ESA Science & Technology30-Jun-2005 16:29:35

Background Science


If you could leave the Earth and travel vertically through its atmosphere, you may expect eventually to reach empty space - a void of darkness where nothing exists. The space around the Earth, however, is far from empty. The solar wind, charged particles (electrons and ions) streaming away from the Sun, envelops our planet and its neighbours. It blows a magnetic bubble out into the region of interstellar space surrounding the Sun. This bubble is called the heliosphere ("Helios" is the ancient Greek word for "sun"). It defines the volume of space over which our Sun's influence predominates.

That influence reaches way beyond what we normally think of as the solar system. The closest boundary of the heliosphere is thought to extend about 100 AU out from the Sun, that is 100 times the distance of the Earth from the Sun and at least twice as far out as the Kuiper belt, which contains the most distant objects in the solar system. NASA's two Voyager spacecraft are expected to reach this boundary before they finally grow too old to transmit signals in about 2020.

The heliosphere acts as a shield, protecting us from the bulk of cosmic rays, very energetic particles generated far away in the universe that would otherwise bombard the Earth continuously, causing damage to living cells. Without the heliosphere, life would certainly have evolved differently - and maybe not at all.

The Sun is not the only star to form a heliosphere. Stellar winds are thought to be common and there is firm evidence for "asterospheres" surrounding several of our closest stellar neighbours, including Sirius and Alpha Centauri.

Schematic of the heliosphere, showing the boundary regions
between the heliosphere and the interstellar medium.


For further information please contact: