I just went through the same process. I went through 130 limpets with the prospector to find the asteroids with it. Finally got the 10 that I needed for the Engineer. I went to a pristine metallic ring as mentioned by other posters. Just be patient and plan on going through a lot of prospector limpets.
My post to be used in conjunction with Minoo's answer.....
Ere yer go Cmdr...... http://edtools.ddns.net/index.php?s=Shinrarta%20Dezhra
With the right setup you can find those 10 tons within 10 minutes:
https://www.youtube.com/watch?v=8anGfouBIRY
I have started mining and found gold/silver etc.. but no Painite or Platinum. any advice would be welcome.
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Thank You,
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Calebe