For the same reason that they de-orbited MIR it will eventually outlive it's purpose.
The modules have a life span, the ISS's mission has already been extended beyond it's original scope, eventually it will come to a point where you can not maintain it and it actually becomes a risk to the astronauts (we didn't had major incidents with the ISS, MIR had a few including a fire, but no major casualties, but after nearly 20 years things can start to break down and a catastrophic failure can very likely result in loss of the entire crew).
The ISS is also limited in terms of what you can bring to it, what type of experiments you can run and how can you extend it so in some sense it limits our capabilities.
When something outlives it's purpose sometimes it's best to discard it because having it and investing resources into it prevents you from expanding and building something new.
We'll eventually have to build a bigger station that could be used for bigger experiments like micro gravity sustainable agriculture, industrial manufacturing and more. As well as potentially actually simulating gravity and other things.
Ideally you'll also want a space station quite further away from earth like maybe a 1/3rd of the distance to the moon which can be used as a launch platform and a future space dock.
Projects like that are going to be very expensive and giving up on the ISS could actually release quite a bit of funding as well as drive the need for a new space station because "well we got the ISS do we really need a new one?" is quite often used as an excuse just like orbiter development was hindered due to the shuttle being in operation for so long.
well the idea that is built with modules should imply that we can swap out the pieces as they age and as the mission changes.
if anything the real reason to abandon the ISS is the same reason we needed to abandon the shuttle program. Nostalgic money pits tend to stick around preventing progress and investment in new and better systems.
I still think one of the best things to happen to NASA in the last ten plus years was having the shuttle program shut down. Yeah it lead to years with lack of capability but it was that shuttle which held back such development and others have stepped in.
I would prefer to get to the moon, having a solid surface under you would have some great benefits, let alone being able to dig deep and hide.
You must remember that a space station is not something like an Army fortification or a building which stands by itself or require very little maintenance, which could be done by a stationed unit.
* The ISS is actually falling back to earth and needs to be lifted from time to time. You need fuel for that.
* The personnel must be changed frequently (too much time in microgravity is harmful to the body). You need to take people in and out.
* The station itself is very little compared to a building. You can't store years of supplies there, just 12 months or so. You need to send up supplies.
So, you must send fuel, people and supplies very often to the ISS. This is very very expensive.
So, it comes a time where you should consider shutting it down or keep using it.
Those are general arguments against pretty much any long term space habitat and they are completely wrong.
The ISS is constantly supplied and refueled that's not a reason why not to maintain it, it's an argument why we should not have built it in the first place which is a very silly argument to make.
We need space stations the more the merrier, China and India are getting into the game, Russia wants a new one of their own, and the public sector is also looking at such platforms for everything from micro-gravity manufacturing of drugs to space hotels.
The argument that people in favor of decommissioning the ISS should make is we need a bigger, better, further away one that we could use to go cheaply to the moon, asteroid belt, mars, and the Jovian and Saturnian moons.
> The argument that people in favor of decommissioning the ISS should make is we need a bigger, better, further away one that we could use to go cheaply to the moon, asteroid belt, mars, and the Jovian and Saturnian moons.
That is the answer to my question. Should we sink the ship because it is old and we have no use for it or should we sink it because we need another one and can't bear the cost of maintaining two?
If someone is willing to use it, then they must pay for it's maintenance. For NASA it's mission was accomplished, and it's time to move on.
Replacing all of the modules including the nodes would cost as much as building a new station.
The specifications are limited, there's a limit to the mass of the modules that can be connected to the nodes (torque is quite real ;)) and considering we don't have the space shuttle to assemble new modules easily (with the exception of Zarya, Unity(node 1), and Zvezda all modules and large parts like the Solar Panel arrays were assembled with the use of the STO) so at best you'll end up with an ISS redux with the same limitations to size, mass, power generation and module compatibility as the current one has.
We need to be able to build bigger modules for bigger experiments, we need more power, we need it to be higher to further avoid atmospheric drag and to be used as a launch platform for further experiments.
There are various proposals for both low orbit space stations like the Russian OPSEK which will be recoverable, and various future uses for the ISS* including using Node 4 and some of the ISS modules for the EGP which will be a refueling station parked around E/M L1 (approx half way to the moon).
The EGP could be build on the existing ISS and launched to L1 since it will have a central node and a service module additional modules even a habitat module could be later added.
* Please note that this will not reuse existing ISS modules in orbit but will reuse built/planned unlaunched parts like Node 4 and Zvezda 2.
These are good points. If I could, I'd revise my post above to say that we shouldn't de-orbit the ISS until we've used it to build something better.
MIR did not de-orbit until the ISS was well underway. More importantly, while building the ISS we had the Space Shuttle, which was like a temporary space station in terms of its habitat. The Space Shuttle pressurized volume was about 75 m^3, and it had an airlock and a big robotic arm.
In contrast, every orbiter currently under development is a little capsule without either. The SpaceX dragon is only 10m^3 of pressurized habitat, and every other is within 2x of that. These are ferries, not platforms for long-term construction projects in orbit. The ISS is the only option for the foreseeable future.
So I hope, if we want to build an interplanetary spacecraft or more-permanent space station, that we find a way to leverage the ISS to reduce the cost and effort.
The modules have a life span, the ISS's mission has already been extended beyond it's original scope, eventually it will come to a point where you can not maintain it and it actually becomes a risk to the astronauts (we didn't had major incidents with the ISS, MIR had a few including a fire, but no major casualties, but after nearly 20 years things can start to break down and a catastrophic failure can very likely result in loss of the entire crew).
The ISS is also limited in terms of what you can bring to it, what type of experiments you can run and how can you extend it so in some sense it limits our capabilities. When something outlives it's purpose sometimes it's best to discard it because having it and investing resources into it prevents you from expanding and building something new.
We'll eventually have to build a bigger station that could be used for bigger experiments like micro gravity sustainable agriculture, industrial manufacturing and more. As well as potentially actually simulating gravity and other things.
Ideally you'll also want a space station quite further away from earth like maybe a 1/3rd of the distance to the moon which can be used as a launch platform and a future space dock.
Projects like that are going to be very expensive and giving up on the ISS could actually release quite a bit of funding as well as drive the need for a new space station because "well we got the ISS do we really need a new one?" is quite often used as an excuse just like orbiter development was hindered due to the shuttle being in operation for so long.