The Sun ejects vast clouds of ionized gas into space; these clouds are known as coronal mass ejections (CMEs). Each CME may carry 1,000,000,000 tonnes of gas into space at speeds that can approach 2000 km/s. When they engulf Earth, CMEs can disrupt power, navigation, communication and satellite control systems. Despite their importance, scientists don't fully understand the origin or evolution of CMEs, nor their structure or extent in interplanetary space.
NASA’s twin spacecraft STEREO mission, launched in October 2006, is providing a totally new perspective on solar phenomena by imaging CMEs and other solar features from two viewpoints simultaneously. The two near-identical spacecraft are in Earth-like orbits around the Sun, displaced from one another, with the STEREO-A spacecraft orbiting the Sun ahead of the Earth and STEREO-B orbiting the Sun behind the Earth. Both spacecraft look at the Sun itself, and also the region of interplanetary space between the Sun and Earth-like distances (1 Astronomical Unit; 1 AU). The unique twin-platform view allows stereoscopic imaging of the Sun and the structure of CMEs, enabling scientists to study their fundamental nature and origin.
It is, in fact, the RAL Space-led Heliospheric Imagers (HIs) on STEREO that observe that region of interplanetary space between the Sun and 1 AU, using wide-angle telescopes. The STEREO/HI instruments are being used to identify and track CMEs as they propagate through interplanetary space, with particular focus on those that are directed towards the Earth.
When combined with data from near-Earth spacecraft, and observatories on the ground, the STEREO data will enable scientists to identify and track in 3D those CMEs that propagate towards Earth and investigate their effects on Earth’s environment, as well as studying the processes on the Sun associated with their launch. In addition to leading the STEREO/HI instruments, all of the imaging instruments aboard both STEREO spacecraft use a CCD-based camera system developed by RAL Space.
(Credit: Backstage Science)