[Zeroisanumber]

What ever happened to their ion engine experiments? I remember that NASA launched a probe equipped with one a while back, but I never heard about how it performed.

[Cincinatus]

It's still in use on several, experimental missions, most notably Deep Space-1, which recently flew by comet Borelly -- click HERE for details.

Ion (solar electric) propulsion is good for unmanned missions which require low thrust, but long, constant operation. Nuclear thermal operates like a conventional engine (high-thrust, short thrust duration), but with about twice the total power of conventional chemical rockets.

[GlesenerL]

Ion engine worked great.

But its for after you've left the atmosphere.