"At the center of NASA’s nuclear rocket program is Bill Emrich, the man who
literally wrote the book on nuclear propulsion. “You can do chemical propulsion to
Mars, but it’s really hard,” says Emrich. “Going further than the moon is much
better with nuclear propulsion.”

Emrich has been researching nuclear propulsion since the early ’90s, but his work
has taken on a sense of urgency as the Trump administration pushes NASA to put
boots on the moon ASAP in preparation for a journey to Mars. Although you don’t
need a nuclear engine to get to the moon, it would be an invaluable testing ground
for the technology, which will almost certainly be used on any crewed mission to
Mars.

Let’s get one thing clear: A nuclear engine won’t hoist a rocket into orbit.
That’s too risky; if a rocket with a hot nuclear reactor blew up on the launch
pad, you could end up with a Chernobyl-scale disaster. Instead, a regular
chemically propelled rocket would hoist a nuclear-powered spacecraft into orbit,
which would only then fire up its nuclear reactor. The massive amount of energy
produced by these reactors could be used to sustain human outposts on other worlds
and cut the travel time to Mars in half."

See:

https://arstechnica.com/science/2019...moon-and-mars/