To find the relative distance from one point to another it is necessary to apply the Relativity equations.
Under the concept of relativity the distance measured from a spatial object is given by the equation
[tex]l = l_0 \sqrt{1-\frac{v^2}{c^2}}[/tex]
Where
[tex]l_0[/tex]= Relative length
v = Velocity of the spaceship
c = Speed of light
Replacing with our values we have that
[tex]l = l_0 \sqrt{1-\frac{v^2}{c^2}}[/tex]
[tex]l = 1.2*10^{11} \sqrt{1-\frac{0.8c^2}{c^2}}[/tex]
[tex]l = 1.2*10^{11} \sqrt{1-0.8^2}[/tex]
[tex]l = 7.2*10^{10}m[/tex]
Therefore the distance between Mars and Venus measured by the Martin is [tex]7.2*10^{10}m[/tex]
