This Satellite Has Been Staring at the Sun for 20 Years
And it's still warning us about Earth-bound coronal mass ejections.
Animation of a solar tsunami, captured by SOHO's EIT telescope. Image: ESA/NASA/SOHO
This space observatory missed the memo that you're not supposed to look directly at the Sun.
The Solar and Heliospheric Observatory (SOHO), a collaboration between NASA and the European Space Agency (ESA), has been doing just that since 1995, marking its 20-year operating anniversary on December 2.
ESA states that this makes it the "longest-lived Sun-watching satellite to date," having observed almost a full two solar cycles, which last 11 years each.
Initially, the main point of SOHO was to find out more about the star at the centre of our Solar System. NASA credits it with producing over 5,000 scientific papers in the field of heliophysics, and with informing us about coronal mass ejections (CMEs), gassy outbursts from the Sun that can drive solar storms that affect the Earth.
Stationed 1.5 million kilometres away from Earth around the first Lagrangian point, SOHO does have some protection against the Sun: it observes CMEs using a coronagraph, an instrument that blocks out the Sun's surface itself so that the fainter corona—the aura around the star's edges—isn't overpowered by its brightness.
But another of SOHO's instruments takes a full picture: the Extreme Ultraviolet Imaging Telescope (EIT). This telescope results in amazing images like the one above, and NASA explains it helps track the direction of CMEs as scientists can observe the ripple of waves—"solar tsunamis"—across the Sun's surface.
We still use SOHO to monitor space weather with three days' notice of Earth-bound disruptions, but since completing its initial two-year planned mission it's also found other callings. As of September 2015, it discovered its 3,000th Sun-grazing comet, continuing its unexpected talent for comet hunting.
SOHO is still going strong, despite having an initial mission length of just two years and a bit of a software wobble in 1998, and has helped shape subsequent solar-focused missions. It's comforting to know we've got an old friend up ready to give us that crucial advance warning of a CME headed our way.