By Paul Rincon
Science reporter, BBC News, Paris Air Show
The US and European space agencies are to co-operate on two missions considered vital for efforts to create a new understanding of the Universe.
Esa's contribution to JWST - as with Hubble - will be 15%
Agency chiefs signed official agreements outlining the partnership in a ceremony at the Paris Air Show.
They will collaborate on the James Webb Space Telescope (JWST), due to launch in 2013, and the Lisa Pathfinder.
The James Webb observatory has been described as the successor to the Hubble Space Telescope.
US space agency (Nasa) administrator, Dr Michael Griffin, and European Space Agency (Esa) director-general, Jean-Jacques Dordain, signed the agreements on Monday at Esa's pavilion.
Dr Griffin said the agreement was "one more step along the path of international co-operation in space science and spaceflight of all kinds".
JWST will operate at visible and infrared wavelengths to shed a new light on the evolution of the Cosmos.
It should be able to see some of the farthest - and therefore the earliest - objects to form in the Universe.
JWST will operate at cryogenic temperatures so that it does not emit its own infrared radiation, swamping faint astronomical signals.
Lisa Pathfinder is designed to try out the technologies required for the Laser Interferometer Space Antenna (Lisa) mission.
Gravitational waves are an inevitable consequence of the Theory of General Relativity
They describe the gravity force as distortions made by matter in the fabric of space-time
Any moving mass will produce waves; they are expected to propagate at the speed of light
Detectable sources should include exploding stars merging black holes and neutron stars
The full Lisa observatory should see remnant radiation from the Big Bang itself
But developing the required technologies has tested the ingenuity of scientists and engineers to the limit, and some issues are still to be resolved.
Lisa itself is a joint Esa/Nasa mission and is intended to test a key prediction of Albert Einstein's Theory of General Relativity - that of gravitational waves.
Currently, our entire understanding of the Universe is based on observation of electromagnetic waves, such as visible light, infrared, radio and X-rays.
Einstein's thinking calls for the existence of "ripples" in space-time. These gravitational waves are expected to criss-cross the Universe at the speed of light; but their existence has yet to be observed directly.
By establishing the presence of these ripples, Lisa will open up a completely new field of astronomy.
A memorandum of understanding is signed by the Nasa boss
Dr Griffin added: "Lisa is going to be a ground-breaking space mission when we finally accomplish it. There are things in astronomy and astrophysics that we cannot see with visible light, radiowaves or infrared.
"But we believe we can see them with gravitational waves. Lisa will detect those and allow us a new window into the Universe."
Modelling studies have suggested that Lisa should be capable of detecting remnant radiation from the Big Bang itself, enabling science to probe the first moments of creation and perhaps sort through some of its contradictory theories.
The mission relies on technologies that have never been built before and cannot be properly verified on the ground. So Esa took the unusual step of pushing forward with a technology demonstration mission - Lisa Pathfinder - before attempting Lisa itself.
Lisa Pathfinder will place two test-masses in a nearly perfect gravitational free-fall, and controlling and measuring their motion with unprecedented precision.
This is achieved through inertial sensors, lasers, a drag-free control system and a remarkable micro-propulsion system.
Professor David Southwood, Esa's director of science told BBC News: "Lisa Pathfinder's been an enormous challenge, it turned out to be much harder than anyone expected. Nobody can deny that.
The JWST will be despatched on the Ariane 5
"We've been trying to do something different and we're still not fully out of the woods. For us, it's a grand goal. I would like to have done it for more like 200m euros; but we don't want to spoil the ship for a ha'pworth of tar.
"We want to get it right now. If anyone had any doubt whether the technology pathfinder was needed, the problems we've had on Lisa Pathfinder prove the case.
"It would have been a catastrophe to run into the problems we've had on the full mission. The taxi meter ticks more heavily when you're building a big mission."
Gravitational waves are thought to move in two different directions - so-called quadripolar motion - and Lisa will have to measure their movement in these two different directions at once. This was no easy task, said Professor Southwood.
Professor Tim Sumner, a mission scientist from Imperial College London, UK, said the prospects of verifying one of the central predictions of general relativity "provides an enormously rich new discovery opportunity".
Lisa Pathfinder was originally slated for a 2008 launch, but is now expected to lift-off in 2010.
Esa's financial contribution to JWST amounts to roughly 400m euros, or 15% of the total $3.5bn budget. Esa has significant involvement on two of the telescope's four onboard instruments.
JWST will be launched aboard a European Arianespace rocket from Kourou in French Guiana.
The cost of Lisa Pathfinder is estimated at about 247m euros. Nasa will contribute systems to the spacecraft.
The industrial lead on Pathfinder was given to EADS Astrium in the UK. The mission is the first major Esa science project to be primed out of Britain since the Giotto comet chaser in the 1980s.
THE JAMES WEBB SPACE TELESCOPE
JWST is named after James E Webb, Nasa Administrator during the Apollo lunar exploration era; he served from 1961 to 1968
It will be placed 1.5m km from Earth, at Lagrange Point 2, an area of gravitational balance that keeps it in a Sun-Earth line
The telescope will be shaded from sunlight by a shield, enabling it to stay cold, increasing its sensitivity to infrared radiation
Three principal instruments will gather images of the Universe in the infrared region of the spectrum
These will yield new information about how stars and galaxies first formed a few hundred million years after the Big Bang