Scientific Method / Science & Exploration

NASA’s Great Observatories after Chandra’s anniversary

Just over ten years ago, the Chandra X-ray Observatory collected its first …

August 19 marked the tenth anniversary of the Chandra X-ray Observatory’s first light, when the space telescope first focused X-ray photons from a cosmic target on its detectors. The target was Cassiopeia A, or Cas A, a previously known X-ray source that had only been visible as a fuzzy blob when imaged by the Uhuru and Einstein X-ray Telescopes. Cas A is thought to be a remnant of a supernova explosion that was visible to the naked eye on Earth back in 1685. It was selected as a test target because its total brightness and the size of the fuzzy blob was well characterized.

Right from the start, Chandra did its job just as expected. With an imaging resolution 30 times better than any previous X-ray telescope, it immediately revealed filamentary clumps in the materials ejected by the dying star. Even more remarkably, the first images taken by Chandra found a bright point source of X-rays at the center of the nebula, marking the first time that a neutron star was ever directly imaged with X-rays.

A thirty-fold improvement in imaging resolution is about the same as what the Hubble Space Telescope (HST) achieved over the best visible and infrared telescopes that had previously existed. Which is appropriate—both Chandra and Hubble are part of NASA’s Great Observatories program, the whole point of which was to revolutionize the observational capabilities across the full range of the electromagnetic spectrum, from far-infrared to gamma-ray. The infrared portion of the task has been handled by the Spitzer Space Telescope (SST) since its launch in 2001; the band from visible to ultraviolet is tasked to Hubble; Chandra does the X-ray observations; and gamma-rays, at the very high end of the spectrum, were handled by the Compton Gamma-ray Observatory (CGO), which operated from 1991 to 2000.

The Great Observatories program actually was, effectively, an afterthought. After the plan to build Hubble was approved, the next in line was supposed to be Chandra. There was a contentious debate over which other telescopes should be built, however, because, back then, it seemed unlikely that Congress would approve multiple space observatories at a time when construction cost overruns in the Hubble program were surfacing.  

But, of course, it was impossible to weigh the merits of one of these telescopes over another, since they covered different portion of the electromagnetic spectrum—their capabilities complement each other, and to get the whole picture, we needed all of them. Thus, a political decision was made to consolidate the plans under a single goal: to cover the whole spectrum. This picked up the name Great Observatories.

Getting the big picture -- three of the Great Observatories worked together in producing this stunning image of Cassiopeia A.  This composite image combines the infrared data from Spitzer in red, visible wavelength data from Hubble in yellow, and X-ray data from Chandra in green and blue.  Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech


The vision was sketched out during a retreat organized by Charlie Pellerin, who oversaw the planning of the telescopes as the director of NASA’s astrophysics division. Pellerin recognized the need to compile a list of fundamental questions that needed to be answered in astrophysics, and asked the scientists what capabilities they would need to make realistic progresses toward answering them.  The answers were clear: we need to revolutionize the observational resolution and sensitivities in all parts of electromagnetic spectrum.

The idea was pitched to the Reagan administration as necessary if the US wanted to remain a leader in observational astronomy. The outcome was the four space-based observatories—without the Great Observatories program, there's a reasonable chance that budget cuts would have taken at least one of the four off the table.   

Seeing all of the sky

The process is a remarkable tale of the leadership needed to advance a large scientific project. For a revolutionary scientific progress, especially in astronomy, we need a comprehensive view of what has yet to be seen. In some cases, this can easily be quantified (e.g., this object in the sky has not been seen at this wavelength or beyond this sensitivity and resolution), which makes identifying common requirements among different observational targets crucial.

The Compton Gamma-ray Observatory was de-orbited in 2000, Spitzer recently ran out of its liquid helium cryogen, and the Hubble Space Telescope saw its last servicing mission, so the Great Observatories program is drawing to a close. Chandra is expected to continue observing the cosmos with its full capabilities for at least five more years, but it is not serviceable and it will eventually shut down. 

Currently, we don't have anything that compares to this vision. There seems to be a consensus among astronomers that the “next big things” in observational astronomy are the Thirty Meter Telescope (TMT), Atacama Large Millimeter-submillimeter Array (ALMA), the James Webb Space Telescope (JWST), and the Large Synoptic Survey Telescope (LSST).

While these telescopes will bring revolutionary progress over what we have today, some of the capabilities we have enjoyed with the Great Observatories program will be lost because these new telescopes are not being designed to specifically replace the capabilities we currently have. TMT and JWST will primarily observe in the infrared, ALMA at radio frequencies. So, although Spitzer’s capabilities will be exceeded by JWST in the infrared, Hubble’s ability to observe across the visible and ultraviolet wavelengths—which are extremely useful for observing planets and hot young stars—will be lost. LSST will observe in the visible light, but its goal is to take a wide-field survey of the sky every night to discover new time-variable phenomena—it is unable to zoom into a single target. While the mission of observing the gamma-ray sky has been carried on by SWIFT and Fermi satellites, there is no plan to replace Chandra anytime in the foreseeable future.

The idea of losing a telescope is actually a relatively new concept in observational astronomy. Before the space age, telescopes were built to last and could undergo numerous upgrades. For example, the five meter Hale Telescope at the Palomar Observatory, built in 1948, remained the biggest effective telescope until 1993, and is still an active research tool armed with the latest state-of-the-art technologies. Thus, sacrificing an old capability to replace it with a revolutionary but different capability is a new paradigm for astronomy, and its implications aren't yet clear. While Hubble, Chandra, and Compton are leaving us with tons of useful data, there will always be more objects to be studied.

Deciding which observational capabilities to maintain, replace, and develop involve strategic scientific policy decisions, and giving up some capabilities is not necessarily a bad thing if we truly believe that reallocation of the resources will help us with long-term goals. That said, no future equivalent of the Great Observatories has emerged, and we are at a stage where programs are competing to steer the future of astronomy as a whole.

But it's worth remembering that the Great Observatories moniker was, after all, an afterthought that was brought about by practical necessity, so the absence of an overarching theme is not necessarily a negative sign at this moment in time. It's possible that a coherent unifying vision will emerge in the near future from a wide mix of currently existing plans. Such a vision will define a new direction in observational astronomy and move us forward to the next revolutionary discoveries.

This report was based on an interview with Wallace Tucker, a member of the Chandra X-ray Observatory team and an author of the book "Revealing the Universe—the Making of the Chandra X-ray Observatory." An article titled “Conceiving and marketing NASA’s Great Observatories” by Martin Harwitt in the February 2009 issue Experimental Astronomy contributed to this report. Brian Siana of the California Institute of Technology and Joseph Meiring of the University of Louisville also provided helpful discussions.

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