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 $6 R$ ? $$ \left[G=6.67 \times 10^{-11} \text { SI unit and } M=6 \times 10^{24} \mathrm{~kg}\right] $$
Given,
$r_p=$ radius of perihelion $=2 R$
$$r_a=\text { radius of apnelion }=6 R$$
Hence, we can write
$$ \begin{aligned} & r_a=a(1+e)=6 R \quad \text{... (i)}\\ & r_p=a(1-e)=2 R \quad \text{... (ii)} \end{aligned}$$
Solving Eqs. (i) and (ii), we get eccentricity, $e=\frac{1}{2}$
By conservation of angular momentum, angular momentum at perigee $=$ angular momentum at apogee
$$\begin{array}{ll} \therefore & m v_p r_p=m v_a r_a \\ \therefore & \frac{v_a}{v_p}=\frac{1}{3} \end{array}$$
where $m$ is mass of the satellite. Applying conservation of energy, energy at perigee $=$ energy at apogee
$$\frac{1}{2} m v_p^2-\frac{G M m}{r_p}=\frac{1}{2} m v_a^2-\frac{G M m}{r_a}$$
where $M$ is the mass of the earth.
$$\begin{aligned} &\begin{aligned} \therefore\quad v_p^2\left(1-\frac{1}{9}\right) & =-2 G M\left(\frac{1}{r_a}-\frac{1}{r_p}\right)=2 G M\left(\frac{1}{r_p}-\frac{1}{r_a}\right) \quad\left(\text { By putting } v_a=\frac{v_p}{3}\right) \\ v_p & =\frac{\left[2 G M\left(\frac{1}{r_p}-\frac{1}{r_a}\right)\right]^{1 / 2}}{\left[1-\left(V_a / V_p\right)^2\right]^{1 / 2}}=\left[\frac{\frac{2 G M}{R}\left(\frac{1}{2}-\frac{1}{6}\right)}{\left(1-\frac{1}{9}\right)}\right]^{1 / 2} \\ & =\left(\frac{2 / 3}{8 / 9} \frac{G M}{R}\right)^{1 / 2}=\sqrt{\frac{3}{4}} \frac{G M}{R}=6.85 \mathrm{~km} / \mathrm{s} \\ v_p & =6.85 \mathrm{~km} / \mathrm{s}, v_a=2.28 \mathrm{~km} / \mathrm{s} \end{aligned} \end{aligned}$$
For circular orbit of radius $r$,
$$\begin{aligned} v_c & =\text { orbital velocity }=\sqrt{\frac{G M}{r}} \\ \text{For}\quad r & =6 R, v_c=\sqrt{\frac{G M}{6 R}}=3.23 \mathrm{~km} / \mathrm{s} \end{aligned}$$
Hence, to transfer to a circular orbit at apogee, we have to boost the velocity by $\Delta=(3.23-2.28)=0.95 \mathrm{~km} / \mathrm{s}$. This can be done by suitably firing rockets from the satellite.