The Rosetta mission to chase down and put a small lander on a comet has lifted off from Kourou in French Guiana.
BBC News Online looks at the aims of the £600m European space adventure.
What exactly is Rosetta?
It is a 3m by 2m spacecraft packed with scientific instruments that will journey across the Solar System to investigate several asteroids and the Comet 67P/Churyumov-Gerasimenko. The mission will take 10 years to complete.
Why is the mission so long?
Scientists want to rendezvous with Churyumov-Gerasimenko out near the orbit of Jupiter.
No rocket technology exists to punt the 3-tonne spacecraft direct to this location with the speed it will need to go into orbit around the comet.
So, Rosetta must use the gravitational attraction of the planets and the Sun to throw itself into the right position. Rosetta will round the Sun four times, flyby Mars once (2007) and the Earth three times (2005, 2007, 2009).
This will see Rosetta enter the asteroid belt twice and reach Churyumov-Gerasimenko in May 2014.
What will Rosetta do in the asteroid belt?
Researchers have identified a number of interesting targets to investigate. One or more will be selected during the course of the mission for close inspection. Asteroids are the debris left over from the building of the planets. These shattered mountains of rock, which can be tens of kilometres across, live between the orbits of Mars and Jupiter. Rosetta will photograph its targets, provide information on their mass and density; measure their subsurface temperature and look for gases and dust around them.
What is the plan for Churyumov-Gerasimenko?
When Rosetta arrives at the comet it will be cold and inactive. The spacecraft will go into orbit around the 4km-wide ball of ice, rock and dust and make a detailed map of its surface. Rosetta will then release a washing-machine-sized lander, called Philae, on to the comet nucleus which will anchor itself down as the comet sweeps into the inner Solar System at speeds up to 135,000km/h.
What will happen as the comet approached the Sun?
The orbiter and lander are expecting a rough ride
The radiation from our Sun will cause the ices on the comet to sublime - they will straight from a solid into a gas. Material will be ejected at supersonic speeds. Gas and dust will be thrown out around the comet to form a coma, and away from the comet to form tails. The Rosetta orbiter and Philae lander will record the whole process and beam images and other data back to Earth.
What will Rosetta tell us?
It will give scientists the best information yet on what comets are made of and how they are put together. Comets are thought to contain materials that have remained largely unchanged since the formation of the Solar System 4.6bn years ago. These cosmic icebergs contain compounds that are rich in carbon, hydrogen, oxygen and nitrogen. Intriguingly these are the elements that make up nucleic and amino acids, the essential ingredients for life as we know it. There are some who believe comet impacts in the early years of the Solar System seeded the Earth with the right chemistry for life.
Why is the mission called Rosetta?
The spacecraft is named after the Rosetta Stone, a basalt slab uncovered by French soldiers in 1799 in the village of Rashid in Egypt's Nile delta. Its markings enabled scholars to unlock ancient Egyptian hieroglyphics. Scientists hope the Rosetta mission will do something similar for their understanding of the evolution of the Solar System. As a connection to the original Rosetta, the spacecraft carries a digital disk containing the opening chapters of the Book of Genesis written in 6,000 languages.