From lost in space to sunshine: SoHO satellite’s incredible 25-year mission

The SoHO satellite has been working in space for far longer than its NASA and ESA engineers imagined. But its mission would have been over before it had really begun were it not for the Saft battery that powers it.

It’s almost been lost twice, and its original two-year mission has now lasted for more than a quarter of a century, but the Solar and Heliospheric Observatory (SoHO) satellite is still orbiting the Sun and sending back vital data for scientists. One of the key reasons for its survival is the Saft battery inside it.

Launched in December 1995 as a joint mission between the European Space Agency (ESA) and NASA, SoHO was designed to orbit the Sun to learn about the structure of the star’s interior and its outer atmosphere, as well as to investigate the behavior of solar winds. It was expected to complete its mission in 1998. Instead, it is still active today, and is now likely to keep working until 2025, when it will have been in space for 30 years.

Surviving in space on Saft's battery power

This outcome would have been unimaginable two-and-a-half years after launch, when the satellite was almost lost forever because of a mistake during a routine maneuver. SoHO is designed to orbit around Lagrange point L1, the spot where the gravitational fields of the Earth and the Sun cancel each other out, which keeps the satellite in line with the two bodies.

However, while calibrating the gyroscopes that hold the satellite in position, the ground crew lost the ability to control it. Worse, the satellite’s solar panels were also out of position and couldn’t provide power. The ground crew worked for months first to determine the position of the satellite and then regain contact. They eventually succeeded three months later, after many scientists had given it up for lost. During that time, only SoHO’s battery was keeping it alive.

That’s precisely why satellites carry a battery. Those orbiting the Earth are out of view of the Sun for a short time during each orbit, so they regularly use battery power. SoHO, however, was designed to keep its solar panels facing the Sun, so the battery was only necessary for emergencies. Fortunately, it did its job.

A space battery is no normal battery. It must be small and light because every extra pound adds to the cost when it comes to firing a rocket into space. It must be able to withstand the vibrations of launch and a wide range of temperatures once it’s in space. The skin of a satellite can reach 250°C on the side that faces the Sun and -150°C on the side facing away. The battery is shielded from these extremes, but still must be able to function in temperatures ranging from -10°C to 40°C. It must be able to do all of this, and if the battery fails the mission is over. There’s no margin for error.

Space batteries: the next generation

The battery powering SoHO is a nickel-based battery – a descendent of the batteries that Saft has been building for space use since the 1960s. The first 10 GPS satellites, which help connect satellite navigation systems across the planet, carried these Saft batteries, for example.

The company began developing SoHO’s battery in 1990 and by the time the satellite went into space, a new generation of batteries was already beginning to replace the nickel-based technology. Lithium-ion batteries have been used since the beginning of 2000 and are now the standard for Saft’s space projects. They offer the same amount of power with a fivefold decrease in weight, which is an invaluable benefit.

Batteries typically outlast the satellites on which they are installed. The satellite is usually retired because it runs out of fuel to adjust its position or because the technology onboard needs replacing. However, SoHO has been fit for purpose for much longer than usual and its lifespan demonstrates the durability of battery technology.

SoHO’s status will be reviewed in 2022, but it’s likely that it will continue to send back its observations until 2025. Information provided by SoHO has led to discoveries about the workings of the Sun, including complex gas currents below the visible surface. It has also become a crucial source of data for the study of space weather – disturbances in solar wind that can trigger geomagnetic storms on Earth.

In all, data from SoHO has been used in more than 6,000 scientific papers. It has also spotted more than 4,000 comets – around half of all known comets – with a new one discovered roughly every three days. When it is eventually retired, SoHO’s contribution to human knowledge will have been vast. But it all could have come to an end after just 18 months if it weren’t for one small battery.