Earth satellites
Earth Satellites Introduction: Earth satellites are objects that revolve around the Earth. These can either be natural or artificial. The Moon is Earth's only natural satellite, which orbits the Earth in an almost circular path. It takes about 27.3 days to complete one revolution around the Earth. Artificial satellites have been launched by several countries, including India. These satellites have various periods of revolution depending on their practical applications, such as navigation, surveillance, communication, space exploration, and weather monitoring.
Types of Satellites
1. Communication Satellites: Communication satellites are typically geostationary satellites. These satellites orbit the Earth in the equatorial plane. They rotate in the same direction and at the same speed as the Earth, with a period of revolution of 24 hours, or one day. Because of this synchronization, they appear stationary when viewed from the Earth’s surface. This characteristic is why they are known as geostationary or geosynchronous satellites. Communication satellites are used for various purposes, including television transmission, telephone communications, and radio wave signal transmission. One notable example is the INSAT group of satellites, which were launched by India.
2. Polar Satellites: Polar satellites are positioned in lower polar orbits, typically at altitudes ranging from 500 km to 800 km above the Earth's surface. These satellites are mainly used for weather forecasting and meteorological purposes, as well as for astronomical observations and the study of solar radiation. The period of revolution for a polar satellite is approximately 85 minutes, which means it can complete 16 orbits around the Earth in one day. These satellites orbit the Earth in a north-south direction while the Earth rotates east-west. Polar satellites are equipped with cameras that capture small strips of the Earth's surface in each orbit. By the end of the day, the satellite can capture the entire Earth's surface, one strip at a time.
Projection of a satellite
The projection of an artificial satellite requires achieving a specific velocity using a minimum two-stage rocket system. A single-stage rocket cannot achieve this because the required velocity for the satellite to enter orbit is greater than what a single-stage rocket can provide.
Stages of Satellite Projection
First Stage (Vertical Lift-Off): The rocket is launched from the Earth's surface. The fuel in the first stage is ignited, generating thrust to move the satellite vertically upwards. The velocity imparted in this stage is known as vertical velocity.
Second Stage (Attaining Orbital Velocity): The first stage of the rocket is detached after reaching the desired altitude. The launcher is rotated 90° to orient the satellite in a horizontal direction using remote control. The second-stage rocket is ignited to impart a specific horizontal velocity (Vh) required for stable orbiting.
Possible cases for projection of satellites
The motion of the satellite depends on the horizontal velocity given at the time of projection. The following cases arise:
Case 1: Vh < Vc
The satellite follows an elliptical orbit with the point of projection as the apogee. Due to atmospheric resistance, it loses energy and spirals back to Earth.
Case 2: Vh = Vc
The satellite moves in a perfect circular orbit around Earth. It remains in a stable orbit without returning to Earth or escaping into space.
Case 3: Vh > Vc
The satellite follows an elliptical orbit with the point of projection as the perigee (closest point to Earth). The satellite remains bound to Earth but moves in an elongated path.
Case 4: Vc < Vh < Ve
The satellite moves along a parabolic trajectory and never returns to Earth. Its velocity reduces to zero at infinity.
Case 5: Vh > Ve
The satellite escapes Earth's gravitational field completely and follows a hyperbolic trajectory. It continues moving indefinitely in space.
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