The ion drive on Deep Space 1 started Tuesday (after refusing to stay on
properly at first, 10 days earlier -- probably due to contamination shorting its
high-voltage accelerating grids), and has been working well ever since.
Acceleration is about 0.00002 gravities, which (if I have it right) works
out to 2085.2 sec to do the standard 1/4 mile drag race, hitting a
sizzling 0.86 miles per hour at the end.
Lest I sound scornful, it also works out to about 5.5 km/sec in a year (although
DS1 only has enough Xe fuel aboard to run about half that long). The Cassini
mission to Saturn was launched on a $400 M Titan IV because of it's huge
(? 5,500 kgm if I recall) launch mass, much of which is rocket propellant needed
to get it into orbit around Saturn. It is in such applications that even the first-
generation DS1 ion drive promises a revolution, especially when combined with
the gravity-assist technique that has been developed to a fine art in the last decade.