This equation computes the change in velocity you would have to cause to decrease a planet's orbit radius around the Sun by a specified distance.
This equation makes use of the equation: Force of Sun's Gravity on a planet
You select a planet and specify the distance you'd like to see the planet's orbit decrease by. The equation then computes a `Delta` velocity that would have to be applied to the planet to make this change in orbital radius.
The basic principle that allows us to perform this calculation so simply is the fact that in a stable orbit, the force of gravity on the orbiting planet (satellite) must balance the centripetal force on the orbiting planet (satellite). So,
`F_("centripetal") = F_("gravity")`
`m_("planet")*v^2/r = F_("gravity")`
`v^2 = F_("gravity")*r/m_("planet")`