The twin STEREO probes (STEREO-Ahead and STEREO-Behind) orbit the Sun with an orbit pretty close to Earth’s — but not exactly. STEREO-A orbits slightly lower, which means it goes around a little quicker and gradually advances ahead of Earth. STEREO-B orbits slightly higher, which means it takes a little longer to go around and gradually falls back behind Earth. They’ve been doing that since 2006, allowing them to observe the Sun from angles not possible from Earth; for a few years now, they’ve enabled NASA to combine their observations with those from SOHO and SDO to produce 24-7 full coverage of the entire Sun for the first time in human history.
The mission is about to enter a new phase now, eight years in, as their motion slowly moves them into conjunction — from our perspective, they will seem to pass behind the Sun. From STEREO’s perspective, of course, we’re the ones who will get eclipsed, and this has a very important impact on the mission — while the Earth appears very close to the Sun from their perspective, communications will be difficult to impossible, partly because of solar radio interference but also because the Sun’s heat becomes dangerously intense on the communication equipment when the parabolic dish is pointed straight at it.
Total communications blackout will begin for STEREO-B next January and will last about two months. STEREO-A will be in conjunction for slightly longer, starting a day after STEREO-B comes out and lasting to mid-July, although STEREO-A will start having communcation difficulties next month, and STEREO-B will not regain full communications until 2016.
The conjunction may be half a year off, but that means it’s time for NASA to start getting ready, preparing for the spacecraft to conduct observations while storing the data for later replay. STEREO-A is out of scientific service right now, and will be for a couple of weeks while engineers conduct tests. STEREO-B will undergo similar testing in the fall. Through the whole phase, at least one of the two spacecraft will be observing the Sun at all times; we just might have to find out about it later. 😉
Goddard Space Flight Center has a great video explaining it all: