Greetings once again,
I posted on here many months ago on a question regarding Earthshine. It involved research for a fantasy novel that I am writing. The responses were very useful. I now find I have a second astronomy question that I'm hoping you all might be able to assist with.
In the second book of the series that I am writing (the first one isn't published, or even finished yet, but I'm planning out the series) the world in question is hit by an interstellar object. This object originated from outside the planet's solar system... it in fact was sent from another planet.
So I have a few questions on the reality of such a scenario.
Would this be classified as a comet? It is not in orbit of any star... as I said it was propelled through space with a particular destination in mind, so what would be the closest classification for such an object. If it matters, it's about half a mile in diameter.
This object's travel time is intended to be about 2,000 years, from a nearby solar system... let's say that it's about 20 light years away. Is it probable that a comet-like object could move fast enough without being overcome by forces (would it even, since space is a vaccum?) to travel that far in that amount of time?
Third question, assuming a size of about 1/2 mile in diameter, how soon do you think it would be before this object could be seen with the naked eye at night? Might I assume that to the uneducated medeival society that it would first appear to be a star, until it got close enough to be larger than 'just a star'?
Last question. The object would need to be large enough to survive atmospheric entry. I'm assuming that 1/2 mile in diameter would be large enough to survive entry without the core of the object being destroyed. I know that would depend on the material that composed the object... I haven't thought about it too much, but it is not an object made of ice, it is essentially rock.
Let me know if I need to provide any more details on this. Like I said, I haven't done a lot of planning of the details, but I'm just doing preliminary research and want to make it fairly accurate.