Staying airborne: How Japan and Europe reformed the LiteBIRD mission
In the summer of 2024, LiteBIRD went blind. The collaboration developing its ultra-sensitive detectors collapsed, leaving the mission unable to hear the faint signal it had to capture.
LiteBIRD—the Lite (Light) spacecraft for the study of B-mode polarisation and Inflation for cosmic background Radiation Detection—is a mission to observe the oldest photons in the Universe and search for an imprint of the cosmos’s earliest moments.
It is thought that in the first instants of existence, the Universe underwent a brief but enormous expansion. This straining of the cosmological fabric would have generated gravitational waves. Such primordial ripples are too weak to be detected directly, but their production should have twisted the polarisation of the light. If this occurred, the polarisation of the oldest photons in the Universe should form a distinctive pattern across the sky, known as primordial B-mode polarisation.
But detecting this polarisation pattern is immensely challenging.
The most ancient light in the Universe is the cosmic microwave background (CMB). After cooling for billions of years, these photons now sit at just 2.725 K (-270°C). In 2006, the Nobel Prize in Physics recognised the detection of tiny temperature variations across the sky at a level of one part in 100,000. The fluctuations in the polarisation that would reveal the signature of inflation are fainter still, by at least another order of magnitude.
In 2014, a ground-based team at the South Pole BICEP2 telescope believed that they had detected the B-mode polarisation signal over a small patch of the sky. At that time, Ludovic Montier was part of the Planck satellite mission, an ESA observatory mapping the CMB to high precision. When Montier saw the result, he suspected this was a false alarm.
“We had high frequency maps of the same sky region from Planck,” explained Montier, now European spokesperson for the LiteBIRD mission. “These were still private, but we had them. I looked at the maps and realised that what they were seeing was not the CMB, but a galactic signal.”
Not only is the polarisation of the CMB extremely difficult to detect, but the signal can be overwhelmed by more recent sources of polarised light, including the dust within our own Galaxy. While galactic dust can mimic the same pattern as primordial polarisation, the strength of the signal changes differently with frequency. The contamination can therefore be identified and removed, so long as the observations are made across a wide enough range of frequencies.
The plans for LiteBIRD changed. The original design consisted of a single telescope with a relatively narrow frequency range. The results from the South Pole and Planck showed that a much broader range was essential. In response, JAXA asked whether Europe could provide additional frequency channels.
The new LiteBIRD carried a three-telescope system. JAXA was to develop the telescope sensitive to low frequencies, while Europe took responsibility for medium- and high-frequency telescopes, giving an order of magnitude frequency coverage with a range from about 40 GHz to 400 GHz.
The need for three separate telescopes came from how the polarisation signal is measured.
“If you have a very tiny lamp very far away in daylight, it’s very hard to tell whether it is on or off,” explains Francesco Piacentini, a professor in astrophysics at Sapienza University in Rome and a member of the European LiteBIRD team. “But if you can switch that lamp on and off at a known frequency, you can see the difference. In physics, this is called modulation.”
LiteBIRD uses an equivalent technique to detect polarisation. A rotating half-wave plate—a birefringent material that rotates the orientation of polarised light—continuously changes the polarisation direction of the incoming signal. The detectors are sensitive only to a particular polarisation direction, and therefore see a signal that rises and falls in time, turning the polarisation into the measurable flicker of a tiny lamp being turned on and off.
Natural birefringent materials, such as sapphire, only work over a limited range of frequencies. To cover the full range required, the LiteBIRD design used three separate telescopes each with its own polarisation modulation units tuned to a different frequency range.
“We knew it was a complicated design,” said Montier. “CNES were already asking whether the design for the medium- and high-frequency telescopes could be simplified. A few options were being considered, and then the detector collaboration encountered severe difficulties.”
To detect the faint whisper of the polarisation signal, LiteBIRD needs detectors with extraordinary sensitivity. The chosen technology is the transition-edge sensor (TES), a cryogenically cooled device made from a superconducting material whose resistance changes sharply at a critical temperature. When a photon hits a TES sensor cooled to fractionally below its critical temperature, the slight heat from the absorbed energy pushes the material out of the superconducting state and causes a rapid increase in resistance that can be precisely measured.
Expertise in TES sensors was concentrated in the United States, where the technology has been extensively developed for ground-based instruments. JAXA initially sought a partnership with NASA, but a change in US policy prevented a collaboration from moving forward. This setback prompted the European LiteBIRD team to explore whether TES detectors could instead be developed in Europe.
“There was a discussion to develop a ‘Plan B’, where detectors could be developed in Europe,” says Piacentini. “With support from ESA, a feasibility study was conducted to assess whether this was possible for LiteBIRD. Then the proposal from KEK came through.”
The High Energy Accelerator Research Organisation (KEK) in Japan proposed a new approach. Under this plan, KEK would partner with US research institutions to deliver the required detectors.
“Compared to the US technology, Europe was still far from being able to provide something viable for LiteBIRD at the time,” says Montier. “The solution from KEK was very convenient because it allowed us to access US technology through Japan. And so the European option was not continued. In hindsight, that was probably a mistake.”
Based on the collaboration with KEK, LiteBIRD passed the Mission Definition Review (MDR) in 2024, a key early milestone in mission progression at ISAS.
Then KEK ran into trouble.
While the United States had considerable experience with TES technology, the detectors had all been developed for ground-based observatories. Operating in space introduced new demands, such as the need to protect the detectors from cosmic rays and the stresses of launch. Meeting these requirements became a serious struggle, and KEK ultimately withdrew from the collaboration.
LiteBIRD’s eyes went dark.
It looked like the end of the LiteBIRD mission. Without detectors, the spacecraft could not see the cosmic microwave background. Neither Japan nor Europe had yet developed the required TES technology. Perhaps this mission was not possible.
Then a new idea emerged.
“It was second generation—the early- and mid-career researchers—who refused to give up,” says Fujimoto Ryuichi, the JAXA LiteBIRD team leader. “LiteBIRD had been in development for years, and during that time these researchers had built up the necessary expertise. They did not propose a quick fix. They proposed a reformation.”
Approximately one month after KEK withdrew, the LiteBIRD team went back to the drawing board. The scientific goal remains unchanged, but every aspect of the mission design was reconsidered. Could recent advances improve the LiteBIRD effective but very complex design?
There was one promising answer.
At Sapienza University, a team led by Giampaolo Pisano had been developing artificial crystals with birefringent properties. Unlike natural materials, these metamaterials could rotate the polarisation of light across a much broader range of frequencies.
“This would allow us to build polarisation modulation unit with a wide enough frequency range to make LiteBIRD with a single telescope,” says Piacentini.
Supported by the ESA, an early version was tested in the laboratory with encouraging results. Further demonstrations are planned in the next few years, including deploying to ground-based and balloon-based instruments.
With the three telescopes consolidated into a single instrument, accompanied by a simpler cryogenic cooling system, the new LiteBIRD design was more streamlined and robust. Europe took the lead on the instrument, while JAXA was responsible for the spacecraft itself. This includes the service module—controlling functions such as the attitude, orbit, power, communications, and data handling—the cryocoolers and thermal shielding that would cool the telescope to below 5 K (-268ºC) and help reduce the temperature around the detectors even lower, and the primary and secondary mirrors of the telescope.
But while detectors were now needed for only one telescope, the question remained: how would the TES sensors be built?
“We returned to the earlier proposal to build the detectors in Europe,” says Piacentini. “And we drafted a new plan and took this to ESA.”
Although TES detectors for the cosmic microwave background had not yet been developed in Europe, there was relevant experience to build upon. The Space Research Organisation Netherlands (SRON) had been developing TES sensors for the future ESA X-ray mission Athena, and had also worked on similar technology for the proposed infrared mission SPICA. Together with the Istituto Nazionale di Fisica Nucleare (INFN) and Sapienza University in Italy, and Cardiff University in the UK, a new consortium formed to develop microwave TES detectors in Europe.
ISAS Director General Fujimoto Masaki travelled to Europe to discuss the situation directly. “I had heard stories that Europe has taken steps toward developing the detectors, but had never been able to complete the process,” he says. “This had happened more than once over the past few decades. LiteBIRD could be the trigger to finally make it happen, not only for this mission, but for future European space telescopes as well. I felt this was a genuine win-win situation, and I had no reluctance in making the proposal to my colleagues at ESA D/TEC.”
Past collaboration between ESA and ISAS had typically been through the Directorate of Science, but more recently this channel had broadened to include the Directorate of Technology, Engineering and Quality (TEC). It was this expanded framework that allowed LiteBIRD to continue. ESA offered their support.
The European Detector Consortium was then established to develop the TES sensors, with strong support by the Italian Space Agency (ASI). The first step is to build detectors for frequencies around 95 GHz and 145 GHz, where the cosmic microwave background can be observed with relatively low contamination from other signals. From there, the technology will be extended to both lower and higher frequencies.
Once demonstration models for the detectors and modulation unit have been completed, the final design and assembly of the flight model will be supervised by CNES in France. This will be another major milestone.
“You cannot just put all the subsystems together like LEGO,” says Montier. “Every time you connect two components, you have to check that everything still works. There are many of testing and validation steps.”
The physical assembly is only part of the challenge. The telescope must be carefully calibrated to verify its performance. Because the LiteBIRD telescope will operate at temperatures below 5 K and the detectors themselves as low as 0.1 K, these tests must be carried out inside large cryogenic vacuum chambers. This is a complex task, and the team is already preparing the facilities required.
“Once it’s in space, it’s gone,” Montier adds. “You cannot go back and measure things again. That is very different from observing from the ground, where you can fix something if it goes poorly. But once it’s in space, it’s done. You have to deal with what you launch.”
In September 2025, the reformed LiteBIRD design passed the Key Decision Point (KDP) at ISAS. The progress was strong enough to move the mission into a detailed study phase, culminating in a second Mission Definition Review which was passed at the end of June. This allowed LiteBIRD to enter mission “Phase A” at JAXA, with the next major assessment provisionally scheduled for 2028.
These milestones are designed to be difficult to achieve. The success of the LiteBIRD mission so far reflects not only the strength of the science, but the determination of a collaboration that refused to let the mission fail. What began as a setback has become a reinvention—one that will shape not only a spacecraft, but the future of space-based cosmology.
Further information:
Cosmos: LiteBIRD aims to find evidence for the greatest expansion in the history of the Universe
Cosmos: A rebel alliance: the small mission collaboration between ESA and ISAS
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