Europe South Asia Asia Pacific Americas Middle East Africa BBC Homepage World Service Education

Front Page



UK Politics







Talking Point

In Depth

On Air

Low Graphics

Thursday, June 24, 1999 Published at 15:11 GMT 16:11 UK


Comet's shadow captured on cosmic screen

The white glow in the left image is the "projection" of the Sun's light

By BBC News Online Science Editor David Whitehouse

The shadow of a comet projected on a "screen" of hydrogen on the other side of the solar system has been seen by scientists.

They have also found a way to peek around the edge of the Sun and predict whether solar storms on its far side will appear on the side facing the Earth.

The discovery could help give warnings when solar storms threaten the Earth.

[ image: Comet Hale Bopp's shadow is in white]
Comet Hale Bopp's shadow is in white
SWAN, short for Solar Wind Anisotropies, is a telescope on board the SOHO satellite that can map the whole sky in ultraviolet light, something that is impossible from the ground.

The solar system is embedded in a huge cloud of thin hydrogen gas that blocks ultaviolet light.

Radiation from the Sun blows a bubble in this cloud. The bubble's inside surface forms a sort of screen on which light from the Sun is projected.

When radiation from solar active regions strikes the screen, the hydrogen gas begins to glow and hot spots are formed.

Because the hydrogen bubble is so large - it is bigger than Earth's orbit around the Sun - it is possible for SWAN to see hot spots caused by active regions on the far side of the sun, simply by looking at the part of the sky which the far side of the sun faces.

"Strong ultraviolet emissions from active regions on the back of the Sun behave like beams from a lighthouse on the landscape," says Jean-Loup Bertaux, of the French Service d'Aéronomie and principal investigator for SWAN.

"They move in the sky in accordance with the Sun's rotation, which takes about 28 days. So we can monitor the activity on the back side of the Sun without looking at it directly."

When comet Hale-Bopp flew near the Sun parading its 100-million-kilometre-long tail in 1997, SWAN spotted a remarkable feature, never before seen by astronomers. A shadow, more than 150 million kilometres long, of a comet projected on the interior of the hydrogen bubble.

Advanced options | Search tips

Back to top | BBC News Home | BBC Homepage | ©

Sci/Tech Contents

Relevant Stories

04 Feb 99 | Sci/Tech
Where the solar wind blows

17 Sep 98 | Sci/Tech
Soho pointing at the Sun again

Internet Links


NASA: Why we study the Sun

The BBC is not responsible for the content of external internet sites.

In this section

World's smallest transistor

Scientists join forces to study Arctic ozone

Mathematicians crack big puzzle

From Business
The growing threat of internet fraud

Who watches the pilots?

From Health
Cold 'cure' comes one step closer