Earth moving in for close encounter with red planet

The Earth is starting to overtake Mars, giving us our two-yearly opportunity to view the red planet from a relatively close vantage point. At present Mars is rising at around 11pm and is now very prominent after midnight in the northeastern sky. On October 31 Mars and Earth will be just 70 million kilometres apart, the closest they have been to each other since August 2003.

As Earth overtakes Mars in its orbit, we reach a point where the Earth, Mars and the Sun are aligned - this will happen on November 7. When Mars and the Sun are seen in opposite directions in the sky this is called "opposition" by astronomers.

Because the planetary orbits are elliptical, opposition does not usually coincide with the point of closest approach.

By the end of October, Mars will be rising at sunset and easily seen all night. At about 12.45am it will be due north and 37 degrees up the sky from the horizon.

When Mars is at opposition, it will appear brighter because it is closer to us. The opposition of August 28, 2003 was the closest one for 60,000 years. However, while this coming one is not quite as favourable, you will have to wait until 2018 for a better view.

Through a good telescope the planet will be about 20 per cent smaller than in 2003, appearing as a small orange disc with dark markings. Usually the telescope will also show a white polar ice cap and, occasionally, clouds in the very thin Martian atmosphere. More rarely the surface markings can be completely obscured by dust storms that can envelop the planet for weeks.

Imaging by spacecraft orbiting the planet has provided a very detailed picture of the Martian surface, some new images showing features as small as 5m across. A few areas of the planet are being scrutinised by two small vehicles crawling over the surface.

The orbital images of Mars show a dry landscape scarred by impact craters. There are a few enormous volcanoes that appear to be extinct, a vast chasm 3000km long and many signs that water once flowed.

Key objectives of the current exploration missions are to understand the history of the planet and build up a detailed picture as a prelude to visits by human explorers.