Both DARPA and NASA are moving forward with plans to develop nuclear thermal rockets.

While nuclear thermal propulsion do not match the efficiencies of electric (Ion, etc.) propulsion (an ISP of about 1000 for nuclear thermal as versus up to 5000 for electric propulsion and about 300-500 for chemical rockets) they provide more efficiency than chemical rockets, and more thrust than electric propulsion. 

It makes a lot of sense for satellites in various earth and lunar orbits where you need to change position rapidly. 

I’m not sure how sanguine I am about the possibility of putting a few hundred pounds of enriched uranium in orbit though.

From the NASA story:

More than sixty years after the U.S. began serious studies into nuclear propulsion for space travel, NASA is taking the first steps on a new path to develop nuclear-powered engines for crewed missions to Mars by the end of the next decade.

The agency is reviewing industry responses to the first phase of a plan with the Energy Department to mature a prototype nuclear thermal propulsion (NTP) reactor and engine design for use in space. The congressionally directed initiative, which is also supported by ongoing NASA/Energy Department research into advanced nuclear fuels, will ultimately lead to the building and testing of demonstrators.

Beyond this, the vision extends to the potential development of a full-scale nuclear-powered system for a crewed mission to Mars that would be launched in 2039. The new capability, which could be based on either NTP or synergistic nuclear electric propulsion (NEP) technology, would also provide power for future crewed and robotic deep-space exploration missions as well as faster, more responsive resupply flights to lunar and Martian outposts.

NEP is about having more energy available for an electric propulsion.

It is still a low thrust system, but provides much more delta V over time.

DARPA’s proposal is more about providing satellites that can move between geostationary orbits and lunar orbits quickly and flexibly to deal with treats presented by some sort of rival in space:

A nebulously named “Deterrence Layer” is on the drawing board for the National Defense Space Architecture, and that could mean the return of a functioning U.S.-operated, nuclear-powered satellite in orbit by 2025 for the first time in 60 years.

The need for the Deterrence Layer may depend on what China and Russia do next. If rival militaries establish a presence in the region of deep space between geostationary and lunar orbits, the U.S. Defense Department believes a future spacecraft—an “advanced maneuvering vehicle” (AMV)—will be needed to charge out as far as the Moon, hopefully just to remind an adversary to keep a tight leash on any nefarious plans in cislunar space.

………

What is envisaged for the AMV is a propulsion system that produces a high amount of thrust compared to its weight yet is significantly more efficient than chemical propulsion.

………

In the NERVA ground tests and SNAP-10A orbital tests, NASA and the military used a fission reactor fueled by highly enriched uranium, the same radioactive material used to make nuclear warheads.

For DRACO, DARPA has specified a technology shift to high-assay, low enriched uranium (HALEU) fuel. Unlike weapons-grade uranium fuel that is typically enriched to 80%, HALEU is by definition only enriched between 5% and 20%—although the exact level is not being released. HALEU in the form of uranium metal will be furnished by the government to GA-EMS for the DRACO reactor.

The choice of HALEU exploits a bureaucratic loophole created by former President Donald Trump. His presidential memorandum signed on Aug. 20 delegates approval for the launch of a spacecraft using uranium that is enriched below the 20% threshold to the head of the sponsoring agency instead of the White House. In effect, the memorandum transfers the launch authority decision for DRACO from the president to the secretary of defense, perhaps along with the risk in case anything goes wrong.

Nuclear loopholes in space.  Now THERE’S a reassuring term.

I understand the advantages of such a system, but I am concerned about the potential safety risks, both for NTP and NEP propulsion.