Free Astronomy Magazine September-October 2014

SMALL BODIES target the comet 46P /Wi r t anen . The mission was consequently re- scheduled and as its final destina- tion was chosen another comet, 67P/Churyumov- Gerasimenko. In having to reach it from a trajectory and with a speed that would allow Rosetta to enter into its orbit, ESA engineers had to work out a long and complex in- t e r p l a n e t a r y route, on which a decisive role was to be had (as often happens) by gravity assists (namely, a close encounter with one or more planets to produce a gravitational slingshot effect and accelerate the probe along the next leg of its journey –a manoeuvre that allows to shorten flight times and save propel- lant). During the five heliocentric orbits after its launch, Rosetta made use of three different gravity assists with Earth and one with Mars, which allowed it to reach a maxi- A bove, comet 67P/Churyu- mov-Gerasimenko size is compared with that of some famous natural and man-made structures. [ESA] Left, the comet photographed in January 2004, after an earlier passage in the inner solar sys- tem. [ESA-ESO]