SPICA: an infrared telescope to look back into the early universe
by Arwen Rimmer
|“The fact is that these questions can only be investigated by observations in the infrared,” said Roelfsema. “You can’t do this from the ground because the Earth’s atmosphere blocks it. So, space-based observations are required.”|
SPICA stands for Space Infrared Telescope for Cosmology and Astrophysics, a next generation observatory with heretofore unseen capability in the far infrared. In theory, it will be able to study stellar nurseries and protoplanetary discs, analyze exoplanets, and shed much needed light on galaxy and planet formation. SPICA will be able to see the adolescent universe at only one billion years of age in high resolution. Dr. Peter Roelfsema, the proposal lead, believes that these capabilities will allow astronomers to shed light on three of the four fundamental questions.
“We've been looking at them as guidelines from the very early days of the project,” he said. “SPICA has three fundamental science pillars which will directly address questions one, two and four; to track the drivers for galaxy evolution over cosmic time, to unravel the physical conditions for planet formation, and to establish the role of magnetic fields on star formation in dusty clouds. The fact is that these questions can only be investigated by observations in the infrared. You can’t do this from the ground because the Earth’s atmosphere blocks it. So, space-based observations are required.”
Measurements in the infrared wavelength illuminate cooler or dimmer matter, which is what is needed to study the early stages of planet and star formation. Previous infrared missions like Herschel have revealed a great deal about what astronomers involved in the mission call the “obscured universe,” but sensitivity has been limited because of the technical difficulties inherent to launching a large cryogenic telescope. SPICA will utilize cutting-edge technology in detectors, passive cooling, and heat shielding to see farther and with better spectral resolution than any infrared mission before it. The mechanical coolant system will keep the entire telescope (2.7-meter mirror, all instruments, and enclosing tube, altogether weighing about 400 kilograms) below 8 K. This is what makes SPICA unique. The major infrared observatories of the past had parts that were kept cold, but not the entire thing. And ISO, Spitzer, and Herschel all used a liquid coolant. This limits the working life of the telescope. And if something goes wrong with the supply, like it did with NASA’s WIRE, the mission can be cut short before its time. Cryogenic stores also create a much heavier payload, which significantly increases costs.
“There is a balance between science goals and programmatic constraints,” Roelfsema said. “But sometimes, these limits can produce great creativity. We had a problem with the projected weight and balance of the satellite in its orbit, which was also pushing costs higher. This was only recently solved in January. Our solution was to go ‘vertical’ with our design. The new alignment allows for a lighter, more stable payload, with the same instrument suite.”
|SPICA will provide the first definitive evidence of how the universe evolved, either proving the models, or disproving them and making way for new ones.|
There are the functional challenges of performing space science, and then there is the push to discovery, which is necessary to make the entire endeavor worthwhile. For SPICA, cryogenics R&D are a major focus at this stage, because none of their science goals are reachable if the telescope can’t be maintained at incredibly cold temperatures. The reason for this is to reduce detector noise below the level of intrinsic background noise. For infrared observations in space, the background limit is set by the instruments themselves, which put out radiation unless cooled to cryogenic temperatures. Dave Clements, a professor at Imperial College London and a member of the collaboration explains it thus:
Imagine doing optical astronomy with a mirror that is glowing red hot. This wouldn’t be easy because the mirror is probably brighter than the sources you’re looking at. This creates what we call a background, and you’ve got to measure everything relative to the background. There’s a famous law in physics called the Stefan-Boltzmann Law. All things being equal, since the SPICA mirror will be ten times cooler than Herschel’s was, it will be able to see objects that are 10,000 times fainter. That makes a lot of difference.
SPICA’s toolkit will peer into the past in a variety of ways. The telescope will be able to see through dust to the ancient convolutions of protoclusters. The mid-IR spectrometer/camera will do spectral mapping and imaging in the wavelength range of 12–36 microns with three channels: low, mid, and high resolution. SAFARI, an advanced spectrometer, will analyze the chemistry, physics, kinematics, and mineralogy of astronomical objects far and near. B-BOP, a polarimeter, will probe the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our galaxy. Each instrument will be able to do much more than was briefly mentioned above, and will also work in concert with each other. This technological synergy will allow the team to approach fundamental topics in astrophysics like never before. Jan Tauber, the ESA study scientist for SPICA, explains how current formation theories are just that: theories. SPICA will provide the first definitive evidence of how the universe evolved, either proving the models, or disproving them and making way for new ones.
“We have a very good theory, overall, which suggests how galaxies and clusters of galaxies formed,” Tauber said. “But the details are not clear. There are issues understanding star formation in the very earliest galaxies using visible-light observations because of dust, which is a major obscuration problem. For example, the evolution of galaxies is believed to be driven by what we call feedback, which is basically a process of matter flowing in and out of galaxies. And there is a competition, if you like, between star formation driven processes and active nuclei processes that we don’t understand very well. These processes cannot be easily distinguished unless we go to the infrared.”
|“In my opinion, the range of science that will be influenced by SPICA is much bigger than the other two [M5] missions,” Clements said.|
If approved, SPICA will launch in the early 2030s. JWST, which is finally slated for launch next year, is also investigating the infrared, but at wavelengths of 0.6 to 28.5 microns, versus the spectral range of 12 to 230 microns for SPICA. NASA is also studying a space observatory called Origins that has similar goals as SPICA, but with a much larger and cooler telescope. If approved, its launch is scheduled for around 2035. So, if SPICA isn’t selected, scientists in the field will still have Origins to root for. Ad if Origins isn’t selected either, all these fundamental questions will just have to wait.
“In my opinion, the range of science that will be influenced by SPICA is much bigger than the other two [M5] missions,” Clements said. “You can send a probe to Venus, and perhaps see evidence of tectonic activity. But between now and when Envision is going to be launched there’s probably a whole bunch of other things going to Venus. Theseus will look for very high redshift gamma ray bursts. But over the life of that mission, they will be lucky to find maybe 120, perhaps less if the predictions are wrong. Whereas SPICA can do everything from solar system objects to the most distant objects in the universe. In my mind, we’ve got greater scientific potential than the other two missions.”
The group meetings all three M5 finalists have scheduled for this year have several vital purposes leading up to the final selection: to fine tune scientific goals, identify problems and figure out how to solve them, update members on any technical or programmatic changes, and delegate responsibilities, both short term and long term. Running an efficient and on-point conference is a perennial challenge no matter the field, and even more so when one has to cancel the in-person event at the last minute and transition to a teleconference. Roelfsema had planned for the meeting to go forward up until the Friday before, when feedback, especially from members in Italy, made it clear that too many people were planning on staying away, either out of a sense of personal precaution or because of direct intervention from their governments.
“Regardless, I am really pleased with how well it went,” Roelfsema said. “Beyond my expectations, possibly even beyond my hope. This was an experiment forced on us. We have never done anything on this scale before. We are quite used to telecoms with 10–15 people or so, because we are from all sides of the world. But on Monday (March 9th), we had almost 140 people in one ‘room.’ Tuesday we had three sessions going parallel. And I think all the information that needed to be conveyed got conveyed.”
But meetings are about more than itineraries and PowerPoints; there’s something both intangible and invaluable that occurs when people are in the same room.
“The clear disadvantage is that we didn’t have the ‘corridor talks’ which are really valuable,” Roelfsema said. “Science is a creative environment. We need to think of solutions for problems that have not been tackled before and may not even have been thought about before. So the personal interactions are important. We will have to have some more face-to-face meetings to catch up, hopefully over the summer. Accepting that challenge, it went really well.”
ESA is the main decision-maker on the continent for space activity and satellite regulations, and so takes over mission responsibilities for instrument housing and launch parameters when a project is approved. Everything they do is by necessity collaborative, bringing together agencies, universities, and industrial partners from all over the world. This can create a bureaucratic tangle, but in 50 years they have sent up almost 100 scientific missions with less than a tenth of NASA’s budget. SPICA will be a joint project with the Japan Aerospace Exploration Agency (JAXA).
Tauber spoke on the costs and benefits that a shared mission like this involves. “Some collaborations are easier than others,” he said. “The form of the involvement is very important. With SPICA, ESA will be in the lead. But we have an almost 50/50 collaboration with JAXA in every respect. This can make decision-making more difficult. Usually, we prefer to be in a position where one of the collaborating agencies is the clear leader, but in this situation, it’s a bit more complicated. The reward is that we can do a ‘large mission,’ like SPICA, with ‘medium-class’ resources.”
February 1, 2021, is the date of final submission for the three finalists, where each of them must deliver a Yellow Book. This is essentially a textbook about every aspect of their mission, with supporting technical documents to be reviewed by independently selected professionals. Pending approval, ESA itself then contracts with industry partners for the construction of said project. But will the deadline hold with COVID-19 disrupting work and travel all over the world?
“It may be that the current situation alters that schedule,” Tauber said. “How this is going to be managed I don't yet know. Some work can go on relatively undisturbed by teleworking, virtual meetings, and so on. But we are already seeing that there will probably be unavoidable delays. Programmatically, the M5 selection is just one cog in a many-wheeled system. Disturbing one part of the system disturbs many others.”
Even though SPICA is still in the design phase, and construction of the payload and instrument suite is more than a decade off, a considerable amount of research and development is done in the lab. This is true for all three finalists.
|“We are resigned to the selection process being delayed at the very least. The circumstances are difficult and we don't know what it will be like when the current difficulty is over,” Clements said.|
“In terms of proving the technology works, which is one thing all three missions need to do, we have to demonstrate a certain technological readiness level,” Clements said. “We need to do lab work for that, and all the labs, such as at University of Cambridge where some of the detector development work is being done, are shut right now. So, ESA may have to delay the deadline, because people haven’t been able to get into their labs for however many months. Or else they will have to allow a lower TRL [technology readiness level], which they are not likely to do, because that brings risks with it. This will probably all be decided at the next Science Programme Committee meeting, which I presume they will be having over Zoom or something similar.”
SPICA was first imagined in 1997 by JAXA scientists, and has adopted scientists from the Herschel and Planck missions along the way who see it as the next logical step in infrared observation. If it is selected next year, astronomers will be able to pull back the veil of the sky like never before. They will be able to see through the dust-obscured universe and into the beginnings of the first galaxies, observing formation and dissolution activity at every possible life-stage for the first time. They will be able to spectroscopically analyze heavy metal distribution, and see where all the water comes from. And SPICA won’t just be able to illuminate far-off locales; it will also be able to shed light on our own galaxy and solar system.
The scientific community has been working for more than 20 years to bring this project to fruition, and now that they are closer to launch than they’ve ever been, COVID-19 happens. Clements remarked on the general attitude of the team members in this unprecedented situation: “We are resigned to the selection process being delayed at the very least. The circumstances are difficult and we don't know what it will be like when the current difficulty is over. This all costs money, and governments may have less of that to spend on space missions by then. All we can do is wait and see.”
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