Let's say you're driving along the highway and come up on another car. Try to get behind him and gently nudge his bumper. (Don't try this in real life.) Not too hard, right? That's docking with the ISS.
Now, do the same thing, except he's stopped on a steep downhill slope, and you're going 70MPH. You can use your brakes, but you have to push them pretty hard if you push them at all. If you come to a stop too early, the hill will accelerate you into his bumper. If you start on the brakes too late, you'll hit too hard. You have to time it precisely.
That's more or less landing on the barge, except you also have to steer in two dimensions, and you're coming it at more like 700MPH.
Let's say you're driving along the highway and come up on another car. Try to get behind him and gently nudge his bumper. (Don't try this in real life.) Not too hard, right? That's docking with the ISS.
Now, do the same thing, except he's stopped on a steep downhill slope, and you're going 70MPH. You can use your brakes, but you have to push them pretty hard if you push them at all. If you come to a stop too early, the hill will accelerate you into his bumper. If you start on the brakes too late, you'll hit too hard. You have to time it precisely.
That's more or less landing on the barge, except you also have to steer in two dimensions, and you're coming it at more like 700MPH.