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Why 2003?

Mars Express sets off during close encounter with Mars

In August 2003 Mars will make its closest approach to Earth for 17 years. Around that time, Mars Express will journey to the red planet. It will be launched in June 2003, before the planet's closest approach, and will head for the position in space where Mars will be the following December. By having the closest approach occur while Mars Express is on its journey, the spacecraft will take the shortest possible route.

Figure 1. Mars 2003 perihelion opposition.

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.

Figure 2. Mars 2010 apohelion.

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 its illuminated side faces us.

If opposition occurs when Mars is at its closest to the Sun (aposition 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. That's what will happen in 2003.

Last update: 13 March 2000

Source: Sci.ESA.Int

More Stuff:
* Mars Observing -- Opposition 2001, 2003
* Appendix 2: Perihelic Oppositions of Mars, 1608--2035

Dan