Two weeks after its Christmas launch, the James Webb Space Telescope finished unwrapping itself today, delighting astronomers in the process.

The deployment of JWST’s 18-segment, 21.3-foot-wide primary mirror marked the end of the riskiest portion of the $10 billion telescope’s mission.

It’s still more than 300,000 miles from its destination, a gravitational balance point known as L2 that’s a million miles from Earth. It still has to fine-tune the orientation of the mirror’s gold-and-beryllium segments, and cool its instruments down to a temperature just a few degrees above absolute zero. But mission controllers at the Space Telescope Science Institute were able to tick off nearly 300 potential points of failure without a hitch.

“We have a fully deployed JWST observatory,” Northrop Grumman’s Paul Reynolds, who led the mission’s deployment operations team, declared during a widely watched webcast.

JWST is designed to be 100 times more sensitive than the 32-year-old Hubble Space Telescope, which is near the end of its longer-than-expected life. Once JWST begins science operations, as early as May, it should bring new revelations about mysteries ranging from the habitability of alien planets, to the nature of black holes and quasars, to the origins of the universe.

Today’s mirror deployment, coming after years of developmental delays and billions of dollars’ worth of cost overruns, was hailed as a signal success by astronomers in the field as well as NASA’s top officials.

“NASA is a place where the impossible becomes possible,” said Bill Nelson, the agency’s administrator.

Mark McCaughrean, senior adviser for science and exploration at the European Space Agency, harked back to Winston Churchill’s wartime aphorisms to describe the moment. “To quote Mr. Churchill, now this is not the end,” McCaughrean tweeted. “It is not even the beginning of the end. But it is, perhaps, the end of the beginning. Five months of cooling, aligning and commissioning remain, hard work for the teams involved, before science begins. But today is a big step.”

It took two weeks to unfold the primary mirror — and JWST’s other components, including a sunshield and the rigging for the secondary mirror — because the telescope was too big to fit in its fully deployed configuration inside the nose cone of the European Ariane 5 rocket on which it was launched.

The sunshield, consisting of five layers of ultra-thin coated plastic, had to be unfurled in space to protect the telescope from solar radiation, in what was considered the riskiest part of the deployment. Then the left and right sides of the telescope had to be drawn up and latched like the sides of a drop-leaf table. The left side was set in place on Friday, and the right side was taken care of today.

“This has been, arguably, the most challenging deployment program ever done by NASA,” NASA mission systems engineer Mike Menzel said during a post-deployment briefing at the Space Telescope Science Institute in Baltimore.

The mission team had made contingency plans for potential glitches, including a procedure for shimmying the spacecraft in space if necessary. But none of those plans had to be used.

“It was not as easy as it looks, but the easiness that you saw … is just a tribute to the folks,” said Bill Ochs, the mission’s project manager. “We went through what I feel now is the exact right amount of testing, the exact right amount of engineering audits, the exact right amount of tweaks to the design.”

Before launch, mission managers said JWST would face 344 potential single points of failure. And because the telescope was destined to operate far beyond Earth orbit, there’d be no opportunity to send out a repair team to fix any flaws — as was the case for Hubble.

Although the riskiest part of the mission is over, some risks remain. The spacecraft will have to fire its thrusters on Jan. 23 to settle into its orbit around the L2 gravitational balance point, and then settle into a routine for the years ahead.

“There are 49 single-point failures out of the original 344 that are not retired, and will not be retired for the duration of the mission,” Menzel said. “These 49 are typical of all missions — things like propulsion tanks.”

The good news is that things have gone surprisingly smoothly so far. So smoothly, in fact, that the telescope has enough surplus propellant to last much longer than its originally expected five to 10 years of operation.

“Roughly speaking, it’s around 20 years of propellant,” Menzel said.

Update for 1:10 a.m. PT Jan. 9: University of Washington astronomer Eric Agol, who plans to use JWST to study the potentially habitable exoplanetary system known as TRAPPIST-1, had this to say about the telescope’s deployment in an email:

“It is a testament to the careful planning that went into engineering JWST that so far everything has gone without a hitch.  That doesn’t mean that we astronomers can relax yet — there are still a lot of things that have to go right for the telescope to work as planned!  But so many critical hurdles have been passed that I feel much more optimistic that in six months we will have a working telescope.”