The general rule on a Stack Exchange site is that it is for questions and answers. “One-way” communications are permitted — provided that they are written in the form of a question and answer  , and the question is otherwise on-topic. However, I do not think most of your examples are a good fit for the Stack Exchange system, even if phrased in the form of a question, because announcements of events inherently become obsolete. A SE site should accumulate a collection of useful information, not of shortly obsolete information.
On the other hand, your third item might be made to fit, as it is a single question*: “What is the current space weather?” A question about the current state of things would generally be frowned upon in most SE sites, but if the community for a particular site chooses to permit a few such questions and continuously maintain their answers, it would likely work well enough.
Also, announcements could be made in a chat room; notably, the “star” and “pin” features allow highlighting particular messages for later visitors. However, the content of chat is not displayed to people merely visiting the site front page.
*: At least, I assume it's a single question; perhaps the topic is far too detailed to discuss in one unit. I don't know yet; I'm supporting this site to ask questions!