SOHO, the two-year mission that forgot to retire, finally faces sunset

ESA and NASA's Solar and Heliospheric Observatory (SOHO) is approaching what could be its final year of operations as it hits the 29th anniversary of its launch.

The mission took off from Cape Canaveral's LC36-B on December 2, 1995, and was designed for a nominal lifetime of two years. The original objective was to answer questions regarding the Sun's interior, corona, and the solar wind.

In later years, SOHO has also become something of a comet hunter, thanks to its location at the L1 Lagrange point, about 1.5 million kilometers from the Earth in the direction of the Sun, and its instruments, such as the Large Angle and Spectrometric Coronagraph Experiment (LASCO).

However, while engineers reckoned in 2020 that the spacecraft would be able to endure for another few years, it is unlikely to remain operational past the 30th anniversary of its launch. A chunk of SOHO's funding comes from the US National Oceanic and Atmospheric Administration (NOAA), which uses data from the spacecraft to monitor solar activity.

While keeping an eye on space weather was not one of the primary goals at launch, it has kept the lights on as the years passed. After all, SOHO's mission started with a duration of two years. Enough consumables were loaded to manage another four years. Thanks to creativity and impressive engineering, the mission has continued for just over 29 years.

• 30 years and still sunbathing: SOHO probe continues work as a space weatherman
• ESA's Solar Orbiter sneaks in bonus science by choking on the dust of a comet tail (again)
• Once considered lost, ESA and NASA's SOHO came back from the brink of death to work even better than it did before
• SOHO solar scope spies 3000th 'sungrazer' comet

However, all good things must come to an end. NOAA's Space Weather Follow On Lagrange 1 (SWFO-L1) mission is due for launch in 2025, rendering SOHO out of a job unless more funding can be found. As it stands, the mission has been extended to the end of 2025 – just in time for its 30th anniversary. ESA is working on the basis that from January 1, 2026, the mission will transition to a "two-year post-operations phase."

SOHO has survived more than a few upsets during its lengthy stint observing the Sun. Despite surviving a near-death experience in 1998 – following a mistake that left the spacecraft out of contact and in a slow spin – as well as the loss of its gyros, and issues with its High Gain Antenna (HGA), the spacecraft remains operational.

The spacecraft even dodged a bullet or two before launch, with a decision to switch out a tape recorder with a newfangled solid-state device that would later prove its worth when SOHO began to experience problems with its HGA, and a replaced component in the Atlas II-AS that could have resulted in catastrophe if flown as it was.

At this point, it seems that it will be the end of funding that finally kills the prolonged expedition once and for all.

So join us in raising a toast to what could be the final launch anniversary for an operational SOHO and celebrate the engineers and scientists who have kept the spacecraft running for nearly three decades. ®