On Aug. 12, 2023, NASA’s STEREO-A spacecraft is expected to pass between the Sun and Earth, which marks the first Earth flyby of the nearly 17-year-old mission.
The visit home presents a special chance for the spacecraft to collaborate with NASA missions near Earth and reveal new insights into our closest star.
STEREO Mission on Space
The twin Solar TErrestrial RElations Observatory (STEREO) spacecraft launched on Oct. 25, 2006, from the Cape Canaveral Air Force Station in Florida.
The two charted Earth-like orbits around the Sun, with STEREO-A (for “Ahead”) advancing the lead on Earth and STEREO-B (for “Behind”) lagging behind.
The first years after launch, the dual-spacecraft mission achieved its landmark goal: providing the first stereoscopic, or multiple-perspective, view of our closest star.
On Feb. 6, 2011, the mission achieved another landmark: STEREO-A and -B reached a 180-degree separation in their orbits, allowing humanity to see the Sun as a complete sphere.
“Prior to that, we were ‘tethered’ to the Sun-Earth line – we only saw one side of the Sun at a time,” said Lika Guhathakurta, STEREO program scientist at NASA Headquarters in Washington, D.C. “STEREO broke that tether and gave us a view of the Sun as a three-dimensional object.”
Over the years, the project performed numerous more scientific breakthroughs, and scientists examined the views from both spacecraft up until 2014, when, following a deliberate reset, mission control lost contact with STEREO-B.
However, STEREO-A keeps traveling while taking views of the sun that are not visible from Earth.
As STEREO-A “laps” us in our orbit around the Sun on August 12, 2023, the distance between the spacecraft and Earth will have increased to one full revolution.
Scientists are utilizing the chance to pose questions that would otherwise be outside the scope of the mission in the few weeks leading up to and following STEREO-A’s flyby.
Viewing the Sun in 3D
During the Earth flyby, STEREO-A will recap its early years: combine views to achieve stereoscopic vision allowing us to extract 3D information from two-dimensional or flat images.
This phenomenon is applied by two eyeballs, looking out at the world from offset locations, creating depth perception.
Your brain compares the images from each eye, and the slight differences between those images reveal which objects are closer or farther away.
STEREO-A will enable such 3D viewing by synthesizing its views with NASA’s and the European Space Agency’s Solar and Heliospheric Observatory (SOHO) and NASA’s Solar Dynamics Observatory (SDO).
STEREO-A’s distance from Earth changes during the flyby, optimizing stereo vision for different solar features, resembling adjusting the focus on a telescope with a million-mile range.
STEREO scientists aim to identify active regions, magnetically complex regions underlying sunspots, to uncover 3D information about their structure, often lost in 2D images.
They’ll also test a new theory that coronal loops – giant arches often seen in close-up images of the Sun – aren’t what they appear to be.[1]
“There is a recent idea that coronal loops might just be optical illusions,” said Terry Kucera, STEREO project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Some scientists have suggested that our limited viewing angles make them appear to have shapes they may not truly have. “If you look at them from multiple points of view, that should become more apparent,” Kucera added.
Inside a Solar Eruption
STEREO-A’s sight and sensation could lead to significant discoveries as it explores Earth’s surface.
When a coronal mass ejection, or CME, from the sun, hits Earth, it can interfere with radio and satellite transmissions and potentially trigger spikes in our power networks.
Or it could not have much of an impact. The magnetic field within it determines everything, and it can shift significantly throughout the 93 million miles between the Sun and Earth.
Scientists build computer models of these solar eruptions to understand how a CME’s magnetic field evolves on the way to Earth, updating them with each new spacecraft observation. But a single spacecraft’s data can only tell us so much.
Any Earth-directed CMEs will pass over STEREO-A and other near-Earth spacecraft in the months leading up to and following STEREO-A’s Earth encounter, providing scientists with crucial multipoint measurements inside a CME.
Experiencing a Different Sun
STEREO-A was also close to Earth in 2006, shortly after launch. That was during the “solar minimum,” the low point in the Sun’s roughly 11-year high and low activity cycle.
Now, as we approach the solar maximum predicted for 2025, the Sun isn’t quite so sleepy.
“In this phase of the solar cycle, STEREO-A is going to experience a fundamentally different Sun,” Guhathakurta said. “There is so much knowledge to be gained from that.”
References
- Miles Hatfield, ‘After Seventeen Years, A Spacecraft Makes Its First Visit Home’, NASA, 10 August 2023, https://www.nasa.gov/feature/goddard/2023/sun/after-seventeen-years-a-spacecraft-makes-its-first-visit-home[↩]
