The ISS is around 400km above us. A rocket needs a speed of at least 8km per second to get to space. If we cut out the acceleration part it could in theory reach the ISS in around 50 seconds. Even if we factor in the acceleration part etc. it should still be very quick up there. Yet the fastest possible time to get to the ISS is 4 hours. That would be an average speed of 100Km/h which is way slower than the speed of the rocket after a few seconds. Why the long journey?
In: Planetary Science
Another factor is that you don’t want to perform a lot of maneuvers near the ISS. All of the gas from the engine exhaust travels far, and could cause unwanted vibration of the station, or directly damage it.
You want to approach slowly, carefully, and with just small puffs of your maneuvering thrusters.
Other comments have addressed the answer. I suggest playing Kerbal Space Program (version 1, not 2). It will give you a good understanding of how orbits work.
Well, it took an SM-3 [a few minutes](https://web.archive.org/web/20120214031001/http://www.mda.mil/system/aegis_one_time_mission.html) to hit a satellite about 220 miles up, so your math seems about right.
Like people say, it’s docking. The real reason is that it’s most fuel efficient to [adjust an orbit halfway around from where you want the effect](https://en.wikipedia.org/wiki/Orbital_mechanics#Orbital_maneuver). Launch – ISS docking takes [a few different orbits](https://www.researchgate.net/publication/260971173/figure/fig1/AS:883542645022721@1587664390571/Profile-of-vehicle-transfer-from-insertion-orbit-to-the-ISS-orbit.png): launch into one, lift up to the right altitude, match position to docking approach, match orbit to ISS. Or something like that, not a rocket scientist. They may be waiting to do each one of those adjustments to the exact right time on the orbit, so up to 45 minute delays or so with each step.
In the simplest terms: getting to the ISS isn’t about traveling the 400 km, it’s about accelerating to 8 km/s (28000 km/h) and ending up in the right place at the right time, so that the relative speed if you and the ISS is about zero. If you went straight up and parked yourself in ISS’s way, you would only get an extremely high speed crash (at Mach 28!) Fun fact, we actually overshoot the station at first and use the magic of orbits to let it catch up to us. Watch [this magnificent bastard explain the current speed record :).](https://www.youtube.com/watch?v=bUi0yWc5Dnw)
I’m gonna add a few other reasons that I don’t think are here yet.
TLDR: There’s nothing in the laws of physics stopping you from launching and docking 15 minutes later, I do it in Kerbal Space Program all the time. However, engineering, risk assessment, timeliness, and biology tend to set us far back from the theoretical minimum.
As you now know, you’ve got to get into a matching orbit. It only takes about 8-12 minutes to get into orbit depending on the vehicle, so in theory you could time your orbit just right to end up right next to the ISS, say 1 kilometer away. It would still take like half an hour to dock and what not unless you’re incredibly cowboy-esque and get right up to the station.
However, most rockets don’t enter orbits that high immediately. The ISS is pretty high up. While you definitely could reprogram the rockets to do that, it would involve a lot of the trajectory being far steeper than it would have otherwise been. If the crew need to abort, they would then be re-entering far more intensely than they would have otherwise, and the spacecraft may not be able to handle that.
This method will also let out a lot of rocket exhaust gas in the vicinity of the station, and the station is coated in a bunch of sensors that won’t like that.
This method also guarantees that a massive second stage booster ends up in an orbit almost identical to the ISS’s orbit. While most modern second stages can de-orbit themselves, what if something goes wrong? It could drift into the ISS and smash stuff a few orbits later. Normally second stages enter lower orbits initially so they pose no risk to the ISS, and the actual spacecraft will complete the rest of the rendezvous.
What if the stage blows up during the last few seconds of the burn and creates a massive debris field that obliterates the ISS?
Due to inclination, you’re only going to get 1 or 2 moments per day when your launch site is directly below the target orbit (unless you are at 0 degrees latitude and the station is in a perfectly equatorial orbit, or you are at +-90 degrees latitude and the station is in a perfectly polar orbit). You want to launch at that time for fuel efficiency reasons. This rarely lines up with the launch time that would put you closest to the station from a distance perspective. You could only launch at these rare times, but that severely limits your launch opportunities. While you definitely can correct for inclination and launch at a better time for distance at the expense of fuel, this makes the rocket software a lot more complicated, increases fuel usage, and really, you’re only losing maybe a day of time at most, the effort to reward ratio isn’t that high.
And from a more human factors perspective, 70-90% of astronauts experience spacesickness, which usually starts shortly after launch and goes away after a while. Most dockings are automatic these days, but you want to have the humans available to do it if the computer fails. Asking a crew to dock while throwing up is a bit much.
Even asking a crew to dock half an hour after launching is a bit much, your mind is all over the place after a launch. Maybe something went a little wrong during the launch that you are still processing. Giving a crew a few hours to clear their minds, allow time for checkouts and window-staring, and giving ground control a few hours to change gears can be a very good thing.
I feel like the complicating factor everyone is missing is the orbital mechanics. Maneuvering in orbit is rather counterintuitive and complicated when it is in reference to another orbiting body.
Getting to the orbital plane of the ISS, 400km, is one thing but once your there you have to reach it too. On a highway you’d accelerate to catch up to them, but in orbit this has the effect of raising your orbit. But you won’t go up in altitude right away, your orbit is raised on the opposite side of the planet and your altitude goes up as you make your way around, however you’ll consequently also start slowing down relative to anything in orbit at the altitude your started at and you’ll actually fall behind your target instead of catch up.
So you see it’s really not as simple as just driving full beans on an intercept course unless you intend to hit it like a missile, which the ISS would really rather you didn’t. Rendezvous with the ISS for docking takes a lot of carefully timed and planned out maneuver to reach the correct side of the ISS at the correct speed and trajectory and with zero chance of accidentally setting yourself on a collision course and that just takes a lot of time.
You’re trying to find a needle in a haystack, except we know exactly where the needle is – it’s moving at 4.76 miles per second towards Earth. Matching the position and speed of the ISS’s orbit is the time consuming part. It’s hard to launch a rocket into orbit, it’s even harder to do it with exact accuracy relational to another object.
ELI5: if you wanted to board a moving train from a car, you wouldn’t just slam into it, that would be the fastest way to get to it but it’s not what you want. You would slowly start to approach it until you are going at the same speed, then you can just hop on.
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