The James Webb Space Telescope (JWST) is on the launchpad and due to take off between 12.20pm and 12.52pm UK time.
The $10 billion (£7.5bn) device is set to take the title of the largest and most powerful telescope ever sent into space when it blasts off on an Ariane 5 rocket from Europe’s Spaceport in French Guiana.
The launch was scheduled for Christmas Eve, but a forecast of high-level winds at the Kourou spaceport forced it to be postponed until December 25.
During a news conference on Tuesday, NASA officials said the rocket and telescope were in good shape, and that the only lingering, though tolerable, problem was an intermittent communication delay between the two.
The issue earlier forced a two-day delay; a clamp that inadvertently jolted the telescope at the launch site had prompted a four-day slip.
If all goes to plan, the instrument will be released from the rocket after a 26-minute ride into space.
It will then take the Webb telescope a month to coast to its destination in solar orbit roughly 1 million miles from Earth – about four times the distance from the moon.
Testing on the joint Nasa and European Space Agency project was finished at the end of August, with shipping preparations finalised in September.
Cosmologists, including those in the UK, will use the telescope to map dark matter around galaxies, with the aim of unlocking the secrets of the mysterious substance that makes up the vast majority of matter in the universe.
Caroline Harmer, head of space science at the UK Space Agency, said: “The James Webb Space Telescope will be the most powerful and complex telescope ever launched.
“It will advance our understanding of the universe by allowing scientists to hunt for the unobserved formation of the first galaxies and look inside dust clouds where stars and planetary systems are forming today.
“The UK has played a crucial role in leading the international consortium that developed the Mid-Infrared Instrument, which will allow us to examine the physical and chemical properties of objects in the early universe in greater detail than ever before.
“As we move one step closer to launch, we are one step closer to understanding the universe and our place in it.”