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Communications with the two solar-observing Stereo spacecraft will be cut off for more than a year, far longer than originally expected, as their orbits move them behind the Sun.
Unexpectedly high temperatures in the high gain antenna feed horns on both STEREO spacecraft will require corrective action in the coming months that will severely limit science operations. The high temperatures are being caused by the small angle between Earth and the Sun as seen from each spacecraft. In other words, pointing the antenna at Earth is putting too much solar heat on the antenna feed horn. To bring down the feed horn temperature, and preserve the spacecraft for years to come, the antennas will be pointed off at an angle from both Earth and the Sun, so that less heat will fall on the feed horns. Communication will still be possible using one of the antenna side lobes, but the telemetry rate will be extremely low. What instrument operations can be supported while the antennas are off-pointed is still being studied.
The spacecrafts’ orbit allows them to observe the back side of the Sun not visible from Earth. Though engineers had expected to lose communications for a period of time when the Sun moved between the spacecraft and the Earth, the blacked out time period has turned out to be much longer than planned. The shutdown will begin later this year with communications not resuming until January 2016.