If you are aiming for the International Space Station and accidentally overshoot it, can you make it back? Or do you just drift off into orbit until someone rescues you?
In: Physics
Others have said why you don’t miss, but if you did… Yes you can make it back. You are still in a low earth orbit if you miss the ISS, not shooting out to the stars. The crewed capsules have enough fuel to do an emergency abort and return to earth if something went wrong on the rendezvous and dock with ISS.
As long as you have remaining fuel you can always change your orbit. But you would not want to carry much extra fuel as every drop of fuel could have been cargo and every extra piece of weight you carry into orbit requires about ten times its weight in fuel to launch into orbit. So whatever maneuver you have to do to change your orbit have to use as little fuel as possible. If you run out of fuel you are not returning to Earth.
Changing the angle of your orbit requires a lot of fuel. So this is something you generally do not do. If the launch rocket did not bring you into the correct orbit you are better off returning home and trying again. But if you end up with the right angles but are just behind, in front, above or bellow your target it is possible to fix this. Firstly the bigger the orbit the longer it takes to complete an orbit. So in order to go forwards or backwards in your orbit you just lower or raise your orbit respectively and wait. But there is a trick to it. If you just burn upwards or downwards you will not actually change the altitude of your orbit, just how round it is. What you need to do to raise your orbit is to accelerate forwards. The centrifugal force will then carry you further from the Earth raising your orbit. Similarly when you want to lower your orbit you have to decelerate. This can be confusing but it makes sense if you think about it.
A typical launch profile to the ISS have the space station pass over the launch site right before launch. But more importantly you wait until the Earth rotates so the launch pad is directly under the track of the space station. Then you launch after the space station making sure to hit the exact angles of the orbit. But instead of trying to get to the exact same altitude as the ISS you aim for an altitude a bit lower then the station. As you are in a lower orbit you go faster then the station which is good because the station is ahead of you. And by going lower you have saved some fuel. The launch rocket is a big powerful rocket that is not perfectly accurate. The spaceship is much more accurate. So after the launch you first have to find out exactly what orbit you ended up in by making lots of measurements. Then you plan how you are going to raise your orbit to that of the ISS. You do not want to raise it too fast as this would slow you down so it takes too long to catch up with in. But you do not want to raise your orbit too slow as this means you will shoot past the station at the lower altitude and have to go to a higher orbit in order to try again, and you do not have much fuel for this. So you raise your orbit at just the right speed so you end up catching the ISS at exactly the right time. This entire procedure will with the current technology and procedures take about a day.
Since other’s have answered the basic question, I thought I’d add a fun related detail.
When Apollo missions went to the moon, they used something called a “free return trajectory”. This meant that if something happened and their rocket wouldn’t fire when they were near the moon, they would fall back to Earth automatically in a way that would allow a safe return. This actually used a little more fuel on launch, but was done for safety.
It’s pretty much impossible to miss like that. The math involved is very straightforward, and you’d know hours in advance whether or not you were going to miss, and could make corrective maneuvers to prevent it.
If things are so bad that you don’t know that you’re going to miss until after it happens, then all bets are off as to what the result would be, because your telemetry equipment has apparently broken down so badly that you don’t know where you are anymore.
You don’t really aim for the ISS. You aim for a similar orbit, and then adjust that orbit so that eventually you’ll wind up near the ISS, at which point you adjust it again to match orbits.
In this sense, you can’t really overshoot – just goof up your adjustments which only requires more adjustments.
Latest Answers