton's third law; multistage design reduces weight step by step.
• Escape velocity of the earth is about 11.2 km/s.
• Chandrayaan-1 (2008) discovered water on the Moon; Mangalyaan reached Mars orbit in September 2014.
• Indian satellite launch centres: Thumba (Thiruvananthapuram), Sriharikota and Chandipur (Odisha).
• The student satellite Swayam (COEP, 2016) weighs about 1 kg and orbits at 515 km.

Solved examples

Example 1. A satellite orbits exactly 35780 km above the earth's surface. Find its tangential (critical) velocity. (G = 6.67 x 10^-11 N m^2/kg^2, M = 6 x 10^24 kg, R = 6400 km)

R + h = 6400 + 35780 = 42180 km = 42180 x 10^3 m.

GM = 6.67 x 10^-11 x 6 x 10^24 = 40.02 x 10^13.

vc = square root of [GM / (R + h)] = square root of [40.02 x 10^13 / (42180 x 10^3)] = square root of (9.488 x 10^6).

vc = 3080 m/s = 3.08 km/s (approximately).

Example 2. How much time will the satellite in the previous example take to complete one revolution?

T = 2 pi (R + h) / v = 2 x 3.14 x 42180 / 3.08 (km and km/s).

T = 86003 s (approximately).

T = 86003 / 3600 = 23.89 hours, about 23 hours 54 minutes, i.e. nearly 24 hours.

Example 3. Calculate the escape velocity from the earth. (G = 6.67 x 10^-11 N m^2/kg^2, M = 6 x 10^24 kg, R = 6.4 x 10^6 m)

vesc = square root of (2GM / R) = square root of (2 x 6.67 x 10^-11 x 6 x 10^24 / 6.4 x 10^6).

= square root of (1.25 x 10^8) = 11.18 x 10^3 m/s.

vesc = 11.18 km/s, about 11.2 km/s.

Example 4. A planet has 8 times the mass of the earth and twice its radius. Find the escape velocity on it. (Escape velocity of earth = 11.2 km/s)

vesc is proportional to square root of (M / R).

For the planet: M / R = 8M / 2R = 4 (M / R) of earth.

So vesc(planet) = square root of 4 x 11.2 = 2 x 11.2 = 22.4 km/s.

Example 5. If the mass of the earth were four times its present value, how long would a satellite at 35780 km above the surface take for one revolution?

vc = square root of [GM / (R + h)], so with 4M the velocity becomes square root of 4 = 2 times: 2 x 3.08 = 6.16 km/s.

The orbit radius is unchanged, so T = 2 pi (R + h) / v becomes half.

T = 86003 / 2 = 43001.5 s, about 11.9 hours (nearly 12 hours).

Example 6. Find the critical velocity of a satellite in a low earth orbit 600 km above the earth's surface. (GM = 40.02 x 10^13 N m^2/kg, R = 6400 km)

R + h = 6400 + 600 = 7000 km = 7 x 10^6 m.

vc = square root of (40.02 x 10^13 / 7 x 10^6) = square root of (5.717 x 10^7).

vc = 7561 m/s, about 7.56 km/s. This is larger than 3.08 km/s for the geosynchronous orbit, showing that critical velocity decreases with height.

Important questions and answers

Q1. Fill in the blank: If the height of the orbit of a satellite from the earth's surface is increased, the tangential velocity of the satellite will ______.

decrease. Since vc = square root of GM/(R + h), a larger R + h gives a smaller critical velocity.

Q2. Fill in the blank: The initial velocity (during launching) of Mangalyaan must be greater than ______ of the earth.

the escape velocity. To leave the earth's gravitational field and travel to Mars, a spacecraft needs an initial velocity greater than about 11.2 km/s.

Q3. True or false: If a spacecraft has to be sent away from the influence of the earth's gravitational field, its velocity must be less than the escape velocity.

False. Its initial velocity must be greater than the escape velocity.

Q4. True or false: The escape velocity on the Moon is less than that on the earth.

True. Escape velocity is square root of 2GM/R, and the ratio M/R for the Moon is much smaller than for the earth, so its escape velocity is smaller.

Q5. True or false: If the height of the orbit of a satellite increases, its velocity must also increase.

False. Critical velocity decreases as the height of the orbit increases.

Q6. What is an artificial satellite? How are satellites classified on the basis of their functions?

A man-made object revolving around the earth or another planet in a fixed orbit is an artificial satellite. By function: weather satellites (study and prediction of weather), communication satellites (communication between places using specific waves), broadcast satellites (telecasting TV programmes), navigational satellites (fixing precise latitude and longitude), military satellites (security information) and earth observation satellites (forests, deserts, oceans, polar ice, natural resources and calamities).

Q7. On what basis are satellite orbits classified? Explain.

They are classified by their height above the earth's surface. High earth orbit: 35780 km or more; geosynchronous satellites used for weather and communication. Medium earth orbit: 2000 km to 35780 km; polar orbits and navigation satellites (about 20,200 km). Low earth orbit: 180 km to 2000 km; about 90 minutes per revolution; scientific and atmospheric studies, ISS and Hubble telescope.

Q8. Why are geostationary satellites not useful for studying polar regions?

Geostationary satellites revolve above the equator and always face the same part of the earth, so they cannot observe the polar regions properly. For this, satellites in elliptical polar orbits passing over the poles are used.

Q9. Why is it beneficial to use satellite launch vehicles with more than one stage?

Fuel forms most of a launch vehicle's weight. In a multistage vehicle, when the fuel of one stage is used up, its empty tank and engine are detached and dropped into the sea or unpopulated land. The vehicle becomes lighter step by step, so the remaining stages can give it a higher speed with less fuel.

Q10. Explain the working of a satellite launch vehicle.

It works on Newton's third law of motion. Fuel burns and produces hot gases that expand and are expelled forcefully through nozzles at the rear. The reaction to this is a forward thrust on the vehicle that drives it up into space. A Diwali rocket and a released inflated balloon work the same way.

Q11. Write a short note on India's Moon and Mars missions.

Chandrayaan-1 was launched by ISRO in 2008 and placed in orbit around the Moon. It sent useful information for about a year, and its most important discovery was the presence of water on the Moon's surface; India was the first country to discover this. Mangalyaan, built by ISRO at minimum expense, was launched in November 2013 and placed in orbit around Mars in September 2014; it gave useful information about the surface and atmosphere of Mars.

Q12. What is space debris? Why is its management necessary?

Space debris consists of non-functional satellites, parts of launchers detached during launching and fragments formed by collisions, all revolving around the earth; about 2 crore pieces longer than 1 cm were estimated in 2016. It can collide with and damage working satellites and spacecraft, and since it is increasing it will make new launches difficult, so it must be managed.

Exam tips

• Learn the derivation of vc = square root of GM/(R + h) by equating centripetal and gravitational forces; it is a common long answer.
• In numericals add R and h first and convert to metres when using G in SI units.
• Remember the three orbit ranges (180 to 2000 km, 2000 to 35780 km, 35780 km and above) with one use each.
• Use ratio methods for "if mass is n times" questions instead of recalculating from scratch.
• Memorise key years and names: Sputnik 1957, Gagarin 1961, Armstrong 1969, Rakesh Sharma 1984, Chandrayaan-1 2008, Mangalyaan 2013/2014.

FAQs

Does a heavier satellite need a larger critical velocity?

No. The mass of the satellite cancels out in the derivation, so the critical velocity depends only on the mass of the earth and the radius of the orbit.

Why does a geosynchronous satellite appear stationary?

It completes one revolution in about 24 hours in an orbit parallel to the equator, the same time the earth takes to rotate once, so relative to a point on earth it does not move.

What is the escape velocity of the earth?

About 11.2 km/s, obtained from vesc = square root of 2GM/R.

Who is the father of the Indian space programme?

Dr. Vikram Sarabhai, who founded the Physical Research Laboratory, helped set up the Thumba launch centre and ISRO, and whose efforts led to the launch of Aryabhatta.

These notes were prepared with AI help from the Balbharati Science and Technology Part 1 textbook (2026-27) and checked against it.