Artificial satellites: helpers in orbit

Natural and artificial satellites

A satellite is an object that goes round a planet. The Moon is the Earth’s natural satellite, taking about a month to go round us. Besides the Moon, many artificial satellites, made and launched by different countries, also orbit the Earth. Most of them are about 800 km above the Earth’s surface, far closer than the Moon, and take only about 100 minutes to go round once.

From the ground, an artificial satellite looks like a tiny point of light moving steadily across the sky. If you have ever seen a ‘moving star’ in the early evening, it was probably a satellite. Its motion is periodic, like the Moon’s: it goes round the same orbit again and again, so it can be predicted.

What satellites do for us

Satellites help in many ways:

• Communication: they relay signals for television, phone and internet links across long distances.
• Navigation: they help find positions on the Earth, which map apps use.
• Weather monitoring: they watch clouds and storms from above, which helps forecasting.
• Disaster management: their images show floods, cyclones and damage, helping relief reach the right places.
• Scientific research: above the air, their instruments can study the Earth, the Sun, stars and other objects in space.

India in space

India’s space agency, ISRO (the Indian Space Research Organisation), has sent up many satellites for these jobs. The Cartosat satellites photograph the Earth in fine detail; the pictures make maps more accurate and help in planning cities and responding to disasters. Bhuvan, ISRO’s online map, uses them to show the shape of the land, soils, how land is used and plant cover. AstroSat is a space observatory that studies stars and other objects in space. Beyond Earth orbit, Chandrayaan 1, 2 and 3 went to the Moon, Mangalyaan went to Mars, and Aditya L1 watches the Sun. Students, too, have built small satellites that ISRO launched, among them AzaadiSat, InspireSat-1 and Jugnu.

Spotting a satellite yourself

Activity 11.4 is a night-sky task. Just before sunrise or soon after sunset, go with an adult to a place with a clear view of the sky, away from trees and tall buildings. Look for a point of light, steady or flickering in brightness, moving fast across the sky; you can see it with your eyes or with binoculars. Mobile apps and websites tell you which satellites will pass over your location and when. The timing matters: at these hours the ground is dark, but a satellite high above is still in sunlight, so it shines by reflected sunlight, just as the Moon does.

Space debris

More and more satellites are launched every year. Once a satellite stops working, it stays in orbit along with spent rocket stages, and all of this turns into space junk, also called space debris. The junk fills up the space around the Earth and may smash into satellites that still work. Small bits burn up in the air when they fall, but bigger ones can hit the ground, so nations are now cooperating to clear it.

The scientist behind India’s space programme

Vikram Ambalal Sarabhai (1919–1971), who did research in space science and nuclear physics, is remembered as the father of India’s space programme. He pioneered India’s effort to launch its first artificial satellites. ISRO’s rocket and launch-vehicle centre in Thiruvananthapuram, the Vikram Sarabhai Space Centre, carries his name.

Read on: phases of the Moon, lunar, solar and luni-solar calendars, the chapter notes and the NCERT solutions.