In a rare event on July 8, 2010, skywatchers will be able to see an asteroid briefly block out the light from a star as it passes in front. It may be the only asteroid 'occultation' this century observable with the naked eye.

Everybody is familiar with a solar eclipse, when our Moon passes in front of the Sun and blocks its light for several minutes.

A similar situation can happen with asteroids, the Sun-orbiting, rocky or metallic objects that are left over from the formation of the Solar System or were formed by collisions between other asteroids.

In total, we know of about 400,000 of these dark bodies, which range in size from a few hundred kilometres to just a few metres. The smaller ones are hard to detect.

While an asteroid is far too small to cover the Sun, one will occasionally move directly in front of one of the many stars in the night sky and block its light from our view, causing a stellar eclipse or occultation.

Since asteroids move relatively fast, these events typically last just a few seconds. Normally the occulted star is so faint the event can only be seen via telescope.

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Roma to pass in front of Delta Ophiuchi

During the night of 8/9 July, however, a star that is visible to the naked eye, Delta Ophiuchi (the fourth-brightest star in the constellation Ophiuchi), will be occulted by asteroid Roma, which has a diameter of about 50 km.

This means the occultation will be visible only along a path about 50 km wide, crossing central Europe, Spain and the Canary Islands (scroll down for details on how to observe this event).

Catching a glimpse

Since asteroids, with very few exceptions, are too small to be resolved with ground-based telescopes, asteroid occultations are the only direct way of measuring the size of such an object. When several observers record such an event, using video cameras with precise timing, the times when they see the occultation help to measure the shape of the asteroid.

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Since we know the speed of the asteroid, the duration of the occultation can be converted directly to a length. This allows scientists to reconstruct the size and shape of the object.

ESA's space hazards programme seeks asteroids

steroids -- in particular those coming close to Earth -- are the focus of the Near Earth Object (NEO) segment of ESA's new Space Situational Awareness Preparatory Programme (SSA-PP).

SSA-PP will provide timely and accurate information, data and services on the space environment, and particularly on hazards to satellites in orbit and infrastructure on the ground.

These hazards stem from possible collisions between objects in orbit, harmful space weather and potential strikes by natural objects such as asteroids that cross Earth's orbit.

Currently, of the 400 000-plus asteroids known in our Solar System; more than 6500 are NEOs -- their orbits come close to that of Earth's.

NEOs could potentially hit our planet and, depending on their size, produce considerable damage. While the chance of a large object hitting the Earth is very small, it would produce a great deal of destruction; thus NEOs merit active detection and tracking.