The successor to the Hubble Space Telescope has arrived in French Guiana to prepare for its 18 December launch.
The $10bn (£7.3bn) James Webb observatory was delivered to South America by ship.
Engineers at Europe's Kourou spaceport will spend the coming weeks undertaking a series of final checks before mating the new telescope to its rocket.
Webb is designed to see deeper into the Universe - and further back in time - than Hubble.
The hope is it will detect the light even from the very first population of stars to switch on more than 13.5 billion years ago.
The James Webb Space Telescope (JWST) is a joint venture between the US, European and Canadian space agencies.
It is an extraordinary machine that will unfurl the biggest astronomical mirror ever sent into orbit - at 6.5m (21ft) in diameter.
All three agencies have contributed instrumentation, with Nasa and its industrial partner, Northrop Grumman, leading the observatory's assembly.
This was completed at a facility in Los Angeles, California, in August, enabling Webb then to be packed off by sea and the Panama Canal to French Guiana.
Wednesday saw the cargo ship MN Colibri enter the Kourou River at high tide to unload the telescope's sealed transport container. A truck took Webb the last few kilometres into the spaceport's cleanroom facilities.
It will be a few days yet, however, before the observatory can be extracted from its bagging and hoisted into position to allow engineers to get to work on it.
They'll want to know nothing has been damaged in transit.
"You've probably seen the animations of how this telescope will unfold once it's in space, but it is coming to Kourou in a folded configuration," explained European Space Agency (Esa) project manager Peter Rumler.
"We don't have the equipment to do any of the deployments here so we'll be restricted just to switch-on and electrical checks to see that everything is OK," he told BBC News.
There is a detailed list of preparation milestones leading up to the scheduled launch in mid-December. These include fuelling Webb so that it can fire thrusters to manoeuvre itself once it gets into space; and, of course, attachment to its rocket, an Ariane 5.
A key event occurs at the end of next week when another Ariane launch is due to take place. Once this gets off the ground, it will free up the big "table" on which rockets at Kourou, including Webb's vehicle, are integrated.
"Right now our biggest concern is the next launch on 22 October," said Peter Rumler. "If that has a problem or is delayed, it affects us. We certainly want to get Webb off before Christmas."
Whenever it occurs, Webb's launch will be one of the very last outings for the Ariane 5. It is being replaced as Europe's premier rocket from next year by an upgraded system, Ariane 6, which should be cheaper to manufacture and operate.
Since its introduction in 1996, Ariane 5 has lofted several high-profile astronomy missions, including the XMM-Newton X-ray telescope, the Herschel infrared observatory and the Planck surveyor. But none of these carried the price tag hanging from Webb.
Indeed, in terms of payload value, JWST will be the most significant launch in the veteran rocket's 25-year history.
Lift-off is likely to occur somewhere between 08:45 and 11:00 local time (07:45-10:00 EST; 12:45-15:00 GMT).
The flight time, up to the point where Webb is ejected from the top of the Ariane, will be just short of 30 minutes.
This will have put Webb on a path to its chosen observing position some 1.5 million km from Earth, a journey that will take roughly a further 30 days.
Controllers back on Earth will need time to set up the telescope for science operations, but the first sample high-resolution images should be released within two or three months.
Astronomers are expecting great things. Webb has been tuned to see precisely those parts of the cosmos that are beyond Hubble's capability.
"Webb will make amazing discoveries in all fields of astrophysics," predicted Esa project scientist Antonella Nota.
"Webb is building on what Hubble has been able to do in its 31 splendid years in orbit. Hubble, in spite of the fact that it is still a relatively small telescope with a 2.4m primary mirror, has been able to push the horizon of the observable Universe all the way back to a few hundred million years from the Big Bang.
"With a factor of 100 increase in sensitivity, Webb will push way through this and will be really able to see how the first galaxies assembled."