39 days is based on some theoretical speed from an engine that hasn’t been built. Besides, even if they could do the trip in 39 days, they would have to wait for a year for the earth and Mars to be close enough for the return trip.
This reminds me of the episode where the test pilots were discussing about getting past the sound barrier in “The Right Stuff” and that it was an impenetrable barrier.
The engine has been bench tested already. It’s going aboard ISS this year for it’s first test in space.
Re: your comment about having to wait a year to return there was an article last year with a plan to use cargo ships to move the necessary supplies to Mars before the Astronauts got there. The cargo ships would use a version of VASIMR also but the transit time would be much longer. Speed wouldn’t be a consideration.
they would have to wait for a year for the earth and Mars to be close enough for the return trip.