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Question: Answered & Verified by Expert
A satellite is in an elliptic orbit around the earth with aphelion of $6 R$ and perihelion of $2 R$ where $R=6400 \mathrm{~km}$ is the radius of the earth. Find eccentricity of the orbit. Find the velocity of the satellite at apogee and perigee. What should be done if this satellite has to be transferred to a circular orbit of radius $6 R$ ?
$\left[G=6.67 \times 10^{-11}\right.$ SI unit and $\left.M=6 \times 10^{24} \mathrm{~kg}\right]$
PhysicsGravitation
Solution:
1838 Upvotes Verified Answer
As given that,
$r_p$ (radius of perihelion $)=2 R$
$r_a$ (radius of apnelion $)=6 R$
Hence,
$$
\begin{gathered}
r_a=a(1+e)=6 R \\
r_p=a(1-e)=2 R
\end{gathered}
$$
On dividing (i) and (ii), we get
$$
\left(\frac{1+e}{1-e}\right)=\frac{6}{2}
$$
eccentricity, $e=\frac{1}{2}$
There is not external force or torque on system.
So, by the law of conservation of angular momentum, angular momentum
at perigee $=$ angular momentum at apogee $L_1=L_2$
$\left(\because m_a=m_p=m\right.$, mass of satellite)
$\therefore m v_p r_p=m v_a r_a$
$\therefore \frac{v_a}{v_p}=\frac{1}{3}$ or $v_a=\left(\frac{v_p}{3}\right)$
where $m$ is mass of the satellite.
Applying conservation of energy,
energy at perigee $=$ energy at apogee
$$
\frac{1}{2} m v_p^2-\frac{G M m}{r_p}=\frac{1}{2} m v_a^2-\frac{G M m}{r_a}
$$
Multiplying $\frac{2}{m}$ to both sides,
$$
\begin{aligned}
&v_p^2-\frac{2 G M}{r_p}=v_a^2-\frac{2 G M}{r_a} \quad\left(\because v_a=\frac{v_p}{3}\right) \\
&v_p^2-\left(\frac{v_p}{3}\right)^2=-2 G M\left(\frac{1}{r_a}-\frac{1}{r_p}\right)
\end{aligned}
$$
where $M$ is the mass of the earth.
$$
\begin{aligned}
&\therefore v_p^2\left(1-\frac{1}{9}\right)=-2 G M\left(\frac{1}{r_a}-\frac{1}{r_p}\right) \\
&\Rightarrow v_p=\left[\frac{\frac{2 G M}{R}\left(\frac{1}{2}-\frac{1}{6}\right)}{\left(1-\frac{1}{9}\right)}\right]^{1 / 2} \\
&\left(\because r_p=2 R \text { and } r_a=6 R\right) \\
&=\left(\frac{2 / 3}{8 / 9} \frac{G M}{R}\right)^{1 / 2}=\sqrt{\frac{3}{4} \frac{G M}{R}} \\
&=\sqrt{\frac{3 \times 6.67 \times 10^{-11} \times 6 \times 10^{24}}{4 \times 6400 \times 10^3}}=6.85 \mathrm{~km} / \mathrm{s} \\
&
\end{aligned}
$$
$$
v_p=6.85 \mathrm{~km} / \mathrm{s}, v_a=\left(\frac{v_p}{3}\right)=2.28 \mathrm{~km} / \mathrm{s}
$$
For circular orbit of radius $r$,
$$
\begin{aligned}
&v_c=\text { orbital velocity }=\sqrt{\frac{G M}{r}} \\
&\text { For } r=6 R, \\
&v_c=\sqrt{\frac{G M}{6 R}} \sqrt{\frac{6.67 \times 10^{-11} \times 6 \times 10^{24}}{6 \times 6.4 \times 10^6}} \\
&=3.23 \mathrm{~km} / \mathrm{s} .
\end{aligned}
$$
Hence, to transfer to a circular orbit at apogee, we have to boost the velocity by $\Delta v=(3.23-2.28)=0.95 \mathrm{~km} / \mathrm{s}$. This can be done by suitably firing rockets from the satellite.

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