The earth surface is 70% water, surely you can aim for an ocean
Atmospheric drag is the main determining factor, and the atmosphere at high levels is thin, but not particularly flat, and difficult to measure. Add in the fact that orbital objects move at ridiculous speeds, and the errors quickly add up.
> The batteries, nine in total, were released on 11 January 2021 and will undergo a natural reentry, which is now predicted for around 18:56 CET on 8 March +/- 0.4 days.
Half a day of uncertainty! This was a day before reentry!
https://www.esa.int/Space_Safety/Space_Debris/Reentry_of_Int...
The batteries from this incident are typically returned to Earth in a controlled manner via a cargo spacecraft, but a failed launch required an alternative plan.
> The uncontrolled disposal of the pallet, however, was not part of the original plan. It was made necessary by a disrupted spacewalking schedule following the failed launch of a Soyuz rocket in 2018, which forced NASA astronaut Nick Hague and Roscosmos cosmonaut Alexey Ovchinin to make an emergency landing in the Kazakh steppe. This event led to a backlog in the disposal of such equipment. Normally, old batteries would be placed inside an HTV and jettisoned from the ISS to burn up on re-entry.
https://gizmodo.com/massive-iss-cargo-pallet-reentry-earth-m...
> "NASA specialists use engineering models to estimate how objects heat up and break apart during atmospheric reentry," they added. "These models require detailed input parameters and are regularly updated when debris is found to have survived atmospheric reentry to the ground."
So, yes, they do rely on stuff burning off as it goes, but with some understanding whether that should actually happen.
Any object coming from the ISS will be going 25,000 km/h. I don't think "aim" is the word since the Earth is moving rapidly beneath the object. There's no stationary point to aim at. Plus once the object of whatever shape in the atmosphere drag which causing friction to burn it up is not controllable. It would break apart and each object with different mass now has its own path and velocity.
I'm amazed any of it works as well as if does now.
I really don’t think that the use of the word “aim” here necessitated this level of pedantry.
Anything released from the ISS will initially be in an orbit very similar to the ISS. That's because the initial difference in velocity between the object and the ISS will be measured in meters per second or less (which is small compared to the orbital velocity measured in kilometers per second).
Then the orbit of the released object will start to slowly decay because of (at that altitude, a very small amount of) air resistance. The details of that depend on the shape, size and mass of the object. It can even depend on unexpected solar activity which can cause the Earth's atmosphere to puff up minutes[1]. The ISS counteracts this by periodically boosting its orbit.
The longer the orbital decay takes, the more the uncertainty adds up. The fix to accurately "aim" an object, is to strap it to a rocket and perform a deorbit burn, to slow down from orbital velocity. This way the reentry process can be more reliably aimed at the middle of the Pacific or something like that. But that's expensive.
[1] https://www.smithsonianmag.com/smart-news/solar-storm-knocks...