What are the Moon, asteroids and comets?

Satellites

Planets move around the Sun, and some objects move around planets. Objects that move around planets are commonly called satellites. They are smaller than the planets they go around. Moons are natural satellites. The Earth has one Moon, Mars has two, and Jupiter, Saturn, Uranus and Neptune have a large number of moons. In general, any object that moves around a much larger object can be called a satellite, so the Earth can be thought of as a satellite of the Sun.

The Moon

The Moon is Earth’s natural satellite and our nearest neighbour in space:

• it is about 3,84,000 km away from the Earth;
• it takes about 27 days to go once around the Earth;
• its diameter is about a quarter of the Earth’s;
• unlike the Earth, it has hardly any atmosphere.

The Moon’s surface has circular, bowl-like hollows called craters. Most of them formed when asteroids or rocks from space hit the Moon. Since there is no atmosphere, water or life on the Moon, these marks stay on its surface for a very long time.

India’s missions to the Moon

India has sent three Chandrayaan missions to study the Moon. Chandrayaan-1 was launched in 2008 and Chandrayaan-2 in 2019. Chandrayaan-3 was launched in July 2023, and its Vikram lander, carrying the Pragyan rover, soft-landed on the Moon on 23 August 2023. India became the first country to land near the little-explored south pole of the Moon, and the Government of India declared 23 August National Space Day. A fourth mission, Chandrayaan-4, is being planned to bring back soil and rock samples from the Moon.

Asteroids

The Sun and the planets are nearly spherical, but the Solar System also has many small objects that are rocky and irregular in shape. These are asteroids. Their sizes range from about 10 m to about 500 km. Many of them revolve around the Sun in paths between those of Mars and Jupiter, a region called the asteroid belt. Occasionally, an asteroid passes very close to the Earth.

Comets

Comets are visitors from the outer regions of the Solar System, made of dust, gases, rocks and ice. As a comet comes close to the Sun, the frozen material in it starts evaporating, and this forms its long tail. (Extra: the tail always points away from the Sun.) As the comet moves away from the Sun it becomes dim, and we can no longer see it with the naked eye.

Many comets revolve around the Sun and return regularly. Halley’s Comet appears about every 76 years; it last appeared in 1986. A few comets escape from the Solar System, and some break up or fall into the Sun or a planet.

In Sanskrit and some other Indian languages a comet is called Dhumaketu, and some tribes in India call it Pucchya-Taro (a star with a tail) or Zendya-Taro (a star like a flag). In many cultures comets were feared as signs of bad luck, but scientists have shown that they are just icy-rocky visitors on a trip close to the Sun.

The Solar System, all together

The Solar System is the Sun, the eight planets, their moons, and many smaller objects including asteroids and comets. The Sun is the largest and heaviest object in it and produces almost all its energy. Everything else, including the Moon, shines by reflecting sunlight. To see how the stars beyond fit in, read stars and constellations.