Mars Express sets off during close encounter with Mars 

In August 2003 Mars made its closest approach to Earth for 17 years. Around that time, Mars Express journeyed to the red planet. It was launched in June 2003, before the planet's closest approach, and headed for the position in space where Mars would be the following December. By having the closest approach occur while Mars Express made its journey, the spacecraft took the shortest possible route.

The distance between Mars and Earth varies mainly because, like all planets, they have elliptical (oval) orbits. Earth's is only slightly elliptical, Mars's is more so. Each planet also takes a different amount of time to travel round its orbit: 687 Earth-days in the case of Mars and 365 days for Earth.

Think of the two planets as athletes on a running track. Earth is travelling faster than Mars on the inside track so will periodically catch it up and overtake it. When Earth is on the point of overtaking, the two planets are lined up with the Sun. This is called an "opposition" because, as seen from Earth, Mars is opposite the Sun in the sky. Mars oppositions occur approximately once every 780 days. They are good times to view the red planet because it is near to its closest approach to Earth and almost all of its illuminated side faces us.

If opposition occurs when Mars is at its closest to the Sun (a position on its elliptical orbit called perihelion) the distance between the two planets will be a minimum - 55 million kilometres. When Mars is at its furthest from the Sun (aphelion) the distance at opposition will be 99 million kilometres.

A spacecraft to Mars can be launched around any opposition, that is about once every 26 months. But the journey will be shortest and use the least fuel around a perihelic opposition, which occurs about once every 17 years. This is what happened in 2003.