Steve Higgins, London asked:
Assuming there's no wind resistance and you had a gun powerful enough, could you shoot a bullet straight up and hit a satellite which is in stationary orbit?
Dave - Do you mean a geo-stationary satellite, straight above your head or just any one in orbit?
Steve - If you knew there was one up there, directly above your head.
Dave - If you fire a bullet up fast enough you would be able to get it past the geo-stationary orbit up at about 36,000km. Basically the orbit takes exactly the same amount of time as a day so it looks like it's above our head all the time. However that still means the satellite is moving at about 11,000km per hour. I don't know if you've ever heard of people trying to shoot ducks or clay pigeons. If something's moving fast you've got to shoot in front of it because it takes a while for the clay pigeon to get to the bullet in front of it. If the satellite was straight above your head it would actually go miles and miles behind it by the time the bullet got there. If you aimed ahead enough and if you calculated it right, if you fired it out fast enough. If the escape velocity is about 11.2km/s – if it's going that fast it'll get out into the Earth's orbit and you should be able to get out of it and hit it.
Chris - But you'd need a pretty powerful gun, wouldn't you Dave?
Dave - No one's built one fast enough yet!