America's Great Observatories - the Hubble, Chandra, Compton and Spitzer space telescopes - have peered into the unknown and made breakthrough discoveries about newborn stars, dark matter and the age of the universe itself.
But these telescopes, whose era began in 1990, are aging, if not already dead, and there is no budget or political will to replace them. This sobering reality was underscored this month when two were beset by technical problems, including the Hubble Space Telescope, that temporarily halted their science.
Shrinking budgets and delayed projects means astronomers will lose some of their key eyes in the skies before NASA can launch new telescopes. It will make some research impossible.
"The unwillingness to invest in substantial science has begun to worry us," said astrophysicist Matt Mountain, president of the Association of Universities for Research in Astronomy, which operates the Hubble telescope on behalf of NASA. "We're facing a very daunting prospect as a community," Mountain said. "Some fields just won't have a telescope. And the science will not be possible to do in any other way."
Some of science's biggest questions - What is dark energy? Does life exist beyond the solar system? - can only be answered by large observatories working in particular parts of the light spectrum.
Whether to invest in pursuing these questions "is a choice for the nation," said Paul Hertz, the astrophysics division director at NASA.
"What missions we do will be influenced by priorities of the community as well as the funding choices made by the political system."
The system prioritized the Great Observatories program when it was conceived in the 1970s and 80s - four telescope missions to cover the entire light spectrum in space. They launched between 1990 and 2003. There was the Compton Gamma Ray Observatory for capturing the most energetic explosions in the universe. The Spitzer Space Telescope to seek out infrared radiation from exoplanets and newborn stars. The Chandra X-ray Observatory could probe the depths of black holes and uncovered evidence for dark matter and dark energy.
The Hubble Space Telescope was the program's crown jewel, whose massive dish for collecting light in the ultraviolet and visible wavelengths helped determine the age of the universe, revealed black holes at the centers of galaxies, and photographed the most distant objects ever seen.
Space telescopes are difficult to engineer and expensive to build. But they are necessary to get a clear glimpse into the cosmos. Even at night, Earth's atmosphere distorts light from space, making images blurry and other signals - particularly gamma rays - impossible to see.
Using the fleet in concert enhances these telescopes' power even further. Last year, observations by Hubble, Chandra, Fermi and dozens of other telescopes allowed astronomers to confirm theories about fundamental physics when they observed two dead stars colliding.
They may not always have that ability.
The Compton telescope was lost in 2001, when a problem with its gyroscope - which allows a telescope to rotate and point at something - meant the space agency had to intentionally ground it or risk the spacecraft plummeting uncontrolled out of the sky.
Spitzer, which has been slowly drifting away from Earth, will end its mission when it loses contact sometime next year. That loss has been expected, but the difficulties with Hubble and Chandra this month were an unanticipated one-two punch.
Neither of the spacecraft's problems are fatal, NASA said. Chandra came back online just days after a glitch with one of its gyroscopes forced the telescope into safe mode. Hubble, which was also hobbled by another gyroscope difficulty, is expected to return to normal operations within a few weeks at the most. NASA expects both telescopes will continue functioning into the 2020s.
Still, the brief brush with oblivion gave astronomers a lasting scare.
"People suddenly realized that Hubble is not going to live forever," said Tom Brown, the Hubble mission head at the Space Telescope Science Institute. Chandra is 19 years old; Hubble is 28.
Brown said frantic fellow astronomers have kept his phone ringing "non-stop" in the past two weeks. "They're thinking about what happens next."
NASA's billion-dollar-a-year astrophysics program currently flies eight major telescopes aimed at studying space beyond the solar system. Of these, all but one are in their "extended missions" - the bonus years beyond the time for which the spacecraft was originally designed.
The only flagship NASA space observatory under construction is the James Webb Space Telescope, whose gold-plated dish is designed to collect infrared radiation from the earliest objects in the universe.
NASA intended for the timing of Webb's mission to overlap significantly with Hubble's. But the launch of the $10 billion behemoth has been repeatedly delayed as the space agency deals with design issues and costly human errors. This year NASA announced that Webb won't launch until 2021 at the earliest, cutting the concerted observing time short.
NASA's next big project would be the Wide Field Infrared Survey Telescope, which is also targeted toward infrared wavelengths and in 2010 was considered the top astrophysics priority of the National Academy of Sciences. But funding for the observatory has been up in the air after President Trump omitted the telescope from his last two budget requests.
The outlook for research across other parts of the light spectrum are fuzzy at best. When Hubble fails, Brown pointed out, there will be no visible or ultraviolet telescopes at that scale. Likewise, NASA has no large X-ray observatories ready to replace Chandra. Compton was succeeded by the smaller Fermi telescope, which has two instruments to Compton's four. Fermi is already 10 years old and exceeded its original mission by five years.
Gamma ray astrophysicist Julie McInery, the project scientist for Fermi, worries that even a temporary gap in telescope coverage could have lasting repercussions in her field.
"You have to have a minimal level of activity in any given telescope area to maintain expertise in the community so you can continue to build instruments," she said.
To prevent such gaps, NASA would have to start developing new missions now; most space telescopes require several decades from conception to launch.
Other nation's space agencies are already working on such programs. The European Space Agency is developing the ATHENA X-ray Observatory for launch in the 2030s. China announced in 2016 it would build its own optical telescope with a field of view 300 times greater than that of Hubble.
"Their budgets are increasing," Mountain said.
In the U.S., NASA's budget hasn't kept up with inflation, and the fraction of federal spending that goes to the agency has been cut in almost half since 1980. The agency has been scrutinized over the Webb telescope's dramatic delays and cost overruns. "The telescope that ate astronomy," it's been bitterly nicknamed.
This spring, NASA announced that it would "narrow the scope" (reduce the budget) of four proposals it is considering for launch in the 2030s.
"We need to ensure we can accomplish breakthrough science while adhering to a realistic, executable scope and budget for the next decade," Hertz, the astrophysics division director, said earlier this year.
But big budgets are necessary to do to "textbook-rewriting science," Mountain said."It's going to be hard to replicate the capabilities of the Great Observatories without expending the equivalent kind of resources." Hubble's cumulative cost has been somewhere around $10 billion. But look what it bought: Nobel Prize-winning research on the accelerating expansion of the universe, among other ground-breaking work.
Hertz acknowledged that building successors to all four Great Observatories would require a much greater investment from the federal government. "There are some science questions that require extremely ambitious observatories and can't be done cheaply," he said.
Detecting life outside our solar system requires a telescope like Webb. Explaining dark energy and why the universe's expansion is speeding up necessitates a huge, sensitive instrument like the Wide Field Infrared Survey.
Hertz said that NASA is trying to make the most of limited resources. During the final servicing mission to Hubble in 2009, astronauts equipped the spacecraft with multiple backups of most essential hardware, adding decades to its life. At 19, Chandra has exceeded the duration of its original mission by more than a decade; this month's gyroscope glitch was the first such problem it has experienced.
Modest "small and middle explorers" - which cost less than $120 million and $200 million, respectively - are constantly in the development. The Transiting Exoplanet Survey Satellite, a "MidEx" project designed to seek out unknown worlds around nearby stars, began science operations this summer.
Even without a gamma ray telescope on the horizon, McInery said she was optimistic.
"Yes, we're moving away from a golden age because this suite of great observatories has gone away," she said. "But there's new things coming to take its place. We're standing on the threshold of an entirely new era."
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