Incidentally, you have the question backwards: no one really cares when it's going to fail. We care when it's not going to fail: will the spacecraft make it to

Incidentally, you have the question backwards: no one really cares when it's going to fail. We care when it's not going to fail: will the spacecraft make it to its destination or not? It doesn't really matter what happens after that. This might seem like a nitpick, but changes in approach and mindset like this are often the difference between success and failure with "impossible" problems like this. So it's critical to get your approach right!