Close encounter with Mars
27 April 1999On a clear night over the coming week, you should be able to spot the planet Mars shining brightly with a reddish glow in the direction of the constellation Virgo in the south of the northern sky. Only the Moon and Venus will outshine it. Mars is making its closest approach to Earth for nine years, so viewing conditions will be good - but not as good as they can be.
In June 2001, Mars will come even nearer and in August 2003 it will make its closest approach to Earth for 17 years. Around that time, the positions of the planets and the Sun will allow a spacecraft to travel from Earth to Mars most efficiently in terms of time and fuel. That's when Mars Express, the Mars exploration mission of the European Space Agency, will set off for the red planet. There's more to launching a spacecraft, though, than aiming at Mars when the planet is at its closest approach to Earth. The journey takes several months and the spacecraft would miss its target. Mars Express 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, which is after the closest approach. By having the closest approach occur while Mars Express is on its journey, the spacecraft will take 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 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. The opposition occurring this week will be somewhere in between.
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. If the skies are grey this week and you miss Mars, take heart - better opportunities for viewing are on the way.