Every decade the United States astronomy community comes together to debate the future of their scientific field. With limited resources, both in research funding and time, astronomers decide what questions they are most interested in pursuing and recommend telescopes that can help them answer those questions. It’s a process by astronomers, for astronomers, but their decisions are primarily funded through federal dollars and lead to scientific discoveries and images, which make their way into public discourse. The Hubble Space Telescope and its breathtaking views of the universe exist in part because of this practice, called the Decadal Survey on Astronomy and Astrophysics. The National Academy of Sciences is due to release their updated Decadal Survey later this year, and the astronomy community anxiously awaits to see the latest recommendations from a 50-plus year process that has led to some of the most groundbreaking and iconic projects in modern astronomical history.

It takes more than one type of telescope to observe all the light and information streaming towards Earth from the near and distant universe. This means that different astronomical questions rely on different types of telescopes to answer them. Want to study planets orbiting other stars? You’ll get the best data flying a telescope on a plane in the high Earth atmosphere or, better yet, launching a telescope into space. Want to study supernovae? The high energy gamma rays created by a supernova explosion can only be studied directly with a space telescope that uses special detectors. How about neutrinos? These elusive particles that can help astronomers discover the sources of cosmic rays are best studied with observatories built deep beneath Earth’s surface. Each area of research requires building very different observatories so astronomers must choose where they want to focus their efforts.

Through the process of prioritizing astronomical questions and instruments, the Decadal Survey serves as a recommendation to Congress on where federal money should be allocated to NASA for astronomical research. Large observatories, whether they are located underground, built on a tall mountain, flown through the atmosphere, or launched into space, are expensive endeavors and take years of development and construction. Because of these constraints, astronomers often have to decide if they prefer several small and mid-range telescopes or one or two extremely large telescopes. In addition to prioritizing future research topics and the required instruments of those projects, astronomers also consider the current state of previously constructed observatories.

The first three decadal surveys prioritized a suite of space-based telescopes that covered the universe from the infrared to gamma rays. These four space telescopes became known as NASA’s Great Observatories and have been the bedrock for astronomical research in space for the last 30 years. The 1970 report prioritized what became the Hubble Space Telescope and a series of high energy astrophysical observatories, which led to the development of the Compton Gamma Ray Observatory. The 1980 report prioritized the Chandra X-ray Observatory and, in the 1990s, the Spitzer Space Telescope and infrared astronomy received the highest ranking. 

In the 2000 and 2010 Decadal Surveys, astronomers focused on continued pressing questions in astronomy, most notably: How did galaxies evolve from birth? How do planetary systems form and evolve? What is dark energy? Where are the potentially habitable planets? Two of the space telescopes prioritized were the Next Generation Space Telescope, now called the James Webb Space Telescope (JWST), and the Wide-Field Infrared Telescope, now called the Nancy Grace Roman Space Telescope. The 2000 and 2010 Decadal Surveys also established a stronger collaboration with international partners in effort to coordinate large-scale projects that are too costly for the U.S. to carry out on its own, a crucial element for JWST, whose budget has increased significantly since its start. After several delays, JWST is currently scheduled to launch in December 2021, after which astronomers hope to fulfill the Decadal Survey’s expectation and make discoveries that will rival those of the Hubble Space Telescope when it was first launched. The Roman Space Telescope is scheduled to be launched in 2025.

Astronomers proposed four space telescopes for consideration in the 2020s Decadal Survey:

• The Lynx X-ray Observatory.  Lynx would be a significant upgrade to the Chandra X-ray Observatory and would reveal finer detail of the violent processes happening around black holes, the explosive nature of supernova, and other high-energy objects and events in the universe.
• The Habitable Exoplanet Observatory (HabEx). HabEx would take direct images of planets around Sun-like stars and could reveal more about potentially habitable worlds around other stars.
• The Large Ultraviolet Optical Infrared Surveyor (LUVOIR). LUVOIR is a scaled-up version of the Hubble Space Telescope and would transform our view of exoplanets, stars, and galaxies.
• The Origins Space Telescope. This telescope would study the infrared universe and expand our understanding of the origin of solar systems, stars, and galaxies.

Each telescope has its strengths, but together they have the potential to form what astronomer Grant Tremblay calls the New Great Observatories. Chances are low that the Decadal Survey will recommend pursuing all four telescopes together, but each poses a unique view of the many mysteries of the universe.

The Decadal Survey on Astronomy and Astrophysics holds immense power in directing future research and discovery. These recommendations to NASA have resulted in budget requests and approvals that have led to huge advances in astronomical research and instrumentation. After the 2020 Decadal Survey is released and prioritizations are made, the next stage of development can begin, where one or more of these telescopes is launched into space in the coming decades.