Mars: at least 56 million kilometres from Earth.
Saturn: 1.2 billion kilometres.
Neptune: 4.3 billion kilometres.
How does a probe get to such distant planets?
Exploring planets, particularly those at the outer reaches of the solar system, poses a number of constraints. Chief among these are distance, energy and journey time.

On Earth, the shortest and most economical route between 2 points is a straight line. And all terrestrial vehicles use their own means of propulsion.
But no matter how far we go, the journeys we are used to making on Earth cannot prepare us for interplanetary space travel. Indeed, they can even lead us into error.

The laws of space are much more subtle and the technical constraints we face there call for quite different strategies.

^{Closest distances to the Sun:
Venus : 107,400,000 km
Mercury : 45,900,000 km
Earth: 147,100,000 km
Mars : 207,000,000 km
Jupiter : 740,000,000 km
Saturn : 1,346,400,000 km
Uranus : 2,742,000,000 km
Neptune : 4,460,000,000 km
Pluto : 4,425,000,000 km}

^{Last updated: May 2003}