Space Nuclear Propulsion for Deep Space Science Missions

The use of nuclear thermal propulsion (NTP) 1 and nuclear electric propulsion (NEP) 2 systems on deep
space science missions to the outer planets and into the interstellar medium 3 can yield significant spacecraft
system and mission performance benefits and improvements relative to the use of conventional chemical
propulsion systems. Several recent and ongoing programs are developing the technologies and systems
required to realize a near-term deep space nuclear propulsion capability.

NTP provides improved propulsion efficiencies compared to chemical propulsion, while also providing
substantial thrust. This combination of high thrust and increased specific impulse (I_sp) provides high
acceleration and extended thrusting periods, enabling greatly reduced trip-times on certain types of
missions compared to various propulsive alternatives. For examples, compared to a baseline mission using
chemical propulsion, NTP-powered missions to Jupiter or Uranus could deliver approximately 2.4-3.6 times
more payload (in the case of Jupiter, the payload delivery is significantly larger than the Juno spacecraft).
In this comparison, the higher end of the payload advantage is obtained when the trip time is held equal for
the NTP-powered vehicle and a vehicle using a chemical propulsion departure stage.

NTP systems are presently under development by multiple government agencies. NASA’s Space Nuclear
Propulsion (SNP) project aims to demonstrate a hydrogen-fed NTP engine at 900 s specific impulse (I_sp)
and approximately 10-15 klb_f of thrust. DARPA’s Demonstration Rocket for Agile Cislunar Operations
(DRACO) program is targeting a demonstration of an NTP system in the cislunar space between the Earth
and the Moon. An appropriately phased development plan that applies the development of the reactor
technology for an NTP engine in this performance class and leverages mature, existing liquid rocket
component hardware provides a path to a lower cost propulsion system that can be realized on a shorter
development schedule.

NEP, with high Isp in the 2,000-8,000 s range, can also provide advantages over chemical propulsion,
including a much greater payload delivery mass and the flexibility for planners to trade between delivered
mass and a wider window of mission trajectory options. Electric propulsion (EP) systems have
demonstrated great utility, performing notably on the Dawn mission to enable rendezvous and orbital
insertion at two separate bodies, Vesta and Ceres. An NEP-powered vehicle would have a similar capability
to visit multiple bodies, loitering at each before moving to the next. A 10 kW_e NEP system provides a power-
rich environment on the spacecraft that is simply not possible using present radioisotope power systems,
giving mission planners more scientific instrument and communication hardware options.

Several programs and projects are presently developing NEP systems and subsystems in the 10 kW_e power
range, leveraging past reactor work and recent nuclear power generation risk-reduction demonstration
activities such as the Demonstration Using Flattop Fission (DUFF) and the Kilopower Reactor Using Stirling
TechnologY (KRUSTY). The goal of the Air Force Research Laboratory’s Joint Energy Technology
Supplying On-Orbit Nuclear Power (JETSON) program is an in-space demonstration vehicle that has a 10
kW_e -class fission power source. These past and present efforts can be combined with the ongoing
development of 10 kW_e -class electric propulsion systems (notably the NEXT-C ion thruster or the Hall-effect
thrusters for Power and Propulsion Element of the Lunar Gateway) to provide a pathway to a low-cost,
reliable NEP system for deep space science application.