The future of space telescopes beyond JWST
by Jeff Foust
|“If we can start a large mission, that large mission is WFIRST,” Hertz said, cautioning there was no guarantee “the people who hold the gateways to our budget” would be willing to do so.|
The problem with WFIRST is not the science, but the funding. NASA’s budgets, and the demands on it, particularly from JWST, have made it difficult for NASA to implement WFIRST and other recommendations from that decadal report. “It is not possible for us to implement the decadal survey as written,” said Paul Hertz, head of NASA’s astrophysics division, in a presentation to a joint meeting of three program analysis groups (PAGs) that provide advice to the agency last month in Long Beach, California. NASA’s astrophysics budget is going up overall, he said, but “the requirement to complete successfully JWST have increased substantially” beyond what was planned when the decadal report was written.
Spending on JWST will ramp down starting in fiscal year 2017, as the telescope nears completion. That opens up a funding “wedge” starting that year for a new astrophysics project. Hertz said that NASA is conducting a number of studies of mission concepts scheduled to be completed by 2015, in time to support a decision on what mission NASA should include in its FY2017 budget request that will be released in early 2016.
Hertz indicated that WFIRST remained the agency’s top choice to be that next mission, but warned that the money—or the willingness to embark on another flagship-scale mission so soon after JWST—may not be there. “If we can start a large mission, that large mission is WFIRST,” Hertz said at the PAG meeting. “We cannot assume that, when we get to 2015, we will get signals from the people who hold the gateways to our budget that they are comfortable with us starting another large mission so quickly after JWST.”
NASA has studied two concepts, or design reference missions (DRMs), for WFIRST. One concept, based on the design in the decadal survey report and designated DRM1, features a 1.3-meter telescope with imaging and spectroscopic instruments, launched to the Earth-Sun L2 point on an Atlas V for a five-year mission. That mission has an estimated cost of about $1.6 billion. An alternative concept, called DRM2, uses a smaller 1.1-meter telescope but more advanced detectors, and could be launched to L2 on a Falcon 9 for a three-year mission. Its estimated cost is $1.1 billion.
There is a third option for WFIRST, though. Last June, NASA announced it had been given, at no cost to the space agency, a pair of 2.4-meter optical systems from the National Reconnaissance Office (NRO). Shortly thereafter, NASA established a new science definition team to examine using one of those NRO telescopes for WFIRST. The WFIRST Astrophysics Focused Telescope Assets (AFTA) team is expected to complete their study of the feasibility and cost of this approach by the end of April.
Using the NRO telescope offers a number of scientific advantages, scientists said during a meeting about WFIRST during the American Astronomical Society (AAS) meeting last month in Long Beach. In addition to the larger mirror, the telescope has a point spread function—a measure of how sharp its images are—up to 2.2 times better than the current two WFIRST DRMs. “I think you’re looking at improved dark energy and microlensing performance,” said David Spergel, a Princeton University astronomer who is co-chairing the WFIRST AFTA study. It would also be very complementary to ESA’s planned Euclid mission, which will also study dark energy at infrared wavelengths; Euclid, he said, would make a wider survey but not as deep as the AFTA version of WFIRST.
Ironically, while Spergel said this version of WFIRST might provide more science for the money than the two other DRMs for the mission, it might not be cheaper. He said having the telescope optics already available would make the mission faster to develop, but not necessary less expensive. “Well, this is a big telescope,” he said, “so it may not be cheaper.” He offered no estimate of how much more expensive the AFTA WFIRST might be to either DRM; an independent cost estimate is slated to be completed at the same time the science team concludes their work in April.
Regardless of the decision on whether or not to use an NRO telescope for WFIRST, there will still be at least one Hubble-class optics system available for another mission. The gifting of these systems to NASA has triggered a wave of interest in the astronomical community on how to use these “free” systems for a wide range of missions, even though there’s no indication of budgets or schedules for flying them.
|“Well, this is a big telescope,” Spergel said of using an NRO telescope for WFIRST, “so it may not be cheaper.”|
In late November, NASA issued a request for information (RFI) for an effort it calls the Study on Applications of Large Space Optics, or SALSO. “The goal of the overall study is to gather and assess concepts for possible utilization of the recently acquired systems for Agency goals aligned with 5 principal areas; space technology, human exploration and operations, heliophysics, planetary science, and astrophysics,” the RFI stated. (It specifically excluded WFIRST, because of the separate ongoing AFTA study.)
“The administrator has said that we’re doing these near-term rapid studies to determine whether or not the agency wants to continue studying and considering keeping them for a future mission,” Hertz said of the NRO telescopes at the PAG meeting last month.
The RFI deadline passed in early January, and NASA selected nearly three dozen concepts to be presented and discussed at a two-day workshop this week in Huntsville. The selected presentations run the gamut from ultraviolet astronomy to studies of and searches for exoplanets to a space situational awareness instrument to track orbital debris.
“Coming out that workshop, a steering committee will identify of order half a dozen interesting, notional ideas to use them for,” Hertz said. NASA would then fund “quick architectural studies” of those concepts that would be completed in time for planning for the fiscal year 2015 budget proposal, to see if further studies of those concepts should be funded.
Hertz cautioned that there is no guarantee that, even if some of the ideas that come out of the SALSO effort are both scientifically interesting and technically feasible, they’ll be funded any time soon. “There is no budget wedge for using these telescopes,” he said. “No money comes along with them, but to see if they help us do more with less, that’s what we’re trying to figure out right now.”
However, even the most innovative uses of the NRO telescopes are unlikely to satisfy astronomers’ desires for bigger telescopes that can collect more light and thus detect fainter objects. While space-based telescopes larger than JWST are, for now, little more than figments in the imaginations of astronomers, some are attempting to lay the technological foundation for such future super space telescopes.
|“Even if this got a green light in 2020,” Postman said of proposed large space telescopes, “you’re still not going to launch this thing until some time in the 2030s.”|
One such effort is called the Optical Testbed and Integration on ISS eXperiment, or OpTIIX (pronounced either “OP-ticks” or “op-TEEKs”.) The program would develop a 1.5-meter telescope using modular components, including a segmented mirror, assembled robotically on the ISS to test technologies and techniques that could be used for future large space telescopes. OpTIIX would not be primarily a scientific observatory, but could be used for some research, as well as educational outreach, once assembled.
While telescopes that may make use of these technologies are many years, perhaps decades into the future, there’s reason now to support efforts like OpTIIX. “If you want to get started on those missions, the next big hurdle is the 2020 decadal, so you want to be able to have some of these technologies ready by then,” said Marc Postman of the Space Telescope Science Institute at a session of the AAS meeting last month. “You also don’t want to lose the opportunity to take advantage of the ISS, which itself is not going to last forever. So it seems like this is a prime decade to make use of that platform.”
Among the technologies that OpTIIX would examine are lightweight “active hybrid” mirrors capable of changing their shape to correct optical errors after launch, yet weigh less than 25 kilograms per square meter. Putting the telescope together on the ISS would test robotic assembly of large telescopes, as well as the ability to service them. Engineers are also looking at ways to add a “decent” camera to the system, said Renaud Goullioud of JPL, perhaps using a detector provided by the Naval Research Laboratory from the recently-canceled Joint Milli-Arcsecond Pathfinder Survey (JMAPS) mission.
Unfortunately for the project’s advocates, OpTIIX is currently on hold. The effort ran out of money at the end of fiscal year 2012, just after successfully completing a preliminary design review. Continuing work on OpTIIX, though the launch of its components on ISS commercial resupply flights, would cost about $125 million and take 40 months.
“In the longer term, it’s a first step towards building some of these large UV/optical telescopes that we think will answer some fundamental astrophysics,” Postman said. But like so many other space projects, patience is advised. “Even if this got a green light in 2020,” Postman said of proposed large space telescopes, “you’re still not going to launch this thing until some time in the 2030s.”