The term ‘satellite’ refers to a natural object such as a moon or spacecraft which is an artificial satellite orbiting a larger astronomical body. Most of the known natural satellites orbit planets, the Earth’s Moon is the most obvious example of it.
All the planets in the solar system generally except Venus and Mercury have natural satellites. More than 160 such objects have so far been discovered in the solar system with Saturn and Jupiter together contributing about two-thirds of the total.
The planets’ natural satellites vary greatly in size and shape as well as colour. A few satellites are larger than Mercury, for example, the planet Saturn’s Titan and Jupiter’s Ganymede each of which is more than 5,000 km that is about 3,100 miles in diameter.
The satellites also usually differ significantly in composition. The satellite moon for example generally consists almost entirely of rocky material. On the other hand, we see that the composition of Saturn’s Enceladus is 50 percent or more ice. Some asteroids are said to have their own tiny moons.
Natural and Artificial Satellites
A natural satellite is a moon that orbits a planet or a star. For example, the moon is a satellite because it orbits the earth. Usually, the word that is "satellite" refers to a machine that is launched into space and moves around the planet Earth or another body in space.
The planet Earth and the satellite moon are examples of natural satellites. There are thousands of artificial or man-made satellites that are orbiting Earth.
In the context of spaceflight, a satellite is said to be an object that has been intentionally placed into orbit. These objects are known as artificial satellites to distinguish them from natural satellites such as planet Earth's Moon.
Some satellites take pictures of the planet earth that helps meteorologists predict weather and track hurricanes. Some of the satellites take pictures of other planets, the sun and the black holes and the dark matter or faraway galaxies. These pictures generally help scientists to understand the solar system and the universe as well.
Still, we can say that the other satellites are used mainly for communications such as beaming TV signals and phone calls that are around the world. A group of more than 20 satellites make up the Global Positioning System or the GPS. If we have a GPS receiver, these satellites can help figure out our exact location.
The satellites come in many sizes and shapes. But most have at least two parts in common - that is a power source and antenna. The antenna usually receives and sends information which is often to and from Earth. The power source can be a panel or solar panel or battery. Solar panels generally make power by turning sunlight into electricity.
Many satellites of NASA carry cameras and scientific sensors. Sometimes these instruments usually point toward the planet Earth to gather information about its land, air and water. Other times they face toward space to collect data from the universe and the solar system.
Satellites can collect more data, more quickly than instruments present on the ground.
The satellites also can see into space better than telescopes at planet Earth's surface. This is because satellites usually fly above the clouds and the dust and molecules in the atmosphere that can block the view from ground level.
Before satellites TV signals didn't go very far. The TV signals only travel in line which is straight. So they would quickly trail off into space instead of following the Earth's curve. Mountains or tall buildings would block them. Phone calls to places which are faraway were also a problem. The setting up of wires of telephone over long distances or underwater is difficult and costs a lot.
With satellites, the signals of TV and phone calls are sent upward to a satellite. Then almost instantly the satellite can send them back down to different locations on Earth.
Satellites Launched into Space
On 4 October 1957, the Soviet Union Russia launched the world's first artificial satellite named Sputnik 1. Since then there are 8,900 satellites from more than 40 countries have been launched. According to the estimates of 2018, there are some 5,000 remaining in orbit. Of those who are about 1,900 were operational while the rest have exceeded their useful lives and become debris space.
In terms of countries with the most satellites, the country USA has the most with 859 satellites. China is said to be the second with 250 and Russia third with 146. These are then followed by India at 118, Japan at 72 and the UK at 52.
A few large stations of space including the International Space Station has been launched in parts and then it is assembled in orbit. Over a dozen probes of space have been placed into orbit around other bodies and become artificial satellites of the Moon, and Mercury, Venus, Mars, Jupiter, Saturn, a few asteroids, a comet and the Sun.