Kourou - The world's most powerful space telescope is set to blast off on Saturday to its outpost 1.5 million kilometres (930,000 miles) from Earth, after several delays caused by technical hitches.
The James Webb Space Telescope, some three decades and billions of dollars in the making, will leave Earth enclosed in its Ariane 5 rocket from Kourou Space Centre in French Guiana.
The launch, scheduled in a brief window after 9:20am (1220 GMT), will send the telescope on a month-long journey to its remote orbit.
It is expected to beam back new clues that will help scientists understand more about the origins of the Universe and Earth-like planets beyond our solar system.
Named after a former Nasa director, Webb follows in the footsteps of the legendary Hubble - but intends to show humans what the Universe looked like even closer to its birth nearly 14 billion years ago.
Speaking on social media, Webb project co-founder John Mather described the telescope's unprecedented sensitivity.
"#JWST can see the heat signature of a bumblebee at the distance of the Moon," he said.
All that power is needed to detect the weak glow emitted billions of years ago by the very first galaxies to exist and the first stars being formed.
'Exceptional measures'
The telescope is unequalled in size and complexity. Its mirror measures 6.5 metres (21 feet) in diameter - three times the size of the Hubble's mirror - and is made of 18 hexagonal sections.
It is so large that it had to be folded to fit into the rocket.
That manoeuvre was laser-guided with NASA imposing strict isolation measures to limit any contact with the telescope's mirrors from particles or even human breath.
Once the rockets have carried Webb 120 kilometres, the protective nose of the craft, called a "fairing", is shed to lighten the load.
To protect the delicate instrument from changes in pressure at that stage, rocket-builder Arianespace installed a custom decompression system.
"Exceptional measures for an exceptional client," said a European Space Agency official in Kourou on Thursday.
Crew on the ground will know whether the first stage of the flight was successful about 27 minutes after launch.
Once it reaches its station, the challenge will be to fully deploy the mirror and a tennis-court-sized sun shield.
That intimidatingly complex process will take two weeks and must be flawless if Webb is to function correctly.
Its orbit will be much farther than Hubble, which has been 600 kilometres above the Earth since 1990.
The location of Webb's orbit is called the Lagrange 2 point and was chosen in part because it will keep the Earth, the Sun and the Moon all on the same side of its sun shield.
Webb is expected to officially enter service in June.
Here is what you need to know:
* The telescope is scheduled to lift off at 7:20am Eastern time on Saturday from a European-managed spaceport in French Guiana on the coast of South America. The launch window lasts 32 minutes, until 7:52am, in case there are any last-minute rocket checks or brief pauses in the countdown. A longer delay means the launch would be postponed to that same time Sunday.
* Nasa, the telescope's primary backer, will host a livestream on its YouTube channel beginning at 6am and on its main Twitter and Facebook accounts beginning at the same time. Agency officials will provide commentary with astronomers leading up to the launch. You can also register for a virtual launch event. If you would rather watch the launch in French or in Spanish, the European Space Agency is also streaming the liftoff in those languages.
* The Webb telescope was designed to probe a crucial stretch of early cosmic history known to astronomers as the dark ages.
* Cosmologists surmise that the first stars appeared when the universe was only about 100 million years old. (Today it is 13.8 billion years old.) The farthest and earliest galaxy seen by astronomers, using the Hubble Space Telescope, dates to when the universe was older, 400 million years after the Big Bang. What happened during those intervening 300 million years when the universe took luminous flight, how the Big Bang turned into a sky full of constellations and life, is a mystery.
* The telescope will also help astronomers better study supermassive black holes at the center of galaxies, and planets orbiting other stars in our galaxy.
* To achieve these scientific observations, the Webb telescope relies on a primary mirror 6.5 meters in diameter, compared with the mirror on the Hubble, which is 2.4 meters. That gives it about seven times as much light-gathering capability and thus the ability to see further into the past.
*Another crucial difference is that it is equipped with cameras and other instruments sensitive to infrared, or "heat," radiation. The expansion of the universe causes the light that would normally be in visible wavelengths to shift into longer infrared wavelengths normally invisible to human eyes.
* Engineers had to invent 10 new technologies along the way to make the telescope more sensitive than the Hubble. Overoptimistic schedule projections, occasional development accidents and disorganized cost reporting dragged the timeline out to 2021 and ballooned the overall cost to $10 billion.
* The European Space Agency does not have an orbital launch site in any of its member countries on the European landmass. Instead, it typically launches from a spaceport in Kourou, French Guiana. Using the Kourou launch site is part of the consortium's 300 million euro contribution to the Webb mission. NASA signed an agreement with ESA to launch Webb on an Ariane 5 rocket from Arianespace, a French rocket-maker, in 2003.
