BERLIN (AP) — How do you land a spacecraft on a comet that is streaking by at 41,000 mph (66,000 kph)? That's a problem scientists have been grappling with for more than a decade as they prepare for one of the most audacious space adventures ever — the European Space Agency's attempt to land a scientific probe on the giant ball of ice and dust known as 67P/Churyumov-Gerasimenko.