It will be the most valuable science launch in European spaceflight history
One of the most important launches in European spaceflight history is about to take place in French Guiana.
Two telescopes with a combined value of 1.9bn euros (£1.7bn) are being sent into orbit on a single Ariane rocket.
The Herschel and Planck observatories will probe the sky to glean new information about the origins and evolution of the cosmos.
The ride into space from the Kourou spaceport should last just under half an hour.
The astronomical satellites will then separate from the rocket more than 11,000km above the Earth and take separate paths out to observing positions some 1.5 million km from the planet on its "night-side".
"This is the result of many years' hard work by thousands of scientists and engineers across Europe," said European Space Agency (Esa) director-general Jean-Jacques Dordain.
"The technology onboard these satellites is unique, and the science these satellites will do is fantastic," he told BBC News on the eve of the launch.
The scheduled lift-off is timed for 1312 GMT (1412 BST; 1512 CEST).
Herschel is named after the 18/19th astronomer who discovered the infrared
Herschel is the largest telescope anyone has yet tried to put in space.
Its 3.5m-diameter primary mirror is one-and-a-half-times the size of Hubble's main reflector.
Such size would ordinarily incur a huge weight penalty but the Herschel mirror has been kept to just 350kg by constructing it from silicon carbide, a novel ceramic material.
HERSCHEL SPACE TELESCOPE
Herschel will be released first from the Ariane rocket's upper-stage
The observatory is tuned to see the Universe in the far-infrared
Its 3.5m diameter mirror will be the largest ever flown in space
Herschel can probe clouds of gas and dust to see stars being born
It will investigate how galaxies have evolved through time
The mission will end when all the superfluid helium boils off
The telescope will be sensitive to far-infrared and sub-millimetre (radio) wavelengths of light, allowing it to peer through clouds of dust and gas to see stars at the moment they are born.
This infrared capability will also enable Herschel to look deep into space, to gaze at those galaxies that thrived when the Universe was roughly a half to a fifth of its present age. It is a period in cosmic history when it is thought star formation was at its most prolific.
"Herschel is going to help us understand much, much better how stars form right now and how they have been forming throughout billions of years of cosmic history; and therefore, indirectly, it's going to help us understand how our own Sun and our own Solar System formed," Dr Göran Pilbratt, Esa's Herschel project scientist, told BBC News.
The spacecraft carries an enormous flask of "superfluid" helium to chill its instruments and detectors close to minus 273C (or "absolute zero", the point beyond which no further cooling is possible).
"We are observing at long wavelengths where all warm objects glow, so we need to cool the telescope and the instruments as much as possible, otherwise the weak signals we are trying to detect from the sky will be totally swamped by radiation emitted by the telescope itself," said Professor Matt Griffin from Herschel's Spire instrument team.
Planck is a survey telescope. It will spin to map the sky at even longer wavelengths of light - in the microwave portion of the electromagnetic spectrum.
It will make the finest ever measurements of what has become known as the Cosmic Microwave Background (CMB).
The CMB is the "oldest light" in the Universe. It is all around us and comes from a time 380,000 years after the Big Bang.
PLANCK SPACE TELESCOPE
Planck is moving at almost 10km/s when ejected
Planck will survey the famous Cosmic Microwave Background
This ancient light's origins date to 380,000 years after the Big Bang
It informs scientists about the age, contents and shape of the cosmos
Planck's measurements will be finer than any previous satellite
The extra detail may confirm inflation, even find new physics
Scientists say there are temperature variations in this ancient heat energy that can give them insights into the early structure of the Universe. Planck will be the third spacecraft to investigate the CMB, after Nasa's COBE and WMAP satellites.
"Planck has the sharpest sight so far; it has the most sensitive instruments and the widest frequency range; and it will therefore make that next big step," explained Esa's project scientist on the mission, Dr Jan Tauber.
"It will allow us to pin down all the basic characteristics of the Universe with very high accuracy - its age, its contents, how it evolved, its geometry, etc."
One key question facing Planck concerns "inflation". This is the faster than light expansion that cosmologists believe the Universe experienced in its first, fleeting moments.
Theory predicts this event ought to be "imprinted" in the CMB and the detail should be retrievable with sufficiently sensitive instruments. Planck is designed to have that capability.
Planck investigator Professor George Efstathiou from Cambridge University, UK, thinks the telescope could throw up fundamentally new discoveries.
"We will be probing regimes that have never been studied before where the physics is very, very uncertain," he said.
"It's possible we could find a signature from before the Big Bang; or it's possible we could find the signature of another Universe and then we'd have experimental evidence that we are part of a multi-verse."
Herschel and Planck are two separate entities but were developed as part of a single programme within the European Space Agency.
Their construction was led by the French-Italian outfit Thales Alenia Space, which sub-contracted work to about 100 other firms including Europe's biggest space company, EADS Astrium.
Göran Pilbratt, Esa's project scientist on Herschel, describes the telescope's aims
Although the formal build of both telescopes only began eight years ago, their genesis goes back much further.
They are both a consequence of Esa's "Horizon 2000" roadmap initiated in 1985 to define the agency's scientific priorities at that time.
Herschel itself can trace its lineage to a 1982 workshop held in Noordwijkerhout, Holland, where the case for a far-infrared space observatory was first sketched out.
Esa's commitment to Herschel and Planck totals 1.2bn euros which includes the cost of the Ariane launcher and the continued operation of the telescopes in the coming years. Agency member states have contributed a further 700m euros for the telescopes' instruments.
The UK, for example, has put additional investment into the Cardiff University-led consortium that built Herschel's SPIRE instrument.
Herschel-Planck project manager Thomas Passvogel explains the telescopes' orbits
The 1.9bn-euro total makes Herschel and Planck the highest value payload Esa's science division has ever put on a single rocket. As is customary with institutional launches, neither telescope is insured.
"Ariane is a superb launcher but it is still a complex machine, and the difference between success and failure can be tiny," said Mr Dordain.
"There is a mixture of serenity - because we have done every test that we can do - but also anxiety - because it is always risky to launch such satellites towards deep space."
Herschel and Planck take many weeks to get to their observation stations
These are at a special location known as the second Lagrange point (L2)
Gravity conditions at L2 allow for cheap (in fuel terms) orbital corrections
Environmental conditions (heat & radiation) are more stable than at Earth
L2 takes its name from its discoverer, Joseph-Louis Lagrange (1736-1813)
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