It's groundhog day in Tehran. Another nuclear scientist, this one identified as Mostafa Ahmadi Roshan, who worked at the Natanz uranium enrichment facility, was killed when an explosive was attached to his car. Several questions arise:
1) Why aren't the Iranians attempting to kill Israeli defense officials? The answer, I believe, has more to do with Iranian technical limitations: Since the Iranian regime has no compunction about killing Israeli civilians (through its proxies Hezbollah and Hamas), I doubt it has reservations about attacking military or intelligence officials. Perhaps one thing holding back Iran, though, is fear that attacks on Israeli officials (or, even more consequentially, American officials -- though of course, Iran is already killing American soldiers in Afghanistan) would prompt an immediate Israeli strike on Natanz, before the regime is able to move its centrifuges to its underground facility at Fordow.
2) Does Israel, or whoever is assassinating Iranian scientists, believe that these killings will actually slow-down Iranian nuclear development? In other words, do the people behind the assassinations believe that Iranian nuclear knowledge is so concentrated in the minds of a few scientists that a limited series of assassinations can cripple the program? This doesn't seem likely, obviously.