As we grow more ambitious in our desires to see further and more precisely in space, the need for larger and larger telescope mirrors becomes inevitable. Only with collection of significantly more photons by a super large mirror can the the quality of the “seeing” significantly improve.
The largest mirror in space now is the Hubble Space Telescope at 2.4 meters (7.9 feet) and that will be overtaken by the long-delayed James Webb Space Telescope (JWST) at 6.5 meters (21.3 feet) when it launches (now scheduled for late 2021.) But already astronomers and space scientists are pressing for larger mirrors to accomplish what the space telescopes of today cannot do.
This is evident in the National Academies of Sciences Decadal Survey underway which features four candidate Flagship-class observatories for the 2030s. Three proposals call for telescope mirrors that are significantly larger than the Hubble’s, and the most ambitious by far is LUVOIR which has been proposed at 15.1 meters (or 50 feet) or at 8 meters (about 30 feet), or maybe something in between. A primary goal of LUVOIR, and the reason for the large size of its mirrors, is that it will be looking for signs of biology on distant exoplanets — an extremely ambitious and challenging goal.
The LUVOIR team would have argued for an even larger telescope mirror except that 15.1 meters is the maximum folded size that would fit into the storage space available on the super heavy lift rockets expected to be ready by the 2030s.
This desire for larger and larger space telescopes has rekindled dormant but long-present interest in having an alternative to sending multi-billion dollar payloads into space via one launch only. The alternative is “in-space assembly,” and NASA has shown increased interest in pushing the idea and technology forward.
Nick Siegler, Chief Technologist of NASA’s Exoplanet Exploration Program at the Jet Propulsion Lab, and others proposed a study of robotic in-space assembly in 2018. The idea was accepted by the NASA Director for Astrophysics Paul Hertz and Siegler said the results are promising.
“For space telescopes larger than LUVOIR, in-space assembly will probably be a necessity because it’s unlikely that heavy-lift rockets will be getting any bigger than what’s being built now,” Siegler said. “And we have enough evidence to suggest there are benefits to assembling in space with smaller telescopes, too. The final conclusion of the study is that even with space telescopes in the range of mirrors of 5-to-7 meters and then above, planners should start thinking of robotic assembling in space to reduce a significant number of risks and maybe cost.”
NASA leadership was impressed by the report and honored team members with a NASA Group Achievement Award for “outstanding leadership of the in-Space Assembled Telescope study providing potentially transformative cost and risk reduction for future large space telescopes.”
My introduction to in-space assembly came from former astronaut and former associate administrator of NASA’s Science Directorate John Grunsfeld. He flew successfully to the Hubble Space Telescope three times for needed repairs and upgrades, and is an advocate for that kind of use of the astronaut corps.
When scientists and engineers were first contemplating the LUVOIR concept, Grunsfeld proposed that it be at least 16 meters in diameter and that it be assembly in space by astronauts. Different components of the observatory would be launched at different times and then the astronauts would fit the pieces together. It was, he told me, as great way to train astronauts for the kind of work they will have to do in the future on Mars and in deep space, and it was perhaps the only way to get a really big telescope into space.
The effort to understand possibilities and challenges associated with in-space assembly led to a formal NASA study in 2018 headed by Siegler, Harley Thronson of the Goddard Space Flight Center and Rudranarayan Mukherjee of NASA’s Jet Propulsion Lab. The report released in 2018 concluded that while in space assembly has a bright future, the Grunsfeld idea of using astronauts to do the assembling — rather than robots — is not considered the way to go now.
The group did conclude, however, that the LUVOIR concept, as well as others, could benefit from in-space assembly rather than fitting everything into one launch on a super heavy-lift rocket that has yet to be completed and flown.
And as part of their effort, they wrote a white paper for the National Academies of Science Astro 2020 Decadal Survey (which will recommend a path for the next NASA Flagship-class mission) describing how and why in-space assembly with multiple launches on smaller rockets may be a good alternative for several of the concepts the Survey panel is reviewing. The in-space assembly white paper had also been green-lighted by NASA’s Hertz.
Siegler said that deciding how the space telescope would be placed into space is not a formal part of the Decadal Survey mission. But it could be and certainly would come up after the Decadal Survey recommendations are sent to NASA and the agency makes its own decisions, which traditionally do follow what the Decadal panel concludes. Siegler said that nobody at NASA has indicated they are inclined to use in-space assembly for the next Flagship-class mission, but both the team and NASA were pleased that agency now has a study detailing what in-space assembly offers.”
A NASA video about in-space assembly of telescopes.
Putting together complex structures in space is not a new idea, The International Space Station, after all, needed more than 40 assembly flights — most of them using the space shuttle — to bring up the parts, and then hours and hours of space-walking astronaut and robotic work to assemble them. But the space station is in low-Earth orbit and space telescopes need to operate in deeper space. No deep-space telescope has ever been delivered in parts and assembled in space.
But the major advance in the capabilities of space robotics seen the past two decades, as well as the declining cost of launches based on the introduction of so many commercial alternatives, has made in-space assembly look more attractive, said Siegler and Mukherjee.
They pointed in particular to the extensive robotic work done in assembling and then docking at the ISS, and also in the success of three robotic rovers on Mars, with a fourth on the way. And then there’s the possibly that the “Gateway” platform project, which is part of NASA’s Artemis program to place astronauts back on the lunar surface, will employ and improve robotic in-space assembly.
The difficulties involved with schedule delays and major cost overruns associated with the JWST, and especially its deployment process after a heavy-duty, single-rocket launch, have no doubt played a role as well in making some observatory planners more receptive to the in-space assembly idea.
Then there is the question of servicing — which has kept the Hubble operating and improving over three decades. While Congress has made serviceability a requirement for future space telescopes when possible, none of the four missions before the Decadal Survey have concrete plans for servicing in space.
“This is because there are no robotic servicers or astronaut missions currently being planned or developed for the deep space environment where these observatories are planned to be operated,” Mukherjee wrote in an email. ”
“Hence, adding the cost of servicing observatories into the mix led the (in-space assembly or iSAT) team to find that in-space assembly may provide cost savings for all large observatories (4-5-meter class or larger diameter of primary mirror) that we studied. This is because we anticipate that the robots that would assemble the observatory from modules would stay with the observatory and be able to service it as needed without needing astronauts or separate robotic servicers.”
Mukherjee, who is a Research Technologist at the Robotics and Mobility Systems section at JPL, has continued working on in-space assembly of large space structures as well as servicing capabilities since the study was completed.
“There was broad consensus among our 70 odd study participants that the (in-space assembly) approach may provide opportunities for cost savings and may also enable the risks or challenges to be better managed than the traditional approaches. This was based on comparing quantitative modeling of the costs of traditional and in-space assembly approaches for four different observatory sizes — observatories with 5-meter, 10-meter, 15-meter, and 20-meter primary mirror diameters.
He said the conclusion was also based on evaluations from experts who had worked on missions such as JWST, the Hubble, the ISS, the space shuttle, the Mars Rover missions,and additional upcoming missions
While in-space robotic assembly would not have the drama of astronauts present, Siegler said there would be plenty of excitement and drama associated with an observatory or the Gateway being put together in deep space.
“There would doubtless be a series of harrowing episodes during assembly and cameras are cheap so they’d be all over the place to capture what’s happening. Everything will be moving at several thousand miles per hour, and the public will be part of the whole experience.”
“I’m optimistic about the future of this approach,” Siegler said.