Mars will make its closest approach to Earth for almost 60,000 years on Wednesday. Dr Robin Catchpole, senior astronomer at the Royal Observatory, Greenwich, London, explains how to witness the event.
Every 26 months, the Earth overtakes Mars on the inside track as they both orbit the Sun.
Every 15 to 17 years, this happens when Mars is closest to the Sun.
On 27 August 2003 they will pass at a distance of 55,760,000 km or 0.3727 AU (1 AU is the distance of the Earth from the Sun), closer than any time during the last 60,000 years.
At its closest, Mars will be 25 seconds of arc in diameter, the size of a tennis ball at a distance of 528 metres - or 1/60 the diameter of the full Moon.
However, on the scale of the tennis ball, this approach is only 18 centimetres closer than in 1924, although 110 metres closer than in 2001.
Throughout July, August and September, Mars will be easy to see with the naked eye. Each night it will rise earlier, until by late August it will be just above the horizon, south of east, soon after sunset.
Mars is already visible with the naked eye
By late August it will be just above the horizon, south of east, soon after sunset
The planet will be the brightest object in the sky, appearing orange red
To see any surface detail requires a good quality telescope
Once risen, it will be the brightest object in the sky, appearing orange red. From the latitude of the UK, it will never rise higher than 22 degrees and you will need an unobstructed view to the south to see it.
Mars will appear at its very best from the Southern Hemisphere, where by the middle of the night it will be high in the sky and its brightness and colour will make it very obvious.
To see any surface detail requires a good quality telescope with a lens or mirror diameter of about 10 cm, on a stable mount. If you don't have one - and few people do - try to visit a public observatory or your local astronomical society.
In the UK, National Astronomy Week will mark the close encounter, and many local astronomical societies will organise public star parties.
Mars, the Roman god of war, has always had a special fascination for us. Indeed, there is even a slight connection as both blood and Mars owe their red colour to iron and oxygen.
Although half the diameter and 1/10 the mass of Earth, it is the planet most like our own. When close, through a good telescope, it is possible to see surface markings, polar ice caps and even thin clouds and frost.
What would our ancestors have made of the bright orange "star"?
We no longer believe in canals and intelligent Martians, as was suggested after the close approach of 1877. But we do think there is a possibility that primitive single celled organisms may have lived, or even still live, below its surface.
Since the 1960s, satellites orbiting, and even probes landing on Mars, have sent back images showing a desert landscape of sand dunes strewn with boulders and rocks and crossed by dry streambeds.
It still remains to be seen if these were carved by water and if so, whether it flowed recently or a thousand million years ago.
A fleet of spacecraft are currently heading to Mars
There are many impact craters on the surface, testifying to the great age and present geological inactivity of the planet.
This was not always the case as Mars is home to the biggest volcano in the Solar System, the now inactive 26-km-high Olympus Mons.
However you watch Mars move through the August sky, imagine a world in many ways familiar and yet very different from our own and know it has not passed quite this close since the Stone Age.