Power free suspended animation.

If a person were to run out of oxygen in a space suit they would die. That is, the brain and body would rot and deteriorate beyond reversal. However if the body was in vacuum rotting would not occur. To rot scavengers or bacteria are required. They are absent in the vacuum of space.

The problem is that to get to vacuum too quickly would cause explosive decompression (rapid expansion) with severe damage. Getting to vacuum too slowly would also be problematic as the rotting would continue in the decreasing residual air pressure.  However a goldilocks (just right) speed of decompression, or entering into vacuum, would leave a body perfectly preserved and capable of reanimation. This would result with storage in the vacuum of space requiring no power regardless of the passage of time.

A seemingly dead person stored in vacuum, if not damaged by explosive decompression, could be suspended indefinitely for a long voyage between stars. (This would be required if time travel is not available.) The problem would be getting the speed of decompression just right, to not spoil or explode. We should also be very careful with any alien bodies found in the vacuum of space as it might be awaiting reanimation.