A spaceship of mass mm circles a planet of mass M in an orbit of radius R. How much energy is required to transfer the spaceship to a circular orbit of radius 3R?

ΔE = GmM/3R

Explanation:

The absolute potential energy of an object in a planet's field is given as:

E = -GmM/2r

where,

E = Potential Energy

G = Universal Gravitational Constant

m = mass of spaceship

M = Mass of Planet

r = distance from surface of planet

Therefore, for initial state:

E = E₁ and r = R

E₁ = - GmM/2R

and for final state:

E = E₂ and r = 3R

E₂ = - GmM/6R

So, the required energy will be:

ΔE = E₂ - E₁ = - GmM/6R + GmM/2R

ΔE = GmM(- 1/6R + 1/2R)

ΔE = GmM/3R

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