Need some maths help with my rocket....

..simulation :)

I'm writing a rocket simulator in Scratch and want the rocket to appear to take off and reach orbit but I need some non-linear function to translate the real distances 0 - 1000000 metres to -170 to 50 (these are Scratch Screen units)

I'm looking for a function that will approach a limit on screen of y=50 when real height is 1000000 but if the rocketeer makes a bigger orbit - I need the function to stop the image disappear off the top of screen :)

My maths mojo has deserted me! help please :)

Si