Launched on 25 October 2006, STEREO (Solar TErrestrial RElations Observatory) consists actually of two nearly identical spacecraft. One of the spacecraft is ahead of Earth in its orbit (STEREO-A), the other trailing behind (STEREO-B), and both are at distances from the Sun similar to Earth's.
Since their launch, both spacecraft gradually moved away from Earth. A first milestone was reached on 6 February 2011, when STEREO-A and –B were separated 180 degrees and thus provided the first ever full view of the entire Sun (see this NASA news item).
Though one of the main objectives of the mission is to study the initiation and characteristics of coronal mass ejections (CME) and their propagation through the heliosphere, it also offers space weather forecasters a formidable tool in distinguishing the direction a CME is headed. In particular, when a halo CME is observed, the coronagraphic images from the STEREO spacecraft unambiguously establish the origin of the CME, i.e. either from the Sun's Earth facing side, or from the Sun's farside. In the first case, the CME can impact Earth's magnetic field, in the other case Earth will not be influenced.
As both spacecraft continue to separate from each other, they are now on a converging course on the other side of the Sun (as seen from Earth), which is scheduled to happen in 2015. Rest assured, the spacecraft will not collide with each other, as they will be at least a million km apart even at their closest point (try out this orbit tool). The STEREO-B spacecraft will then be ahead of Earth, and STEREO-A behind. It's very likely the name of the spacecraft will be switched to avoid any confusion amongst the users.
A communication problem will arise at some point during this encounter, as the spacecraft will be occulted by the Sun as seen from Earth. Hence, for some period of time in 2015, Earth will not be able to receive data and imagery from the spacecraft. The STEREO Science Centre has a dedicated website for this event.
The out-of-contact periods will also depend critically on how much the radio noise from the Sun interferes with safe communication with the spacecraft. Before the interference from the Sun gets too large, the spacecraft must be put into a safe mode, and then operators have to wait until it is back out of the interference zone to wake it back up again. For STEREO-A, this period is currently scheduled for 11 February–8 July 2015, while STEREO-B is expected to be in the interference zone from 14 January-19 March 2015, and again between 25 August-19 October 2015, a result due to Earth's non-circular orbit (see the latest planning). If the interference zone turns out to be not as bad as currently estimated, then this second non-contact period may be greatly reduced in duration, or even eliminated.
Just last week, William Thompson from NASA's Goddard Space Flight Center, wrote about another upcoming disruption that will affect the reception of space weather data from STEREO. As he wrote:
"... Now that the STEREO spacecraft are on the far side of the Sun, pointing the high gain antennae (HGA) at Earth means that they're also pointed pretty close to the Sun. As a result, the temperatures have been rising. To protect the spacecraft, we're soon going to have to off-point the HGAs to keep their temperatures down. This is expected to happen as early as mid-August, and will continue for over a year, until the spacecraft have passed behind the Sun and gone far enough around for this not to be a problem anymore, around early 2016."
Fortunately, testing with the three 70 meter radio telescopes from the Deep Space Network (DSN) has indicated that some communication with STEREO may remain possible. These DSN-telescopes are located at Madrid (Spain), Canberra (Australia), and Goldstone (USA). William Thompson is hopeful:
"... Currently, we expect that we will be able to bring down some real-time space weather beacon data for a short period each day, possibly an hour. Which instruments will be participating, however, will depend on whether sufficient instrument housekeeping data can be brought down to ensure instrument safety. This is a rapidly developing situation, and the final result isn't clear yet. ..."
So it looks that for the next year and a half, space weather forecasters will have to do with a very reduced availability of data from STEREO as these spacecraft will pass the other side of the Sun. Space weather forecasting seems to become quite a bit more cumbersome till 2016.
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