Solar and Heliospheric Observatory (SOHO)  is a project of international collaboration between NASA and ESA to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind, the stream of highly ionized gas that blows continuously outward through the Solar System.

The spacecraft was launched on an Atlas II-AS rocket from the Cape Canaveral Air Station in Florida on December 2, 1995.

It moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep the observatory in an orbit locked to the Earth-Sun line. The L1 point is approximately 1.5 million km away from Earth, in the direction of the Sun.

Clues on the solar interior come from studying seismic waves that are produced in the turbulent outer shell of the Sun and which appear as ripples on its surface.

“Waves are traveling and bouncing around inside the Sun, and if your eyes were sensitive enough they could actually see this,” said Dr. Alex Young, associate director for science in the Heliophysics Science Division at NASA’s Goddard Space Flight Center.

“We don’t have straightforward ways to look inside the Sun. We don’t have a microscope to zoom inside the Sun.”

“So using a star or the Sun’s vibrations allows us to see inside of it.”

These vibrations allow solar physicists to study a range of complex motions inside the Sun, from solar flares to coronal mass ejections.

“We can see huge rivers of solar material flowing around,” Dr. Young said.

“We are finally starting to understand the layers of the Sun and the complexity.”

“That simple sound is giving us a probe inside of a star. I think that’s a pretty cool thing.”

Besides watching the Sun, SOHO has become the most prolific discoverer of comets in astronomical history: as of September 2017,more that 3,300 comets had been found by the observatory .