Unrealistic cost estimates could make an ambitious NASA space mission to study dark energy no better than ground-based projects, experts warn.
The agency is preparing a call for proposals for the Joint Dark Energy Mission (JDEM), as part of its Beyond Einstein cosmology programme, which was revived after it had been put under review in 2006 for budgetary reasons.
Dark energy is the mysterious force that appears to make up about 70% of the energy density of the cosmos and is causing the expansion of space to speed up.
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JDEM鈥檚 main aim is to measure dark energy鈥檚 effect through cosmic history, which can be done by studying the universe at ever greater distances to see how its expansion rate has changed.
If successful, the mission could help determine whether the mysterious force is in fact Einstein鈥檚 cosmological constant 鈥� a fixed, 鈥榲acuum鈥� energy of space itself 鈥� or something that might vary with time.
While NASA has conceived of JDEM as a 鈥楶robe-class鈥� mission costing roughly $600 million, in 2007 an independent advisory committee put the costs at more than $1 billion, excluding the cost of launching the satellite.
Now, some members of the disbanded committee are worried that NASA may attempt to stick with the original budget. And NASA admits that if none of the proposals can realistically fit within the budget, the project may be subjected to further reviews and other projects may take higher priority.
鈥淓ither this will lead to an inferior mission or else it will lead to a great delay,鈥� says Joel Primack, a physicist at the University of California, Santa Cruz, who served on the advisory committee. 鈥淚t is very much a possibility that it would result in cancellation of the whole thing.鈥�
Precision measurement
The advisory committee, part of the US , was charged with reviewing NASA鈥檚 Beyond Einstein programme. It picked JDEM as a high-priority project, but it found the mission would cost in excess of $1 billion.
The committee used a different technique than NASA does to estimate costs 鈥� one that incorporated information on the total budgets of past missions and how their original cost estimates ballooned over time.
鈥淪pace projects tend to overrun,鈥� says committee chair Charles Kennel of the Scripps Institution of Oceanography in San Diego, California, US. 鈥淲e thought that these cost estimates would give the decision-makers a sort of plus-or-minus 10% range for what the cost would end up being.鈥�
JDEM aims to measure dark energy parameters with 1% precision. But former committee members say if NASA sticks to its original cost estimate, the mission has little chance of achieving such precision. Instead, JDEM may achieve an accuracy close to that of ground-based telescopes, which are approaching 3% precision.
Tight budget
鈥淵ou don鈥檛 recommend a space mission unless you can do better than you can do from the ground,鈥� Kennel told New 杏吧原创. The committee recommended the project, but with this revised budget in mind. 鈥淲e felt if they wanted it, they should pay for it.鈥�
NASA recently issued a 鈥� including cost estimates 鈥� that cites its original $600 million figure. But it is unclear whether or by how much JDEM鈥檚 cost cap will change when the official request for mission proposals is issued later this year.
鈥淲e will adjust as necessary and as we are able to within available resources,鈥� says Jon Morse, director of NASA鈥檚 astrophysics division in Washington, DC, US. But, he adds: 鈥淲e believe we have a good indication of how much the mission should cost.鈥�
With launch costs, NASA hopes the total budget for JDEM will fall under $1 billion. But if JDEM looks to be more expensive, the project may have to be postponed until it can be evaluated and ranked in an upcoming decadal review of astronomy and astrophysics projects, he said. Morse added that NASA鈥檚 budget will not support such a large mission until after the James Webb Space Telescope launches in 2013.
No guarantees
NASA has funded studies of three different mission concepts that are the primary candidates for a future JDEM telescope. All have been planning for a budget of about $600 million. 鈥淲e will make a mission that will work within whatever NASA says is the cost cap,鈥� says astronomer Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona, US.
Lauer, who serves as principal investigator for the JDEM candidate mission DESTINY, adds that the National Academies committee made some errors in estimating DESTINY鈥檚 total cost.
JDEM was originally planned to start in 2009. But NASA鈥檚 budget for that year is still undecided, so it is unclear what will happen with the mission. Also unclear is how much the US Department of Energy, which will partner with NASA on this project, will contribute.
鈥淲e鈥檙e hoping to have a chance to propose [a mission], but there鈥檚 no guarantee in any of this,鈥� Lauer told New 杏吧原创. 鈥淲e鈥檒l just have to see how it shakes out.鈥�
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