By Jonathan Amos
Science correspondent, BBC News
Euclid would map the distribution of dark matter
The competition to find the next great European space mission has seen three ideas move to the front of the field.
The European Space Agency's (Esa) Science Programme Committee (SPC) will meet next month to consider the current status of the candidates.
The committee members will have in front of them a report from an advisory group that has done a ranking on science.
A final decision is unlikely to be made for a year or so.
And to secure the opportunities available, the various competitors will have to show not only that their science is compelling but that their technological ambition is realistic and affordable.
Even then, only two missions can be afforded out of the three.
The concepts include a satellite that would map the "dark Universe" (called Euclid), a probe to study the Sun up-close (Solar Orbiter), and a telescope to find distant planets (Plato).
At the moment, the trio are in the lead, jockeying for position for the earliest launch opportunities in Esa's Cosmic Vision programme. The first launch could come in 2017.
However, the missions still have much to prove if they are ever to be flown by the agency.
A further concept known as Spica (a contribution to a Japanese-led infrared telescope that builds on Europe's heritage with its Herschel observatory) is relatively low budget, and again has compelling science.
However, its special nature means it may now be treated in a separate way, depending on how the partner state's side of the project evolves and the budget that ultimately becomes available.
The scientific community ran the rule over the concepts in December
Two other ideas put forward for consideration in the competition have not been put out of the long-term picture. They seem though to be too far from the SPC's budget constraints to progress in this round.
They are Marco Polo (an asteroid sample-return mission) and Cross-Scale (to study space plasmas). Again, these concepts were considered to have excellent science credentials, according to the Space Science Advisory Committee.
But the high prospective cost of implementing these concepts means they would have to acquire additional partners to bring their foreseen budgets back into contention.
Very similar ideas to Marco Polo and Cross-Scale could well return to run in future competitions if the right international cooperation can be found.
Esa has budgeted up to 475 million euros (£430m) (at 2010 prices) for its contribution to the Cosmic Vision winners. The expectation is that the cost of the instruments that fly on the spacecraft will be borne directly by the member-states that provide them.
All the concepts were presented to the scientific community in December in what amounted to a kind of space "beauty contest".
The six concepts in more detail:
EUCLID - MAPPING THE 'DARK UNIVERSE'
Euclid would map the distribution of "dark matter", the matter that cannot be detected directly but which astronomers know is there because of its gravitational effects on the matter we can see. Hubble has done this for a tiny portion of sky measuring two square degrees. Euclid would do it across 20,000 square degrees of sky. The study would yield information also on "dark energy", the mysterious phenomenon thought to be accelerating the expansion of the cosmos.
SOLAR ORBITER - GETTING UP CLOSE TO THE SUN'S 'ENGINE'
This would be a joint venture with the US. Solar Orbiter would circle the Sun, flying to within 35 million km of our star to make detailed measurements of the activity from the equator to the poles. The multi-instrumented probe would both observe the Sun and take in-situ measurements of its environment.
PLATO - SEARCHING FOR PLANETS LIKE OURS
A spacecraft incorporating a suite of telescopes to hunt for planets around nearby bright stars. Crucially, these would include many rocky planets in the "habitable zone" - the region around a star where water can keep a liquid state. Plato would find these worlds by monitoring stars for the tiny dips in light that occur when planets move across their faces.
SPICA - TO FILL THE 'INFRARED GAP'
A joint mission with the Japanese space agency (Jaxa) to send the next generation of infrared telescope into orbit. Europe's contribution would include the 3.5m primary mirror and an instrument. Spica would see targets beyond the vision of the current state-of-the art infrared observatories - Esa's new Herschel telescope and Nasa's soon-to-launch James Webb telescope.
CROSS-SCALE - SAMPLING THE SPACE AROUND US
A constellation of spacecraft that would fly around Earth to sample the charged gas, or plasma, that envelops our world. Esa would provide seven spacecraft; Japan and Canada are considering their own mission (Scope) which could bring an additional five satellites. Together they would sample the plasma and detail its behaviour in three dimensions.
MARCO POLO - GRABBING SAMPLES FROM AN ASTEROID
A mission to a near-Earth asteroid to grab a handful of dust and pebbles off its surface to bring back to Earth labs for study. Marco Polo is a spacecraft that would land on the asteroid to try to drill or scoop up what would be perhaps just tens of grams of material. But even this small sample could give scientists invaluable insight into the formation of the Solar System.