&gt;  What happens if someone has scheduled time with the telescope. Do they get bumped? Who has priority?<p>Yes they generally get bumped and they will generally get that time back later on in the telescope schedule. Folks who do these &quot;target of opportunity&quot; proposals apply like usual, but don&#x27;t specify targets. If awarded time, they get the ability to &quot;trigger&quot; their observations once an interesting target appears. So their time allocation is (generally) capped like everyone else&#x27;s, but isn&#x27;t scheduled. So presumably the people who get bumped will have their observations rescheduled for later.<p>In detail the question of priority is a bit more complicated, and sometimes the people observing can refuse to be bumped for a TOO, but that varies with the observatory and their rules.<p>Edit: Also, people are frequently happy to even &quot;donate&quot; a little bit of their telesocpe time to observe interesting transient sources (sometimes in exchange for co-authorship or acknowledgements in papers, sometimes not). This is partly because astronomy is a reasonably friendly field. And it&#x27;s also because we can&#x27;t design and carry out experiments like other fields of science – we have to take what the Universe gives us. So it&#x27;s good to take advantage of opportunities to view interesting and new sources when they happen.