Well, I'm in a challenging starting position. The Crystal Empire is a long way from Everfree Forest. Then again, there might be a way if this is taking place as far ahead as Season 8.

I like to imagine Astral Gleam knows Sunburst. Not as a friend really, more like a... professional acquaintance. They're both academics and both live in the Crystal Empire, it's possible they've met professionally a couple of times. And then there's Twilight's School, what if Astral Gleam were visiting it as a Guest Professor, probably to give a few lectures about the history of the Crystal Empire (a pony who has actually lived through some of it would be rather qualified for such a job), and he was essentially invited on Sunburst's recommendation? Sunburst could have also had an ulterior motive, sending him to Twilight's School hoping it would inspire him to make a few attitude adjustments (Astral is still known for being a bit of a cynical prick after all). But it's while staying in Ponyville that he spots the crashing ships that night through his handheld telescope and this gives him adequate means to be near the plot as it begins.