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Report charts new course for US astronomy


Dark energy and exoplanets prioritized by decadal survey.

The proposed Large Synoptic Survey Telescope (LSST) was the number one choice for a new ground based astronomical facility in the 2010 decadal survey. Credit: Todd Mason, Mason Productions Inc. / LSST Corporation

In a report that marries cosmic curiosity with down-to-Earth pragmatism, an expert committee has delivered its game plan for the future of US astronomy.

The much-anticipated 'decadal survey', kept tightly under wraps until its release today, recommends which astronomy and astrophysics projects NASA, the National Science Foundation (NSF) and the Department of Energy (DOE) should fund over the next ten years (see 'Programmes recommended by the survey'). It also reflects how the landscape of astronomy has changed in the past decade, by placing heavy emphasis on dark energy, a mysterious phenomenon responsible for accelerating the expansion of the Universe, and extrasolar planets — two fields that barely existed at the time of the previous survey, in 2001.

"What we've seen over the last decade and can confidently expect for the next is unscripted discovery," says astronomer Roger Blandford of Stanford University in California, chair of the 23-person committee charged with writing the report for the National Academy of Sciences.

But although the scientific questions raised in the report are no surprise — what is dark energy, and how common are Earth-like worlds? — its strategy for answering them is likely to raise eyebrows. Constrained by a fiscal climate that is drastically different from the one their predecessors faced ten years ago, committee members have avoided recommending specialized projects to tackle astronomy's two biggest questions. Instead, they have given highest priority to a new space telescope that can go some way towards addressing both.

WFIRST, or the Wide Field Infrared Survey Telescope, is a proposed 1.5-metre imaging telescope that would map the sky in the near infrared — a wavelength range also accessible with the James Webb Space Telescope (JWST), scheduled for launch in 2014. The difference is that, whereas the JWST is designed to gaze deeper into the Universe than ever before, WFIRST would look more widely, acquiring a massive data set that spans the sky. Such data would contain subtle clues — in the distance-brightness relationships of supernovae, the weak gravitational lensing, or the bending of light, by background galaxies and the three-dimensional clustering of matter in space — that can independently measure the influence of dark energy on the expansion of the Universe.

WFIRST priority

Although the acronym is new, WFIRST looks remarkably like a version of the Joint Dark Energy Mission, a concept developed in recent years by NASA and the DOE. In this new incarnation, the US$1.6-billion space telescope would be used to broader purpose by spotting 'microlensing' events caused when exoplanets briefly pass in front of background stars in the central bulge of the Milky Way — a region containing the highest density of stars in the Galaxy. This method is not suitable for studying individual solar systems in detail, but promises to provide an unbiased sample of what kinds of planetary systems are prevalent in the Galaxy through the sheer number of its discoveries.

Unseen but pervasive, dark energy is thought to be accelerating the expansion of the universe. A better understanding of its nature through astronomical observations could have a major impact on fundamental physics. Credit: Sloan Digital Sky Survey

The decision to prioritize WFIRST will probably have a significant impact on a similar project, called Euclid, that is being developed by the European Space Agency. Astronomers working on Euclid were expecting some US participation, but the decadal survey now calls on the United States to play a leading part in a potential collaboration that would combine Euclid with WFIRST.

First-place ranking for ground-based instruments went to the Large Synoptic Survey Telescope (LSST), an 8.4-metre optical telescope to be located under the almost perpetually clear skies of the Chilean Andes. When it is completed in 2015, the 3,000-megapixel camera on the LSST will image the entire sky every three nights, providing a massive database that will complement WFIRST on dark energy, while advancing research in diverse areas from γ-ray bursts to exoplanets to near-Earth asteroids.

A second priority identified for research from both the ground and space isn't a large mission at all. Rather, the survey recommends that money be allocated to fund proposals for small and medium-scale experiments and facilities that can respond to new discoveries. In the past, smaller directed missions, such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Swift probe, have yielded groundbreaking results at relatively low cost.

The report pointedly stresses the need for funding agencies to select between the two large-scale, US-led facilities currently under development — the proposed Thirty Meter Telescope (TMT) on Mauna Kea in Hawaii and the Giant Magellan Telescope (GMT) envisioned for Las Campanas in Chile. Both are supported by private money and will require significant resources simply to keep them running. Given that Europe has also prioritized a 40-metre telescope, a swift decision on which of the two US initiatives should receive federal support would help avoid a potential stalemate for both.

Further down the rankings, a proposed mission to study nearby Earth-sized planets that are likely to be discovered in the coming decade has been given tentative support as a technology development programme — but not on a fast track to the launch pad. Meanwhile, a space-based laser interferometer (the Laser Interferometer Space Antenna, LISA) that would detect gravity waves has moved up in ranking, now coming in ahead of an international X-ray space telescope that had been ranked higher in the past.

In choosing which projects should be promoted and which fell by the wayside, Blandford's committee relied on input from the entire astronomical community. Starting five years ago, thousands of scientists met at 17 town-hall meetings to produce informal reports. The committee also received more than 340 scientific white papers proposing different ideas, which were picked over by nine expert panels and boiled down to their essential components. "We went out of our way to engage the community very broadly throughout the process, ensuring that we got a diverse picture of where the field stands today," says Michael Moloney, study director for this survey at the National Research Council (NRC).

Besides widespread engagement, the survey aims to minimize cost overruns. In part, this can be traced back to the fact that only the first recommendation of the 2001 survey, the JWST, is under construction, after going $3.5 billion and five years over budget.

"I think at the time of the previous decadal survey, people didn't appreciate the importance of taking a second look at the cost of things and not just taking the word of the people submitting the projects," says astronomer Claire Max of the University of California in Santa Cruz, a member of the final survey committee. This time around, the panel hired an outside expert to help estimate the funding and technical risk of each project. The report also sets up an annual panel called the Decadal Survey Implementation Advisory Committee, which will convene regularly and decide whether or not to continue funding troubled projects.


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Mann, A. Report charts new course for US astronomy. Nature (2010).

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