It’s the same reason that getting on a flight doesn’t take, say, 30 minutes when the airport is 15 miles away. You have to go through all sorts of extra things to get there that you wouldn’t have to worry about if you’re going in a straight line without stopping.
Also, the ISS is 250 miles up, but it’s much, much, much, much more distance farther away even just minutes later because it’s going so fast.
So imagine this: you want to get on a bus that’s on a racetrack and is zipping around at 100 miles per hour. When it’s at its closest, it’s just 50 feet away from you. Now picture yourself getting in another bus, and that bus has to accelerate to 100 miles per hour. It has to start accelerating at the right time to end up going 100 miles per hour at about the same place on the racetrack as the other bus, else you’d have to travel a good distance to catch up to the other bus.
All that time accelerating, catching up, mostly matching speed, then slowly coming up to the other bus takes a LOT more time than just walking 50 feet.
Latest Answers