Glossary

When an object moves around a planet due to gravitational force between them, it is called a ‘satellite’ or a ‘Moon’ of that planet. Apart from Mercury and Venus, all other planets have satellites. The Earth has only one Moon, and Mars has two satellites. So far, we have found 67 satellites of Jupiter. Four of these are big: Ganymede is bigger than Mercury, Io and Callisto are bigger than our Moon, and Europa is slightly smaller than our Moon. Galileo Galilei was the first one to see them when he watched Jupiter through a telescope in 1610 (hence they are called the Galilean Moons). He identified them as the satellites of Jupiter. This was the first time when we learned that other planets also have satellites.

Saturn is known to have 62 satellites (only one of them is as big as the Moon). Uranus has at least 27 satellites, and Neptune has 14. Imagine the sight of multiple Moons being visible at a time from these planets! Refer to Figure 1 for selected Moons in the solar system.
 

Figure 1: Sizes of selected satellites in the solar system as compared to the Earth
(Credit: By Originally uploaded from NASA by Bricktop; edited by Deuar, KFP, TotoBaggins - solarsystem.nasa.gov, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1641353)

Artificial Satellites
When human beings launch objects into the orbit of any planet, they are known as artificial satellites. The Earth has many artificial satellites revolving around it. Artificial satellites are useful in weather forecasting, in Tele and Radio communication and GPS (global positioning system), which is used in mobiles to determine our position. The Indian Space Research Organization (ISRO)  has been successfully launching satellites since 1975. A rocket is needed to carry the artificial satellite into its orbit and launch it. We have launched many artificial satellites using our own rockets from the Satish Dhawan Space Centre in Sriharikota, Andhra Pradesh. Among them, a satellite named Astrosat was launched on September 28, 2015 to carry out research in astronomy (Figure 2a). Artificial satellites have been launched to revolve around other planets as well. Mangalyaan (Mars Orbiter Mission) is one such satellite launched by ISRO on 5 November 2013.
 

Figure 2: Indian Artificial Satellites
 

Figure 2a: Astrosat - India’s first dedicated multi wavelength space observatory.
(Credit: ISRO http://www.isro.gov.in/astrosat/astrosat-gallery)

Figure 2b: Indian scientist working on Mangalyaan (Mars Orbiter Mission)
(Credit: ISRO http://www.isro.gov.in/pslv-c25-mars-orbiter-mission/pslv-c25-mars-orbiter-mission-gallery)