This image shows a bow shock around the very young star, LL Ori. It is located in the intense star-forming region known as the Great Nebula in the constellation Orion.
A bow shock can be created in space when two streams of gas collide. LL Ori emits a vigorous stellar wind, a stream of charged particles moving rapidly outward from the star. This stellar wind collides with slow-moving gas evaporating away from the center of the Orion Nebula, which is located to the lower left in this image. The surface where the two winds collide is the crescent-shaped bow shock seen in the image. A second, fainter bow shock can also be seen around a star near the upper left-hand corner of the image.
ESA's Cluster was in the right place and time to make this shocking discovery. The four spacecraft encountered a shock wave that kept breaking and reforming - predicted only in theory.
On 24 January 2001, Cluster's spacecraft observed shock reformation in the Earth's magnetosphere, predicted only in theory, over 20 years ago. Cluster provided the first opportunity ever to observe such an event, the details of which have been published in a paper on 9 March this year.
The shock wave that sits above the Earth's surface is a natural phenomenon. It is located on the side facing the Sun, at approximately one quarter of the distance to the Moon, and is caused by the flow of electrically charged particles from the Sun.