This phase is a direct result of the orbits for STEREO, which is short for the Solar Terrestrial Relations Observatory.
The spacecraft travel at different speeds. This means that over time, the satellites become increasingly out of sync, appearing from Earth's perspective to drift farther apart, able to observe first the sides and eventually the far side of the sun. For the first time ever, thanks to STEREO and near-Earth solar telescopes such as NASA’s Solar Dynamics Observatory the human race has had its first 360-degree view of the sun.
The orbits have continued to cause the STEREO spacecraft's position to change, however, and now they are nearing each other once again, this time on the other side of the sun. During this period when the sun blocks Earth's view — a geometrical position known in astronomy as a superior conjunction — radio receivers on Earth will not be able to distinguish STEREO's signal from the sun's radiation. Communication with the spacecraft will cease and the satellites will both go into safe mode without collecting data for a time. This will happen for STEREO-Ahead from March 24 to July 7, 2015. STEREO-Behind will be in superior conjunction from Jan. 22 to March 23, 2015. At least one spacecraft, therefore, will always be collecting data.
Before this occurs, the heating from the sun will also begin to affect – though not shut down — data collection. From wherever they are in space, the STEREO spacecraft aim their antenna toward Earth to send down data. This position puts the antenna fairly close to pointing at the sun, exposing the instruments to more heat than it can safely bear. The antenna can be adjusted to point in different directions, but the signal coming to Earth will be much fainter and won't allow for as much data to be downloaded. This antenna adjustment will begin on Aug. 20, 2014, for the STEREO-Ahead spacecraft and on Dec. 1, 2014, for STEREO-Behind.
During this phase, STEREO instruments will continue to run 24 hours a day, but they will gather lower-resolution data than usual. Some of this data will be downloaded whenever STEREO can link up with an Earth receiver. The rest of the data will be stored on board to be downloaded when the spacecraft reach a more auspicious geometrical position in early 2016.
To test for this off-pointing from the sun, STEREO-Ahead will undergo tests and not be collecting data from July 6-12, 2014. The same tests will be performed on STEREO-Behind from Sept. 29 – Oct. 6, 2014.
Throughout this entire phase until 2016, at least one STEREO spacecraft will be capturing data at any one time, so scientists will have an uninterrupted record of events on the sun to coordinate with the observations of solar telescopes on the Earth side. Real time monitoring of the sun, its flares and coronal mass ejections – information used by the US National Oceanic and Atmospheric Administration to help forecast space weather — will also continue via a fleet of NASA spacecraft closer to Earth.