On Hubble’s 25th, looking at the next 25 years
by Jeff Foust
|While Hubble dominated the last 25 years of space astronomy, no single observatory is likely to have the same dominance in the next 25.|
Those celebrations are, of course, well deserved. Hubble was not the first, or the only, space telescope, but it has become far and away the most famous. Launched with flawed optics that made it the butt of jokes a quarter century ago, a 1993 servicing mission restored its vision and turned it into an engine of astronomical discovery from our own solar system to the distant universe, including work that helped win a Nobel Prize in Physics. It became so popular that there was a public outcry when NASA announced in 2004 it would cancel a final planned shuttle servicing mission, a backlash that played a role in NASA’s later decision to restore that mission.
Yet, Hubble’s life is finite. With the shuttles retired, there is no capability left to service the space telescope, and eventually—no sooner than 2020, astronomers hope—it will fail, either from the gradual demise of its instruments or key subsystems, or a catastrophic failure of some kind. And while Hubble dominated the last 25 years of space astronomy, no single observatory is likely to have the same dominance in the next 25.
The telescope usually billed as the successor to Hubble is NASA’s James Webb Space Telescope (JWST). Scheduled for launch in the fall of 2018 on an Ariane 5 provided by Europe in exchange for a share of time on the telescope, JWST—or simply “Webb”—will place a telescope with a 6.5-meter mirror at the Earth-Sun L2 Lagrange point, 1.5 million kilometers from the Earth. With that large mirror and instruments tuned to infrared wavelengths, JWST promises to peer farther back into the early universe and scrutinize potentially habitable exoplanets, among other uses.
But JWST has become best known not for its scientific potential but its management problems. The telescope has suffered years of delays and billions of dollars of overruns, raising doubts by some in Congress of NASA’s ability to carry out the mission at all. For the last few years, NASA has been on the defensive about JWST.
Now, the agency and the telescope’s prime contractor, Northrop Grumman, say the worst of those problems are behind it and that JWST is back on track. “2014 was a very successful year for the program,” said Eric Smith, NASA program manager for JWST, during a “town hall” session about the space telescope at a meeting of the American Astronomical Society (AAS) in Seattle earlier this year.
During 2015, he said, major elements of the telescope will start coming together. “2015 is really the year of the telescope,” he said, as key elements are assembled and tested. “We are staying within the replanned budget and are on schedule for an October 2018 launch.”
Observers of the program, including the Government Accountability Office (GAO), have kept a close eye on the amount of schedule margin available to accommodate problems or other delays in the spacecraft’s development. That margin has gradually diminished to nine and three-quarters months, NASA associate administrator for science John Grunsfeld said at a meeting of the National Research Council’s Space Studies Board in Washington April 23.
That decrease in schedule margin, though, has alarmed neither NASA nor Northrop Grumman. “We will be able to launch in October 2018 if all keeps going as it has,” Grunsfeld said.
“I like the schedule that we have laid out,” said Scott Willoughby, Northrop’s JWST program manager, in an interview during the AAS conference. The margin in the schedule includes four months at the very end of integration and testing of the telescope in addition to schedule reserve allocated to various parts of development and testing. “I’m very comfortable with where the schedule is at.”
|“Many of you have heard about the cryocooler. We’re making progress there,” Grunsfeld said last week. “It’s not off our hit list yet, but hopefully it will be this summer.”|
Recent attention has focused on one problematic element of JWST, a cryocooler used for one of the spacecraft’s instruments. Northrop, building that cryocooler under a separate contract from the prime JWST contract, has encountered technical problems that has delayed its completion from early last year into as late as November of this year. A GAO report last year identified the cryocooler as the top schedule risk for the program.
NASA leadership, though, is confident that the problems with it will soon be overcome. “Many of you have heard about the cryocooler. We’re making progress there,” Grunsfeld said last week. “It’s not off our hit list yet, but hopefully it will be this summer.”
The cryocooler also came up at separate House and Senate hearings on NASA’s proposed 2016 budget on April 16. “We are concerned about the technological challenge of the cyrocooler, but we think that’s getting back on track,” NASA administrator Charles Bolden said when asked about the telescope’s status by Sen. Barbara Mikulski (D-MD), the ranking member of the Senate Appropriations Committee. Bolden added he has a “monthly tag-up” with the CEO of Northrop Grumman, Wes Bush, to discuss progress on JWST.
An illustration of the WFIRST-AFTA observatory as currently proposed. NASA hopes that the program will get a “new start” in the fiscal year 2017 budget proposal. (credit: NASA/GSFC)
As NASA works to get JWST ready for launch in three and a half years, it’s also thinking ahead to the next major space telescope. The agency hopes to get the green light to start work on the Wide-Field Infrared Survey Telescope (WFIRST) as a formal project when the administration releases its fiscal year 2017 budget request early next year.
WFIRST, the top-ranked flagship mission in the 2010 astrophysics decadal survey, has evolved since that study. In particular, WFIRST is now almost certain to use a 2.4-meter telescope that the National Reconnaissance Office donated to NASA in 2012. This so-called Astrophysics Focused Telescope Asset (AFTA), one of two provided to NASA by the NRO, is significantly larger than the original telescope proposed for WFIRST; the mission is thus sometimes called WFIRST-AFTA.
Last month, a study team issued a report on the WFIRST-AFTA concept that supported the use of the AFTA telescope for the mission. The use of the larger mirror “enables a truly compelling version” of WFIRST, the report concluded, capable of studies of the origins of the universe, the nature of dark matter and dark energy, and the formation of galaxies.
If WFIRST-AFTA is outfitted with a coronagraph, as some propose, the spacecraft could also be a tool in studies of exoplanets: the coronagraph would block light from a star, allowing astronomers to better observe planets orbiting them. The coronagraph is still considered a technology risk, though, compared to other elements of the spacecraft, and studies have required that it not drive the design of the overall observatory.
|“The science community has no shortage of ways of how they would like to spend the [JWST budget] wedge,” Bolden said.|
As currently envisioned, WFIRST-AFTA would weigh a little more than 4,000 kilograms, plus about a 25 percent margin. Unlike other space telescopes that operate in low Earth orbit or deep space, the spacecraft would fly in an inclined geosynchronous orbit. Operating closer to Earth, the report notes, improves the downlink data rate versus operating at L2, like JWST. That outweighs the better thermal environment at L2, at least at this stage of the analysis. Operating in Earth orbit could also make easier for the spacecraft to be serviced.
The study assumes WFIRST-AFTA would be ready for launch in October 2024, and operate for a minimum of six years (a lifetime that presumably could be significantly extended if the spacecraft is able to be serviced.) The baseline launch vehicle for WFIRST-AFTA is a Delta IV Heavy, although given that vehicle’s uncertain long-term future as United Launch Alliance seeks to retire the Delta IV, WFIRST-AFTA might have to move to another vehicle, like a SpaceX Falcon Heavy or NASA’s own Space Launch System.
The latest study doesn’t indicate what WFIRST-AFTA, in this configuration, would cost yet. “The life cycle cost estimate for the current baseline (with coronagraph) is in process,” the report states. The use of the AFTA telescope offers, at least in theory, a cost savings over earlier designs with a smaller, custom-built telescope; adding a coronagraph, though, could offset at least in part those savings.
WFIRST advocates hope to get a “new start” for the mission in 2017 budget proposal NASA will release in early 2016. Congress has, so far, been favorably inclined to WFIRST, allocating study funds above what NASA has requested. WFIRST, by starting in fiscal year 2017, would be able to make use of the “wedge” of funding in the agency’s budget freed up as JWST development ramps down.
However, at a House hearing earlier this month about the NASA budget, Bolden suggested there was no decision made yet on whether that JWST funding wedge would be available for WFIRST. “There are a number of projects being considered,” he said, naming not just WFIRST-AFTA but also “telescopes on the Moon,” a concept that’s long been of interest to some astronomers but not something NASA has been actively pursuing recently.
“The science community has no shortage of ways of how they would like to spend the wedge,” he said.
Then there’s the question of what comes after WFIRST. At the AAS meeting earlier this year, Paul Hertz, head of NASA’s astrophysics division, said he was soliciting ideas for what could be considered as the top-ranking flagship mission in the 2020 astrophysics survey. Those general concepts include telescopes operating from the far infrared to x-ray wavelengths, including an exoplanet imaging mission (see “Debating the future of exoplanet missions concepts and community”, The Space Review, January 19, 2015).
At the AAS meeting, Hertz said he wants to develop, by the end of this year, a shortlist of three or four mission concepts that would receive funding for studies to flesh out both their scientific rationale and technology requirements. Those studies would then be available to the scientists working on the decadal survey in 2018.
Whatever mission “wins” the competition to become the top-ranked large mission in that 2020 report would likely have to wait several years to get a new start, just as WFIRST, the top choice in 2010, has to wait until at least fiscal year 2017. That would push out the launch until some time in the 2030s, meaning it would be operational when astronomers celebrate the 50th anniversary of Hubble’s launch in 2040.
That seems a long way off, but Hertz said it’s time to start planning now for whatever mission that will be. “It’s not too early,” he said. “We need to identify some missions for after WFIRST in order to start making technology investments.”
And what of Hubble itself? The space telescope is largely as good as new—better, actually, given its corrected optics and improved instruments—25 years after its launch. “Right now the telescope is in fantastic shape,” said Jennifer Wiseman of NASA Goddard at a seminar about Hubble during the AAS meeting.
Ultimately, though, something will go wrong. “It’s very hard to predict when things start to fail,” she said. “Right now, we’re pretty confident that… the telescope will work well into the 2020s, at least.”
|“I’m rooting for way longer than 2020, myself, until we get a UV replacement,” McGrath said of Hubble’s lifetime.|
Kenneth Sembach of the Space Telescope Science Institute said at the seminar that the telescope’s instruments are still in good shape more than five and a half years after the final servicing mission. That means there’s no sign of “infant mortality” issues that would cause them to fail in the near future.
The gyroscopes used to orient the telescope, which have caused problems in the past, “are not now a life-limiting factor,” Sembach said. Five of the six gyros are working today, and he said it’s possible for Hubble to work on a single gyro. “We should easily be able to get into the early 2020s with the complement of gyros we have,” he said.
Scientists have talked about their “2020 vision” for Hubble, keeping the spacecraft in operation at least until 2020 in order to allow it to overlap with JWST. That could include joint operations of the two observatories. “We’re in the process of putting together policies that’s going to ensure that’s possible,” said Jason Kalirai of the Space Telescope Science Institute.
Some astronomers are hopeful that Hubble lasts well beyond 2020 because it will be able to do things that neither JWST nor WFIRST will be capable of. “There is no plan to replace the ultraviolet capability” of Hubble, said Melissa McGrath of NASA Marshall, as those future telescopes are designed to operate in the infrared. “I’m rooting for way longer than 2020, myself, until we get a UV replacement.”
Even when Hubble does meet its end, the science from it won’t end. Wiseman said astronomers will be able to mine a vast data archive from decades of Hubble observations. “For many, many years beyond when Hubble is operating,” she said, “we’ll still be making Hubble discoveries.”