As well as keeping their fingers crossed for good weather during next week’s close approach of Mars, astronomers are hoping that the skies will be clear on Mars itself.

During National Astronomy Week – Saturday 23 to Saturday 30 August – observing events will be held across the UK where the public can view Mars through telescopes. It is expected that thousands of people will turn out to see the planet closer than it has been for almost 60,000 years. But it”s not just our own familiar clouds that could spoil the view. There is a risk that a major dust storm on Mars could wipe out any of the surface details visible on the planet.

Mars’s orbit round the Sun is not circular, and its distance from the Sun varies by over 20 per cent. So when it is close to the Sun, as it is now, the additional solar heating can whip up dust storms that can cover huge areas. The most severe can cover the entire planet. This happened in 1971, for instance, just as the US Mariner 9 space probe arrived at the planet. Its cameras revealed a barren disc, with no detail at all. Mars’s atmosphere was filled with fine dust from pole to pole. Observers on Earth confirmed that no details could be seen for a whole month.

At Mars’s last close approach in 2001, another planet-wide storm occurred, and pictures from the Hubble Space Telescope showed the planet looking like a red billiard ball, with the polar caps and markings appearing only dimly.

Already this year, a dust storm has obscured detail on Mars. In July, winds whipped up dust in the Hellas region of Mars to cover an area greater than 1000 miles. They subsided after a week or so, and currently the Martian atmosphere is clear again.

”We are all hoping that Mars will stay dust-free during National Astronomy Week,” said Robin Scagell, NAW publicity officer. ”It would be a great disappointment if all people could see was an orange blob.”

Scagell says that observers would actually prefer slightly hazy weather on Earth for good Mars observation. ”Mars will be very low in the sky this year. Slight mist often helps to reduce atmospheric turbulence. On clear nights when the stars are twinkling like mad, the planet is all over the place and it”s hard to see any detail.”

Details of the observing events around the country can be seen at the National Astronomy Week web site: www.astronomyweek.org.uk

Hubble pictures of the 2001 dust storm are available at http://hubblesite.org/newscenter/archive/2001/31/
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