Regardless of race, nationality, gender or faith, one thing has often bound us together as the human race: our fascination with the heavens. Books, television shows and films romanticise travelling through our solar system and the galaxy to explore and discover. Some of us may spend our free time stargazing, while others work to become the next class of astronauts.
Curiosity and discovery run through our veins and space is the next frontier. Since the Moon landing, the US and others have dreamed of becoming an interplanetary species by landing on Mars. But what would this take? And can we do it?
Travel to Mars currently is projected between six and 12 months each way, meaning up to two years of travelling alone to reach the Red Planet. This is not just a problem for cargo and communication—it is a problem for the astronauts’ health.
Exposure to radiation for as long as a year causes damage to the human body. American astronaut Scott Kelly spent a year in space on the International Space Station. While researchers found that most of the damage Kelly incurred from space was reversed upon his return to Earth, they are not sure astronauts will be so lucky on trips to Mars.
The potential to not only curb radiation exposure for the crew, but also achieve what will be humanity’s greatest accomplishment of the millennium, depends on what propulsion system is chosen to travel to Mars. The most common form of space propulsion is chemical. In fact, it is the only form of propulsion that is capable of lifting off the Earth’s surface. Used both for human and scientific missions, chemical propulsion is the oldest method of space travel.
Once off the Earth’s surface and in space, the choices are much less limited. Both crewed and scientific missions can continue with chemical propulsion or they can switch to electric, thermal or a hybrid option. In the past, human missions have often used chemical for lift-off and for in-space travel. Scientific and commercial missions often use solar electric once in space to save money and increase efficiency.
So, what will get us to Mars? The most promising solution is one that has never flown before. Nuclear electric propulsion (NEP) is a proposed form of travel that could potentially land humans on Mars in less than 40 days.
For dreamers like Dr Franklin Chang-Díaz, the answer is not if we can go to Mars, but when. From his doctoral days at the Massachusetts Institute of Technology, he has been studying space propulsion. After years as a physicist and National Aeronautics and Space Administration (NASA) astronaut, Chang-Díaz founded the Ad Astra Rocket Company to build the VASIMR engine, a magnetoplasma rocket thruster designed to ultimately pair with a nuclear reactor and enable nuclear electric propulsion for travel in space.
“Chemical propulsion will continue to be important for launching and landing, surface-to-space transportation, but for deep-space missions to Mars and beyond, there is no way in my mind that we can do this sustainably and safely without nuclear electric propulsion,” Chang-Díaz says.
NEP extracts heat from a nuclear reactor to produce electricity and ionise plasma that is expelled from the back of the rocket. Cutting travel time from months to days would save the industry millions and eliminate a host of problems surrounding cargo storage, crew physiology and radiation exposure.
The need to develop an efficient and safe method of space travel is not just technical. It is also human.
“On your way to Mars, you are going to see a little point of light—that is going to be the Earth—and then you are going to see another point of light, and that is going to be Mars. And then you will see millions of other tiny points of light.
You are going to redefine loneliness. You are going to see what it is like to be alone in such a journey. The perspective of an astronaut perhaps helps in understanding the need for the propulsion to be robust, fast and resilient. What got me started was exactly that: the ability to give astronauts a fighting chance.”
But while physicists like veteran astronaut Chang-Díaz believe NEP is the only hope in getting humans to Mars, other experts say otherwise. Solar electric propulsion utilises solar panels to generate electricity and convert it into power for the spacecraft. Hybrid propulsion utilises both chemical and solar electric to create an efficient method of in-space travel for specific missions.
While all of these choices have been proposed and considered for Mars, they come with limitations. Chemical propulsion in space acts like a slingshot. It is difficult to return if a problem occurs onboard. Solar electric is more efficient than chemical, but comes at the cost of much longer travel times. For human missions, increasing the trip time could put the crew at risk.
Dr Ronald Litchford, who currently serves as principal technologist for propulsion for NASA’s Space Technology Mission Directorate, believes that while NEP has practical use for scientific missions, it may not be essential to get humans on Mars.
“We can do Mars without NEP. You can do it with chemical,” Litchford states. But if NEP were pursued, he suggests building a nuclear thermal propulsion (NTP) system first. NTP is the other form of nuclear propulsion, often confused with NEP. In NTP, hydrogen is passed through the hot nuclear core of the reactor. The extracted heat then expands the hydrogen through a conventional rocket nozzle, similar to a chemical rocket.
“My perspective would be that you would first build an NTP system just to do a basic mission,” Litchford suggests. “Then, once you think about long-term sustainability, asking, ‘how can we adapt this NTP reactor so that we can build a vehicle that has this nuclear electric reactor in it with high-efficiency thrusters?’ Low-cost infrastructure that would allow us to continue a human presence on Mars on a sustainable basis.”
But just as the process with NTP is similar to chemical, so are the limitations.
“The materials cannot withstand those kinds of temperatures. You can only go so far with the exhaust performance,” Chang-Díaz explains. “So, you end up with a nuclear rocket that is only twice as good as a chemical rocket, but not more. That is a lot of effort to only get a factor of two benefit. That is not the best use of nuclear power, so nuclear electric is the way to go.”
And while NEP may not be critical in Litchford’s eyes, he would choose it above other options, like solar electric or nuclear thermal propulsion, if a workable solution existed today. “You would choose NEP, I think. There are very sound reasons for going nuclear when you are talking Mars. You can do chemical, but it is like people just saying, ‘[NEP] is too hard’.”
One of the main reasons NEP is considered too hard is not scientific.
The largest obstacle in engineering nuclear electric propulsion so that it is capable of space travel to Mars is the fear of nuclear power. Although nuclear power remains one of the safest forms of energy harvesting, many fear it could harm both life and property if a rocket launch were to fail.
“It is important to inform,” Chang-Díaz notes. “We cannot just dismiss [the concerns]. But the nuclear reactor for space will most likely be assembled in space, in the vicinity of the Moon, not near Earth. When we launch the components of the nuclear reactor, including the nuclear material, it would be substantially harmless.
The problem with nuclear reactors is not when the reactor has not started. It is only when the reactor spends time burning nuclear fuel that it produces the decay products. The concept is that you would not start this reactor until you are far away from Earth. There is no chance that the reactor would crash on Earth.”
Beyond misinformation, funding for NASA is another barrier. Although the budget for NASA increased by USD1 billion in fiscal year 2018, the agency still only receives 0.5 percent of the American annual budget. A nuclear reactor for NEP must perform on the order of 10mw.
Such a level of power is possible on the ground, where American reactors are capable of producing between 500mw and 1,000mw of power. However, such an output from an in-space nuclear power source is not yet safe or space-rated, and will take time, interest and funding to develop.
And for NEP to be funded and developed to the level that Mars requires, it must be selected as the propulsion choice for Mars. It is impossible to determine what year a human landing on Mars could happen when we do not know how we will get there.
NASA and the White House have yet to select which propulsion system will be used and other space agencies are further behind in plans to get to Mars than NASA. The first step in committing to reaching the Red Planet must be committing to the propulsion method. And even beyond space travel, investing and committing to NEP might have benefits here on Earth. Chang-Díaz sees NEP being useful in addressing climate change too.
“Nuclear power for humans in space is essentially like the invention of fire. If we do not have fire, we die. It is as simple as that. “Right now, the only energy we get on planet Earth is the Sun, which happens to be a nuclear reactor. As long as we stay here and suck up energy from the Sun, I guess we are okay. We are on our way to messing up our planet though. In a way, we are destined to perish on this little planet because we cannot carry the fire anywhere else. Nuclear power is essentially that—the ability to bring our own fire to survive.”
He believes humans are meant to explore, not just to survive if Earth is no longer livable, but because we have always done so. “Humans are destined to get off Earth. If we want to survive, we cannot all be here. I often tell people that I dream Earth will be humanity’s national park, and most of humanity will be living somewhere else: Mars, the moons of Jupiter, the moons of Saturn, colonies on the Moon and other places in our solar system.”
The future Chang-Díaz dreams of is one many of us have been dreaming of since our youth. Perhaps one day, in the near future, our dreams will be realised. And when it is, it will likely be because of nuclear electric propulsion.