NASA’s STEREO-A (Solar TErrestrial Relations Observatory Ahead) space probe has resumed contact with the Earth a few days ago after more than three months and on July 15 sent new photographs of the other side of the Sun from the Earth. Its EUVI (Extreme UltraViolet Imager) instrument was used to take photographs at a wavelength of 171 angstroms, invisible the human eye, then colorized in blue to allow us to appreciate them.
The STEREO mission is a NASA Sun-observing project. Two nearly identical spacecraft were launched in 2006 into a heliocentric orbit. This means that they orbit the Sun just as the Earth but the orbit chosen for the two probes is slightly smaller and this means that they travel at a slightly higher speed.
The orbit of the two STEREO probes is out of phase to allow them to keep the Sun under observation as completely as possible. Solar activity is also being studied in the opposite side fom the Earth, allowing to have among other things the possibility of identifying earlier coronal mass ejections, potentially dangerous for satellites and in extreme cases even for the electrical grids.
Unfortunately, the STEREO-B (Solar TErrestrial Relations Observatory Behind) space probe stopped answering contact attempts on October 1, 2014 during a reset that was intended to test some of the spacecraft systems. NASA hasn’t lost hope of restoring communications but they’re decreasing.
Meanwhile, the STEREO-A spacecraft spent time on the other side of the Sun from the Earth. The spacecraft was physically invisible for just a few days but its position was such that from March 24 to July 8, 2015 the Sun interfered with its signals. For this reason, it was placed in the “safe mode”, a state in which it merely collect data with some instruments.
The STEREO-A space probe resumed regular contact with NASA’s mission control center and is continuing its mission on its own. In recent days, all of its instruments have returned to normal operation and was able to send this image of the Sun at ultraviolets.
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