Time Period And Energy Of A Satellite

Time Period And Energy Of A Satellite

Vishal kumarUpdated on 02 Jul 2025, 07:57 PM IST

Satellites, whether natural like the Moon or man-made, orbit Earth due to a delicate balance of gravitational pull and velocity. The time period of a satellite, which is the time it takes to complete one full orbit, is a fundamental concept in understanding its motion. This period is influenced by the satellite's altitude and the mass of the Earth. The energy of a satellite, comprising both kinetic and potential energy, determines its speed and orbit stability. In real life, these principles are crucial for the operation of GPS satellites that guide our navigation systems, communications satellites that enable global connectivity, and weather satellites that monitor Earth's climate. Understanding the time period and energy of satellites helps engineers design efficient and reliable systems that are integral to modern technology, ensuring everything from accurate weather forecasts to uninterrupted phone calls.

This Story also Contains

  1. The Time Period of the Satellite
  2. Height of Satellite
  3. The Energy of Satellite
  4. Binding Energy (B.E.)
  5. Solved Examples Based on Time Period And Energy of a Satellite
Time Period And Energy Of A Satellite
Time Period And Energy Of A Satellite

The Time Period of the Satellite

The time period of a satellite is the duration it takes for the satellite to complete one full orbit around a celestial body, such as Earth. This time period is determined by the satellite's altitude and the mass of the celestial body it orbits. According to Kepler's laws of planetary motion, the time period is directly related to the size of the orbit—the higher the satellite, the longer the time period.

The time period (T) of the satellite is given by
T=2πrv=2πrrGM[ As v=GMr]T=2πr3GM=2πr3gR2[ As GM=gR2]T=2π(R+h)3gR2=2πRg(1+hR)3/2[ As r=R+h]
Where
r= radius of orbit
T Time period
M Mass of planet
If the satellite is very close to the Earth's surface,
i.e., h≪≪R,

T=2πRg84.6 minutes then or T1.4hr

The time period of a satellite in terms of density
T=3πGρ

ρ Density of planet
T Time period
G Gravitational constant

ρ=5478.4Kg/mfor earth 3

For a satellite, the time interval between the two consecutive appearances overhead

If a satellite in the equilateral planes moves from west to east Angular velocity of the satellite with respect to an observer on earth will be (ωSωE)
ωS Satellite angular velocity
ωE earth angular velocity
So T=2πωSωE=TSTETETS
if ωS=ωE,T=

means the satellite will appear stationary relative to Earth.

Height of Satellite

As we know, time period of satellite T=2πr3GM=2π(R+h)3gR2 By squaring and rearranging both sides qR2T24π2=(R+h)3 h=(T2gR24π2)1/3R

Putting the value of the period in the above formula we can calculate the height of the satellite from the surface of the earth.

The Energy of Satellite

When a satellite revolves around a planet in its orbit, it possesses both kinetic energy (due to orbital motion) and potential energy (due to its position against the gravitational pull of Earth). These energies are given by

Potential energy: U=mV=GMmGMm=L2mr2
Kinetic energy : K=12mv2=GMm2r=mL22mr2
Total energy : E=U+K=GMmr+(2Mm2r=GMm2r=L22mr2
Where

M mass of planet m mass of satellite
And

K=EU=2EU=2K

Energy Graph of Satellite

Where

E Energy of satellite
K Kinetic energy
U Potential energy

Energy Distribution in an Elliptical Orbit



In this Total Energy
E=GMm2a= const
Where a= semi-major axis

Binding Energy (B.E.)

The minimum energy required to remove the satellite from its orbit to infinity is called Binding Energy.

And It is given by
BE=GMm2r

where
B.E Binding energy
M mass of planet
m mass of satellite

Work done in changing the orbit

When the satellite is transferred to a higher orbit i.e. (r2>r1) as shown in the figure.


W=E2E1W=GMm2[1r11r2]
Where
W work done
r1 radius of 1st orbit
r2 radius of 2nd orbit

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Solved Examples Based on Time Period And Energy of a Satellite

Example 1: A very long (length L) cylindrical galaxy is made of uniformly distributed mass and has a radius R (R<<L). A star outside the galaxy is orbiting the galaxy in a plane perpendicular to the galaxy and passing through its centre. If the period of the star is T and its distance from the Galaxies axis is r, then :

1) T2αr3
2) Tαr2
3) Tαr
4) Tαr

Solution:

The period of satellite
T=2πr3GMr= radius of orbit T Period M Mass of planet T= Circum ference of orbit orbital velocity F=2GMLrm,F=(kr)m
Here K is some constant

(mv2r)=kmrv=constantT=2πrvTr

Hence, the answer is the option (3).

Example 2: The relative uncertainty in the period of a satellite orbiting around the earth is 102. If the relative uncertainty in the radius of the orbit is negligible, the relative uncertainty in the mass of the earth is

1) 102
2) 2×102
3) 3×102
4) 6×102

Solution:

Period of satellite

T=2πr3GMr= radius of orbit T Time period M Mass of planet wherein T= Circum ference of orbit orbital velocity v=(QMR)T=2πRv=2π(4m)R3/2M=4π24R3T2ΔMM=3ΔRR+2ΔTT=3.0+2×102ΔMM=2×102

Hence, the answer is the option (2).

Example 3: A spaceship orbits around a planet at a height of 20 Km from its surface. Assuming that only the gravitational field of the planet acts on the spaceship, what will be the number of complete revolutions made by the spaceship in 24 hours around the planet? (Given : Mass of planet =8×1022Kg, Radius of planet =2×106 m Gravitational Constant G=6.67×1011Nm2Kg2 )

1) 11

2) 17

3) 13

4) 9

Solution:

( T= time,r= radius, G= gravitational force,m= mass of planet )
using r=R+h

T=2πr3GM=2π(2.02×106)36.671011×8×1022=7800sec. Now, No of revolution =24×36007800=11.07=11

Hence, the answer is the Option (1).

Example 4: If the satellite whose mass m is revolving in a circular orbit of radius r around the earth (mass of earth =M). The time of the revolution of satellites is:

1) Tαr3GM
2) Tr32GM
3) TαGMr3
4) TαGMr32

Solution:

T=2πrv0=2πrGMrT=2πrGMrTr32GM

Hence, the answer is the option (2).

Example 5: If the density of the planet is ρ and a satellite is revolving in a circular orbit of radius r then the time of revolution varies with density ρ as :

1) Tρ
2) T1ρ
3) Tρ
4) T1ρ

Solution

The time period of a satellite in terms of density
T=3πGρ

ρ Density of planet
T Time period
G Gravitational constant
wherein

ρ=5478.4Kg/m3 for earth
As we known

T=2πr32GMM=ρ43πr3T=2πr32Gρ43πR3 so, T=3πGρ

T1ρ

Hence, the answer is the option (4).

Summary

The time period of a satellite depends on its orbit radius and the mass of the planet, following Kepler's laws. It is influenced by gravitational forces and can be calculated using specific formulas that account for satellite altitude and Earth's mass. The satellite's energy consists of kinetic and potential energy, which together determine orbit stability. Understanding these factors helps in applications like GPS, communication, and weather satellite

Frequently Asked Questions (FAQs)

Q: What is the "Hill sphere," and how does it relate to a satellite's orbital stability?
A:
The Hill sphere is the region around a celestial body where its gravitational influence dominates over other nearby massive bodies. For Earth satellites, it defines the approximate volume where stable orbits are possible. Beyond the Hill sphere, a satellite's orbit becomes unst
Q: How does the "Kozai mechanism" affect the long-term evolution of satellite orbits and energy?
A:
The Kozai mechanism is a gravitational effect that causes coupled oscillations in the eccentricity and inclination of a satellite's orbit due to the gravitational influence of a third body (like the Moon for Earth satellites). While conserving the orbit's semi-major axis and energy, it can lead to significant long-term changes in the orbit's shape and orientation.
Q: What is meant by "orbital injection," and how does it relate to a satellite's initial energy?
A:
Orbital injection is the process of placing a satellite into its intended orbit. It involves providing the satellite with the correct velocity and position to achieve the desired orbital parameters. The energy required for orbital injection depends on the target orbit's altitude and shape. Higher or more eccentric orbits generally require more energy for injection.
Q: How does the concept of "gravitational assist" relate to changing a satellite's energy?
A:
Gravitational assist, or gravity assist, is a technique where a satellite uses the gravity of a planet or moon to change its trajectory and speed. This maneuver can increase or decrease the satellite's kinetic energy and alter its direction without using propellant. It's commonly used for interplanetary missions to save fuel and achieve velocities that would be impractical with propulsion alone.
Q: How does the concept of "orbital velocity" relate to a satellite's energy?
A:
Orbital velocity is directly related to a satellite's kinetic energy. For a circular orbit, the velocity is given by v = √(GM/r), where G is the gravitational constant, M is Earth's mass, and r is the orbital radius. As the orbit's radius increases, the velocity decreases, but the total energy (kinetic + potential) increases due to the greater increase in potential energy.
Q: What is the significance of the "semi-major axis" in determining a satellite's orbital energy?
A:
The semi-major axis is a key parameter in orbital mechanics, directly related to a satellite's total energy. For elliptical orbits, it's half the longest diameter of the ellipse. The specific orbital energy is given by ε = -μ/(2a), where μ is Earth's standard gravitational parameter and a is the semi-major axis. This relationship shows that larger orbits have higher (less negative) total energy.
Q: How does the concept of "orbital resonance" affect the long-term stability and energy of satellite orbits?
A:
Orbital resonance occurs when the orbital periods of a satellite and another body (or Earth's rotation) have a simple integer ratio. Some resonances can lead to stable configurations, while others can cause orbital instabilities. Resonances can either help maintain a satellite's orbit with minimal energy input or require additional energy for station-keeping to avoid undesirable effects.
Q: What is the "frozen orbit" concept, and how does it relate to satellite energy conservation?
A:
A frozen orbit is designed to minimize variations in orbital elements over time due to perturbations like Earth's oblateness. By carefully selecting the eccentricity, inclination, and argument of perigee, these orbits maintain nearly constant average altitude and ground track. This stability helps conserve satellite energy by reducing the need for frequent orbital corrections.
Q: How does atmospheric drag affect the energy of satellites in very low Earth orbits?
A:
Atmospheric drag causes satellites in very low Earth orbits to lose energy continuously. This energy loss manifests as a decrease in orbital altitude and an increase in orbital velocity (as the satellite falls into a lower orbit). Without intervention, this leads to a spiral decay of the orbit and eventually atmospheric re-entry.
Q: What is the significance of "Lagrange points" in terms of satellite energy?
A:
Lagrange points are locations in space where a small object can maintain a stable position relative to two larger bodies. At these points, the gravitational and centrifugal forces balance, creating a local energy minimum. Satellites placed at Lagrange points require minimal energy for station-keeping, making them ideal for certain space observation missions.