Reply #8: It still seems to be xenon

[muriel_volestrangler]

according to this Google cache of the project diaries (complete with pictures of celebrating scientists!)

http://64.233.183.104/search?q=cache:W3pk38pXA_MJ:www.esa.int/gsp/ACT/propulsion/safe_test_diaries_wk4.htm+DS4G+xenon&hl=en&client=firefox-a

"1% as heavy as"? Do you mean "1% heavier than" - ie per atom?

I think the ionization energy may not be an important consideration for ion engines - they are always wasteful of energy, but very efficient in terms of reaction mass. Since they expel the exhaust at such high speeds, nearly all of the kinetic energy goes to the exhaust, rather than the craft. But the advantage of using so little mass it worth it if you have an energy source outside the reaction mass - solar, or nuclear. So taking up a compound as the reaction mass would mean either a waste product, or a second element to ionize - possibly needing a second ion engine.