Spacecraft prepares for rare Earth flyby 17 years after it left planet

Nasa's Solar Terrestrial Relations Observatory (STEREO-A) spacecraft is set to make its first Earth flyby on August 12, 2023, nearly 17 years after its launch.

This event will provide a unique opportunity for the spacecraft to collaborate with other missions near Earth and offer new insights into our Sun.

Launched from Cape Canaveral on October 25, 2006, the twin STEREO spacecraft, STEREO-A, and STEREO-B, embarked on their mission to chart Earth-like orbits around the Sun.

Their primary objective was to provide the first stereoscopic, or multiple-perspective, view of our closest star.

On February 6, 2011, the mission achieved a significant milestone when the two spacecraft reached a 180-degree separation in their orbits, allowing humanity to see the Sun as a complete sphere for the first time.

During the upcoming Earth flyby, STEREO-A will once again combine views to achieve stereoscopic vision, a technique that allows the extraction of 3D information from two-dimensional images. This process is similar to how our two eyes, looking out at the world from different locations, create depth perception.

STEREO-A will synthesize its views with those from Nasa's and the European Space Agency’s Solar and Heliospheric Observatory (SOHO) and Nasa’s Solar Dynamics Observatory (SDO).

As STEREO-A's distance from Earth changes throughout the flyby, it will optimize its stereo vision for different-sized solar features at different times, akin to adjusting the focus on a several million-mile-wide telescope.

Scientists are also planning to use this opportunity to identify active regions underlying sunspots and uncover 3D information about their structure. They will also test a new theory suggesting that coronal loops, giant arches often seen in close-up images of the Sun, might be optical illusions.

Moreover, STEREO-A's flyby will allow scientists to understand how a coronal mass ejection's (CME) magnetic field evolves on its way to Earth.

CMEs, plumes of solar material, can disrupt satellite and radio signals or even cause surges in our power grids. By obtaining multipoint measurements from inside a CME, scientists hope to improve their computer models of these solar eruptions.

The upcoming flyby marks a stark contrast to STEREO-A's early days in 2006, during the solar minimum when the Sun was relatively quiet. Now, as the Sun approaches the solar maximum predicted for 2025, it is far more active, offering a fundamentally different perspective for STEREO-A's observations.