During its 10-year journey to Comet 67P/Churyumov-Gerasimenko, Rosetta will circle the Sun almost four times. It will also cross the asteroid belt twice and gain velocity from gravitational "kicks" provided by close swing-bys of Mars (2007) and Earth (2005, 2007 and 2009).
The gravity assist is the use of the gravity of a planet or other astronomical objects to alter the path and the speed of a spacecraft. The velocity of the spacecraft and the trajectory are changed by entering the gravitational field of a planet. The spacecraft accelerates as it approaches the planet and decelerates while escaping its gravitational pull. To accelerate, the spacecraft go with the movement of the planet and to decelerate, the spacecraft go against the movement of the planet.
But the spacecraft is also propelled by 24 paired bipropellant 10 Newton thrusters. Bipropellant thrusters are used for precision trajectory and orbit of large satellites and deeps space probes. The fuel used is the monomethylhydrazine (MMH) CH6N2, it is easily stored in outer space providing moderate performance for every low fuel tank system weight. There are also oxidizers like N2O4, MON-1, MON-3 The combusting chamber allows operational temperatures up to 1,500°C that enable maximum thruster performance. The thruster is provided with trimming orifices upstream of the propellant control valves. The thruster can also be fitted with heaters and thermistors for thermal control.