Primary Mirror Of NASA's James Webb Space Telescope Finally Completed

A robotic arm was used to install each of the 18 segments. NASA/Chris Gunn

It is several years behind schedule and billions of dollars over budget, but there is finally light at the end of the tunnel for NASA’s James Webb Space Telescope (JWST). The primary mirror of the giant space telescope is now complete – a significant step towards launching in 2018.

On Wednesday, February 3, the 18th and final hexagonal segment of the primary mirror was installed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Each of the segments weighs a hefty 40 kilograms (88 pounds) and measures 1.3 meters (4.2 feet) across. Together, the whole primary mirror measures 6.5 meters (21.3 feet) in diameter, about three times larger than Hubble's, making this the most powerful space telescope ever built.

"Completing the assembly of the primary mirror is a very significant milestone and the culmination of over a decade of design, manufacturing, testing and now assembly of the primary mirror system," said Lee Feinberg, optical telescope element manager at Goddard, in a statement. "There is a huge team across the country who contributed to this achievement."

The mirrors took months to install, as they are incredibly sensitive. Coated in gold, they will focus the light from the distant universe onto a secondary mirror and then the on-board instruments. With the mirror complete, assembly of the rest of the telescope can begin. The mirror itself will sit on top of a vast sun shade, the size of a tennis court, while a complex instrumentation package will probe its views of the deep universe.

The telescope will be positioned beyond the orbit of the Moon. NASA

JWST is often billed as the successor to the Hubble Space Telescope, although the two are quite different. The latter is located in Earth orbit and observes the universe in visible wavelengths; JWST will be located 1.5 million kilometers (1 million miles) from Earth, beyond the orbit of the Moon, and will observe in the infrared. This will allow it to peer further back in the universe than Hubble can.

Among the science possible with the mission, JWST will study planets in solar systems beyond our own, and may even be able to characterize the atmospheres of some planets within 100 light-years. It will also attempt to look for the galaxies and stars that first arose after the Big Bang.

At the moment, the $9 billion (£6.2 billion) JWST is scheduled to launch in 2018 from French Guiana on an Ariane 5 rocket. Given that it was originally planned to cost $1.6 billion (£1.1 billion) and launch in 2011, organizers will be hoping for no more setbacks in this ambitious mission. 

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