P3_7 Why Does Venus Spin Backwards?

Abstract

Venus has a very slow, retrograde rotation compared to the 24 hour prograde rotations of
Earth and Mars. We find that for an impacting asteroid to have changed Venus' rotation from
an Earth-like period and direction to its current state, a minimum asteroid mass of 2x10^29kg
would be required, and that such a collision would destroy Venus. This makes it unlikely that
such a collision is responsible for the odd Venusian rotation.