I think that Robert Zubrin proposed an atomic engine to go to Mars early in the 1990', that would cut the travel time from one year to several weeks.

https://en.wikipedia.org/wiki/Robert_Zubrin

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

An astronaut, Franklin Chang Diaz, proposed Vasimir, a ion engine that would take 39 days to go to Mars.

https://www.bbc.com/news/science-environment-48912458

Zubrin criticized the Vasimir proposal:

https://spacenews.com/vasimr-hoax/

But it seems development on Vasimir is still ongoing:

https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magn...

Looks like click bait. Nowhere is there a clear link to how this drive will cut a Mars transit to 45 days (not the 5 days the title here states). Nuclear drives in space could solve a lot of problems. Building a compact controlled fission device is a bit of a challenge and getting it into space safely every time is another.

Hate to be that guy, but the article says 45 days not 5 days...