The JWST mission was a bigger gamble than most people thought, simulation alleviated a lot of the risk.
Disclosure: Shawn Wasserman is a former employee of Ansys Inc. who owns minor Ansys company stock.
Early this year, there was a collective sigh of relief as the James Webb Space Telescope (JWST) successfully maneuvered into its observational orbit and started its final alignments, calibrations and cooldowns. But the truth is, most people—even many in the engineering community—have no idea how much of a gamble this mission has been since it started.
The total cost to build, launch and commission the space-based observatory came to $9.7 billion dollars and required equipment and procedures that were still theoretical at the time of the project’s inception. If anything goes wrong with these new systems, there is no way to fix anything. Though this has been the norm for Hubble—which floats 350 miles above Earth—Webb will sit at the Earth and Sun’s second Lagrange point (L2) which is about a million miles away.
“They made this very bold decision early in the mission that we were making a big telescope. We’re making it segmented. We’re going to fold it up, deploy it in space and that is the way our missions are going to be from now on,” said Erin Elliott, Principle R&D Optical Engineer, at Ansys Zemax. “But all that technology was new because we hadn’t done any of that before. There was constant debate within the community over whether that was the right thing to do.”
Elliott knows all about the challenges faced by the various JWST teams. Before she was at Ansys, she spent over ten years working on Webb. “It was a bold decision and I think it looks like it’s really going to pay off. The technologies are all new and that’s always risky, but it’s also going to set us up for the next bigger, better thing,” she added.
Another expert, James Woodburn, has been the Chief Orbital Scientist at Ansys Government Initiatives (the backronym given to AGI after its 2020 acquisition) for 27 years. He put it this way: “From the orbital trajectory point of view, the biggest risk has been the deployment sequence; luckily, the vast majority of those steps are now accomplished. We’re just down to having to calibrate the instrument at this point. The mechanical deployment of the spacecraft had a huge number of single point failures. It was by far the biggest risk. I don’t know of anything that compares to it.”
With all of this risk involved with the JWST, how were the engineering teams that worked on it able to ensure success? They could build prototypes, but it wasn’t as though they could turn off gravity or easily recreate the harsh environment of space. However, they could simulate the system on Earth, verify it with a prototype on the ground and then change the environment within the simulation to be comparable to the void the telescope would actually experience. “Then we can say, okay, now we trust this model to tell us what’s really happening while in orbit,” said Elliott.
For the orbital team, Woodburn explained, this wasn’t an option. “For the orbit, we have no way to do that on the ground,” he explained. “This has been historically the case. We have to precompute the orbits on the ground to know how the mission will fly. Doing a ton of simulations is critical for all space missions as the cost to redo is incredibly high.” Since the project had this added challenge, let’s start by digging a little further into the simulation of the orbit.
Simulating the Orbit of the JWST
One of the biggest advantages to the JWST is that it is passively cooled. The infrared observation tools are required to operate at 40 K, not far from absolute zero. If the mission opted to utilize coolant to achieve this, then the mission would be over as soon as that resource ran out. Instead, JWST includes a massive sunshield to keep its instruments at optimal temperature. (We dig into that simulation in the article: The James Webb Space Telescope Is Deployed: But How Did Engineers Design for its Biggest Challenge?)
Though this solved the need for massive amounts of coolant, it posed a challenge when calculating the spaceship’s trajectory. “The light from the sun carries momentum and can push the spacecraft. It’s not a huge driver for most trajectories, but you don’t want to ignore it either.” said Woodburn.
There are two major areas where the sun pressure can come into play.
First, as the JWST is traveling to its observation point, it unfolds and deploys. The more surface area facing the sun after each deployment, the larger that sun pressure is going to be. To address this, the JWST team had to produce a different sub-model for each stage of the deployment.
“Each stage has a different physical configuration. We’re going to get a different acceleration due to the solar pressure,” he explained. By modeling how the solar pressure affects the trajectory at each stage of deployment, the team can better estimate any trajectory adjustments needed to get the JWST to its final location.
“Ansys’ Orbit Determination Tool Kit (ODTK) is a software product that provides orbit determination capabilities. ODTK was used for pre-flight navigation analyses, based on simulated observation schedules and data, and is used operationally to fly JWST,” added Woodburn. “Two of the big factors that made ODTK attractive for operational use on JWST were the option for NASA to insert their own custom solar pressure model and the ability of the optimal sequential filter in ODTK to process data seamlessly across spacecraft maneuvers.”
The second area where sun pressure can affect the mission is when the satellite is at its final L2 destination. Woodburn explains that this is technically an unstable LaGrange point. This means that without intervention, the satellite will eventually drift out of orbit. To combat this, the JWST has extra fuel to keep it in place for over ten years. But this drift, and the amount of orbit control needed, are comparable to the push from the solar radiation.
“Solar radiation pressure is important to model the L2 Orbit correctly so that we can properly gauge the amount of control we’re going to need to employ to keep the spacecraft where it’s supposed to be,” said Woodburn. “Our flagship product is Systems Tool Kit (STK) has a module called Astrogator that supports spacecraft trajectory design. STK/Astrogator, which has a long history in the design of Lagrange point missions, was used for much of the early design work including the generation of design reference missions (DRM) for JWST.”
DRMs are representative missions that are performed before the final mission is decided upon. They help teams optimize and narrow down the steps of any given scheduled mission. DRMs also help these teams study and adapt any scenarios that could arise during the mission.
“You work through all the mission conflicts and build towards something plausible in the DRM from which you can pull together an actual mission that could be flown,” said Woodburn. “You can bring in the flight, hardware and science teams to say, ‘If we did it like this what would happen?’ Then you can look at another option and build them all up into a study to see if you should do it this way or that way.”
Now that we understand how NASA plotted the course of the JWST, how do they ensure the 18 sections of its complex mirror will work in unison?
Simulating the Mirror Alignment of the JWST
Traditional space telescopes will utilize a single mirror to channel all the light it collects into one location. However, for the 6.5 meter, in diameter, sized mirror of the JWST to be able to fit into a rocket, it needed to be produced in segments that could fold up into the allotted launch space. Then, once the satellite was in place, these 18 segments needed to be aligned, to within tens of nanometers, to work as one single mirror. This is where Elliott’s team came into play with respect to the design of the satellite.
Simulated wavefront map of the JWST primary mirror with piston errors in the mirror positions. (Image courtesy of Ansys.)
“We didn’t quite know how to do that when we started,” said Elliott. “We built a test bed one-seventh the scale, which had 18 hexagonal segments and the same kind of actuators as the flight system. We connected our simulations to the test bed so we could try everything out. There are a bunch of steps in the mirror alignment and with each one we could simulate something, come up with a plan, try it out on the test bed and then scale it to work in flight. After we did all that, there were 10 steps in the final alignment process.”
Developing the alignment steps and testing them out wasn’t a guess and check process. “We leaned heavily on simulation to develop the software needed to optimize and align each step. We used Zemax OpticStudio, and several other tools, for that,” Elliott explained.
The team not only tested the most likely scenarios that would happen after the spacecraft unfolded, but the simulation tool also enabled them to test best and worse case scenarios. This is important, because at a million miles away, there would be no second chance to get the alignment right.
The team also knew that all the testing on the ground wasn’t going to translate to what the mirrors would experience in space. To account for this, they re-ran the simulations after inputting the environment within space.
But that wasn’t enough, Elliott notes. “The shape of the mirrors is interesting, as we needed to know what they would be in Zero G but we only had what it was like in gravity. So, we did a FEA simulation to predict how the mirrors would change shape at a Zero G and a 40 K environment.”
With knowledge of the shape of the mirrors in that environment input into the final simulation, the team was able to optimize the alignment protocol.
What the JWST Means for Space Exploration
At the time of interview, it was unknown what the final fate of the JWST would be. As it is now finally fully deployed in its L2 observation point, after some remaining setup, its mission will be to explore our cosmic history.
When the universe was young and the first light sources illuminated, light started to move in every direction. Because of the expansion of the universe, the longer light traveled the more redshifted it would become. Some of that light is still traveling, and whatever ends up at the L2 point can be captured by the JWST. That is also why the system is aiming to collect infrared light—to observe what the universe looked like back when it was just starting to form.
“It’s a failure of our brains to understand the extent of the universe and how big it is,” said Elliott. “It’s so big that light has been traveling for billions of years and some of that light was emitted at the dawning of the universe and it’s just getting to us now. In general, in infrared, you’re seeing older things, so it takes you closer to the dark ages where nothing in the universe was luminated. The colder infrared will allow us to see farther back in time.”
Some might ask why this couldn’t be done on land, or with the existing Hubble telescope? Elliott puts it quite elegantly. “Hubble revolutionized astronomy, and is still making discoveries based on its data. I don’t want to downplay it. But it launched in the 90s, so it is over 30-year-old technology. The mirror is lightweight, but the technology was largely the same as ground space telescopes. It’s no longer pushing the technology further, and it has a lot of problems due to aging. It’s limping along, and it only sees in the visual spectrum of light.”
While Hubble can’t see the infrared spectrum, what about ground telescopes? Why were these not an option? “For infrared telescopes on the ground, the Earth is glowing in this spectrum and the atmosphere emits and absorbs readily in infrared too,” Elliott adds. “It causes all kinds of problems. It’s a struggle to pick out the signal from the noise.” This also explains why the faraway location at L2 was chosen for the JWST. This way, the infrared radiation from the Earth, Moon and Sun won’t make as much signal noise.
“Infrared telescopes in space where they can be cold, like the JWST running at 40 K, it’s going to see things we haven’t seen before, Elliott added. “It’s a low noise environment, an 6.5-meter telescope with high resolution. It’s a big light bucket that sees faint things. For infrared astronomers, it’s a dream come true.”
In other words, when we start to get images this summer from the JWST, it’s sure to make a Big Bang in the world of astronomy.
For more on this, read: The James Webb Space Telescope Is Deployed: But How Did Engineers Design for its Biggest Challenge?.
