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The centerpiece of the James Webb Space Telescope has arrived at NASA’s Johnson Space Center in Houston for a three-month test to ensure the observatory’s sensors and gold-coated mirrors work in the frigid temperatures of deep space.

While the assembly and initial checks of the telescope’s durability have uncovered no major problems, NASA officials caution that the $10 billion, multi-national project is on the cusp of some of its most critical tests leading up to a planned launch in October 2018.

Over the next few months, technicians will unpack the telescope and install it inside a thermal vacuum chamber at the space center, then pump air out of the test facility as helium and liquid nitrogen chills the hardware to a temperature colder than minus 370 degrees Fahrenheit, or about 50 Kelvin, a measure above absolute zero.

Parts of the telescope, the largest ever built to fly in space, will be even colder during the 93-day freeze to verify the sensitivity of JWST’s infrared detectors, part of an end-to-end test of the observatory’s optics to ensure they will function in space.

The mission’s four science instruments already went through three cryogenic tests at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, but officials had to ship the fully-assembled telescope to Houston to subject the complete structure to the conditions it will encounter in space.

“The Webb telescope is about to embark on its next step in reaching the stars as it has successfully completed its integration and testing at Goddard. It has taken a tremendous team of talented individuals to get to this point from all across NASA, our industry and international partners, and academia,” said Bill Ochs, NASA’s Webb telescope project manager, in a press release. “It is also a sad time as we say goodbye to the Webb Telescope at Goddard, but are excited to begin cryogenic testing at Johnson.”

JWST’s suite of instrumentation peer back in time, resolving the first stars and galaxies that formed in the dark early universe around 13.5 billion years ago, a few hundred million years after the Big Bang.

In the Milky Way Galaxy, the observatory will see into clouds of gas and dust where stars and planets are forming. JWST will determine that chemical make-up of atmospheres of planets around other stars, producing data on which worlds might be habitable.

Some of the observatory’s scientific targets are among the coldest spots in the known universe, requiring the telescope’s sensors to be even colder to register infrared light coming from faraway stars and galaxies.

The cryogenic test at Johnson Space Center will occur inside “Chamber A,” the largest facility in the world designed to mimic the cold, airless conditions of deep space.

NASA tested Apollo spacecraft inside Chamber A, a National Historic Landmark, before sending astronauts to the moon.

Eric Smith, director of the JWST program at NASA Headquarters, said last month that the coming months include “a lot of very challenging activities that are new, and they carry a lot of risk with them.”

The “serialized” test and assembly steps planned over the next year mean each milestone is vital for JWST to remain on schedule for its October 2018 launch on an Ariane 5 rocket from Kourou, French Guiana.

Instead of accomplishing parallel work on JWST’s instruments, mirrors, thermal sunshield and spacecraft components, big pieces of the observatory are coming together. Smith told NASA’s Astrophysics Advisory Committee on April 24 that the program is down to two hardware flows now.

“If something gets delayed, you don’t have progress you can make with another piece of the schedule,” Smith said. “It’s all kind of lining up in a row.”

Since December, technicians at Goddard put the telescope and instrument segment of JWST through vibration and acoustic testing to ensure its sensitive components, such as internal camera micro-shutters, will survive the rough conditions of launch. Engineers halted vibration testing more than a month after ground crews heard an unexpected noise during shake testing on the telescope.

Smith said the problem was traced to a latching mechanism that did not seat properly before the test. The latch holds one of the telescope’s wings folded up for launch to fit inside the Ariane 5’s payload fairing, then releases once JWST is in space to fully unfurl its 21.3-foot-diameter (6.5-meter) primary mirror.

Two plates in the latching mechanism clapped together to generate the noise, but Smith said technicians found no sign of damage to the telescope. The problematic latch was a test unit, and was already due to be replaced with a space-qualified latch before the mission’s launch.

Engineers wrapped up vibration and acoustic tests in March, then spent several weeks re-checking the telescope’s alignment and curvature to make sure it withstood the testing unscathed.

“Before and after these environmental tests took place, optical engineers set up an interferometer, the main device used to measure the shape of the Webb telescope’s mirror,” NASA said in a statement. “An interferometer gets its name from the process of recording and measuring the ripple patterns that result when different beams of light mix and their waves combine or ‘interfere.'”

The primary mirror is made of 18 gold-coated beryllium hexagonal mirror segments made by Ball Aerospace. Each segment more than half the size of the mirror on the Hubble Space Telescope.

The primary mirror segments and JWST’s secondary mirror will adjust their focus with the help of tiny mechanical motors once the telescope is in space.

“Some people thought it would not be possible to measure beryllium mirrors of this size and complexity in a clean room to these levels but the team was incredibly ingenious in how they performed these measurements and the results give us great confidence we have a fantastic primary mirror,” said Lee Feinberg, Webb’s telescope optical element manager.

“The measurements are very stringent for us,” Smith said. “We have to measure our surface figure within a fraction of (the width of) a bacterium, and keep it clean … We have to keep the optics clean, which is especially challenging for a telescope that has naked optics.”

“It’s pretty exciting to see $4 billion worth of hardware being shaken like that, but it passed the vibration testing,” Smith said.

The methodical checks of JWST’s optics after vibration testing, and in cryogenic conditions, are aimed at avoiding a repeat of the mirror deformity that plagued the Hubble Space Telescope after its launch in 1990. Space shuttle astronauts had to add corrective vision aids to the orbiting observatory in 1993 to fix the problem.

No such astronauts visits are planned to JWST, which is not designed to be serviced in space.

Smith said the observatory program, a partnership between NASA, the European Space Agency and the Canadian Space Agency, has around four-and-three-quarters months of slack in the schedule leading up to the October 2018 launch window.

The vibration test glitch in December consumed more than a month of schedule margin.

The super-cold test in Houston will be the last time the telescope experience something like its operating environment until launch. The entire observatory cannot be tested in such a way once the telescope element is connected to the mission’s spacecraft module at Northrop Grumman Corp. in Southern California.

“Why do we do these tests? There’s a lot of verification that has to go on, and unlike most NASA missions, we can’t test like we fly,” Smith said. “We’re just too big. There’s no vacuum chamber big enough in the world.”

“We have to do our testing piecemeal, and then use analysis to make sure that it all comes together in the end,” Smith said.

During the cryogenic test in Houston, which should begin in July, engineers will trace the path light from cosmic targets will take from JWST’s primary mirror, through secondary and tertiary mirrors, and finally to the instrument’s detectors.

Engineers put models of the telescope into the cryogenic vacuum chamber in Houston over the last few years to practice for the upcoming test.

The telescope flew from Andrews Air Force Base, Maryland, to Ellington Field near the Johnson Space Center over the weekend aboard a U.S. Air Force C-5 cargo plane.

During the next few weeks, workers will unpack the telescope from its shipping container and roll it into the cavernous vacuum chamber. It will take around 30 days to chill the chamber to JWST’s operating temperature.

In the meantime, engineers at Northrop Grumman’s satellite factory in Redondo Beach, California, are putting together the spacecraft that will host the telescope. The platform will provide propulsion, electricity, cooling and pointing for JWST at its operating post at the L2 Lagrange point, a gravitational balance point located around 930,000 miles (1.5 million kilometers) from Earth in the direction opposite the sun.

JWST’s sunshield, comprising five membrane layers of kapton each as thin as a human hair, is also being attached to the spacecraft in Southern California.

Spacecraft assembly tasks have also been delayed a few months recently for technicians to re-weld transducers into the propulsion system to replace units damaged during testing.

Workers will install deployable radiators on the spacecraft and put the platform through an acoustic test in July. The spacecraft’s own cryogenic thermal vacuum test is scheduled for September in Redondo Beach, around the same time the telescope is undergoing its freeze test in Houston.

The telescope join the other half of JWST in California around November for a final sequence of combined vibration and acoustic tests, making it perhaps the most tested vehicle ever sent into space, according to Smith.

Another crucial milestone scheduled for early 2018 will be the full deployment of the observatory into its flight configuration. Because of the dimensions of the Ariane 5’s nose cone — the largest payload shroud currently available — JWST will launch with its mirrors, sunshield and solar panels folded up origami-style.

Depending on how you count, Smith said JWST will have more than 300 deployments after it separates from from the upper stage of the Ariane 5 launcher. Counting steps in a similar way, the Curiosity Mars rover had around 70 deployments, according to Smith.

A complicated apparatus of hoists, cranes and lift fixtures will counteract the effect of gravity during the deployment testing. JWST’s mechanisms are designed to function in microgravity, not the 1g environment on Earth.

The spacecraft will travel by boat from Southern California through the Panama Canal to the Ariane 5 launch base in Kourou, French Guiana, in mid-2018 for fueling and final launch preparations.

“We are still on track budget- and schedule-wise, but we are moving into a very challenging period with enormous tests of very complex hardware,” Smith said.

The JWST program escaped the danger of being canceled in 2011 as delays mounted and costs skyrocketed. NASA officials re-planned the program’s budget and schedule at that time, committing to launching the mission by October 2018 at a cost of $8.84 billion to the U.S. space agency.

Adding the contributions of European and Canadian partners, including the Ariane 5 launcher, pushes the mission’s total cost to around $10 billion.

But any significant problems during the rapid-fire test campaign over the next year could delay the launch date.

“People are naturally going to be very cautious if they see any anomalies,” Smith said.

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