Mankind's quest to peer ever deeper into the cosmos gained a massive boost on Thursday as Europe released two mighty observatories into space and the US pressed ahead with refurbishing the acclaimed Hubble telescope.

The two European instruments comprise Herschel, the most powerful infrared space telescope ever built, and Planck, designed to delve into the remnants of the "Big Bang" that created the Universe some 14 billion years ago.

The pair were hauled aloft by an Ariane 5 ECA heavy rocket from the European Space Agency's launch pad in Kourou, French Guiana.

Relieved engineers gave a round of applause after the 1.8-billion-euro (2.44-billion-dollar) payload -- Europe's most expensive gamble in space astronomy -- escaped Earth's gravity and separated from the rocket in low orbit.

"ESA is en route to the origin of the Universe," declared Jean-Jacques Dordain, the agency's director general.

"Let's imagine that we shall see the first light after the 'Big Bang'," he added.

The two unmanned craft will separately make their way over the coming weeks to a spot called the second Lagrangian point (L2), around 1.5 million kilometres (937,000 miles) from Earth.

Herschel is designed to look into deep space to explore how stars and galaxies accrete from clouds of gas and dust.

It boasts a primary mirror of 3.5 metres (11.37 feet) across, more than four times larger than any other previous infrared space telescope.

Planck is a 1.5-metre (4.8-feet) telescope with two ultra-sensitive detectors of Cosmic Microwave Background (CMB) radiation, as the backwash of energy from the "Big Bang" is known.

With luck, Planck will provide "the sharpest picture ever" of the Universe, providing a snapshot of cosmic physics when the heavens were just 380,000 years old, ESA says.

It could help explain why the Universe's expansion is accelerating and throw up clues on "dark matter," which is believed to account for around 23 percent of all the stuff in the Universe but has never been directly detected.

Meanwhile, two astronauts from the US shuttle Atlantis began the first of five space walks to overhaul the Hubble, a 13.2-metre (43-feet) instrument lauded for revolutionising knowledge of space.

"It's an unbelievably beautiful sight," said astronaut John Grunsfeld, a 50-year-old astronomer, as he pored over Hubble's exterior, which seemed in good shape despite years of exposure to ultraviolet radiation and space debris.

Fitted with a new science computer and a new-generation camera designed to capture light that may have been emitted as far back as 500 million years after the Big Bang, the Hubble may be able to extend its operations by at least five years, NASA hopes.

That will give the agency time to deploy a more powerful successor, the James Webb Space Telescope.

Space telescopes have a big advantage over Earth-based instruments because they are not affected by local light pollution, nor are they inhibited by Earth's atmosphere, which disturbs visible light and can block other parts of the energy spectrum.

These include X-rays and gamma-rays which are the hallmarks of powerful phenomena such as neutron stars, pulsars and black holes.

The first space observatory was a little British probe, Ariel 1, launched in April 1962 to study solar ultraviolet and X-rays.

There are more than three dozen other space telescopes operating today, specialising in variously monitoring microwave, X-ray, gamma-ray, visible light, radio or ultraviolet energy.

The Hubble, launched in 1990, is the best known of all, with instruments designed to detect emissions from infrared to ultraviolet.

It has provided stunning images of star clusters and emerging galaxies, and in 1994 yielded the first conclusive evidence of the existence of a black hole, ending a theoretical debate that had raged for decades.