Good question... but what is the correct answer?
Is the de-orbit of the ISS planned?
No correct answer as of yet. Should be interesting to see when all of the international partners agree to no longer support the effort.
Kenneth - N5VHO
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Could you please add an option - before 2015 ie deorbited due to onboard accident?
The reason why you need to de-orbit space stations like MIR and the ISS is because they take a lot of maintenance to keep them in their proper orbits and you absolutely do not want bits of space station landing in anyone's back yard.
In the case of ISS it needs regular re-boosts to keep its orbit from decaying, this comes from visiting space craft or burning the limited supplies of fuel on board. Additionally the attitude of the station is controlled by fly wheels that have a limited life.
Essentially if you don't bring the ISS down in a controlled way they it will come down on its own when the systems that keep the ISS in its stable orbit fail. Needless to say this is not a good scenario.
As to taking a trip to Mars, its a nice idea but would require a big propulsion module as the thrusters on the ISS are only designed for maintaining the stations orbit. There is also the little issue of space radiation, the ISS gets a lot of protection from Earths magnetic field in LEO and does not have enough shielding for deep space missions.