Lesson 10: Space Missions

Rule 1: Two kinds of space missions — Space missions are of two types: placing artificial satellites in orbits around the earth for research and useful applications, and sending spacecraft into outer space for close observation of objects in and beyond the solar system. Space missions help communication, the internet, advance alerts of calamities, aerial surveillance and exploring fossil reserves and minerals.

• Easy : Easy: a TV channel from another country reaches you through a communication satellite.
• Medium : Medium: the cyclone warning that gives people time to move to safe places comes from weather satellites.
• Exam-type : Exam: difference – satellite missions stay in orbit around the earth; deep-space missions travel to the Moon, planets or comets.

Rule 2: Firsts in space — Yuri Gagarin of the USSR was the first person in space (1961); Neil Armstrong of the USA first stepped on the Moon (1969). Rakesh Sharma orbited the earth in 1984 in a Russian spacecraft for 8 days. Kalpana Chawla died when Columbia exploded on 1 February 2003. Sunita Williams stayed 192 days in space and visited the International Space Station in 2006 in Discovery. The moon is the only natural satellite of the earth. The first artificial satellite, Sputnik, was sent by the Soviet Union in 1957. Satellites work on solar energy through solar photovoltaic panels attached like wings.

In the figure : In the figure firsts in space exploration.

• Easy : Easy: Sputnik (1957) came before Gagarin (1961), and Armstrong (1969) came after both.
• Medium : Medium: Gagarin's flight (1961) to Armstrong's Moon landing (1969) took 1969 − 1961 = 8 years.
• Exam-type : Exam: why do satellites carry solar panels? They have no other power supply in space, so they use sunlight.

Spot the difference

Rule 1: Types of satellites — Weather satellites (INSAT, GSAT; launcher GSLV) study and predict weather. Communication and broadcast satellites (INSAT, GSAT; GSLV) connect distant places and telecast television. Navigational satellites (IRNSS; PSLV) fix the precise latitude and longitude of a place. Military satellites collect information for security. Earth observation satellites (IRS; PSLV) study forests, deserts, oceans and polar ice, manage natural resources and guide during floods and earthquakes.

In the figure : In the figure what satellites are used for.

• Easy : Easy: the map app finds your place with a navigational satellite system (IRNSS for India).
• Medium : Medium: images of a flooded river from an IRS satellite help rescue teams plan routes.
• Exam-type : Exam: match – weather: INSAT; navigation: IRNSS; earth observation: IRS.

Rule 2: Abbreviations — INSAT = Indian National Satellite; GSAT = Geosynchronous Satellite; IRNSS = Indian Regional Navigation Satellite System; IRS = Indian Remote Sensing Satellite; GSLV = Geosynchronous Satellite Launch Vehicle; PSLV = Polar Satellite Launch Vehicle. EDUSAT is used specially for education.

• Easy : Easy: EDUSAT sends lessons to remote classrooms.
• Medium : Medium: PSLV launches navigation and earth observation satellites; GSLV launches the heavier communication satellites.
• Exam-type : Exam: expand IRS and IRNSS.

Spot the difference

Rule 1: Critical velocity — A satellite is carried to the required height by a launcher and then given a specific tangential velocity, the critical velocity v_c = √(GM/(R+h)). The centripetal force is provided by the earth's gravity. v_c does not depend on the mass of the satellite and decreases as the height of the orbit increases.

• Easy : Easy: a 500 kg and a 1500 kg satellite at the same height need the same critical velocity, because v_c does not depend on mass.
• Medium : Medium: a satellite at a higher orbit moves slower than one at a lower orbit, since v_c decreases with height.
• Exam-type : Exam: gravity provides the centripetal force that keeps the satellite in orbit; without it the satellite would fly off in a straight line.

Rule 2: Orbits of satellites — High earth orbit (35,780 km or more): the satellite takes about 24 hours for a revolution, so it appears stationary (geosynchronous); used for meteorology, telephone, television and radio. Medium earth orbit (2000–35,780 km): polar orbits are elliptical and take 2 to 24 hours; global positioning satellites are at about 20,200 km. Low earth orbit (180–2000 km): revolution in about 90 minutes; used for scientific experiments, atmospheric studies, the International Space Station and the Hubble telescope.

In the figure : In the figure satellite orbits at different heights.

• Easy : Easy: a low-orbit satellite takes 90 minutes per revolution, so in a day it makes 24 × 60 ÷ 90 = 16 revolutions.
• Medium : Medium: a satellite at 515 km (Swayam) lies in the low earth orbit range of 180–2000 km.
• Exam-type : Exam: why does a geosynchronous satellite look still? Its period (24 hours) equals the earth's rotation period, so it stays over the same place.

Spot the difference

Rule 1: How launch vehicles work — Launch vehicles work on Newton's third law: hot gas expelled backwards through the nozzles produces a forward thrust. Fuel is a major part of the weight, so vehicles with two or more stages (टप्पे) are used; the empty first stage detaches and falls into the sea or on unpopulated land, reducing the weight. The structure and fuel depend on the weight of the satellite and the type of orbit.

In the figure : In the figure a rocket moves forward because gas is pushed backwards.

• Easy : Easy: a rocket's gas goes backwards (action) and the rocket goes forward (reaction).
• Medium : Medium: when the first stage is dropped, the rocket becomes lighter, so the remaining fuel can push it farther.
• Exam-type : Exam: a balloon with its mouth open flies forward as air is pushed backwards – an example of Newton's third law.

In the figure : In the figure dropping empty stages saves weight.

Rule 2: Cost and escape velocity — Launch vehicles are costly as they are used once; the USA developed the reusable space shuttle (except the fuel tank). A Diwali rocket or a released balloon works on the same law. To leave the earth, a spacecraft needs an initial velocity greater than the escape velocity v_esc = √(2GM/R) = 11.2 km/s. The shortest time taken to reach the moon so far is 8 hours 36 minutes.

• Easy : Easy: 11.2 km/s = 11.2 × 1000 = 11,200 m/s.
• Medium : Medium: at 11.2 km/s a spacecraft covers 11.2 × 60 = 672 km every minute.
• Exam-type : Exam: 8 hours 36 minutes = 8 × 60 + 36 = 516 minutes.

Spot the difference

Rule 1: Missions to the Moon and Mars — Moon: Luna 2 (USSR, 1959) was the first craft to reach near the moon; USA missions ran from 1962 to 1972; India's Chandrayaan-1 (2008) orbited the moon for about a year and discovered water on the moon's surface. Mars: almost half the missions failed, but ISRO's low-cost Mangalyaan was launched in November 2013 and entered Mars orbit in September 2014. Spacecraft also study other planets, asteroids and comets.

In the figure : In the figure Indian missions to the Moon and Mars.

• Easy : Easy: Mangalyaan was launched in 2013 and reached Mars orbit in 2014, in about 10 months.
• Medium : Medium: Chandrayaan-1 (2008) found water on the moon's surface, an important discovery.
• Exam-type : Exam: why was Mangalyaan special? It was a low-cost mission that entered Mars orbit in its first attempt.

Rule 2: The Indian space programme and space debris — PSLV and GSLV can place satellites up to 2500 kg in all orbits. INSAT/GSAT give telecommunication, TV and weather services; IRS manages natural resources and disasters; IRNSS gives location. Launch centres: Thumba, Sriharikota and Chandipur (Odisha). Dr Vikram Sarabhai is the father of the Indian space programme, and India's first satellite was Aryabhatta. Space debris is increasing; about 2 crore pieces longer than 1 cm were estimated in 2016. It can damage spacecraft, so it must be managed.

• Easy : Easy: India's first satellite was Aryabhatta, and the father of the Indian space programme is Dr Vikram Sarabhai.
• Medium : Medium: 2 crore pieces of debris = 2 × 10,000,000 = 20,000,000 pieces.
• Exam-type : Exam: space debris is dangerous because even a small fragment moving at very high speed can damage a working satellite.

Spot the difference