Contact was reestablished on Sunday with one of NASA's Solar Terrestrial Relations Observatories, known as the STEREO-B spacecraft, after communication was lost on October 1, 2014, the US space agency said in a statement on Monday.
However, the health of the observatory is not yet known.
Over the course of the 22 months, the STEREO team had worked to attempt contact with the spacecraft.
Most recently, they attempted a monthly recovery operation using NASA's Deep Space Network, or DSN, which tracks and communicates with missions throughout space.
"The STEREO Missions Operations team plans further recovery processes to assess observatory health, re-establish attitude control, and evaluate all subsystems and instruments," the statement added.
Communication with STEREO-B was lost during a test of the spacecraft's command loss timer, a hard reset that is triggered after the spacecraft goes without communications from Earth for 72 hours.
The STEREO team was testing this function in preparation for something known as solar conjunction, when STEREO-B's line of sight to Earth -- and therefore all communication -- was blocked by the Sun.
The spacecraft's twin, STEREO-A, is still working normally, Space.com? reported.
Launched in October 2006, STEREO is the third mission in NASA's Solar Terrestrial Probes programme (STP).
It employs two nearly identical space-based observatories -- one ahead of Earth in its orbit, the other trailing behind -- to provide the first-ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections, or CMEs.
The two STEREO spacecraft were originally designed to complete a two-year mission, ending in 2008.
But -- like many NASA spacecraft -- they lasted much longer. The long life of the two STEREO spacecraft has been a boon for scientists studying the Sun and its influence throughout the solar system, the US space agency said in a statement earlier.
The two STEREOs slowly drifted away from Earth as they orbited the Sun, one ahead and one behind our home planet, giving scientists constantly-improving views of the Sun's far side, allowing us for the first time to see the whole Sun at once, it added.
