Nuclear Isotopes or Solar Panels? Here's How We'll Power a Manned Spacecraft on the Journey to Mars.

Electric-Powered Ion Engines

Previously, I discussed how traditional liquid-fueled spacecraft propulsion isn't a realistic means of getting humans to Mars. The weight requirement of using traditional liquid-fueled propulsion to get a spacecraft to Mars would be unworldly—every added liter of fuel increase the total weight of the spacecraft, and every added kilogram of weight increases the amount of fuel needed in order to get to Mars. 

In order to reach the red planet within a reasonable time-frame, we'll need to utilize and enhance what is currently a cutting-edge propulsion system: electric-powered ion engines that use 95% less fuel than rocket engines.

But in addition to propellant, ion engines require a constant flow of electricity in order to produce propulsion. The larger the engine, the greater the thrust, but the more electricity it needs. NASA doesn't yet have an energy system capable of generating enough electricity to power a sufficiently large ion engine. And in order to ferry humans to Mars, the next-generation Orion spacecraft will need an energy source capable of producing several times more energy than any previous manned spacecraft has used before—and it must be sustainable over long periods of time. 

Artist's concept of a Mars mission in 1963. Soon, NASA, soon...
Image Credit: NASA/Aeronutronic Divison of Philco Corp

Just how much energy? On the low-end, a 100-kilowatt ion engine may be able to get a small manned spacecraft to Mars in about 9 months each way—if the spacecraft were kept light enough. But when factoring in crew size (preferably 4+ astronauts), a deep-space habitat module, a Mars descent/ascent vehicle, and enough provisions to keep the crew alive, 100 kilowatts may not be enough.

For a comprehensive and more reasonably-timed mission, a spacecraft capable of generating as much as 500 kilowatts worth of electricity (spread among instrumentation and ion engines) may be more suitable for manned missions to Mars. And with the majority of that energy dedicated to propulsion, a trip to Mars could be accomplished in as little as 3 months each way—a significant improvement over liquid-fueled propulsion in terms of transit time.

Energy Systems Past, Present, and Future

The Apollo command and lunar modules each carried hundreds of kilograms worth of fuel cells and batteries to get to the Moon and back. The journey typically lasted between 8 to 12 days, during which the Apollo command module could generate up to 2.3 kilowatts to power its various instruments—about as much energy as an oven uses in a modern home. The computers of the Apollo area were rudimentary and didn't require much power in order to run, and the engines were powered by rocket-fuel instead of electricity. In the 1960's spacecraft, 2.3 kilowatts went a long way.

Picture taken of the Apollo command and service module in orbit around the Moon by Apollo 15 astronauts while inside of the Lunar landing module.
Image Credit: NASA

Beginning in the 1980's, the Space Shuttle used a next-generation version of the Apollo fuel cells, generating an average of 7 kilowatts and as much as 12 kilowatts of power for the duration of  its mission—about as much as an average household at peak energy usage (lights, stove, computer, TV, refrigerator, charging electronics, etc). But, as with its Apollo predecessor, all Space Shuttle missions were generally concluded within one or two weeks. 

For the past fifteen years, the International Space Station (ISS) has been permanently occupied by a rotating human crew in low-Earth orbit. It's powered by eight independent solar array 'wings,' each capable of moving independently in order to best capture sunlight. With each wing measuring 35 meters long by 11 meters wide, they weigh a combined 8,700 kilograms and provide a maximum power output of 120 kilowatts, covering over half the area of a football field.

While the ISS generates enough electricity to power a smaller ion engine, much of that power is already allocated to running its many computers and instruments—although more panels could theoretically be added, the craft is already far too bulky to be used as an interplanetary vehicle. To make matters worse, the effectiveness of solar panels drops off drastically as distance from the sun increases

A complication in using solar power for deep-space missions is that, as distance from the sun increases by a factor of two, the amount of solar energy available for harvest decreases by a factor of four. This means that any spacecraft traveling to Mars would require more than twice the amount of solar panels in order to operate at this greater distance.

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Even using electric ion engines, the journey to Mars could take 9 months (one way), and the total mission timeline could extend to as long as two years. Fuel cells and batteries fail to provide a sustainable source of energy, but the solar panels installed on the ISS are far too bulky and may not be able to provide enough sustainable energy. 

Since NASA plans to send humans to Mars as soon as the 2030's, it's unlikely that any game-changing energy tech will emerge and become viable between then and now. In order to reach the red planet, engineers will need to use the technologies already available to them in order to produce the most amount of energy with the least amount of added weight (in order to maximize thrust and reduce transit time). Two of these potential technologies are:

  1. Radioisotope Thermoelectric Generators (RTG's)
  2. Advanced Solar Arrays

The twin Spirit and Opportunity rovers that landed on Mars back in 2004 were equipped with solar panels which, at peak sunlight levels, were capable of generating as much as 140 watts (0.14 kilowatts) of power for up to four hours per day. As time went on, solar cell degradation and dust accumulation on the panels began to limit the amount of energy the rovers could generate. And since the little robots require 100 watts of power just to rove around, a lack of consistent power supply meant that their ability to do science on Mars was limited by the effectiveness of their solar arrays. 

Image in which Curiosity's RTG is clearly visible hanging off of its rear-end.
Image Credit: NASA/JPL-Caltech

When Curiosity landed on Mars back in 2012, NASA made sure it was equipped with a Radioisotope Thermoelectric Generator (RTG)—a device which generates a constant and reliable power supply from the slow decay of the highly radioactive Plutonium-238. In 2012, Curiosity was generating a constant 110 watts of power day and night. Two years on, it was still producing a constant 100 watts of power (as the isotope decays, less of it becomes available to continue to decay, gradually lowering its power output). 

Equipped with an RTG, Curiosity is producing five times more energy per day than the solar panels of its predecessors, allowing it to embark on a full suite of activities regardless of dust and sunlight conditions. And if the rover really wanted to, it could rove on into an unlit Martian cavern and continue doing science in perpetual darkness.

That little trick could have really helped the ESA's Philae lander, which ran out of power when it attempted to land on a comet in 2014 and bounced into a rocky outcrop that blocked sunlight from hitting its solar arrays. 

RTG's also provide a great solution to the problem of distance, and nearly every robotic probe that's ventured to the outer planets has used decaying isotopes as its primary energy source. The New Horizons spacecraft that recently flew past Pluto uses an RTG capable of generating a constant 228 watts of power. And, since 90% of the energy generated by an RTG is radiated as heat, the RTG also prevents vital components from freezing and disabling the probe in the cold regions of the outer solar system. 

Image of an RTG mounted to the side of New Horizons. It remains insulated from the spacecraft in order to allow excess heat to slowly radiate into space.
Image Credit: NASA

Had New Horizons been using solar panels to produce electricity as far out as Pluto, it would have needed three times more panels than the ISS just to generate its required 228 watts at a distance of 35 AU (5.2 billion kilometers, 35 times further from the Sun than Earth is).

In other words, solar arrays that could have generated 360 kilowatts of energy in orbit around Earth would only be capable of pulling in 228 watts (0.228 kilowatts) of energy at Pluto. Because of this reliable supply of electrical and thermal energy, RTG's become most cost-effective as their operational distance from the sun increases. 

RTG's and Ion Engines?

In order to build an RTG large enough to power a large electric-ion engine (as previously determined, around 500 kilowatts of available power would be ideal), it would need to be significantly larger than any previously designed. And because of this, weight becomes a problem—as does the amount of thermal energy released by radioactive decay. 

The Cassini spacecraft generates about 800 watts using three RTG's simultaneously, each weighing 56 kilograms. Cassini's RTG's are state of the art, but scaling them up to produce 500 kilowatts of electricity would require a 625-fold increase in power output. 

625 x 3 RTG's x 56kg = 105,000 Kilograms

In order to produce 500 kilowatts, a manned mission to Mars would require 105 thousand kilograms worth of radioisotope thermoelectric generators. That's the weight of about 20 fully grown adult African elephants. Just in radioactive generators. That's more than twice the total weight of the Apollo spacecraft that went to the Moon and back in the 1960's—the combined weight of command module, service module, and lunar lander came in at around 45,000 kilograms. 

One of Cassini's RTG's before launch. Capable of generating about 266 watts, a Mars-bound spacecraft would need 1,880 of these in order to generate its 500 kilowatts.
Image Credit: NASA

This immense size would become problematic: and RTG is a thermo-electric generator that produces about ten times more energy as heat than it does as electricity (in The Martian, Mark Watney uses an RTG as a heater for his Mars rover). And although the vacuum of space is quite cold, the scarcity of free-floating particles means that radiating heat out and away from the spacecraft is an extremely long and slow process. 

If we continue to use Cassini as our scale model, its three RTG's produce a total of 13,182 watts of energy as heat. This converts to nearly 45,000 BTU's per hour.

And since a manned Mars-bound spacecraft would need 625 times Cassini's electricity output, the amount of heat radiated from its power source would bump up to over 28 million BTU's per hour.

To put that in perspective, burning one kilogram of wood releases about 7,500 BTU's. Therefore, a 500 kilowatt RTG would produce heat at a rate equivalent to burning over 3,700 kilograms of wood every hour for the entire duration of a mission to Mars.

That's like a raging forest fire—on a space ship.

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An additional problem is that of fuel. Because the RTG is powered by the radioactive decay of Plutonium-238, its use in deep-space missions is limited by the problem of supply. Cassini uses a total of 33 kilograms of Plutonium-238 in return for a maximum output of 800 watts (slowly diminishing year over year).

Cassini carried enough Plutonium-238 on board that, if it were to have re-entered and disintegrated in Earth's atmosphere or suffered a launch failure, radioactive material exposure could have resulted in hundreds—or thousands—of deaths by cancer depending on its location. A 500 kilowatt version of this reactor would carry over 20,000 kilograms of radioactive Plutonium-238, essentially turning the launch vehicle into a nuclear missile. With this much Plutonium, a launch failure or accidental re-entry of the spacecraft could result in millions of deaths and a new irradiated dead-zone.

Inside the RTG: radioactive Plutonium-238. Looks hot, is hot.
Image Credit: Wikimedia Commons

Unfortunately (or luckily?), the reality is that Earth's entire stockpile of Plutonium-238 comes nowhere close to the amount required for an RTG of that size. As of 2013, the total U.S. stockpile of Plutonium-238 amounted to only 35 kilograms, half of which was already too far decayed to be viable for deep-space missions. The total annual production goal for Plutonium-238 currently sits at only 1.1 kilograms per year, enough to support one of NASA's robotic probes every five years or so—anymore than that has been deemed unnecessary for the agency's long-term plans. 

Ultimately, RTG's are not a viable option for manned exploration missions to the surface of Mars—the engineering challenges presented by heat radiation and the limited supply of materials make this technology a write-off. 

If there were a perfect model spacecraft for designing a mission to Mars, it would be the ISS. With 15 years of flawless performance and over 3 billion kilometers traveled through space, it could have already traveled to Mars and back multiple times over (had it been equipped with a large enough engine... and some additional radiation shielding for good measure). 

The International Space Station in low-Earth orbit.
Image Credit: NASA

With a volume comparable to the size of a six-bedroom home and hordes of computers and instruments running around the clock, the amount of power generated by the ISS' solar arrays typically hovers around 80 kilowatts—plus or minus about 30 kilowatts depending on the orientation of the arrays. The ISS' annual power generation is equivalent to the average annual power consumption of 70 North American homes. But for the journey to Mars, we'll need even more. 

More Energy Than Ever Before

The ISS' 80 kilowatts of energy production is spent exclusively on running its hardware—dozens of computers controlling every aspect of the spacecraft, hundreds of simultaneous experiments being conducted on-board, and so on.

For a Mars-bound spacecraft equipped with electric-ion engines, a 500 kilowatt power source would only spend about 10% of its power budget on running hardware and instrumentation—the rest would be dedicated to powering the engines around the clock. In order to generate 500 kilowatts of electricity from solar power, the spacecraft would require about 36,000 kilograms worth of ISS-like solar panels, or more than four times as many as are installed on the space station today (this weight does not include other electrical components and structural supports).

But distance also complicates things. With Mars averaging 1.5 times further from the sun than the ISS, a spacecraft in orbit around the red planet receives only 44% as much sunlight per square meter as it would in orbit around Earth. This means that a 500-kilowatt solar panel array at Earth would only produce 220 kilowatts of electricity at Mars.

In order to continue generating 500 kilowatts at the red planet, the spacecraft would require 2.3 times more ISS-like solar panels, bringing their total weight to around 82,000 kilograms—significantly more than the total weight of NASA's Orion spacecraft at liftoff, and 9 times more panels than are currently attached to the space station. 

The great distances between the planets are problematic for solar panel-equipped spacecraft, which is why NASA primarily uses RTG's to power its deep-space missions. But there are exceptions: NASA's Juno spacecraft, currently en route to Jupiter, will reach a distance five times further from the Sun than Earth, therefore receiving only 4%, or 1/25th, as much sunlight. At Jupiter, Juno's 60-square-meters worth of solar arrays will only produce around 420 watts of power. Had they been designed for use around Earth, they would be producing closer to 12,000 watts of power—12 kilowatts. 

Next-Generation Solar Panels

Solar panel technology has improved rapidly over the past few decades. Whereas the ISS' decades-old panels contain around 100 photovoltaic cells per square meterJuno's modern solar panels contain around 300 photovoltaic cells per square meter—and have greater overall efficiency. 

Looking decades into the future, a Mars-bound spacecraft could be equipped with solar panels that are significantly improved over current iterations. The ISS' 14% efficient solar panels designed two decades ago are no longer cutting edge technology; NASA's next-generation Orion spacecraft uses a system of folding solar panels that can convert 30% of sunlight to energy and weigh 1/4 as much as conventional solar arrays

Whereas Juno's 60 square-meters worth of solar panels could generate 12 kilowatts at Earth, Orion is able to generate 12 kilowatts using only 26.3 square-meters (spread among two 5.8 meter wide circular arrays)—more than a two-fold improvement.

Artist's concept of the Orion spacecraft's circular solar arrays.
Image Credit: NASA

Scaling this up to a 500 kilowatt version, using Orion-like solar panels would only require 1100 square-meters of area, or only 44% as much area as is taken up by the ISS' solar arrays. And at 1/4th the weight, this Mars-bound spacecraft would only be adding a few thousand kilograms worth of solar panels to its total weight (keep in mind that this is with 2015 technology, not 2030's technology). 

Potential Solutions to the Problem of Distance

Even with the lightweight and efficient solar panels currently being developed by NASA, there's still the problem of distance to contend with. A solar-powered spacecraft around Mars will be operating with half as much energy as it had around Earth. Here are five potential solutions:

1. Add additional solar panels. With Orion-like solar panels generating 500 kilowatts of energy with only 44% as much collection area as ISS-like panels, and Mars only receiving 44% as much sunlight as Earth, then a Mars-bound spacecraft would need 2500 square meters of Orion-like panels—the same as the ISS—in order to produce 500 kilowatts at Mars. The total added weight would be as low as 1,000 kilograms, and they could remain folded up for the majority of the mission (and would therefore serve the dual-purpose of being a backup in case one of the primary arrays fails). 

2. A 'solar sail' capable of being deployed at Mars to turn additional sunlight into propulsion. Although the amount of thrust gained by a solar sail would be minimal unless it were quite large, it may be able to provide enough of a boost so as to make a small difference in terms of Mars-Earth transit times. 

3. A battery capable of storing days—or weeks—worth of propulsion energy for part of the voyage home. With current battery technologies, a 200+ kilowatt rated battery would need to be massive. But there's still the chance that at some point in the next two decades battery technology could progress to the point of making this a viable option. Perhaps it could even be sent on a separate probe and be placed in Martian orbit awaiting its rendezvous with the Mars-bound spacecraft prior to its return home. 

4. A large orbital sunlight reflector attached to a probe. Having a sufficiently large mirror in orbit around Mars that could focus additional sunlight onto a spacecraft's solar arrays may allow them to generate significantly more energy than they otherwise would. The complexity of having such an object capable of independent positioning and even following a return-vehicle for some time (in order to continue providing focused sunlight) would be a massive undertaking, but this type of probe could probably be built using current technology. And if it were to fail, it wouldn't be as catastrophic as an out-of-control solar sail attached to a spacecraft or a malfunctioning battery that could release dangerous chemicals. 

5. Just return home anyways. Even though the electric-ion engines would only be operating with about 44% as much energy as before, they would still be able to provide sufficient thrust to return the spacecraft to Earth—eventually. Also, as the spacecraft left Mars' orbit and moved closer to the sun, more solar energy would be made available for propulsion. The craft would gradually gain more energy and more thrust as it journeyed towards Earth, resulting in the slight inconvenience of a longer return voyage but otherwise having avoided the need of complicating an already complex mission with additional hardware. 

Finding The Right Formula

In order to best ensure the success of a manned mission to Mars powered by electric-ion engines, NASA will need to determine the winning formula of spacecraft weight—available thrust—available energy

Artist's concept of a mission to Mars. Needs more solar panels...
Image Credit: NASA/Pat Rawlings

It may be possible to reach Mars using only a 100-kilowatt electric-ion engine, assuming spacecraft weight was sufficiently low and longer transit times were planned for. An engine of this size would likely take 9 months to a year to travel from Earth to Mars, perhaps allowing for a multi-month stay on the red planet before astronauts would need to head back to Earth. Total mission duration would probably be two years or more.

But mission times could be significantly cut down by using a 500-kilowatt electric-ion engine, with Mars transit times being reduced to as little as 3-4 months. This would allow much greater flexibility with regards to length-of-stay on Mars and the window of opportunity for returning to Earth. Total mission duration could be kept under one year, but a faster Mars-Earth transit time also means greater flexibility with scheduling the return voyage—missions could be made longer or shorter depending on objectives, and astronauts would always have the option of quickly returning to Earth (relatively speaking) in case of emergency.

NASA's already invested heavily in improving the design of current solar-electric spacecraft, with current non-experimental versions reaching power levels of around 7 kilowatts. There's still a ways to go, but in the meantime, low-powered versions of ion engines could become viable for sending humans back to the Moon in the near future. 

This is part three of a three-part series. Check the rest out below:


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