ARTIFICIAL SATELLITE: know types, launching and uses of Satellites

INTRODUCTION OF ARTIFICIAL SATELLITE

Moon is Earth’s only satellite. As discussed earlier, the motion of various planets or satellites can be determined from Newton’s law of motion and Newton’s law of gravitation. From this, scientists believe that if any object launched from the earth can be given the proper speed at the proper distance from the earth, then the object will orbit the earth like the moon. Putting this idea into practice led to the launch of the first artificial satellite of human kind (Sputnik I) on October 4, 1957. Today, technology has advanced so much that it is possible to place artificial satellites around not only the Earth, but also other planets.

SIZES AND ALTITUDES OF SATELLITE

Spacecraft vary in size and cost depending on their application; some are small enough to grasp in your hand, while others, like as Hubble, are the size of a school bus. Small satellites (SmallSats) are spacecraft with a mass less than 180 kilogrammes and a size comparable to a big kitchen refrigerator. Even with small spacecraft, a wide range of sizes and masses can be distinguished.

Minisatellite, 100-180 kg
Microsatellite, 10–100 kg
Nanosatellite, 1–10 kg
Picosatellite, 0.01 to 1 kg
Femtosatellite, 0.001-0.01 kg

Low Earth Orbit (LEO)

A low earth orbit (LEO) satellite is an object, typically a piece of electronic equipment, that orbits the Earth at lower altitudes than geosynchronous satellites. low earth orbit satellites orbit between 2,00 and 2,000 kilometers above Earth. low earth orbit satellites are widely utilized for communication, military reconnaissance, spying, and other imaging purposes.

Medium Earth Orbit (MEO)

A medium earth orbit (MEO) satellite orbits the Earth at a higher height than a low earth orbit (LEO) satellite, but lower than a geostationary earth orbit (GEO). Medium Earth Orbit satellites orbit the Earth at least twice a day, some have completely circular orbits, while others track elliptically, but all maintain the same orbit once formed. This satellites are operating at altitudes ranging from 1,000 to 22,000 miles.

Geosynchronous Satellite

If the orbit of a satellite is inclined to the equator and the satellite orbits the earth once in 24 hours in the direction of the earth’s rotation, then the values of the angular velocities of the earth and the satellite are equal. Seen from Earth’s surface, it appears as if the artificial satellite is moving north-south along a fixed line of longitude, completing one complete oscillation in 24 hours. Such satellites are called Geosynchronous satellites.

Polar Satellite

Satellites placed about 700-800 km above the Earth’s surface in polar midplane instead of equatorial midplane are called polar satellites. It therefore takes about 1/100 of a second for a signal to travel from one point on Earth to another; Even a telephone conversation is not difficult for such a short time interval. Apart from that, the problem of signal reception or transmission in polar regions is also eliminated. Naturally, they are not geosynchronous satellites, that is, they are not in an apparent fixed position on the Earth’s surface. As can be seen by calculation, these satellites take about 2 hr to orbit the Earth once.

LAUNCHING OF ARTIFICIAL SATELLITE

A satellite is launched vertically or eastwards from the Earth’s surface by means of a rocket. With the help of rockets arranged on the back of the satellite, its direction of motion is changed in such a way that the satellite can achieve the right horizontal velocity by reaching a certain height. Only then it is placed in a certain orbit and starts orbiting the earth.

Naturally, any artificial satellite obeys Kepler’s laws. Hence, the orbits of artificial satellites, like the orbits of the planets around the Sun or the orbits of a planet’s satellites around it, can also be elliptical or circular. However, since the eccentricity of the satellite’s orbit is quite low in most cases, even if the orbit is assumed to be circular, there is no special error in the calculation.

ORBITAL VELOCITY AND PERIOD OF REVOLUTION OF AN ARTIFICIAL SATELLITE

The speed at which the satellite orbits the Earth is called the orbital speed of the artificial satellite. Let the mass of earth be M, radius of earth (OP) = R, mass of satellite m, orbital speed of satellite v and altitude of orbit above earth (PA) = h. Hence, the distance of the satellite from the center of the earth i.e. the radius of the orbit, r = R+h.

Assuming the orbit to be circular, centripetal force = mv²/r = mv²/R+h.

The mutual gravitational force between Earth and the satellite provides this centripetal force. This gravitational force is = GMm/r²

So, mv²/r = GMm/r² or, v² = GM/r = GM/R+h or, v = √GM/R+h …….(I)

If the surface value of gravitational acceleration is g,

g = GM/R² or, GM = gR²

Substituting this value into equation (I),

v = √gR²/r = R√g/r ……..(II)

If the orbital period of the satellite is T, the distance travelled by the satellite in this time T = Circumference of the orbit = 2πr.

So, T = 2πr/v = 2πr·1/R·√r/g = 2π/R·√r³/g …….(III)

Equations (II) and (III) above can be written in terms of the height (h) of the artificial satellite above the Earth’s surface,

v = √(gR²/R+h) = R√(g/R+h) ……..(IV)

And, T = 2π/R·√(R+h)³/g = 2π/R·√R³(1+h/R)³/g = 2π/R·R√R(1+h/R)³/g = 2π√R/g(1+h/R)^3/2……(V)

Equations (I) and (IV) mentioned above clearly prove that the orbital speed and orbital period of an artificial satellite do not depend at all on the mass of the satellite.

Incidentally, if the orbit is an ellipse with center ‘e’, then the maximum and minimum orbital speed of the artificial satellite will be respectively.

v_max= √GM/a(1-e) and v_min= √GM/a(1+e)

Where a is the semi-radius of the orbit.

ARTIFICIAL SATELLITE VERY CLOSE TO THE EARTH

Equations (IV) and (V) above show that the shorter the orbital distance (h) of an artificial satellite from the Earth’s surface, the higher its orbital speed (v), on the other hand, the shorter the orbital period (T). Therefore, to place a satellite in a very close orbit, it needs to be given a very high velocity. For such an orbit, the value of h is very small compared to the radius R of the Earth. That is, R+h ≈ R, so putting h=0 in equation (IV) or (V) does not cause any particular error. Hence,

orbital velocity, v = √gR ………(VI)

and orbital period, T = 2π√R/g ……(VII).

The orbital period of such artificial satellite depends only on the density of the earth. We know,

ρ = 3g/4πGR or, R/g = 3/4πρG.

(VII) Substituting the R/g-value in equation no, T = 2π√3/4πρG i.e., T ∝ 1/√ρ

Radius of Earth R = 6400 km = 6.4 X 10⁶ m, Gravitational Acceleration of Earth’s Surface g=9.8 m/s². Substituting these values into Equations (VI) and (VII) respectively,

v = √(9.8 X 6.4 X 10⁶) = 7.9 X 10³ m/s = 7.9 km/s

And, T = 2π√(6.4 X 10⁶/9.8) = 5078 s (Approx.) = 1 h 24 min 38 s.

Here is another important point, we know the release velocity of an object on Earth surface is v_e = √2gR = 11.2 km/s. In comparison, the orbital speed of the nearest artificial satellite (v = √gR = 7.9 km/s) is less than the release speed at the Earth’s surface.

The ratio of these two velocities is v/v_e = √gR/√2gR = 1/√2 = 0.707.

Increasing the value of h decreases the value of rotational velocity v, i.e. the rotational velocity decreases more than the release velocity. Therefore, to place an artificial satellite around the Earth, it is never necessary to give it a velocity equal to or greater than the release velocity.

USE OF ARTIFICIAL SATELLITE

Determination of pressure, altitude and composition of the upper atmosphere.
Weather monitoring and forecasting.
Protective surveillance.
A survey of the shape and size of the earth.
Telecommunication (We can watch sports live on television in different parts of the world by exchanging signals with the help of artificial satellites, we can communicate with people in different parts of the world by telephone.)
Collection of data on ionosphere of upper space, cosmic ray radiation, Van Allen Radiation Belt of charged particles, effects of solar radiation etc.