There is a 1% chance your Sim is abducted. Minimum. This is our base chance - if absolutely nothing else happens, there is just a 1% chance.
If someone on the lot is using a telescope, this is increased by 14%. Each Space Rock found* will increase the chance by 15%, to a maximum of 60%. This means that only four rocks will work -- any more is pointless.
So, if a Sim has four space rocks and looks through a telescope, there is a 75% chance he or she will be abducted, assuming they can be abducted (some Sims can't).
* "Found" is directly from the code, I'm not sure exactly what it's referring to, and I've just taken a look at the scripting that controls alien abductions as well as the tuning.