Gravitation

Two satellites \(S_1\) and \(S_2\) are revolving round a planet in coplanar circular orbits of radii \(r_1\) and \(r_2\) in the same direction, respectively. Their respective periods of revolution are \(1 \mathrm{~h}\) and \(8 \mathrm{~h}\). The radius of orbit of satellite \(S_1\) is equal to \(10^4 \mathrm{~km}\). What will be their relative speed (in \(\mathrm{km} / \mathrm{h}\)) when they are closest?