So it seems you can farm the queen missions as they appear. Make sure to keep an eye on the left side of the screen when you open the map to warp to other destinations. Seems they drop legendary items a lot, so it's a good way to farm for armor or ascendant shards. Just keep in mind that they can be difficult, but if your level is high enough you should do fine. I soloed a mission just now as a level 28 Warlock. But it was still a pain. I posted a link to an image of items that can be earned from these missions.