Published online 16 January 2009 | Nature | doi:10.1038/news.2009.38

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ESA seeks to join US dark energy mission

Deal would boost JDEM budget but scupper Europe's Euclid.

SupernovaStudies of supernovae provided some of the first evidence for the existence of dark energy.MPIA/NASA/Calar Alto Observatory

The European Space Agency (ESA) is seeking a slot as the third partner in the Joint Dark Energy Mission (JDEM), alongside two US agencies already involved — NASA and the Department of Energy (DoE). A deal would boost the budget as well as the flight chances for the space telescope, which, launched sometime next decade, would peer beyond the Milky Way to pin down the mysterious force which is accelerating the universe's expansion. However, a merger would spell the end for Euclid, a proposed stand-alone ESA dark energy mission.

"Given the economy, it's clear that flying two individual missions, one in Europe, one in the US, may be seen as an extravagance," says Bob Nichol, a Euclid study scientist from the University of Portsmouth, UK, who said his team was worried about the merger not being an "equal marriage".

David Southwood, ESA's director of science and robotic exploration, says he hopes to broker a deal by the time he meets with an ESA science advisory committee at the beginning of February. On the negotiating table for ESA, he says, is money equal to or less than the €400 million to €450 million that Euclid was competing for as a part of ESA's Cosmic Vision programme. Additionally, he says, one or two ESA member states will likely want to contribute money towards building a detector, or the telescope itself.

Budget bonus?

A deal could help bring JDEM closer to the price tag scientists want; a 2007 National Academies report that endorsed the mission found that the three design proposals would each cost at least $1.3 billion. However, Jon Morse, NASA's astrophysics division director, has said that the mission shouldn't cost more than $600 million plus launch costs. DoE officials have said that they would pay for 25% of the mission.

Fabio Favata, ESA's coordinator for astronomy and fundamental physics missions, met with Morse, his counterpart, in Washington, DC, last week, and Morse is expected to go to Europe later this month. Both say that any money from ESA would be a chance to lower the contributions from both agencies.

The JDEM design process is already well underway. An initial design, expected within the next month, would include a 1.5-metre-wide optical and infrared telescope. Three distinct methods offer ways of measuring the rate of the universe's expansion: the cataloguing of distant supernovae; a search for tiny distortions in the shape of distant galaxies caused by the gravitational lensing of dark matter; and the measurement of ripples in galaxy clusters caused by sound waves from the Big Bang.

No competition

Scientists were expecting to pit the three methods against each other in a competition to design the telescope, but in September, NASA and DoE said they would develop a reference design that could incorporate any of the three methods.

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The concept for Euclid was a 1.2-metre telescope that would have emphasized two of the three methods: weak lensing and baryonic acoustic oscillations. Euclid was itself a compromise between two earlier mission concepts — the Dark Universe Explorer (DUNE) and the Spectroscopic All-sky Cosmic Explorer (SPACE) — that merged last May. "People have spent several years of their lives already working on the proposals," says Rene Laureijs, the Euclid science study leader, based at ESA's European Space Research and Technology Centre in the Netherlands. "They would like to have some stability."

While Euclid may lose its independence, some on the team realise that, by signing on with JDEM, they'll avoid competing with four other medium-sized missions in the Cosmic Vision programme. "That's the carrot being dangled in front of us," says Nichol. 

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