NASA’s next flagship observatory, the James Webb Space Telescope, is gearing up for its launch to space on Friday morning — finally. The Webb telescope is the biggest observatory built for launch into space. Its 18 gold-plated mirrors make for a system that is far more sensitive than the Hubble Space Telescope, which it will succeed as humanity’s most powerful scientific instrument for studying the formation of our universe and distant worlds in our galaxy.

But the Webb, with a price tag of some $10 billion, has trudged through one of the most fraught development timelines of any space program, lasting over two decades and costing billions more than its original estimate.

“The stuff they faced was what a lot of space programs face, because everything has to be perfect on a spacecraft like that — you can’t go fix it after launch,” said Cristina Chaplain, who for roughly a decade led audits of the James Webb Space Telescope at the Government Accountability Office, Congress’ watchdog agency.

“It’s very complex and fragile,” she said. “There’s going to be mistakes, but on a program like that, one little teensy thing can have dramatic consequences.”

Here’s a look back at some of the loose screws, cost overruns, stealthy trips at sea and political controversies that the James Webb Space Telescope and its supporters endured on their way to the launchpad.

$1 billion and launching in 2010.

Planning for a telescope to come after Hubble began in 1996, but the Webb did not get its current name until 2002. NASA picked Northrop Grumman to build it, estimating costs from $1 billion to $3.5 billion. Mission managers expected it to launch as early as 2010.

Construction of Webb’s most complex structures — its main science instruments and the massive 18-plate mirror — began in 2004. In 2005, a review prompted redesigns to scale back its technical complexity.

$4.5 billion and launching in 2013.

Though less complex, the telescope became more expensive, with the price tag swelling to $4.5 billion, and NASA officials estimated a new launch date in 2013.

Well into the telescope’s construction around 2009, engineers and NASA officials began to grapple with the difficulty of inventing, building and testing cutting-edge technologies.

One challenge was developing the observatory’s “cryo-cooler” to keep Webb’s ultrasensitive infrared sensors and computers from overheating in space. Developing the telescope’s micro shutter array, a small device crucial to surveying massive swaths of the sky, was also difficult. The device, the size of a postage stamp, contains some 248,000 tiny shutters, or windows — each only a few times larger than a human hair — that open and close to allow light in.

It became clear that the telescope could not be built for the amount of money Congress had appropriated.

$8.8 billion and launching in 2018.

An independent review of the program ordered by Congress in 2010 “found that the program was in a lot of trouble, and it wasn’t going to meet its cost and schedule deadlines, and it was not being funded appropriately, and there were a lot of management and oversight issues that were called out,” Ms. Chaplain said.

“I think it was a bit of a surprise,” she said. “It hit Congress pretty hard.”

The review estimated a new cost of $6.5 billion and a launch date of September 2015. In response, some lawmakers proposed a bill that would have canceled the telescope entirely.

But NASA vowed to get the program back on track, and prepared new estimates: an $8.8 billion total charge, including development and managing the telescope after its launch, with an October 2018 launch date.

To keep NASA in check, Congress capped the cost of the program’s development at $8 billion and required Ms. Chaplain’s team at the G.A.O. to conduct annual audits. It “was probably the first time we were asked to look at a major NASA program every year,” she said.

$9.6 billion and launching in 2021.

The telescope’s construction was completed in 2016. That’s when NASA and Northrop Grumman discovered a new set of bugs.

In 2017, NASA announced it would need to launch the telescope in 2019, because “integration of the various spacecraft elements is taking longer than expected,” the agency’s science chief, Thomas Zurbuchen, said in a statement at the time, stressing the change was not the result of any accident. No boosts to the program’s budget were needed, the agency indicated.

Then, an independent review in 2018 found that a handful of human errors had caused more delays and cost increases. The telescope’s propulsion valves were damaged when engineers used the wrong solvent to clean them. Dozens of screws that fastened the telescope’s massive sunshield came loose during vibration tests. And faulty wiring during tests sent excess voltage into the observatory’s transducers.

“The error should have been detected by the inspector, who did not inspect, but relied on the technician’s word that he had done the wiring correctly,” the 2018 report said.

Fears that the testing mishaps would lead NASA to breach its $8 billion development funding cap grew. The report said human errors cost the program $600 million and caused 18 months of delays. Then, in the summer, NASA announced a new date, acting on the report’s recommendations: Webb would launch on Mar. 30, 2021, Jim Bridenstine, President Trump’s NASA administrator, announced on Twitter.

The agency also concluded that the new development cost would be $8.8 billion, breaching its cap by $800 million. The program’s total cost, including post-launch operations, rose to $9.6 billion.

Last-minute jitters on Webb’s long journey.

Schedule disruptions caused by the coronavirus pandemic further delayed the launch of Webb in 2021.

At the same time, another stumbling block sprouted: The telescope’s name was called into question. James Webb, the NASA administrator who played a central role in the Apollo program, also served as the under secretary of state in the Truman administration. During his tenure, thousands of gay men and lesbians were ousted from government jobs in a period known as the Lavender Scare. NASA ultimately refused to rename the telescope.

In June, four months before Webb was expected to launch, NASA and ESA officials further delayed the launch to review the successful operation of the Ariane 5 rocket.

Once these concerns were resolved, the agencies set a Dec. 18 launch date. The telescope was ferried from California to French Guiana in October during a 16-day trek that passed through the Panama Canal. It was done in secret, in part out of concerns over piracy.

After two decades of tumultuous delays and cost overruns, the telescope had finally reached its launch site. The telescope, however, could not escape some late performance anxiety.

The Dec. 18 launch date shifted to Dec. 22 in early November after a clamp band, which had been helping secure the telescope to its launch mount, unexpectedly came undone, shaking up the telescope and causing worries but no damage. The Dec. 22 launch was then pushed to Dec. 24 last week after there were glitches with a cable that helped the telescope communicate to ground systems.

Greg Robinson, NASA’s program director for the telescope, told reporters on Tuesday that the issue persisted, but that he expected it would be fixed once the Webb and its rocket were wheeled out to the launchpad.

Whenever that happens is up to the weather. The Dec. 24 launch plans were pushed to Dec. 25 because of high winds near the launch site.

Now Christmas morning awaits, the climactic launch date for NASA’s most powerful space telescope.