Europe's Herschel and Planck telescopes have blasted into space on an Ariane 5 rocket from Kourou in French Guiana.
The satellites are being sent into orbit to gather fundamental new insights into the nature of the cosmos.
The Ariane thundered clear of the launch pad at 1312 GMT (1412 BST) - its flight lasting just under half an hour.
Mission controllers in Germany made contact with the telescopes over the Indian Ocean once they had separated from the rocket's upper-stage.
The acquisition of the signals, relayed through ground stations in Australia, will have been a moment of huge relief for everyone connected with the two observatory projects.
Their combined programme cost is 1.9bn euros (£1.7bn; $2.5bn), which made Herschel and Planck the highest value payload the European Space Agency's (Esa) science division had ever put on a single rocket.
"The launch today is just one step in a long chain of decisions and a fantastic amount of work by thousands of scientists and engineers," said agency director-general Jean-Jacques Dordain.
"All these scientists and engineers have worked together with just one objective, which is to discover the unknown and to make the technologies that are necessary to make this scientific progress."
The ascent through the Earth's atmosphere was just the first stage in what will be a long journey for the astronomical satellites.
They will spend the next two-to-three months making their way out to observation positions some 1.5 million km from Earth on its "night side".
The long cruise will allow engineers to check out sub-systems and commission the telescopes' instruments. One of the first things controllers have to do is to get a precise description of the telescopes' orbits.
The Ariane is an excellent rocket but the satellites will almost certainly need some correction to their flight paths to be sure of making the proper transition to deep space. To save fuel, the controllers would like to make these correction manoeuvres as early as possible, perhaps on Friday.
Birth of stars
Herschel is the largest telescope anyone has 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, an advanced ceramic material.
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.
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."