Gegenschein over ALMA

To the right of this image captured in northern Chile, the eye-catching arc of the Milky Way soars above the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner. But despite the breathtaking view, the glow of our home galaxy is here rivalled by another type of interesting light. 

The image also shows, on the arc to the left, a faint brightening of the night sky in the region of the ecliptic directly opposite the Sun. This phenomenon is known as gegenschein (German for “countershine”) and is created as interplanetary dust in the outer part of the Solar System — from comets and broken-up asteroids — scatters incoming sunlight. 

In most parts of the world, phenomena such as gegenschein, or the stunning arc of the Milky Way, are masked by light pollution from artificial light sources. Some estimates indicate that over 80% of the world’s population lives under light-polluted skies, making sites like the Atacama Desert, where ALMA is located, all the more unique.