How it works:
1. Via special commands menu, you choose "create asteroid", this will prompt you to pick a sector, and location to place the new rock (works like placing a station with the numpad arrows).
2. Select ore, silicon, or nividium.
3. You input your yield (I've done yields up to 300 with no problems, be warned tho, these will take A LOT of ecells to mine).
4. After inputting the yield you want, you will recieve an instant message, containing one of two messages:
a. An "estimate" of the cost of the asteroid (not cheap*), with yes/no options.
b. A message stating you do not have the required credits for that yield (will kill the script).
If you have enough in your account and choose "yes", a new asteroid will be created. You can then place a mine on it, and start mining.

Asteroid Creator
*Each asteroid's price is based on 100000 to 110000 credits per 1 yield. So, a 10 yield asteroid, will run you between 1M and 1.1M credits.