Extending the Planck mission was judged a top priority in NASA's senior review. Credit: D. Ducros/ESA

Bigger is better, according to NASA's latest rankings of its astrophysics satellites, released on 5 May.

The assessment gave the three missions with the biggest budgets the highest ratings, and deemed them most deserving of continued funding. Planck, which maps radiation left over from the Big Bang, the Chandra X-ray observatory and the infrared Spitzer Space Telescope were judged to be the best of the 11 missions considered (see table). The external 'senior review' of astrophysics missions based on science return per dollar takes place every two years.

The astrophysics division faces flat funding in the years ahead, so the review committee had to make some hard choices. It reinforced NASA plans to stop funding the three lowest-ranked missions: The Rossi X-ray Timing Explorer (RXTE), launched in 1995; the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), a European Space Agency (ESA)-led mission launched in 2002; and a proposal that would extend the Wide-field Infrared Survey Explorer (WISE).

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Click for a larger version of this table.

In estimating 'science per dollar', the review committee looked at the quality and quantity of science publications and the numbers of astronomers wanting to use the telescopes. It also tried to gauge the capabilities of the satellites for the next two years. "This sort of process of peer review is not perfect, but like democracy, it's a heck of a lot better than the next best thing," says J. Craig Wheeler, an astronomer at the University of Texas at Austin and chairman of the 12-person committee.

Recently launched missions, such as the exoplanet-hunting Kepler space observatory, were excluded from the exercise. The newly refurbished Hubble Space Telescope also got a free pass. The top-ranked mission, the ESA-led Planck observatory, launched in 2009 but was reviewed because the mission team had requested an extension.

Consolation prize

Although US President Barack Obama's administration remains locked in a dogfight with Congress over the future of human spaceflight at NASA, a proposed budget boost for NASA science seems more certain. However, most of that boost will go to Earth-science programmes — the astrophysics division faces a far tougher outlook. The constraints faced this year were more stringent than in the 2008 senior review, says Jaya Bajpayee, the NASA programme executive in charge of the review. In the agency's 2011 budget request, about US$160 million was available for the 11 missions considered by the panel. But $16 million more would be needed to operate all of them, she says. Two of those missions, Spitzer and Chandra, suck up just over half of the $160 million.

This sort of process of peer review is not perfect, but like democracy, it's a heck of a lot better than the next best thing. ,

For most of the missions, the committee's recommendations agree with the agency's budget plans — notably, in not funding RXTE, INTEGRAL and the WISE extension. But there are exceptions. The Galaxy Evolution Explorer, an ultraviolet survey telescope, was ranked third in the 2008 senior review and given an extension. However, the new review puts it in eighth place, and recommends that the agency's 2011 budget allocation for it be reduced by $1 million. Conversely, the ESA X-ray satellite, XMM-Newton, was ranked at seven in 2008 and NASA had zeroed its 2011 budget. But the 2010 review ranks it fifth and recommends giving it $1 million to support US astronomers who want to use it.

Wheeler notes that all of the missions are doing valuable science and ought to be funded if the money were available. That was not the case for Gravity Probe B in 2008, when the senior review committee ranked it last and recommended its closure. They noted that the $750-million mission, conceived in the 1960s to test an aspect of the theory of general relativity, had been "somewhat overtaken by events".