Great Observatories: Past, Present, and FutureJ. Tumlinson (tumlinson[at]stsci.edu)
NASA developed the concept of the "Great Observatories"—a multi-wavelength fleet of space telescopes operating simultaneously—throughout the 1970s and 1980s. Hubble was the first to launch, followed by Compton, Chandra, and Spitzer over the next 13 years. Compton was de-orbited in the year 2000, and the X‑ray/UV/optical/infrared set of Hubble, Chandra, and Spitzer operated together from 2003 until Spitzer's decommissioning in 2020, while the first two continue on today (Fig. 1).
With the eventual retirement of the last two Great Observatories in the foreseeable future, NASA's Cosmic Origins Program Analysis Committee (COPAG) began a study to assess their joint scientific accomplishments and the key questions that might motivate such multi-wavelength coverage over the next two decades. The COPAG-chartered "Science Analysis Group 10," co-chaired by Lee Armus (IPAC) and Tom Megeath (Toledo), published a comprehensive report in 2019 that surveyed the scientific impact of the original Great Observatories on our understanding of galaxies, stars, planets, and fundamental physics. Some of the highlights are identified are in Figure 2, drawn from the SAG-10 report.
After surveying the science of the Great Observatories, the report concludes:
"The scientific legacy of the Great Observatories has demonstrated the importance of sensitive, panchromatic observations for progress in astrophysics, as well as the ability of NASA and its partners to provide concurrent and sustained access to a large part of the electromagnetic spectrum from space. The Great Observatories became a deliberate NASA agency program that transcended individual missions and wavelength regimes."
With the first generation of Great Observatories now fading away, we can ask whether and when a new generation will come to pass. In 2020, we look forward to the joint operation of the James Webb Space Telescope (from 2021) and the Nancy Grace Roman Space Telescope (from 2025), together covering the 1–35 μm region of the infrared spectrum. In preparation for the Astro2020 decadal survey, NASA and the community studied four flagship mission concepts from the X‑ray to the far infrared that could serve as the basis of a new generation of Great Observatories. The Lynx telescope would observe the hot and high-energy Universe with 100 times Chandra's X‑ray sensitivity at high spatial and spectral resolution. The Origins Space Telescope would operate at 4.5K and explore the cold and dusty Universe from 3 to 600 μm. To cover the UVOIR, NASA commissioned two studies combining exoplanet direct imaging with frontier general astrophysics: the Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/Infrared Surveyor (LUVOIR) explore the wide range of architectures from 2.4 to 15 meters in aperture, with the expectation that the next UVOIR flagship will settle somewhere in this range.
By virtue of their ambitious multi-wavelength science goals and great leaps in capability, these mission concepts could form the basis of a second generation of Great Observatories. But can they operate simultaneously? They might, starting in the late 2030s or early 2040s, if there is sufficiently strong will on the part of the community, expressed through and building on its Decadal Survey, and on the part of NASA and Congress to support the budgets and schedules that would be necessary to make it happen.
Following this advice, and to serve as a resource for the community who are interested in this idea, the www.greatobservatories.org website has been created by members of the flagship Science and Technology Definition Teams (STDTs) to serve as a single-source stop for the four STDT reports, science highlights from the SAG‑10 report, budget information, and other useful items. In the words of the SAG‑10 report, the first Great Observatories "point the way to a future where … our understanding of the Universe continues through the development of the next generation of space observatories that will inspire further giant leaps in astrophysics in the coming decades." As NASA's Astrophysics Director Paul Hertz put it to the Astro2020 Steering Committee: "Carpe Posterum!" (Seize the Future!)