Any award for the most productive observatory in history would certainly go to the Hubble Space Telescope. But the Hubble's days are numbered—its instruments and orbit continue to degrade—and its inevitable demise will result in a significant data-collection gap for astrophysics and cosmology. Because Earth's atmosphere filters out most ultraviolet wavelengths, they are accessible only from space, where Hubble lives. Neither of NASA's next-generation observatories—the 6.5-meter James Webb Space Telescope and a 2.4-meter repurposed infrared spy satellite called WFIRST—will fill these wavelength gaps. “When Hubble goes, it goes,” says John Mather, a Nobel laureate astrophysicist at the NASA Goddard Space Flight Center. “And we don't have anything else on the books that does what it does.”

Mather and other astronomers are proposing a supersize successor with a mirror 10 to 12 meters in diameter—four to five times larger than Hubble's. That would be big enough to fulfill several high-priority items on astronomers' wish lists, revolutionizing studies of faraway galaxies, observations of planets in the outer solar system and searches for life on Earth-like exoplanets. Provisionally called the High-Definition Space Telescope, or HDST, the proposed telescope would observe, as Hubble does, at optical, ultraviolet and near-infrared wavelengths. Befitting its high-definition moniker, HDST's mirror could resolve structures about 300 light-years across in galaxies on the opposite side of the visible universe—something useful for understanding star formation, as well as the nature of dark matter and dark energy. And it would allow astronomers to closely examine dozens of potentially Earth-like exoplanets for signs of alien life. The plan appears in a summer report from the Association of Universities for Research in Astronomy.

Some researchers involved with HDST worry, however, that no matter how broadly appealing such a powerful instrument might be, any proposal for a supersize space telescope is destined to be a nonstarter: although giant observatories are astronomically useful for researchers, they also tend to be deemed astronomically expensive, especially lately. “NASA's gotten more conservative since we started the Webb,” says Mather, the Webb's senior project scientist. The Webb was originally targeted for a 2011 launch and an estimated cost of $1.6 billion, but current estimates aim for a launch no earlier than October 2018, with a cost that has swelled to nearly $9 billion. “After the telescope was nearly killed because of cost overruns,” Mather says, “no one wants to think big anymore.”

No astronomer involved with the HDST report will publicly hazard a guess at the required budget for a telescope of this magnitude—only that it would be quite large. Skeptical of the financial feasibility of HDST, critics suggest that a somewhat smaller, Webb-size broadband telescope would better serve the community. Others say a new generation of ground-based 30-meter-class observatories now under construction could do much of the same science for a fraction of the cost.

But those approaches are unlikely to deliver the answers space scientists are looking for, points out Marc Postman, an astronomer and HDST report co-author at the Space Telescope Science Institute. Trapped below Earth's ocean of air, even the largest ground-based observatories will be stymied by starlight-warping turbulence and by airglow, faint light emitted by atmospheric chemical reactions that can corrupt delicate observations. Further, neither they nor the Webb can directly image and investigate large numbers of exoplanets, which decreases the odds of finding any that support life. For some questions, only a large, broadband space telescope offers hope of answers.

The dream telescope could head for the skies as soon as the early 2030s, the report authors say, but only if NASA and other space agencies begin planning for it now. Such a long incubation for HDST may seem excessive but is actually an improvement over Hubble's, which began in 1946 with a visionary report from astronomer Lyman Spitzer. Transformative astrophysics leaps such as those that Hubble provided, and that its eventual successor also could offer, will require big investments not only of money but of time, Postman explains. “You don't make revolutionary changes in our understanding of the cosmos by taking small, incremental steps.”