Nearly 20 years after its engineers began construction, Nasa began testing this week on the largest space telescope ever built, a 21ft arrangement of mirrors and instruments that is meant to succeed the Hubble telescope and look through 13bn years of space.

Nasa administrator Charles Bolden announced that the agency had finished construction earlier this week on the James Webb Space Telescope, an $8.8bn giant whose 18 gold-coated, hexagonal plates that span the length of a tennis court and which, from bottom to top, is as tall as a three-storey building.

“We’ve done two decades of innovation and hard work, and this is the result,” project scientist John Mather said at a press conference on Wednesday. “We’re opening up a whole new territory of astronomy.”

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The telescope is designed to be far more powerful than Nasa’s famous but aging Hubble telescope, which has for 26 years revealed auroras and supernovas, mapped the sky for dark matter, and discovered billions of stars, galaxies and planets, including those in habitable zones.

The James Webb Space Telescope (JWST) has a giant sunshield that divides the craft in two: one side facing the sun, which will power the device, and another to keep the instruments at an operating temperature of -220C, cold enough to minimize the effect of the telescope’s own heat on its observations. The telescope has a primary mirror more than five times larger than Hubble’s, meaning the JWST will be able to spot fainter, more distant objects.

The JWST’s greater power will also mean that it can see planets in greater detail, letting scientists look for atmospheres, seasons and even weather or signs of life. Mather said the telescope will have “seven times the collecting area” of Hubble, and enough power to detect the body heat of a bumblebee on the moon.

“We’d like to know if another planet out there has enough water to have an ocean, and we think we can do that,” Mather said.

The new telescope will have infrared instruments, to reflect and collect infrared wavelengths, meaning it can look farther back in time – at the stars and galaxies hurtling away into the expanding reaches of the universe, from where it takes light billions of years to reach the Earth. Those stars’ light has stretched into wavelengths so red (“redshifted”) it has entered the infrared spectrum, past the range of Hubble’s view.

The telescope will now be subjected to two years of tests before its planned launch in 2018. Unlike Hubble, it will be sent far from where astronauts could repair it, and Nasa said it will first test the JWST for the “violent sound and vibration environments” of a rocket launch, including tests of forces 10 times stronger than gravity and blasts of sound to mimic rocket explosions. Then they will test for the conditions the JWST will face during two weeks of delicate unfurling in space and years in orbit.

In February, the scientists will send it to a chamber at the Johnson Space Center that can be lowered to -220C (about -370F), and Bolden said it is designed to survive collisions with space debris and small holes in its mirrors, and that the telescope will need six months in space before it’s ready for work.

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He added that scientists need to “get it right here on the ground” to avoid finding, “as we did with Hubble, that we don’t have the ability to do what we thought it was going to be able to do”.

The JWST was originally meant to launch in 2014, but the project was nearly canceled by Congress in 2011 due to delays and budget problems.

“Our lessons learned from the Hubble were, if you really care about something, you’ve got to measure it at least twice,” Mather added, alluding to repairs astronauts had to make after launch.

Bolden said the telescope will help scientists see stars and planets from the earliest epoch of the universe. “Nasa has always sought to unravel the mysteries of our universe; to find out where we come from, where we are going, and whether we are alone in the universe,” he said in a later statement. “We are building the James Webb Space Telescope to answer these age-old questions.”