Unlike earth-orbiting satellites, a spacecraft designed for a flight to another planet, must first escape from the earth. It has to achieve velocity greater than Earth’s escape velocity which is 11.2 kilometres per second. This, however, is only part of the problem. Other factors must be considered, particularly the Sun’s gravitational field and the motion of the Earth around the Sun.
Before launching, the rocket is moving with the Earth’s around the Sun—about 30.5 kilometres per second. If it is fired in the same direction as the Earth’s orbital motion it will obviously have an independent velocity around the Sun greater than that of the Earth’s itself. It will then take up an orbit such as A, shown in the above diagram. If properly launched, it could reach the outer planets Mars, Jupiter or Saturn.
What if the rocket is launched ‘backward’, i.e. in the opposite direction of the Earth’s orbital motion around the Sun? In such a case, it will assume an independent velocity less than that of the Earth. It will move on orbit like B, indicated in the diagram. It could reach inner planets Venus and Mercury. So not only the velocity but also the direction of launching determined whether the spacecraft will be heading for the outer planets or the inner planets. The minimum launch velocities as well as transit times or reach all planets are given in the accompanying table,