Mars, Humans, and Nuclear Power

Getting humans to Mars will be hard. Getting them back safely to Earth will be harder. Let’s look at what it will take. We’re going to get successively more technical as we go, so if you want to bail out at any point, I won’t be offended.

WHY IT’S HARD

More than 50 years ago, NASA sent nine three-person crews to the moon, landing six two-person spacecraft on the surface. All of those crews returned safely to Earth. A trip from Earth to the moon takes about three days out and three days back, not including time on the surface. The longest Apollo mission was the last, Apollo 17, lasting a little over 12 ½ days. The Apollo command module and lunar lander could keep three men alive for that long.

It’s important to understand that this diagram is not to scale. This one is.

This will give you some idea of how much farther away Mars is, even at its closest approach. And of course, spacecraft do not travel at the speed of light.

Mars is much farther away than the moon, and it will take much longer to get there.

GETTING THERE

We routinely send robot spacecraft to Mars, so why not humans? Well…humans are considerably more fragile than metal and silicon, and require things like food, water, breathable air, and a fairly narrow temperature range. Add to that the debilitating effects of long periods in weightlessness. (I refuse to use the NASA term of “microgravity.” Sue me.)

And then there’s the radiation.

Astronauts in the International Space Station are exposed to more radiation than you and I, protected as we are by the atmosphere. But the astronauts are protected by the Earth’s magnetic field, that redirects most of the worst of solar radiation around the planet.

Once beyond this protective blanket, space travelers are exposed to the full force of solar radiation.

Robot spacecraft take about seven months to get to Mars. For them, of course, it’s a one-way trip. We’re going to assume that human voyagers would like to return home. That’s another seven months for the return trip, plus time spent on a surface that doesn’t offer the same radiation protection as Earth’s. (Mars’s magnetic field is much weaker and its atmosphere much thinner.)

That’s a LONG time to spend in a hostile environment. Can we get there faster?

ROCKET SCIENCE 101

Rockets work by pushing stuff out the back, which makes the rocket go forward. It’s Isaac Newton’s third law of motion.

The more stuff you throw out the back, and the faster you throw it, the greater the force, and the faster the rocket will pick up speed. Rocket launches from Earth burn fuel in a combustion chemical reaction to create very hot gases that shoot out from a nozzle at high speed. The most powerful rocket successfully launched to date is NASA’s Space Launch System (SLS), whose combination of liquid fueled center stage and solid fueled side boosters provide a pretty spectacular show.

What’s coming out of those nozzles? Well, for the solid rocket boosters, there’s a rather nasty mix of gases and particulate aluminum oxide. These range from relatively light molecules like carbon monoxide to the much heavier aluminum oxide. A heavier molecule will not move as fast as a light one with the same application of force.

How about if you don’t burn anything at all? How about if you just get a propellant really hot and eject it through a rocket nozzle designed to make it go even faster? And how about if you use a very light molecule for the propellant, so it will go especially fast?

Welcome to the concept of nuclear thermal propulsion.

ROCKET SCIENCE 201

The most efficient rocket engines using chemical propulsion are those that burn liquid hydrogen for the fuel with liquid oxygen for the oxidizer. The product of this reaction is water. That’s a relatively light molecule that can leave the rocket nozzle at a high velocity, imparting a high velocity to the rocket itself.

A rocket engine’s efficiency is measured by the rather obscure parameter of specific impulse, or I_sp. (If you care, the simplest definition is how long one pound of propellant cam generate one pound of thrust.) I_sp is measured in seconds. The higher the number, the more efficient the rocket engine.

Solid rocket engines have I_sp = 250 seconds, and liquid hydrogen fueled engines = 450 seconds. For why you don’t use liquid hydrogen exclusively, check out this older StarStruck post.

ROCKET SCIENCE 301

A nuclear thermal rocket engine doesn’t burn anything to generate hot gases, so it doesn’t need to carry oxygen to complete the combustion reaction. Rather, it heats hydrogen gas propellant in a nuclear reactor. The reactor can get very hot, and hydrogen is the lightest molecule of all. It therefore leaves the rocket nozzle very fast, and I_sp values of more than 1000 seconds are feasible—more than twice as efficient as the best chemically fueled engines.

This isn’t a new idea. NASA tested nuclear rocket engines beginning in the 1950s and found that they could both fly humans safely and could provide significant advantages to any space program. The project was cancelled in 1973, not for technical reasons—the project was successful—but as a political casualty of partisan infighting and constraints imposed by the Vietnam War.

In 2023, it’s a new story. In January 2023, NASA and the Defense Advanced Research Projects Agency (DARPA) announced that they would collaborate on the development of a nuclear thermal rocket engine that would be tested in space to develop nuclear propulsion capability for use in crewed NASA missions to Mars. In July 2023, DARPA announced that the Demonstration Rocket for Agile Cislunar Operations (DRACO) reactor and fuel would be supplied by BWXT, located right here in Lynchburg, Virginia.

ROCKET SCIENCE 401 (WHERE WE GO DEEP INTO THE WEEDS)

What will that engine look like? There are some things we know about it, some things that have yet to be decided, and some constraints imposed by basic nuclear physics. BWXT has not released anything more than very basic information, so some of what follows is necessarily speculative. But some information is public, and rational nuclear reactor design dictates the probabilities.

First let’s start with the fuel, where there are several clever innovations that increase both efficiency and safety. BWXT has said that the reactor will use High Assay Low Enriched Uranium (HALEU) fuel. This is uranium that has a percentage of the U-235 isotope that is most easily fissioned enriched from the natural 0.72% to somewhere between 5% and 20%. The fuel used in this reactor will likely be enriched to just under 20%; commercial power reactors typically use fuel enriched in U-235 to between 3% and 5%. The higher enrichment lets the reactor be smaller. You don’t want to be carrying something the size of a commercial nuclear power plant to space!

Next is the chemical form of the uranium; it isn’t just pure uranium metal. The chemical form most preferred by NASA is uranium nitride, a ceramic compound with a very high melting point and very good thermal conductivity. In other words, it can safely get very hot and can efficiently transfer that heat to the hydrogen propellant.

Lastly, there is the physical form of the fuel. TRISO fuel particles (TRi-structural ISOtropic) put that uranium fuel inside several layers of material. First there is a porous buffer layer made of carbon that absorbs fission product recoils, followed by a dense inner layer of protective pyrolytic carbon (PyC), followed by a ceramic layer of zirconium carbide (ZrC) to retain fission products at elevated temperatures and to give the TRISO particle more structural integrity, followed by a dense outer layer of PyC.

This particle is less than a millimeter in diameter! The term of choice is “poppy seed.” 15,000 of these are then encapsulated in a “pebble” made of graphite, pure carbon.

Let’s stop for a minute to catch up. The purpose of the nuclear reactor in this rocket engine is simply to generate heat—a lot of heat. It does so by splitting (fissioning) uranium nuclei with neutrons. Each nucleus that splits releases two or three neutrons, which can go on to cause more fissions in a chain reaction. Those neutrons are initially too energetic to be easily absorbed by the uranium, and need to be slowed down (moderated). And because we want to keep the fission reactions at a steady rate and not running out of control, a means of absorbing some of the neutrons is required (control rods). And—yet another consideration—for the reactor to be more efficient and to protect a human crew from the effects of neutron radiation, we’ll need a reflector to prevent neutrons from escaping the reactor altogether. It’s a delicate balance!

This isn’t necessarily the physical arrangement that will be used in space, but it gives you some idea of how these fuel elements can be arranged.

Finally, here is a fairly generic image of a nuclear thermal rocket engine, based on ones from those early designs of the 1960s.

Short of warp engines, this will be the fastest means of interplanetary travel for the foreseeable future. From BWXT’s press release: “The spacecraft is targeted for a 2027 launch from Earth in “cold” status (meaning that the reactor is turned off as a part of launch safety protocols) by a conventional rocket, and then the reactor will be powered on once the spacecraft attains an appropriate location above low earth orbit.”