NASA Nuclear Space Tug: A Game Changer for Mars Missions

NASA Nuclear Space Tug: A Game Changer for Mars Missions

Space is big, Mars is far, and conventional chemical rocket propulsion systems just aren’t cutting it when it comes to going long distances in a hurry. This is a problem if you want to become a multi-planetary species because conventional thrust technology potentially puts astronauts’ health at risk due to years in zero-G and long exposure to deep space radiation during the transit to and from Mars. Astronauts also like to do silly things like breathe, eat, drink and bring along supplies which get a bit out of hand when a trip to mars takes the better part of the year each way.  

One potential solution is for NASA to consider building and flying a nuclear-powered space tug (for lack of a better word) that shortens the time to Mars dramatically while also allowing government agencies to ensure the safety of nuclear systems much as they do in the terrestrial maritime realm.

So in alignment with our Critical Six factors to a successful Mars Mission/Settlement the the Mars Blueprint is humbly suggesting that NASA should construct and operated a Nuclear Space Tug for Mars missions.

In this concept, a space based “tug” should be operated by NASA for lunar and deep space manned missions that require speed and active maneuvering.  To continue cultivating the commercial sector’s clear progress towards reusable spacecraft designed to go to and from a planet (aka SpaceX Starship) the nuclear-powered space tug should be a true space-based craft optimized for deep space and not designed to make atmospheric entry but instead intended to stay in orbit while the towed vessel transits to and from the target planetary body. From a safety perspective this plan lowers the risk of nuclear material contaminating the atmosphere by keeping it confined to the space-based craft and not on a lander.  The nuclear space tug should also be platform agnostic and be designed to tow a range of craft just like a maritime tug on earth. This would allow Starship, Orion, Starliner, Soyuz etc. to dock and be and accelerated/ deaccelerated to and from the destination in much shorter times without any of the docked spacecraft’s own propulsion systems required.    

This arrangement also solves several problems.  First, placing the space tug under NASA and commanded by a NASA astronaut solves the tricky nuclear regulatory and procurement problems by keeping a national agency in charge. This also encourages civilian providers to continue designing and building their own spacecraft for manned planetary missions (for government or private customers) that need reduced transit times to keep the astronauts healthy.  And finally, making it a NASA led program that supports both public and private missions using nuclear power is economically viable as a reactor is typically designed to last years if not decades, and would mainly just require the liquid propellant and other consumables to be refilled prior to each journey. This makes the initial investment in the materials and construction of the spacecraft the primary up front cost lowering lifetime operating costs.

Illustration of a spacecraft with a nuclear-powered propulsion system. Credit: NASA

So why is there a need for a nuclear space tug when SpaceX is so close to having their Starship program up and running? It comes down to the distance astronauts must travel away from Earth. While Mars is relatively close to Earth in our solar neighborhood it is still REALLY far away using conventional propulsion technology.  To understand just how far away Mars really is, we recommend you check out this Space.com article that does a great job of breaking it down in simple terms. To summarize, Mars is on average about 140 million miles from Earth, but even that varies greatly depending on the orbits and can be as near as ~40 million miles and as far as 250 million miles.  The closest point of approach occurs every 25-26 months allowing a roughly 9-month transit time which can be as short as 5 months or as long as 11 months depending on planetary alignment and insertion trajectory.  If you still don’t believe us, check out the graphic below that depicts the transit times for most of the missions to Mars to date and note that the majority of them take between 200-300 days ONE WAY to arrive at the red planet.  

A detailed list of historical missions to Mars and their respective transit times. Credit: Future

All of this distance means more time enroute in deep space exposing astronauts to even more risk than a low earth orbit or even a lunar mission.  Specifically in looking at going to Mars, NASA identified five key spaceflight risks presented to humans which can by summed up by the acronym “RIDGE,” which stands for Space Radiation, Isolation and Confinement, Distance from Earth, Gravity fields, and Hostile/Closed Environments.  The key point here is that the less time an astronaut spends travelling from Earth to Mars (and back) the less overall exposure they have to all of these risks. Therefore the goal for any successful Mars mission should start with getting across the distance between planets as quickly as possible to lessen the risk.

The key part to this time-distance equation is that the math for conventional rocket propulsion that plays out in the graphic above assumes that nearly all the thrust for this journey is applied at the beginning and then the journey itself is largely unpowered. This is because nearly half of the fuel to get to Mars is expended by the rocket just getting out of Earth’s gravity well. Even if a fuel transfer was done as SpaceX proposes for starship, traditional chemical rockets are simply hungry beasts who will burn up all the fuel at the start of the transit and empty the tanks within minutes.  This is where the nuclear tug concept pays HUGE dividends by shortening the time required to cross the distance by using constant low thrust or periodic high thrust throughout the journey, accelerating/deaccelerating the entire way in order to shorten the journey from 200-300 days down to less than 100. This would reduce requirements for consumables like food, air and water, and increases the health of astronauts by limiting zero-G time and reducing the crew’s deep space radiation exposure by up to 40%.

Given the recent success with SpaceX and even more small-scale wins at smaller companies like Rocket Lab and Intuitive Machines why should NASA run this instead of a civilian provider?  The simple answer comes down to Nuclear power.  Using Earth based maritime examples as a surrogate, history clearly shows that using a nuclear propulsion plant significantly increases the hurdles in building and fueling the vessel AND dramatically increases the regulations in operating it.  To date there have only been a handful of civilian nuclear maritime ships ever built and the small number still in operation today are mainly limited to a few Russian icebreakers in the artic. Compare that to the over 160 nuclear vessels and 200+ reactors owned and operated by the militaries of the U.S.A, Russia, China, U.K., France and India clearly shows that allowing a government organization to run the nuclear spacecraft (at least initially) dramatically increases the odds that it will actually be built/operated.   As a government agency, NASA is in the position to procure nuclear material for the vessel just as it has done for the Radioisotope thermoelectric generators (RTG) on many of its unmanned spacecraft and rovers. Also, NASA is already working with the department of energy on space nuclear propulsion concepts, so the groundwork is already in place.

The really exciting part is that this concept isn’t just a theoretical one as several major components of the technology is nearing commercial viability as we speak! 

Space Nuclear Propulsion (SNP) has been tested in two distinct types of systems: Nuclear Thermal Propulsion (NTP) and Nuclear Electric Propulsion (NEP).  Each system uses nuclear power to produce thrust in very different ways with equally different pros and cons. Below is a break down of the nuclear options for a space based propulsion system and a clear outline of the pros, cons and time constraints on implementing each for future missions. For a more detailed deep dive comparison between NEP and NTP check out this article by Neal Lachman.

One quick clarification on terms that will be key in this discussion is Specific Impulse (I_SP) vs. Thrust.  Thrust is the force that moves an object, while specific impulse is a measure of how efficiently that force is produced.  The two are related directly as I_SP = Thrust/Weight flow rate of the fuel. This article by Thanansan Kuganesan gives a much more detailed explanation but the image he created below gives a great overview and is a solid 101 visual for this article.

Ranked from most likely to fly in the next 10 years to least here are the current SNP programs that are currently in the works:

Nuclear Electric Propulsion (NEP): Most likely to fly – commercial partnerships are already operating functional prototypes.

NEP works by pairing a nuclear reactor that is configured to produce electricity with a high powered electric rocket engine or thruster that produces plasma or ions which are ejected to produce thrust.

An overview of a proposed spacecraft utilizing Nuclear Electric Propulsion. Credit: Ad Astra

There are several commercially viable electric thrusters on the market right now that include ion thrusters, hall thrusters and the more advanced Variable Specific Impulse Magnetoplasma Rocket (VASIMR).  VASIMR is currently the most powerful/scalable as it has no moving parts and offers extremely high and variable I_SP and thrust at constant power and is the engine that will be focused on for this space tug concept.

Aside from its performance it is also the most likely engine for this type of vessel because Ad Astra (the company who is developing VASIMR) is pursuing NEP and recently announced a partnership with the Space Nuclear Power Corporation (SpaceNukes) to use their newly developed space rated reactor as a power source.  SpaceNukes system called the Kilowatt Reactor Using Stirling TechnologY (KRUSTY) was originally developed by NASA and the U.S. Department of Energy National Nuclear Security Administration as part of the Kilopower project, produces up to 10 kW of electrical energy using a fission reactor combined with a Stirling engine and is the first nuclear-powered operation of any truly new reactor concept in the United States in over 40 years.

Pros:

• Fully functional VASIMR 200 kW prototype and a 5 kW KRUSTY prototype exist and work is underway to expand both to higher output that would enable a mars transit in under 100 days.
• SpaceNukes’ Gen1 reactors based on the KRUSTY design is a fully functioning system that SpaceNukes considers “ready for flight,” is passively safe under all nominal and accident conditions and can be fielded in a 1 to 20 kilowatt electric configuration to power a NEP system within the next three years.
• VASIMR uses readily available Argon or Krypton and is extremely efficient with a 73% power efficiency rating while also producing extremely high I_SP which can be throttled to 5000+ I_SP (best chemical rockets are sub 450 I_SP). It can also operate continuously for weeks or months using propellant extremely efficiently as needed and can be scaled up by increasing electrical power and/or by adding more thrusters. Current systems are operating in the kW range but future MW range systems would cut transit times in half once the power production capacity improves.
• VASIMR uses Argon propellant which is easy to store in relatively standard gas cylinders at ambient temperatures and more compact/less mass consuming spaces than NTR systems which require specialized cryogenic tanks to store hydrogen in liquid form.

Cons:

• While VASIMR does produce very high I_SP the thrust is currently in the 6-10 N range compared to 40,000 N to 70,000 N range from chemical or NTP systems. While still a downside the offset is that the 6-10 N can be constant for the entire transit. Because there is no friction in space to slow the spacecraft down the constant low thrust has an additive effect resulting in the same reduced transit time as NTP while using far less propellant.

Nuclear Thermal Propulsion (NTP): Long shot – NTP has potential promise of very high thrust but major near-term development is needed and operational prototypes are still unproven.

NTP uses a nuclear reactor to create heat which is transferred to a liquid propellant. In most current designs that liquid is hydrogen which then undergoes a phase change into a gas which heats up and expands through a nozzle at very high velocity to create thrust.

The concept is well understood and builds upon the Project Rover and NERVA Programs which began in the late 1940’s and continued on until their cancellation in the early 1970’s.  Based on these designs the system can produce an incredible (estimated) thrust of 44,000 N – 67,000 N for periods of typically less than 15 minutes but up to an hour which reduces transits to as low as 60 days one way. General Atomics, Lockheed Martin and other contractors have been recently contracted by NASA and DARPA to conduct studies and test new configurations of a NTP system with some limited success in recent years.

Pros:

•  NTP produces very high amounts of thrust and nearly double the I_SP (estimated at ~900 I_SP) of chemical rockets. This is achieved using the very high temperatures achieved by the fission of radioactive materials in the reactor which heats the propellant (usually hydrogen) to high temperatures and releasing it as exhaust.
• The NTP design is also being studied to be used as a Bimodal NTP/NEP system to provide both power and propulsion and leverage the benefits of both.

Cons:

• No viable operational system is available or planned to be available in the near future. While some progress is being made by General Atomics on new types of fuel, the DARPA-NASA nuclear thermal propulsion project was put on hold in January 2025 due to safety concerns in the design phase.  Due to the design requirement of flowing liquid Hydrogen across the fissionable material of the reactor past designs have been susceptible to reactor material erosion and damage.
• Hydrogen has the best performance in an NTP system but hydrogen fuel tanks are bulky and heavy because they must keep the Hydrogen in a liquid form at very cold temperatures. Hydrogen is also prone to boiling off making long term storage a challenge.
• Radiation from the reactor is an increased risk to the crew and added shielding adds extra mass to the spacecraft that can impact the design of the tug.

The future of interplanetary travel has never been brighter.  As we publish this article SpaceX is preparing for their 8^th test flight of the Starship and super heavy booster system laying the groundwork for a whole new class of spaceship the likes of which has only been seen in sci-fi up till now.  What SpaceX lacks is a propulsion system to deliver Starship and its crew to Mars in a reasonable timeframe.  We encourage the Trump administration and NASA to empower SpaceX to bring Starship up to full operational capability, BUT at the same time invest national resources in a Space Nuclear Propulsion “tug” system to be ready by the time Starship is rated for a manned mission. This two-fold approach ensures a U.S. mission to Mars is successful and sets the foundation for a truly multiplanetary society.