James Webb Space Telescope
The James Webb Space Telescope reached the halfway mark of its construction in December 2015. Pictured are the nine assembled mirrors in a clean room at NASA's Goddard Space Flight Center. NASA's Goddard Space Flight Center/Chris Gunn

Nine down, nine more mirrors to go for the James Webb Space Telescope. NASA announced Monday it reached the halfway mark for completion of its $8.8 billion successor to the Hubble Space Telescope. The Webb Telescope is on schedule to launch in October 2018.

Engineers inside the clean room at NASA's Goddard Space Flight Center, in Greenbelt, Maryland, have been carefully assembling the mirror segments that will make up the Webb Telescope's primary mirror since Nov. 25. Each mirror weighs 88 pounds and measures 4.2 feet across. Each of the hexagonal segments will be pieced together to create a 21.3-foot mirror. For a comparison, Hubble's primary mirror is eight feet in diameter.

"The James Webb Space Telescope will be the premier astronomical observatory of the next decade,” John Grunsfeld, astronaut and associate administrator of the Science Mission Directorate at NASA, said in a statement after the installation of the Webb Telescope's first mirror. NASA expects to complete the installation of all 18 mirrors by early 2016.

Dubbed the "most powerful space telescope ever built," JWST will operate in visible, near and mid-infrared light using the Integrated Science Instrument Module -- which houses four scientific instruments -- and the Optical Telescope Element -- the primary mirror.

The Webb Telescope can explore the early universe and observe galaxies and stars formed just after the universe began 13.7 billion years ago. Using the scientific instruments aboard Webb will provide new insights into the structure and evolution of these distant galaxies and stars.

Stars and planets are formed in large clouds of dust that light cannot penetrate, which is why infrared observations are so important for the Webb Telescope. The space telescope will also spend plenty of time observing the solar system.

After a rocky start -- and a potential casualty of budget cuts in 2011 -- the Webb Telescope is on pace to stay within the $8.8 billion budget and launch in 2018. The European Space Agency -- a Webb partner -- signed an agreement to launch the telescope aboard an Ariane 5 rocket from Kourou, French Guiana in October 2018.

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