I've got my distinct suspicions that they've simply made a mistake there, they are trying to take the word "encounter" and make an action of it I believe - this should lead to "encountering" rather than dropping the "er" from it before adding the "ing". It wouldn't be the first or last time that there will be a failure in the proof reading section. It is I suppose just about possible that there is a specific use of the word in astrophysics but that means that it can only be correct in a very specific context.