The biggest challenge of our Mars journey is the journey time (9 months). Now I have seen news coming stating that atomic or nuclear rockets would cut the journey time in half. Is that right? When I googled for nuclear rocket, I found that this technology has been known since the 1960s. What took it so long to be considered a better technology than the chemical rockets?
6$\begingroup$ news [...] stating that atomic or nuclear rockets would cut the journey time in half. Can you add references? $\endgroup$– user10509Feb 16, 2018 at 10:17
10$\begingroup$ this technology has been known since the 1960s On paper yes, but not in practice $\endgroup$– user10509Feb 16, 2018 at 10:17
2$\begingroup$ The Apollo programme had active plans to use a nuclear rocket on the S-IVB stage, but that got shelved because of all sorts of concerns. $\endgroup$– GdDFeb 16, 2018 at 10:38
6$\begingroup$ @CodeIt, simply, (1) "nuclear rockets" have been fully explored, since back in the 60s. (In many ways they are just not very good.) Note that (2) these articles are "non-news" items. NASA happened to spend a tiny amount of $ on a study, involving the word "nuclear". By sheer coincidence, a newspaper noticed this and presented it as a "clickbait" headline. (Other newspapers then copy.) It is nothing. Zero. NASA continually does tiny studies like this on all sorts of things. You will see clickbait headlines like "NASA antimatter! NASA sex in space!" etc. Means, precisely, nothing! $\endgroup$– FattieFeb 16, 2018 at 14:38
2$\begingroup$ @CodeIt: The articles are all a good six months after the actual contract, which is for a ridiculously tiny $19 million. And they all repeat the same mistaken talking points about "last used in the 70s" (wrong: NASA nuclear tests occurred also in the 90s — and, yes, BWXT's ancestor company was involved then too). $\endgroup$– Nathan TuggyFeb 16, 2018 at 17:14
The biggest challenge of our Mars journey is the journey time (9 months).
Well, maybe. More likely, the biggest challenge is making it economically worthwhile to support a viable colony. But if you're just talking about the problems of transport, assuming the will to do it, sure, that's plausible enough.
Now I have seen news coming stating that atomic or nuclear rockets would cut the journey time in half. Is that right?
It's right, or even understating the advantage, but this is not news at all. This has been known since the designs were drawn up in the first place, and the advantages fairly well understood.
When I googled for nuclear rocket, i found that this technology has been known since the 1960s. What took it so long to be considered a better technology than the chemical rockets?
Very simply, it didn't take so long. It's always been considered technologically superior, and several concepts are comparatively straightforward to develop or have already been mostly worked out (Orion, in particular, got quite far before being shut down hard, and Timberwind/NERVA got even farther). The problem has been that no one wants to deal with the risks, public perception, or definite hazards associated with putting high-powered nuclear reactors in space. Or even putting nuclear reactors in space at all. (Let's not even mention nuclear bombs every second, Orion/Medusa style.)
Until you can say "1 MW nuclear-thermal engine" to a US senator without them reaching for their smelling salts or their shotgun, this is a non-starter.
See also Are nuclear-powered engines the way to go for space exploration? for a slightly different perspective on the issues.
5$\begingroup$ "high-powered nuclear reactors in space" Nuclear reactors are already in space! radioisotope thermoelectric generators are on many satellites and probes, and the TOPAZ nuclear reactor for example: en.wikipedia.org/wiki/Nuclear_power_in_space Projects like Orion are hardly reactors though. More like giant nuclear bomb bays detonating their contents in sequence... $\endgroup$ Feb 16, 2018 at 11:59
4$\begingroup$ Or in getting the reactor/fuel safely into space in the first place. $\endgroup$ Feb 16, 2018 at 12:10
10$\begingroup$ @Baldrickk: RTGs are not reactors. TOPAZ and SNAP-10 and company aren't flying in new missions. (And I don't think it needs to be said that if there's public resistance to nuclear reactors, the resistance is that much greater to bomb propulsion.) $\endgroup$ Feb 16, 2018 at 15:10
9$\begingroup$ GalacticCowboy has the right idea. I don't think many lawmakers care much about reactors in space. They care about a launch disaster spewing radioactive fuel over millions of square miles of ocean off the Florida coast that we rely on for food, or winds wafting that junk over populated areas such as Orlando. Move the launch site to Edwards AFB and now you risk one of the largest metropolitan areas in the nation if the launch fails during a critical stage. None of this helps reelection. $\endgroup$– user5932Feb 16, 2018 at 15:53
6$\begingroup$ I feel compelled to point out that nuclear reactors are not a significant radiological hazard UNTIL THEY ARE OPERATED. Launch it before it's operated and you reduce your risks near to zero. Before startup, your fuel is enriched uranium, chemically bound as an oxide or carbide (not water soluble). Pretty tame stuff. A rocket with a freshly-built reactor exploding on launch and somehow rupturing the reactor presents nearly zero risk to the population. AFTER it's been run it starts accumulating fission products, like water-soluble iodine and bone-loving cesium. $\endgroup$ Feb 16, 2018 at 20:39
Cut the journey in half compared to what? As far as I’m aware there is no physical limit for chemical rocket velocity or acceleration (except for the speed of light of course). With a huge enough rocket, thrust and enough stages you could go as fast as you want.
Nuclear rockets have a much higher efficiency of about ~800s of specific impulse compared to the ~400s of chemical rockets. So it’s true that it would take about half as much fuel for the same change in velocity. However, the most limiting factor in rocket flight is cost  and I doubt that a nuclear rockets would be very competitive in that regard.
3$\begingroup$ Specific impulse has a distinctly non-linear (exponential, in fact) effect on total fuel mass. If your fuel fraction is, say, 90% (common enough for practical orbital rockets), halving specific impulse from 800 to 400 gets you a rocket that can be as much as ten times the mass, and that's before you increase the dry mass to account for the larger engines you need to maintain the thrust to weight. See e.g. quantumg.net/rocketeq.html to run the numbers yourself. (m0 is the wet mass, m1 the dry. For orbit, 9k or 10k m/s delta-V is necessary; for Mars trip, closer to 16k at minimum.) $\endgroup$ Feb 16, 2018 at 17:06
Current flights from the Earth to Mars are routes that take the least amount of fuel (energy) to get there. If you have plenty of fuel (money) you could fly a route that goes directly to Mars, no need to wait for a launch window or anything.