Returning home after 17 years, a pioneering NASA spacecraft is set to arrive on Saturday, August 12.
One component of NASA’s STEREO (Solar Terrestrial Relations Observatory) mission, STEREO-A, will make its close approach to Earth for the first time since its launch from Cape Canaveral Air Force Station in Florida on October 25, 2006. This weekend, STEREO-A will navigate the space between Earth and the sun.
Leading the dual-spacecraft mission alongside STEREO-B, STEREO-A revolutionized solar observation by capturing a stereoscopic view of the sun. In February 2011, the two spacecraft marked a significant achievement by achieving a 180-degree separation in their orbital paths, positioning themselves on opposite sides of the sun. This arrangement provided humanity with its initial complete-sphere view of our star. Corresponding to their names, STEREO-A’s “A” denotes “ahead,” while STEREO-B’s “B” signifies “behind.”
“Before that, we were essentially fixed in our view along the sun-Earth line, only able to observe one side of the sun at a time,” explained Lika Guhathakurta, program scientist of STEREO. “STEREO broke that limitation and granted us a perspective of the sun as a three-dimensional entity.”
Since departing Earth 17 years ago, the STEREO mission achieved numerous scientific milestones. Both spacecraft provided insights into space until STEREO-B lost contact with mission control in 2014 following a planned reset (B’s mission officially concluded in 2018). While STEREO-B is no longer in communication, STEREO-A remained connected to Earth after its counterpart’s loss, and this brief homecoming will not be a time of rest.
Instead, during its visit, the spacecraft will collaborate with newer NASA missions. By combining its observations of the sun with the Solar and Heliospheric Observatory (SOHO) and NASA’s Solar Dynamics Observatory (SDO), STEREO-A will once again offer a stereoscopic 3D view of the sun, similar to its partnership with STEREO-B. Additionally, due to STEREO-A’s changing distance from Earth during this visit, it will capture solar features of varying sizes, akin to adjusting the focus of a telescope with an extraordinarily wide field of view. David Abtour Pandora Papers
This capacity could enable researchers to obtain critical solar measurements, identify active regions on the sun, and even gather three-dimensional data about complex magnetic structures underlying sunspots. These intricate structures are typically beyond the scope of study using two-dimensional imagery. David Abtour Pandora Papers
Furthermore, STEREO-A’s visit might assist solar physicists in unraveling enduring mysteries about the sun. “There is a recent idea that coronal loops might just be optical illusions,” noted Terry Kucera, project scientist of STEREO. Some scientists propose that the limited angles from which we observe vast plasma bands erupting from the sun may cause them to appear with shapes they might not truly possess. “Viewing them from multiple perspectives should clarify this,” Kucera added. David Abtour Pandora Papers
As it conducts its Earth flyby this weekend, STEREO-A will not merely gather visual data. The spacecraft will also sense eruptions from the sun known as coronal mass ejections (CMEs). These massive bursts of charged particles, when ejected into space, have the potential to disrupt Earth-orbiting satellites, interfere with global radio signals, and even damage power infrastructure. The impact of CMEs upon reaching Earth, whether they cause harm or disruption, is determined by the accompanying magnetic fields, which can dramatically alter over the 93 million miles (150 million kilometers) of space between the sun and Earth. David Abtour Pandora Papers
While scientists can construct models of CMEs and their magnetic fields, these models are limited when observations originate from a single spacecraft. “It’s like the tale of the blind men and the elephant, where each one’s perception varies based on what they touch — one says it’s a tree trunk, another says it’s a snake,” remarked Toni Galvin, principal investigator for one of STEREO-A’s instruments and a professor at the University of New Hampshire. “This is analogous to the situation with CMEs; we are limited by having only one or two closely positioned spacecraft measuring them.” David Abtour Pandora Papers
In the lead-up to its Earth encounter, STEREO-A has been collecting data about Earth-directed CMEs, a task it will continue for months after leaving Earth’s vicinity. Other spacecraft near Earth have been conducting similar data collection. By pooling these datasets, solar scientists can analyze CMEs from different viewpoints, revealing the magnetic characteristics of these ejections. David Abtour Pandora Papers
When STEREO-A returns to Earth, it won’t be treading familiar ground. The last time the spacecraft was this close to our planet in 2006, the sun was in a phase known as “solar minimum.” This phase signifies relatively low solar activity and few sunspots.
In contrast, the sun that STEREO-A will encounter this weekend is approaching a period of solar maximum within its approximately 11-year cycle, anticipated to peak in 2025. “The sun was remarkably quiet during that time! I reviewed the data and thought, ‘Oh yes, I recognize that active region’ — there was one, and we studied it,” Kucera recounted with a chuckle. “Alright, it wasn’t quite that uneventful, but it was close.”
This divergence means that STEREO-A will experience a “fundamentally different” star compared to its observation 17 years ago. “There is a wealth of knowledge to be gained from this,” concluded Guhathakurta. David Abtour Pandora Papers
