With a combined cost of 1.6 billion euros (2.17 billion dollars), the Herschel and Planck telescopes represent Europe's greatest-ever investment in orbital astronomy.

But the price tag is on a scale with the mysteries that these machines hope to pierce.

How do stars and galaxies form? When did the Big Bang -- the instant that kickstarted the cosmos -- occur? What are dark matter and dark energy? And will the Universe expand forever, or will it one day implode in a gigantic "Big Crunch"?

The largest reflector ever to be launched into space, Herschel will scan never-before-detected infrared radiation wavelengths for clues on how stars and galaxies were formed, and whether they continue to come into being.

Even the coldest of space objects give off thermal radiation, which means they can be "seen" by infrared telescopes even if they are invisible to the human eye.

Deployed in a distant orbit beyond Earth's obstructing atmosphere, Herschel will be the only instrument in space able to cover the farthest reaches of the infrared part of the energy spectrum.

It can also penetrate massive clouds of dust, allowing it to delve into deep space, into regions never before explored.

The new data could show whether all galaxies were created at the same time, as some astronomers argue, or whether there is a non-stop galaxy-making mechanism still at work.

They may also tell us which came first, galaxies or the stars of which they are composed.

The instruments onboard Herschel must be cooled to temperatures just above -273.15 degrees Celsius (-459 Fahrenheit), also known as absolute zero, otherwise the heat they generate would ruin the observations.

Herschel has a co-passenger aboard the Ariane-5 ECA heavy rocket, scheduled to lift off at 1312 GMT on Thursday from the European Space Agency (ESA) launch pad in Kourou, French Guiana.

It is Planck, a 1.5-meter (five-feet) telescope with two ultra-sensitive detectors of cosmic microwave background radiation, or CMB.

Discovered by chance in 1965 by two radio astronomers in the United States, CMB was quickly recognized as the best proof available of the primaeval explosion that created the Universe as we know it.

CMB is the cooled remnant of the first blast that travelled freely across the heavens, a kind of "fossil" radiation released shortly after the so-called Big Bang.

In detecting this radiation -- omnipresent but with no particular source -- Planck will be seeing the Universe as it was almost at its origin.

It will observe the microwaves by measuring minuscule differences in temperature across patches in space, thought to be the imprints left by CMB by the earliest seeds of today's huge concentrations of matter, such as galaxies.

The probe may also yield clues on the mysterious "dark matter" that scientists say accounts for around 23 percent of all the stuff in the Universe, but which has never been directly detected.

Likewise for "dark energy," a theoretical phenomenon that could account for 72 percent of all energy in the Universe. It could explain why the Universe is expanding at an accelerating rate, rather than slowing down.

Herschel is a feat of engineering, combining the need for lightness with optical precision.

Its huge primary mirror -- 3.5 metres (11.4 feet) across -- features an unprecedented fusion of 12 silicon carbide petals into a single piece. The mirror has had to be polished to eliminate any "bumps" bigger than a few thousandths of a millimetre high. Planck's two detectors hold a claim on being the most sensitive CMB sensors ever built.

Herschel is scheduled to work for three years. Its mission will end when the helium used to cool its instruments runs out. Planck is set to yield data for at least 15 months, perhaps longer depending on its own cooling capacity.

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