NASA’s Solar Terrestrial Relations Observatory (STEREO) mission has the goal of studying the Sun and the nature of Coronal Mass Ejections (CMEs). It employs two spacecrafts, one orbiting ahead of Earth (STEREO-A), and the other orbiting behind Earth (STEREO-B).
One of the main activity of the satellites is to trace the flow of energy and matter from the Sun. Then, using the images of the far side of the Sun provided from STEREO, together with images from SOHO (SOlar and Heliospheric Observatory), it is possible to measure the CMEs propagation.
Communications with STEREO-B were interrupted in October 2014, after a planned reset of the spacecraft. An anomaly was detected and after that, no further communications have been successful.
In February 2015 the instruments on STEREO-A were turned off in preparation for solar conjunction, and in March they were switched into safe-mode. This was necessary because the spacecraft was in the line of sight of the Sun, and communications with it were impossible due to the strong radio interference from our star.
The very good news is that now, after over 3 months, STEREO-A has started sending images of the Sun again! Now it will be possible once more to take more accurate CME measurements using both STEREO-A and SOHO images.