By Dr David Whitehouse
BBC News Online science editor
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A huge dust storm is raging over a third of Mars, obscuring many surface features. Experts hope it will subside by 27 August when Mars will be at its brightest and closer to Earth than it has ever been in recorded history.
The dust storm between 4-6 July
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The dust storm began in the Hellas basin, one of the lowest regions of Mars, and a frequent breeding ground for such events.
Since then, it has expanded with winds carrying dust into the thin atmosphere for several thousand kilometres.
Dust storms occur during most Martian southern hemisphere summers, because at that time of year sunlight is strong enough to heat the ground and alter atmospheric circulation.
Summer event
On 1 July observers noticed that one of Mars' most prominent features, the dark Syrtis Major region, was weaker than normal.
Over the course of the next few days, it became clear that a major dust storm had erupted on the planet and was growing rapidly.
Astronomers began monitoring Mars to see how quickly the storm would spread, and if it would cover the whole planet as a similar storm did in 2001.
Richard McKim's drawings of Mars
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However, Mars observers doubt the current disturbance - which stretches a third of the way around the planet - will get much bigger than it is now and say it is likely to decline over the next few weeks.
Richard McKim, director of the Mars section of the British Astronomical Association, said: "This dust storm is a typical one. It is probably not going to reach planet-encircling size like the 2001 dust storm."
At the end of August, Mars and Earth will be a mere 55,760,000 kilometres apart. They have not been so close in nearly 60,000 years.
The Red Planet will appear especially large in diameter, so it will be relatively easy to make out surface features with a large telescope. Also, Mars will appear exceptionally bright, and its orange colour will be very conspicuous.