Reply To: What if?
OK, so if you were standing on Pluto looking at Charon and saw something walking around on the surface, aimed you 308 and fired, how long would it take for the bullet to get there and how much would you have to lead the target?
Answer, you can’t do it.
http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html
The escape velocity of Pluto is 1.2 km/second which works out to a bit over 3900 feet per second. .308 ammo clocks in at 2700 fps or so. It will return to Pluto.
OK that’s the easy answer. But is it possible that by shooting straight up, it will climb to the height of Charon before returning? OK, time to drag out the orbital mechanics books.
2700 fps = .82296 km/sec.
The circular orbital velocity is always 1/sqrt(2) times the escape velocity. So given an escape velocity of 1200 meters per second, circular velocity is 848 meters per second. In other words, if you fired a rifle where the bullet traveled at that speed, horizontally on the planet, it would skim along the surface and be in orbit (until it hit a hill somewhere). Now this is not much higher than the speed of .308. So let’s figure I am shooting a slightly more potent, specially loaded round that moves at 2782 feet per second (the same as 848 meters/sec).
A circular orbit has a semi-major axis of whatever Pluto’s radius is. However, here’s another fact about orbital mechanics. If you are a given distance from the focus of the orbit, and moving at a certain speed, it doesn’t matter what direction you’re going, your orbit will have the same semi-major axis. It might be circular, a fat ellipse or an extremely narrow ellipse. And of course it might intersect the surface of the primary, which would mean it’s not really an orbit, but mathematically the part of the trajectory above the surface behaves like one.
So firing that slightly hot .308 round at 2782 feet straight up gives you an *extremely* narrow ellipse with a semi major axis the same as Pluto’s radius. Given that the center of pluto has to be inside that ellipse, you can’t possibly get even that hot 308 round out to more than double the distance from Pluto’s center. Pluto’s radius is 1185 kilometers (according to that fact sheet), so that’s how far above the surface the bullet can go. Not enough to reach Charon.
But it is kind of cool that a .308 round can go into orbit, if you fire it from the highest spot on the planet.
