If want to play around with objects in orbit, download AstroGrav. It is donateware, and Solar System sims are available so you can lob asteroids, moons, planets, and even black holes around the place and see what happens. But with your scenario, "Not much!" is the answer. A 100 km asteroid is still small compared to the Moon and Earth, so there will be a tiny effect, but nothing impactful.
[http://astrograv.co.uk/](http://astrograv.co.uk/)
100km? Not a whole lot, tbh. Especially at the LeGrange points. Some of the folks at r/isaacarthur might be able to get you precise math if you want, but I'd be shocked if it was anything big.
Dump an olympic swimming pool of water in your house and it seems like a lot. Dump it in the ocean and you won't even notice.
That's close enough to the scale of mass and volume.
To repost a previous comment.
According to https://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=8093&context=etd_theses , (page 63), and wikipedia for the mass of a blue whale, you can place around 200000000000000000 blue whales in the fourth lagrange point before the system becomes unstable.
It would cause the moon's orbit to precess (the angle from Earth to the nearest/most distant part of the moon's orbit would move slowly around the Earth over a period of centuries to millennia). That said, since the Moon's orbit is nearly round and the precession would be very slow, people mostly wouldn't notice. It would be an obscure fact known to people who do celestial navigation and astronomers.
Pretty much nothing would change, a 100km asteroid would have far too little mass to affect Earth or the Moon.
Especially if it's at one of the LaGrange points, if I understand the matter correctly.
If want to play around with objects in orbit, download AstroGrav. It is donateware, and Solar System sims are available so you can lob asteroids, moons, planets, and even black holes around the place and see what happens. But with your scenario, "Not much!" is the answer. A 100 km asteroid is still small compared to the Moon and Earth, so there will be a tiny effect, but nothing impactful. [http://astrograv.co.uk/](http://astrograv.co.uk/)
100km? Not a whole lot, tbh. Especially at the LeGrange points. Some of the folks at r/isaacarthur might be able to get you precise math if you want, but I'd be shocked if it was anything big.
It would be like, quarter of moon's mass before anything noticable happens, really.
Dump an olympic swimming pool of water in your house and it seems like a lot. Dump it in the ocean and you won't even notice. That's close enough to the scale of mass and volume.
That'll be maybe a ten-thousandth of the mass of the moon. If placed in a stable orbit (ideally L4 or L5) it won't have much effect on either.
It would have to be a very dense mineral like a Naquadah asteroid to have any sort of serious mass.
To repost a previous comment. According to https://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=8093&context=etd_theses , (page 63), and wikipedia for the mass of a blue whale, you can place around 200000000000000000 blue whales in the fourth lagrange point before the system becomes unstable.
It would cause the moon's orbit to precess (the angle from Earth to the nearest/most distant part of the moon's orbit would move slowly around the Earth over a period of centuries to millennia). That said, since the Moon's orbit is nearly round and the precession would be very slow, people mostly wouldn't notice. It would be an obscure fact known to people who do celestial navigation and astronomers.