to find out more about this incredible advance in space exploration, we are joined via skype by dave brodyand technology writer. thanks so much for chatting with us. let's start by looking at why this is such a significant achievement. and, of course, what needs to be done to make sure that it's a successful mission? >> sure. well, in order for it to be a complete success, rosetta actually has to get to the comet. what happens a little later on this evening is a firing of the engine for about six and a half minutes. rosetta will begin a very slow beautiful dance down toward the nucleus of the comet, this will take place over a long time, it will take months to happen. by october, they expect to be about 30 kilometers away. by november, 10 clicks, they may get as close as 2000 meters. that's important, they want to take very detailed images of the surface of that comet, so they know exactly where to land the lander. that's when the fun begins. then we'll start actually knowing what this comet is truly made of. >> yeah, and i mean, that's the main question. what have we learned from doing th