Just some random space debris? To advanced for them to penetrate? Some new alloy that wasn't known when they departed?
If they sent it on purpose to them with fuel rods, it would have been nice to engrave some instructions on the shell.
Right, they left it completely unexplained; nothing, not even a word about them not being able to penetrate it.
shareIt was clearly the product of an intelligent design, not random space debris. And it wasn't man-made. If it was man made and had been intended to deliver fuel to the astray Aniara, it would've had some kind of clear cut instructions (either printed on its hull or a radio message emitting off it from inside) on how to open it and would have also not been impossible to penetrate.
You could say it was some probe or unknown alien artifact. The chances of running into such an object in space is astronomically minuscule to being practically impossible (unless it had been designed to detect intelligent devices such as the Aniara and navigate towards them).
This movie had some interesting bits, but it had no character development, highly improbable scenarios and huge plot holes. Was OK as a one time watch, but doubt I'll be re-watching down the line, even though I love space-based sci fi flicks with decent visuals. 6.5/10 for me.
The best possible explanation is some kind of science probe or military weapon, which would mostly account for it not being meant to be opened, although not really explaining why it wasn't possible to get into it at all or that it was made from unobtanium materials.
The general plot purpose -- false hope of a rescue -- was fine, but leaving it a total mystery or potentially interstellar in origin was just needless ambiguity.