James Webb Has 344 Single-Point Failures. Here Are the 5 Most Critical Elements
After more than a decade of delays, the James Webb Space Telescope is finally ready to look deeper into the universe — and farther back in time — than any telescope ever has. All that's left to do is to get Webb into space and safely into its orbit…a million miles away.
Unfortunately, that's not a simple process. Patrick McNally, an aerospace engineer and managing director of the Space Physics Research Laboratory at the University of Michigan, told Interesting Engineering in an interview that Webb might be, “the most challenging thing we've ever put into space.” If not, it’s “right up there with the Space Station [and] the Hubble."
But unlike those missions, Webb has to traverse these final steps of its journey without any help from humans. Once it launches, it's on its own.
When creating something like the Webb telescope, designers maintain a list of elements that would have severe, mission-disrupting consequences if any one of them were to fail, according to McNally, who does not work on the Webb mission. The goal is to keep that list of "single-point failures" as short as possible by developing backup systems and redundancies, but design constraints and expense often mean that some items have to stay on the list.
The Galileo probe, which McNally helped NASA send to Jupiter in the 1980s, had about 30 single-point failures. A Mars landing has more than 100. Webb’s list of single-point failures is 344 items long. “Those who are not worried or even terrified about this are not understanding what we are trying to do,” Thomas Zurbuchen, NASA’s Associate Administrator for the Science Mission Directorate, wrote in a blog post.
One reason Webb has so many points of failure lies in the nature of the instrument itself. McNally notes that designers were faced with a monumental challenge when it came to getting Webb into its orbit, which will orbit at the Earth-Sun L2 Lagrange point, one million miles (1.6 million km) away, compared to the Hubble's 340 miles (550 km). The team had to balance the requirements of a powerful infrared telescope with the limitations of the rockets currently available to carry it into orbit. The approach they settled on is to fold Webb into the nose of an Ariane 5 rocket for launch and then have it unfold once in space, kind of like a Transformer.
Ultimately though, McNally says that one of the biggest challenges facing Webb isn't the sheer number of single-point failures, it's that so many of them are interconnected. "You've got one part that has to fold out properly so that another part can fold out properly," he said.
But it’s far from a shot in the dark. Engineers and technicians have spent years imagining scenarios, running simulations, and testing every last component. More than 10,000 people have worked on Webb in some capacity since the project began, according to The Washington Post. And yet, no amount of preparation can guarantee the safety of such an ambitious bid for the stars. So, despite the $10B budget (and the hopes, dreams, and careers of a generation of astrophysicists) on the line, the Webb team had no choice but to accept the extremely long list of make-or-break moments.
Here’s an overview of the top challenges Webb must overcome before the telescope can start sending images back to Earth.
Webb’s distant, lonely orbit
As previously noted, Webb won't orbit Earth like Hubble does, a mere 340 miles (550 km) from above the planet. Instead, Webb will keep a tight orbit around a point called L2, which is 930,000 miles (1.5 million km) from home. L2 is one location where the gravity of the Sun and Earth combine in a way that makes it possible for a satellite to stay in the same position relative to the Earth. There are actually five such points, but astronomers chose L2 because it protects Webb from as much infrared radiation (otherwise known as heat) as possible.
Its far-flung location in the Solar System means once Webb is out in space, it's designed to operate completely on its own. That's especially daunting given that the Hubble had to be serviced several times before engineers could get it in proper working order. It "was designed for servicing," McNally said, "but there's no way we knew that it was going to require so much servicing." NASA says that won't be an option for Webb. "There is currently no servicing capability that can be used for missions orbiting L2, and therefore the Webb mission design does not rely upon a servicing option," according to the Agency. On this point, McNally is more optimistic. "It's not designed for that," he said, but "I would never underestimate the ingenuity of the space enterprise," he said.
Webb's largest surface is its sunshield. At more than 3,000 square feet (about 280 square meters) once fully deployed, the sunshield will protect Webb from infrared radiation emanating from the Sun, Earth, and Moon. Its five thin layers of Kapton, a silvery lightweight material with special thermal properties, will create a temperature difference of about 570 °F (316 °C) between its warm side, which faces us on Earth, and the cold side that faces the telescope. Three days after launch, the two "pallets'' containing the tightly folded sunshade material will unfurl on either side of the telescope, like a pair of giant wings. "Those have to get out perfectly in order to be in a secure location where the whole shape can be deployed out," McNally said, and added, "I'm very interested to see how that very large, thin material deploys.”
If the pallets fail to lock into place exactly as designed, problems could quickly "cascade" during the rest of the unfolding sequence.
The thin fabric of the sunshade will begin unfolding on day five, once the pallets are in position. While small holes might not cause major problems, McNally noted that debris or snags could put an additional load on other mechanisms, preventing a successful deployment. The challenge is heightened because it's difficult to simulate how such thin materials act, he says, so “it's hard to get really good confidence that everything is going to work."
The primary mirror
The primary mirror will deploy on day 13. Coated with gold and shaped like a honeycomb, it's the largest mirror that's ever been put into space, with a collecting area more than six times larger than the Hubble. Webb’s science mission required such a large mirror to catch as much light as possible, but this proved a massive engineering challenge. The solution is a mirror consisting of 18 individual hexagons, each one 4.3 feet (1.3 meters) in diameter. Its weight per unit area is about one-tenth that of the Hubble’s! When folded up inside the rocket fairing, Webb’s mirror is divided into three parts that will extend during Webb’s second week in space.
Each of the beryllium segments is attached to a series of small motors, called actuators, that will enable the craft to focus light at exactly the right angles. There is extraordinarily little room for error here. Even the slightest scratch could cause irreparable damage and destroy our ability to peer across the cosmos. The Hubble was kept offline for years while crews made up for a manufacturing error that left its mirror misshapen by about one-fiftieth the width of a human hair. Unfortunately, as previously noted, no such repairs will not be possible with Webb, due to its extreme orbit.
The secondary mirror
The secondary reflector mirror isn't as flashy as the honeycomb surface, but it's just as important. Mounted onto three 25-foot-long (7.6 meters) arms, this mirror directs the photons gathered by the primary mirror into the telescope itself, where cameras and spectrographs will be waiting to transform the physical properties of the light into data. The arms, which are tubes made from a composite material just one millimeter thick, will deploy around 10 days after blastoff.
McNally says the secondary mirror is similar to — though bigger than — mirrors used on other telescopes, but that doesn't mean successful deployment will be easy. "These big surfaces that have something delicate on top give me concern," he said. The biggest challenge is getting the arms to position the mirror. "You need to have a certain amount of momentum built up... in order to drive [the components] to snap into place" so the mirror is positioned precisely where it needs to be, he said. Any type of misalignment that causes even a tiny change in the mirror’s final position could render Webb seriously dysfunctional.
The idea of 344 single-point failures is eye-popping, but the reason engineers make lists is to limit and mitigate the risk of failure. They can test components for thousands of hours under an array of conditions. They can run simulations to watch how different processes will unfold under various circumstances — some of them likely not and many of them will not allow the mirror to unfold. What's more difficult to prepare for is the unexpected.
For example, it's possible that the string of delays, which were meant to ensure Webb's safety, could actually put its success in peril. "Each one of those may introduce a little bit of uncertainty," McNally said.
The Galileo probe also faced years of setbacks before it was launched. After the Challenger accident, the probe was transported and placed in storage. At some point in that process, lubricant was rubbed off of three ribs that enabled one of the craft's antennas to extend. When Galileo tried to deploy the antenna, it got stuck. Scientists and engineers spent nearly four years devising a workaround. "Nobody said, 'we're storing it or moving it back and forth, that lubrication is going to be gone.' It was something that they found after the fact."
And yet, it's likely to work
It’s true that Webb has a lot of single-point failures. A lot. However, it’s important to note that single-point failures aren’t necessarily bad. A car, for instance, only has one way to steer. “If the steering wheel goes out, we don't have any way to maneuver the car," McNally said. It's a single-point failure we accept because carmakers "make the steering system very reliable." The brakes, by contrast, are supported by a backup system: the emergency brake.
"You have to accept that these failures can affect overall performance and mission success," McNally said, but “you go through reliability testing [and] simulation analysis... in order to gain confidence that that's not going to happen.” Most of the decisions, judgment calls, and tests that will collectively determine Webb’s success have already been made. Now, all we can do is sit tight, hold our breath, and wait for the images that may — just maybe — help researchers rewrite the history of our universe and reimagine our place in the cosmos.