European space prepares to make a really big decision

More than 300 of Europe's leading space scientists gathered in Paris this week to discuss how to spend more than a billion euros. The options? Well, try to choose between these three:

IXO would be compressed into a smaller shape to fit in its launch rocket

(1) a 20m-long telescope called IXO that could see the very "edge" of a black hole; or (2) a trio of satellites collectively known as LISA which might be able to detect the ripples in space-time left by the moment of creation itself; or (3) a pair of spacecraft that would visit two of the most promising locations for life beyond Earth in our Solar System. This is called EJSM/Laplace.

The European Space Agency is working through the process of selecting a large mission to do something extraordinary, with the idea of launching the venture in 2020 or soon after. The start of the next decade might seem a long way away, but in the business of space this type of extended planning is very common.

The mission concepts being considered in this instance stretch what we know scientifically and challenge what we think we're capable of achieving technologically. And the reality is that in the case of two of the three missions I'll discuss on this page, several million euros will be spent just to say "no, we're not going to do that this time".

This week's meeting was a beauty pageant, if you like. It was a chance for the proponents of each mission concept to sell their idea to the wider community, and, very importantly, to the committee members in the audience who will make the final decision.

So what exactly is on offer?

The meeting was held in the grand surroundings of the Institut Oceanographique in Paris

IXO [7MB PDF]: The International X-ray Observatory would be another of the grand telescopes, in the mould of Hubble, Herschel and James Webb. Like James Webb, it would be so big that it would need to be compressed, accordion-like, to fit inside its Atlas 5 launch rocket.

Only when it got into orbit could this 6.5 tonne beast extend to full length. Carrying advanced optics, it would deliver sensitivity and resolution 10 to a 100 times better than the current state-of-the art machines - Nasa's Chandra telescope and Esa's XMM. What could it do? Well, X-rays are a signal from the energetic Universe - from places where matter is being accelerated to great speeds, heated, or even torn apart.

To a science journalist one might even say it's a signal from the "exciting Universe" because the sources of X-rays are often those staples of gripping astronomy stories - black holes. Indeed, IXO would allow us to probe these objects in ways the current generations of astronomers could only dream of.

IXO would hunt for the first supermassive black holes to form in the Universe, and learn how they evolved through cosmic time. It would also allow us to peer right at the event horizons of black holes, locations where some really weird relativistic effects are predicted to occur as matter is pulled "inside". Paul Nandra, from the Max Planck Institute for Extraterrestrial Physics in Garching, is an IXO champion. He told me:

"We think we've already seen some of these effects with the current generation of telescopes; evidence that time slows down close to a black hole. That causes the light to shift. But even weirder things happen when you get close to a black hole: you get effects that light is bent so that you can almost see the back of your head. That sort of thing can become observable if you've got enough sensitivity like you'll have with IXO. So, we want to see these effects; we've got hints of them already. But now we know we are close to a breakthrough and that if we get this increase in sensitivity from IXO, we can see these effects predicted by Einstein's general relativity."

The LISA satellites would fire lasers across five million km of space

LISA [12MB PDF]: The Laser Interferometer Space Antenna has been studied as a mission concept in some form for at least 18 years. Its purpose would be to detect gravitational waves. The movement of truly massive bodies, such as merging black holes, are expected to disturb the space-time around them, sending this energy radiating outwards. It's a very small signal, however, and to identify it requires extraordinary sensitivity.

LISA would fly three satellites five million km apart in an equilateral triangle formation. Laser beams travelling between the spacecraft would measure distances between free-floating gold blocks. The trick to detecting a wave washing over the observatory would be to see the laser beams deviate in a very characteristic way.

Measuring that, though, means observing changes as small as about 10 picometers, or 10 million millionths of a metre, a length smaller than the diameter of the smallest atom. Astonishing. But if this is possible - and everyone seems to think it is - it will turn astronomy on its head because it means we will be able to probe the Universe in ways that do not depend on detecting light. Professor Bernie Schutz from the Max Planck Institute for Gravitational Physics in Potsdam told me:

"Light is a wonderful medium for exploring the Universe and our own neighbourhoods, but the problem with light is that it's pretty easy to block it; and when you're talking about getting light from very distant regions of the Universe, there are too many things in the way. The light gets scattered or absorbed. Gravitational waves don't do that; they go through absolutely everything. Ordinary gravity does; you can't screen gravity out. You know that you weigh as much when you're standing inside a building as when you're standing outside. You can always block radio waves and the transmission to your phone - that's electromagnetic waves, that's light. But you can't do that with gravity. So if we can detect gravitational waves then we can observe things that we can't reach any other way."

Like IXO, LISA will open up black holes to study in ways that are simply not possible currently, but what really fascinates me is what it could do for the study of the really early Universe.

There should be a background of gravitational waves rippling across the Universe from the Big Bang. LISA just might have the sensitivity to pick this up, or certainly some of the other key events predicted to have occurred in the first fractions of a second after the cosmos came into being.

Europe would concentrate on Ganymede, putting a spacecraft in orbit around the moon

EJSM/Laplace [9MB PDF]: This is a two-spacecraft mission that would go out to Jupiter, to study the planet and its Galilean moons. Particular emphasis would be paid to Europa and to Ganymede. I'll talk to the importance of international collaboration in just a moment, but this endeavour would see the Americans concentrate on Europa with one spacecraft and the Europeans concentrate on Ganymede with the other.

Each satellite would conduct a stream of independent science in the Jupiter system. BUT, the two spacecraft would also work in tandem, gathering data together from different standpoints around the gas giant that would give scientists a totally new perspective on the Solar System's biggest planet.

Jupiter's significance has grown in recent years as we've discovered more and more planets around distant stars. Jupiter is an archetype, a model, for those far-flung systems, not least because it is on the rocky and icy moons of giants planets that life may exist. And this is the real draw of going to Europa and Ganymede. These two bodies probably harbour deep sub-surface layers of liquid water, and, as such, are considered prime locations for biology to perhaps take hold. Professor Michele Dougherty from Imperial College London said:

"You need essential elements; you need water; you need stability over time; and you need energy as well. What we want to do is to try to understand the details of those four different areas [at these moons]. And you can't do that if you have flybys. Nasa's Galileo spacecraft spent years in the vicinity of Jupiter but it didn't spend more than three or four hours on a flyby of each of the moons. To separate out all of the different effects, you need to spend time in orbit. You need to be able to see the same piece of surface time and time again, to see how it might change."

IXO, LISA and EJSM/Laplace would cost well in excess of a billion euros to implement. The European Space Agency says it can spend no more than 700m euros on any one venture.

That's realistic if the member states of the agency pick up the costs of building instruments (which they would normally do) and Nasa (and Japan in the case of IXO) also joins the party. And here's the tricky part. While Esa works through its selection process, the US is also working through a separate selection process, too.

The priorities of the two agencies - or at least their scientific communities - have to align; so too do the timelines for making decisions.

It's rather like organising a multi-billion euro wedding and trying to get the bride and groom to the church on the same day to say "I do". But watch out in June because we should at least get some indication then from the Science Programme Committee of Esa on how it views the big choices above.