Not a very SMART article ;-)
ESA was there first with the SMART probe to the moon!
Once in space the best form of acceleration is gravity which is why it's used by all projects: free and very powerful
Interestingly ion drives are one of the areas of patent contention between NASA and ESA with presumably the US department of defence not keen on any "competitor" having access to the same technology. I thought the US version used a nuclear reactor rather than solar panels to energise the propellent.