The planet Mars has a mass of$6.4\times {10}^{23}\mathrm{kg}$and a diameter of $6790 \mathrm{km}$. Suggest why it has been proposed that, for a successful mission to Mars, the craft that takes the astronauts to Mars will be assembled at a space station in Earth orbit and launched from there, rather than from the Earth's surface.

This diagram shows the gravitational potential near a planet of mass M and radius R.

On a copy of the diagram, draw similar curves. For a planet of the same radius but of mass $\mathit{2}M\mathit{-}$label 
this i. 

This diagram shows the gravitational potential near a planet of mass M and radius R.

On a copy of the diagram, draw similar curves. For a planet of the same radius but of radius $\mathit{2}R\mathit{-}$label this ii. 

Explain what is meant by the gravitational field strength at a point.This diagram shows the dwarf planet, Pluto, and its moon, Charon.These can be considered to be a double planetary system orbiting each other about their joint centre of mass.