WASHINGTON NASA plans to begin preliminary work this fall on a dark energy-mapping observatory recommended in the National Research Council’s latest 10-year plan for space- and ground-based astronomy, though full-scale development of the new flagship-class mission will have to wait until the agency launches its $5 billion James Webb Space Telescope (JWST), according to officials.

Jon Morse, astrophysics division director at NASA headquarters here, said the Wide-Field Infrared Survey Telescope (WFIRST) proposed in the decadal survey is not likely to launch before 2022, some seven years after NASA hopes to loft JWST to the second Lagrange point — a gravitationally stable spot 1.5 million kilometers from Earth — in 2014.

“The pacing of WFIRST’s development time is JWST, plain and simple. We can’t afford to do them simultaneously,” Morse said during a two-day public meeting of the NASA Advisory Council’s astrophysics subcommittee here Sept. 16-17, adding that the division has seen reduced budgets in recent years and expects only flat funding through 2015. “When you’re trying to execute a new flagship, you are in line behind the previous flagship, that’s the key message here.”

Morse said the Astro2010 decadal survey envisions a WFIRST launch as early as 2020 under the panel’s most optimistic funding scenario. But a launch in 2022 or beyond is more likely, he said, given that JWST is undergoing several programmatic reviews that could reveal additional development costs and potential schedule delays.

Ed Weiler, NASA associate administrator for science, said the JWST reviews are expected to wrap up next month and that a final decision on the program’s future should come by November’s end. Weiler declined to speculate on cost-growth specifics, but said, “It’s going to cost more. That’s about all I can say.”

In the meantime, Morse said NASA is proposing to increase a planned investment in Euclid, a dark energy mapper designed by the European Space Agency (ESA). Morse said Euclid is “similar to WFIRST and could potentially be well aligned” with the highest science priorities laid out in the Astro2010 decadal survey.

“We’re trying to prudently plan for the future such that the U.S. would have a leading role in a dark energy program that would get us to data this decade,” Morse said, adding that Euclid’s projected launch near the end of this decade could provide the U.S. astronomy community with access to dark energy data sooner than WFIRST.

Euclid is one of three so-called M-class missions vying for funding under the ESA’s Cosmic Vision program for separate launch opportunities in 2017 and 2018. A decision is expected in mid-2011 on which two of three finalists proceed toward launch. Morse said Euclid — which is expected to cost roughly 500 million euros ($650 million) — shares a number of scientific goals with WFIRST. He views a proposed 33 percent investment in Euclid as a near-term down payment on the decadal survey’s top science objective.

Released Aug. 13, the Astro2010 decadal survey — formally titled “New Worlds, New Horizons in Astronomy and Astrophysics” — designated WFIRST as the top priority for large missions for the decade ahead. The survey envisions WFIRST being developed by NASA in partnership with the U.S. Department of Energy and ESA at an estimated cost of $1.6 billion to study dark energy, hunt for Earth-like planets and advance scientific understanding of the nature and evolution of galaxies.

Morse said ESA invited NASA earlier this year to make a 20 percent contribution to Euclid. But he said increasing the investment to 33 percent would afford greater scientific opportunities. Under the proposed arrangement, NASA would select four  U.S. scientists to join Euclid’s 12-member science team and ask Europe to consider a reciprocal investment in WFIRST if Euclid is selected, including commensurate representation on the science definition team.

“So we’re looking at a two-mission program to implement Astro2010’s recommendations and prioritizing these science goals of WFIRST by partnering with [Europe] on Euclid and WFIRST,” Morse said.

During the two-day meeting, Weiler and Morse fielded probing questions from NASA Advisory Council members concerned there is too much overlap between the two dark energy missions.

Astro2010 chairman Roger Blandford, who also spoke during the session, said the report recommended a combined mission only if it fully supports all of the WFIRST science goals and leads to cost savings with the U.S. playing a leading role.

Morse said NASA proposed a combined mission that could potentially merge Euclid and WFIRST during a bilateral meeting with ESA in early September.

But Fabio Favata, head of science planning and community coordination for ESA, said altering Euclid’s scale or scope would derail the competitive process already under way.

“At this stage this would be very difficult,” he said during a Sept. 17 conference call with the NASA advisory body. “This would effectively mean to start from scratch a design phase now.”

Morse assured the subcommittee that NASA’s increased investment in Euclid would not pose any threat to WFIRST’s development timeline, and that NASA intends to move “full-speed ahead” on the flagship observatory, starting with assembling a science definition team this fall.

“WFIRST is not in line behind Euclid,” he said. “We’re going forward with WFIRST, and Euclid is sort of a down payment on that science, but it’s expected under the current conditions to come years before WFIRST would launch.”