Celestial fireworks

The "shooting star month" has arrived. During the first two weeks, and especially between 8 and 12 August, the Earth ploughs through a stream of tiny, sooty particles in space shed by Comet Swift-Tuttle III. These streak into the upper atmosphere and burn up by friction at a height of about 60 kilometres (37.5 miles) - the familiar "falling star" effect.

The Perseids pose no threat to the Earth, and provide a lovely celestial fireworks display that this year won't be drowned by moonlight. The real danger, according to a delegate at the Spaceguard UK Conference in Cambridge earlier this month, is not "Wysiwyg", but "What you don't see is what you get".

He was referring to a recently-recognised menace, in the form of bigger chunks of Solar System debris. Astronomers have known for years that our planetary system contains leftover material from the "cosmic building- site". And the spectacular collision of Comet Shoemaker-Levy 9 with Jupiter in July 1994 showed that comets can still hit planets. Scientists reckon there are more than 2,000 pieces of debris with the potential to do calamitous damage to Earth. Almost certainly, the dinosaurs were wiped out by such an impact 65 million years ago. Earth scientists and archaeologists are finding that impacts from space have shaped our planet's history: even the Moon was probably formed by a sidelong impact, early in the Earth's history, as reported in The Independent yesterday.

The threat from space prompted scientists in America to form "Project Spaceguard" a few years ago. UK scientists held a conference this month to evaluate the dangers. Though a large comet, like Hale-Bopp or Halley, may be expected to strike Earth only once in a billion years, small asteroids less than one kilometre across could hit us over timescales of 100,000 years.

The risk is small, but the outcome for the Earth is catastrophic. And it will happen; the question is, when? To get a better idea of the magnitude of the threat, astronomers need to log the positions and orbits of the unseen "near-Earth asteroids". If an asteroid were found to be coming straight at us, exploding a nuclear warhead close to the object might knock it off course.

There is no funding provision for this hands-on research. Since the bottom line here is public safety, astronomers believe the Government should pay for it. As a result, quite a few MPs may be surprised in the next few weeks to receive unusual letters outlining the threat to planet Earth, and how British astronomers can help.

Stars and planets

The "summer triangle" of Vega, Deneb and Altair rides high in the sky, and bright planets are on view. Venus is in the west, setting an hour after the Sun; Mars, in the south west, is drawing away from Earth and setting a couple of hours after sunset. Saturn rises in the east around 10pm BST (look for the Moon close to Saturn on 22 August). Mighty Jupiter is at opposition this month, and shines brilliantly all night long - use binoculars to spot its four brightest moons, or a small telescope to pick out its ochre-tinged cloud beltsn

August diary (24-hour, BST)

3 09.14 new Moon

9 opposition of Jupiter

11 13.43 Moon at first quarter

12 16.00 maximum of Perseids meteor shower

18 11.56 full Moon

25 03.24 Moon at last quarter