<font color="yellow"><br />But operating in the soup of the Earth's atmosphere shadow shielding isn't good enough. Some of the radiation from the engine will reflect off of the air surrounding the spacecraft thereby bypassing the shadow shield. The crew would need a full sphere of heavy shielding to protect them from the radiation.<br /></font><br /><br />Are you really sure about this? I have some practical experience with x-ray and neutron shielding techniques, nothing like the same intensity of course. As I understand it, gamma rays do <b>not</b> reflect off anything to much extent, certainly not anything as thin as the atmosphere. Neutrons do bounce around a bit - again the atmosphere isn't thick enough for them to bounce around very much within a short enough distance that a lot of them would return to the spacecraft - but it's easy enough to shield neutrons anyway, in fact a hydrogen tank combined with a little boron is ideal for this.<br /><br />Certainly radiation shielding must be considered but I don't think it's a show-stopper, and I don't think the atmosphere makes it any worse. If I'm wrong I'd love to know more about this reflection phenomenon! <br /><br /><br />Edit: Crap, gunsandrockets, I just noticed your private message... sorry, I've been away for the last few weeks. Haven't done much further work on that NEP tug study either, apologies for my laziness. Things are pretty tight for the next few days but I'll try to at least put together what work I have done into something presentable, and send it to you.