Nuclear energy has played a key supporting role in historic missions to Mars, Pluto, and across the Solar System for the last 50 years. On January 1 2019, the nuclear-powered New Horizons flew by the most distant object ever observed up close – Ultima Thule, after it having already flown by Pluto in 2015.
Nuclear energy for space applications is nothing new. The past 50 years have focused on robotic exploration and usually involved providing a few hundred watts for a computer and communication system. However, the next 50 years will involve providing power for human settlements and will require kilowatt and megawatt power systems for life support, propulsion, and industry. While solar is an alternative power source and works well in many locations, nuclear energy is a necessity for locations far from the sun or places like the moon which has long periods of darkness.
The most common type of nuclear technology used today is the ). RTGs use the heat produced by radioactive material (usually Pu-238) decaying into stable state. RTGs are often called nuclear batteries because they can be modularized almost like AA or AAA batteries. RTGs have played a vital role in robotic science missions including the Curiosity Rover, Cassini, and the Voyager probes. Voyager 1 and 2 have left the Solar System and are still communicating with Earth after over 40 years and billions of miles distance. RTGs will continue to play an important role in science missions such as the Mars 2020 Rover.
However, RTGs will are not suited to supply the kilowatt- and megawatt-scale power needs of future human spaceflight. There is a second type of nuclear energy called fission that can achieve high power density and can scale to power levels capable of supporting human operations.
Power Regimes for Solar, RTG, and Fission
Power Required
(kWe)
0-1
1-100
100-1,000
>1,000
Sunlight Available
Solar
Solar
Solar or Fission
Fission
Sunlight Unavailable
RTG
RTG
Fission
Fission
Nuclear fission requires a “critical mass” of material which means that the reactor needs to be of a certain size before it can generate heat, but once it reaches that critical mass it can produce nearly as much power as desired on demand. Fission power in space is nothing new. In 1965 the SNAP10A space reactor was successfully launched and operated in space. Russia has extensive experience with having launched over 30 fission powered spacecraft. In addition to these space launched systems, there were many ground tests. In the 1960s NASA successfully ground tested over a dozen nuclear rocket engine in a program called NERVA.
More recently in spring of 2018 NASA ground tested the “Kilopower” space fission reactor. The reactor core was cast from a Uranium Molybdenum metal alloy and connected to heat pipes and a Stirling engine in a vacuum chamber. The tests showed great performance and it has prepared the way for a flight prototype to be built. Kilopower has been success if not for no other reason than showing that development of nuclear technology can be cost effective. The entire ground test campaign for Kilopower was completed in three years and on a shoestring total budget of around 20 million dollars.
Example of Surface Power Needs for Crewed Mars Mission
Four 10-kWe Kilopower units would provide up to 35 kWe continuous power for all mission phases
Peak Power Needed
(kW)
Keep-Alive Power Needed
(kW)
Element
Cargo Phase
Crew Phase
Cargo Phase
Crew Phase
In-Situ Resource Utilization (ISRU)
(Extracts CO2 from Martian atmosphere and processes into LOX propellant)
19.7
0.0
19.7
0.0
Mars Ascent Vehicle (MAV)
6.7
6.7
6.7
6.7
Surface Habitat
0.0
14.9
0.0
8.0
Science Laboratory
0.0
9.5
0.0
0.2
Total
26.4
31.1
26.4
14.8
Source: Based on NASA Solar vs. Fission trade studies and use of Kilopower fission system, high-density battery technology, and more efficient solar arrays.
Solar vs. Fission Surface Power for Mars. (2016, September). NASA.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160011275.pdf
NASA has also been supporting nuclear rocket development over the last few years. NASA gave 18.8 million dollar contract to BWXT to design and manufacture prototypes of a new, low-enriched uranium fuel element made from a tungsten uranium dioxide cermet. The current house appropriations bill has allocated “$150,000,000 is for continued development and demonstration of a nuclear thermal propulsion system.” with similar language in the senate version. While the bill may change, it can be clearly seen that congress is supportive the development of space nuclear technologies.
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The gateway to space is opening, however before we can think about sustainable human presence in space, there critical technologies that are needed to operate beyond the gateway. Nuclear technology is certainly one of the critical technologies. Nuclear fission can rise to meet the needs of commercial and government customers. When humans are ready to live and work in space, nuclear energy must be ready as well.