A lost Nasa spacecraft “phoned home” after 22 months of silence this week. Stereo-B (Solar Terrestrial Relations Observatory) is a twin to Stereo-A, and together they have been studying the Sun since 2006.

Although planned to last just two years, the mission had been working so well that ground controllers and scientists kept the spacecraft going. The data returned has given unprecedented insights into solar storms, which can threaten the electrical systems of satellites around Earth.

The twin spacecraft gave different views of the Sun, one being sent ahead of Earth while the other lagged behind. As the years have gone by, so the spacecraft have drifted further apart. It was during preparations for them to pass behind the Sun and out of communications with Earth that the malfunction took place.

To test the computer reboot system that kicks in when the spacecraft are out of range, mission controllers purposely triggered the response on 14 October 2014. Although Stereo-A came back on line, Stereo-B sent back fragmentary messages and then fell silent.

Piecing data together, engineers traced the malfunction to a guidance unit that was erroneously convinced that the spacecraft was tumbling. This made thrusters fire, which indeed set the spacecraft rolling.

Having diagnosed the problem, engineers have been sending commands “blind” to tell the on-board computer to disregard the signals from the faulty guidance unit, to conserve battery power and finally to try transmitting back to Earth.

On Monday evening, Nasa’s Deep Space Network locked on to the signal from Stereo-B. Now the spacecraft is back, engineers are performing a complete check-out of its systems and instruments.