conservation of momentum

I came up with this while driving home yesterday and being annoyed by a fly. You are driving down on a straight country highway at 100km/h (60mph) relative to the ground. A fly on the back window takes off and starts to fly towards the windscreen at about 10km/h (6mph) relative to the car...

Someone on another forum is very pleased with themselves at having come up with a fool-proof demolition of the "official story" and believe that it is refuted by applying the conservation of momentum. They also link to some execrable page from the execrable "What Really Happened" website. But...

I' m currently studying the conservation of momentum (Barely started about an three hours ago). I have no idea what is. Googling it explains it to me and I somehwat understand it. Can anyone give me any articles or reports on the law and why it wasn't violated so I can understand it better. Thanks.