[2] Two nearly identical spacecraft (STEREO-A, STEREO-B) were launched in 2006 into orbits around the Sun that cause them to respectively pull farther ahead of and fall gradually behind the Earth.

Contact with STEREO-B was lost in 2014 after it entered an uncontrolled spin preventing its solar panels from generating enough power.

[3] Even as the angle increases, the addition of an Earth-based view, e.g., from the Solar Dynamics Observatory, still provided full-Sun observations for several years.

[4] On October 1, 2014, contact was lost with STEREO-B during a planned reset to test the craft's automation, in anticipation of the aforementioned solar "conjunction" period.

The team originally thought that the spacecraft had begun to spin, decreasing the amount of power that could be generated by the solar panels.

Later analysis of the received telemetry concluded that the spacecraft was in an uncontrolled spin of about 3° per second; this was too rapid to be immediately corrected using its reaction wheels, which would become oversaturated.

[4] After a silence of 22 months, contact was regained at 22:27 UTC on August 21, 2016, when the Deep Space Network established a lock on STEREO-B for 2.4 hours.

Engineers determined that during an attempt to despin the spacecraft, a frozen thruster fuel valve probably led to the spin increasing rather than decreasing.

Four years after the initial loss of contact, NASA terminated periodic recovery operations effective October 17, 2018.

[11] STEREO program scientist Madhulika Guhathakurta expected "great advances" in theoretical solar physics and space weather forecasting with the advent of constant 360° views of the Sun.

[12] STEREO's observations are incorporated into forecasts of solar activity for airlines, power companies, satellite operators, and others.

[20][21] The STEREO spacecraft are 3-axis-stabilized, and each has a primary and backup miniature inertial measurement unit (MIMU) provided by Honeywell.

[24] STEREO's onboard computer systems are based on the Integrated Electronics Module (IEM), a device that combines core avionics in a single box.

Its main processor collects and stores on the recorder images and other data from STEREO's instruments, which can then be sent back to Earth.