Satellites we put in the sky

Look at the sky in the early evening and you may see what look like moving stars. The Moon is the Earth's natural satellite, but many artificial (man-made) satellites from different countries also go round the Earth, and those are what you're seeing.

They look like tiny moving specks. Most orbit about 800 km above the ground and go once round the Earth in roughly 100 minutes, which is why they move so fast across the sky.

Satellites help with communication, navigation, weather forecasting, disaster management and scientific research. India's space agency, ISRO (Indian Space Research Organisation), has launched many.

ISRO also launches small satellites built by students, such as AzaadiSat, InspireSat-1 and Jugnu.

Spot one (Activity 11.4). Just before sunrise or 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, moving fast across the sky. You can see them with your eyes or binoculars, and phone apps or websites tell you which satellites will pass over you, and when.

There's a cost. When satellites stop working, many of them and their rocket parts stay up as space junk (space debris). It crowds space and can crash into working satellites. Small pieces burn up as they fall through the air, but big ones can hit the ground. Countries are now working together to clear it.

The Father of the Indian Space Programme was Vikram Ambalal Sarabhai, a researcher in space science and nuclear physics who led the effort to launch India's first satellites. The Vikram Sarabhai Space Centre in Thiruvananthapuram, which develops rockets and launch vehicles, is named after him.