James Webb Space Telescope is finally complete! Set to launch in 2018.

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After more than 20 years of construction, the James Webb Space Telescope (JWST) is complete and, following in-depth testing, the largest-ever space telescope is expected to launch within two years, NASA officials announced on November 2 at Goddard Space Flight Center, Maryland.

JWST is considered the successor to NASA’s iconic Hubble Space Telescope. The telescope will be much more powerful than even Hubble for two main reasons. First, it will be the biggest telescope mirror to fly in space. And second, it is designed to collect infrared light, which Hubble is not very sensitive to.

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Earth’s atmosphere glows in the infrared, so such measurements can’t be made from the ground. Hubble emits its own heat, which would obscure infrared readings. JWST will run close to absolute zero in temperature and rest at a point in space called the Lagrange Point 2, which is directly behind the Earth from the sun’s perspective. That way, the Earth can shield the telescope from the sun’s infrared emission, and the sun shield can protect the telescope from both bodies’ heat.

The telescope’s infrared view will pierce through the obscuring cosmic dust to reveal the universe’s first galaxies and spy on newly forming planetary systems. It also will be sensitive enough to analyze the atmospheres of exoplanets that pass in front of their stars, perhaps to search for signs of life, according to NASA officials.

The telescope was originally scheduled to launch in 2014, at a cost of about $5 billion, but a series of setbacks and budget constraints delayed and nearly canceled the project. Now the telescope costs $8.7 billion and is expected for an October 2018 launch on an Ariane 5 rocket. The project is led by NASA but supported by international partners, including the European Space Agency and the Canadian Space Agency.

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The telescope’s testing at Goddard, which has already begun, will ensure that it can withstand the shaking and loud noise of a rocket launch. Then, it will be moved to Texas, where its focus will be tested, and then to California for some final assembly. The testing is particularly high-stakes, because unlike Hubble, which was repaired and refocused in orbit by astronauts, this telescope is not intended to be repaired by humans. Each of JWST’s mirrors is individually tunable so that they can be adjusted without a corrective lens like the one astronauts put on Hubble.

Researchers will make observations with the telescope for at least 5 years, and will carry enough fuel for 10 years — if they’re lucky, JWST will last even longer. Regarding space debris, of course, JWST will be hit over the course of its lifespan but it is designed to function fine with small holes in its mirrors.

The full telescope, with a 21.3-foot (6.5 meters) mirror assembly, is too large to launch fully extended, so the telescope will be carefully furled during launch and will have to unfold over the course of two weeks once it’s in the air. After that, the sun shield will be extended carefully, and the telescope will be given time to cool down. Finally, it will be focused. By six months after launch, the telescope will be ready to begin doing science. NASA reminded the ‘7 minute of terror‘ of Curiosity Mars rover’s automated landing in 2012. “This is six months of terror,” NASA official said.

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