Published online 4 February 2011 | Nature | doi:10.1038/news.2011.74

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ESA on countdown to flagship mission selection

International collaborations at risk as three candidates vie for one spot.

EJSM-LaplaceArtistic impression of the EJSM-Laplace mission.ESA/NASA/M. Carroll

In June, the Science Programme Committee of the European Space Agency (ESA) will pick the next large European space mission, the scientific flagship of the agency's Cosmic Vision 2015–2025 plan. It will not be an easy decision.

At a meeting in Paris on 3 February, project teams for the three competing missions presented their ambitious plans: revealing the secrets of Jupiter's moons; detecting X-rays in the Universe; or hunting for gravitational waves. What makes the choice even more difficult for ESA officials is that all three projects are international collaborations, so their decision will have consequences for other countries' space programmes.

For instance, the Europa Jupiter Systems Mission (EJSM–Laplace) consists of two separate spacecraft — one European and one from the United States. Both will first loop around the giant planet Jupiter, with each later going on to orbit one of the four large Jovian moons. NASA's craft will focus on icy Europa, which is thought to harbour an ocean beneath its frozen crust, whereas Europe's probe will study Ganymede — the largest moon in the Solar System.

"The overarching theme of the mission is the emergence of habitable worlds around gas-giant planets," says space physicist Michele Dougherty of Imperial College London, referring to the remote possibility that microbial life might lurk in subsurface oceans on both Europa and Ganymede. Having two craft also creates unique opportunities to study Jupiter's atmosphere and magnetosphere in three dimensions, adds physicist Emma Bunce of the University of Leicester, UK.

According to ESA's Christian Erd, who was the European project manager of the Indian-led Chandrayaan-1 mission to the Moon, the mission faces no technical obstacles, although it will struggle to meet the original launch window of 2020. And even if NASA were unable to hold up its end of the bargain, not too much would be lost. "The European Jupiter Ganymede Orbiter is an outstanding mission in its own right," says Dougherty.

International commitments

IXOArtist´s conception of the IXO spacecraft.ESA/NASA

This is not the case for the two other proposed missions, the International X-ray Observatory (IXO) and the Laser Interferometer Space Antenna (LISA). They are just too big, too complex and too expensive to be built by one agency, and can't be split in two.

IXO relies on contributions from both NASA and the Japanese space agency JAXA: after launch, the giant space telescope will be extended to its final length of 20 metres by a technique being developed by JAXA.

But ESA still has to validate the novel design of the X-ray optics, and according to Wilton Sanders, an official in NASA's Astrophysics Division, IXO's technical readiness — or lack thereof — was the main reason that the decadal survey carried out by the US National Academy of Sciences ranked the mission below LISA in its recommendations for US space-science priorities last year.

However, study team member Kirpal Nandra, who directs the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, strongly believes that these hurdles will be overcome, and that the results will be worth it. "IXO will provide the sharpest and most sensitive X-ray view of the Universe ever," he says. In particular, the space telescope is expected to reveal what happens in the immediate vicinity of black holes.

LISA goes a few steps further, according to Bernard Schutz, a director at the Albert Einstein Institute in Golm, Germany. Being able to 'hear' the gravitational waves (tiny ripples in space-time) of binary white dwarf stars or merging black holes, he says, will be like adding sound to a silent film of a walk through the jungle. "It's going be our first look through a whole new window," says Schutz. "There are bound to be many things we didn't even expect."

LISAArtist's impression of the three LISA spacecraft.ESA/EADS Astrium

But flying three laser-coupled spacecraft in formation five million kilometres apart will be no mean feat. ESA has already spent a lot of money on technology validation through the LISA Pathfinder mission, launch of which has now been delayed to late 2013. In addition, cost estimates for the European part of LISA are already substantially higher than for EJSM–Laplace or IXO, according to Frédéric Safa, a space scientist who heads ESA's Advanced Studies and Technology Preparation Division.

In the end, the selection process for a new flagship mission may turn out to be an even bigger headache for Europe's partners than for ESA. Depending on what ESA's Science Programme Committee decides four months from now, the US and Japanese space-science communities may need to reconsider their own rankings of astrophysics missions.

"ESA is in the driver's seat right now," says Sanders. "We'll have to wait and see." 

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