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The asteroid and the disappearing star


Asteroids come in many shapes and sizes
 
Asteroids come in many shapes and sizes
 
 
5 July 2010
 
On 8 July, a fairly bright star, known as Delta Ophiuchi, will disappear from the night sky. However, there is no need to worry. The naked eye star is not being eaten or dying – it is simply being hidden by a fast-moving asteroid.
 
For a few seconds, this small chunk of rock, known as (472) Roma, will be in just the right place at the right time to block the starlight coming to Earth. This is a rare event - the only time this century when it will be possible to see an asteroid eclipse a star without the help of binoculars or a telescope.

Everybody is familiar with a solar eclipse, when our Moon passes in front of the Sun and the sky goes dark for several minutes. Although the Moon is 400 times smaller than the Sun, it is also 400 times nearer, so it is able to cover the entire disk of our nearest star.
 
 
A grazing occultation of Rhea, a moon of Saturn, by another moon
   
A grazing occultation of Rhea, a moon of Saturn, by another moon, Dione
 
The same thing can happen with more distant stars. A 50 km wide asteroid such as Roma can briefly hide a huge star such as Delta Ophiuchi because the space rock is much closer to us. Events such as this are important because they enable astronomers to work out the size and shape of asteroids – including some that may one day collide with Earth.

The star’s disappearing act will only be visible from a path about 50 km wide that links Stockholm, Copenhagen, Bremen, Nantes, Bilbao and the Canary Islands. It will take place at about 23:57 CEST, although the exact time will vary from place to place.
 
 
Star chart
 
Star chart showing the sky from central Europe
 
 
 

Asteroids

 •  Exploring asteroids (http://www.esa.int/esaKIDSen/SEMD29WJD1E_OurUniverse_0.html)
 •  Meteorites (http://www.esa.int/esaKIDSen/SEMN99WJD1E_OurUniverse_0.html)
 •  Shooting stars (http://www.esa.int/esaKIDSen/SEM059WJD1E_OurUniverse_0.html)
 •  Comets (http://www.esa.int/esaKIDSen/SEMYC9WJD1E_OurUniverse_0.html)