17 years into its mission, NASA’s STEREO spacecraft visits home

NASA’s STEREO-A spacecraft made its first flyby of Earth, 17 years after it was launched with its twin STEREO-B to make observations of the Sun.

The flyby came just one day before our nearest planetary neighbour Venus passed between the Sun and Earth.

STEREO (Solar TErrestrial RElations Observatory) was designed to give stereoscopic measurements of the Sun and the nature of its coronal mass ejections (CMEs) with one of the two spacecraft orbiting the Sun ahead of the Earth’s orbit (STEREO-A) and one trailing behind the Earth (STEREO-B). This stereo vision allowed the mission to extract 3D information about the Sun, like two human eyes giving depth perception.

The instruments aboard the space observatories include ultraviolet and white-light imagers.

The mission gave humanity the first ever image of the Sun as a complete sphere in 2011 when the orbits of the twin spacecraft were separated by 180°.

“Prior to that we were ‘tethered’ to the Sun-Earth line – we only saw one side of the Sun at a time,” says Lika Guhathakurta, STEREO program scientist. “STEREO broke that tether and gave us a view of the Sun as a three-dimensional object.”

The mission has given scientists valuable information over the years. However, in 2014 mission control lost contact with STEREO-B, leaving its twin alone to gather solar views unavailable from Earth.

STEREO-A will continue to give three-dimensional data by syncing up its views with those of NASA and the European Space Agency’s Solar and Heliospheric Observatory (SOHO) and NASA’s Solar Dynamics Observatory (SDO).

Astronomers and space weather experts are hoping that such 3D information about solar active regions, the magnetically complex regions underlying sunspots, to uncover details about their structure lost in 2D images.

Scientists will use the data to test a new theory that coronal loops – giant arches rising thousands of kilometres above the Sun’s surface – aren’t what they appear to be.

“There is a recent idea that coronal loops might just be optical illusions,” comments Terry Kucera, STEREO project scientist. Some scientists suggest these structures may not have the shapes we might think. “If you look at them from multiple points of view, that should become more apparent.”

STEREO will also encounter plumes of solar material known as coronal mass ejections, or CMEs. These events can disrupt satellites, radio signals and power grids on and around Earth.

Any CMEs headed to Earth during STEREO-A’s flyby will pass over the spacecraft giving scientists valuable data from inside the plume. The last time STEREO-A was close to Earth, in 2006 after it was launched, was during “solar minimum” – the lull in the Sun’s 11-year cycle.

As we approach solar maximum predicted for 2025, the Sun’s activity is increasing.

“In this phase of the solar cycle, STEREO-A is going to experience a fundamentally different Sun. There is so much knowledge to be gained from that,” Guhathakurta says.