NASA's James Webb Space Telescope is one step closer to completion after the primary mirrors passed a crucial test, the space agency announced Dec. 21. Webb is the successor to the Hubble Telescope, and once in place, it will be able to peer into the very earliest galaxies at the edge of the known universe.

"With the completion of all mirror cryogenic testing, the toughest challenge since the beginning of the program is now completely behind us.," Lee Feinberg, NASA Optical Telescope Element manager for the Webb telescope at the agency's Goddard Space Flight Center in Greenbelt, Md said in a statement.

Cryogenic testing for the final six primary mirror segments and a secondary mirror is now complete. Building these mirrors took years and broke new ground in manufacturing and testing large mirrors, NASA said in the release. The Webb telescope has 21 mirrors, with 18 mirror segments working together as a large 21.3-foot (6.5-meter) primary mirror. Each individual mirror segment now has been successfully tested to operate at 40 Kelvin (-387 Fahrenheit or -233 Celsius), the temperature they will operate at in space.

"Mirrors need to be cold so their own heat does not drown out the very faint infrared images," Feinberg said.

A ten-week test series cooled the primary mirror segments to -379 degrees Fahrenheit. During two test cycles, telescope engineers took very detailed measurements of how each mirror's shape changed as it cooled. Testing verified each mirror changed shape with temperature as expected and each one will be the correct shape upon deployment. The Webb telescope is the world's next-generation space observatory and will be the most powerful space telescope ever built. Furthermore, it will provide images of the first galaxies ever formed, and explore planets around distant stars. It's a joint project of NASA, the European Space Agency and the Canadian Space Agency.