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
The ISS is moving at 28,000km/hr.
You don’t just hit it with a rocket and say job done.
You need to enter orbit (moving fast enough sideways so when the spacecraft falls it doesnt hit the earth) AND match velocity with the ISS so that you can dock to the station without destroying anything.
Checking trajectory, equipment, and communications takes time. Over-do a burn and you have to waste more precious fuel to slow back down.
Yes, one could “technically” do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way.
The people in the ISS want the rocket to approach them at approximately 0 speed, relative to the ISS (which is of course moving really fast). If you approach them at high speed, there is a collision and everybody dies.
It takes time to get into the right orbit and then very slowly approach the ISS.
The ISS is not static. It’s orbiting, which means anything that wants to get to it and not just ram it, needs to match its trajectory. So the rocket is actually getting into a (largely) circular orbit itself, such that it will intercept ISS, adjust and match.
Put differently if the only thing the rocket did was travel 400 km upwards it’d fall right down
So the ISS is moving very fast, if you want to safely dock it, you need to have the right velocity, going too fast would be catastrophic, since you would slam into the ISS and break it, on the other hand going too slow would mean you wouldn’t catch up to it. So they fire the rockets so that the astronauts can go into orbit, where they can safely make sure that they get the right velocity, which takes time.
The Russians have done it in about 3 hours (Soyuz MS-17), but generally it takes longer, since it is much safer
hitting it directly would be impossible
Most of getting to orbit is reaching that 8km/s, that’s very fast
Most of the docking with the ISS is planning your rendezvous, so it happens at a nice and slow speed, and you use as little fuel as possible.
We can’t just say “the ISS is this way, so let’s go this way” we actually need to be in a slightly higher or lower orbit, depending on if we are ahead of or behind the ISS. We want our difference in speed to be very small, so we need our difference in orbit to also be very small, so we need to make multiple orbits with that small difference until we reach the ISS. Going near the ISS with a large speed difference can be very catastrophic
I believe that at least some launches will also deliberately ‘miss’ the ISS. That way if there is a failure later on in the launch (after getting into orbit), the vehicle/payload will not be aimed directly at the ISS.
You technically could launch pretty much directly to the ISS and ger there quickly… but there’s no reason to hurry, so you do it step by step, launch close, check everything good, get closer, check everything is good, repeat.
orbit means lateral speed, not just altitude. you could strap on your bionic legs and leap 400 km into the air. then wave at the space station as it goes whizzing by, but you would then fall back to roughly the same place you jumped from. in order to be in orbit, you have to be moving parallel to the earth surface, faster than you are falling.
It’s not about getting to the ISS. It’s about getting there while both are moving at the same velocity.
If you wanted to pass a glass to somebody on the far side of the room, the quickest way would be to just hurl it at them as hard as you can.
But since you probably don’t want to shatter a glass in their face, you’re likely to take the slower option which gets the glass there at a more acceptable speed.
Imagine you’re driving a car to the next city and you only have your feet to slow down. And you have to come in at exactly the right angle, or else you’ll overshoot or use up all of your fuel trying to correct the mistake.
I’m sure you could shoot a missile at the ISS and have it reach it that fast, but when it got there the two objects would be flying perpendicular to each other at like 30K miles an hour.
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