Armageddon
It is certain that events in space can cause life on Earth to come to an end. There are many scenarios that have been suggested that could happen in the future. Some are certain to happen and will almost certainly end humanity, but may only happen on a very long timescale. Others are likely to happen on a shorter timescale, but will probably not completely destroy civilization.
If an asteroid or comet would collide with our planet and wipe out its life, we can be certain that governments will find out earlier than the rest of the population. The end of planet Earth is not necessarily synonymous with the extinction of humanity. It is possible for humanity in the future to find other places to live. However, suppose governments found out, but too late to do anything about it. In such a situation, would secrecy be the best policy or deserve people to be informed about their imminent Armageddon?