The Moon or Mars debate continues despite every single report or recommendation from NASA, NRC or other independent study that point to the Moon as the next logical destination for human space exploration and settlement. Once we hone the technologies to live there, "this time to stay" as the Bush administration of yore put it, we would have all the tools to live on Mars, return resources from the asteroids, homestead on Ceres or even the much prettier outer gems in our solar system like the satellites of Jupiter or Saturn, where the vistas are far more spectacular and seasonal changes more dynamic than anything that Mars or Venus can offer.

The physical facts are right above us in the skies every night, right in front of our eyes, for those doubting Thomases. The Moon is our closest celestial neighbor, a lifeless and barren continent, that orbits the Earth, just a quarter million miles away whereas planet Mars is at least five hundred times more distant, depending on orbital alignments. Literally and symbolically the Moon is a highly visible orb in our skies, compared to a peach pale dot that planet Mars is, that many who advocate cannot even locate in the empyrean.

Current technology allows us to ply rocketships in cislunar space, i.e., between the Earth and the Moon, every day while there are only very limited windows of opportunity to depart Earth to go to Mars. Rocketships to the Moon are much smaller, ten to hundred times smaller, depending on what and how many crew you wish to carry on expeditions, especially propellant, food and potable water. And mission control can keep check almost instantaneously round the clock. We can even mount rescue or emergency missions in short order, should the need arise. We cannot do this for Mars missions using current technology. The communications time lag during most of the Mars expedition is such that mission control on Earth can do nothing to help in an emergency. Even prayers can take 30 minutes or more to reach a transiting Mars crew in trouble. We can resupply Moon missions every day, if we so wish, but Mars crew are stuck with what they have onboard for the length of their journey that may last five to six months. And imagine this: floating around in weightlessness for five or six months, and then all of a sudden, crew are subject to gravity forces upon landing on the Martian surface. Even crew returning from much shorter trips to the ISS need a lot of time to regain their muscle and bone strength once back on Earth.

The saving grace about Mars is that they will experience less than half their body weight on Earth and be able to adjust to a similar diurnal rhythm of approximately 12 hours of night and day. But what use is that when you need to be fully suited and unable to breathe the almost pure CO2 atmosphere, and that too, at such a low pressure as to be of no use at all, not to mention the dust storms that can mask the sun for months at a time. Solar photovoltaic arrays that power the ISS today have been the mainstay for space systems and satellites since the dawn of the space age and this technology will not suffice for Mars habitats because of dust storms in the thin atmosphere block out sunlight, and nuclear power and propulsion systems are decades away from certification by NASA. We could use mature and reliable space qualified photovoltaics in those polar regions of the Moon almost perpetually while we learn to deploy, operate and service nuclear reactors that could be commissioned later on as these systems are proven on the lunar surface, on Mars and other destinations further out in the solar system, where the sunlight gets progressively dimmer and solar power becomes untenable.

The emerging robotic construction technology has huge ramifications for planetary infrastructure establishment, and that is especially true for the Moon. It is now possible to erect or build entire habitable structures, certify and commission them before humans arrive at the destination to occupy them. Lunar settlements and associated infrastructure elements like landing pads, roads, storage hangars and even component manufacturing factories and their supply chains may all be built and serviced from Earth. Robotics technologies have advanced so far that robots landed on the Moon, may be controlled from Earth, using advanced telerobotic systems and technologies that are already playing a vital role here on Earth. It is much more challenging to build infrastructure on Mars this way, let alone steer a rover that is hundreds of million miles away because of time delay associated with command and control signals.

Why is NASA's Mars plans always thirty years away ? This is a question often asked in policy meetings but never even brought up in any technical gatherings. The reason is simple. We do not have the technologies currently to keep people alive and well for the long duration missions through the deadly radiation environment that pervades interplanetary space, especially in our neighborhood close to the sun, where life evolved and we live happily, thanks to the protection offered by Earth's magnetic field and the thick blanket that is the Earth's atmosphere.

Even the International Space Station is protected by the Earth's magnetic field, blanketed by the Van Allen belts where the often lethal solar storms are moderated and much of the fury of our sun is quenched. Neither Mars nor our Moon have such a protective field which is responsible for planet Earth holding an atmosphere, and that is very troubling for permanent extraterrestrial settlements. Artist impressions of people flying around in low gravity with magnificent vistas in the background may need serious revision. Space architects think that all extraterrestrial settlements may be deep underground with few observation towers and habitable facilities on the lunar or Martian surface which may be human tended only for very short periods in order to avoid excessive exposure and consequent radiation damage to human tissue.

Now compounding this natural phenomenon is yet another. It is called galactic cosmic radiation, abbreviated as GCR. Particles, mostly made of iron ions, some that pack energies comparable to baseball fast pitches, are constantly zooming through interplanetary space. Thought to originate at the death knell of stars or supernovae, their energies are several orders of magnitude more than what we can generate here on Earth, even in the most advanced accelerators ever built. These particles are so powerful that they go right through spacecraft and human tissue alike, but they also generate secondary particles upon collision with spacecraft material, and these much slower secondary particles, especially neutrons, are the real culprits that can be lethal to astronauts. Ways exist to deflect charged particles like high energy protons and solar alpha particles through active shielding technologies i.e., creating electromagnetic fields around the spacecraft artificially that can deflect them, but we have yet to devise ways protect us from neutral particles created by spallation, those energetic neutrons that are generated by secondary radiation.

Most talented engineers who build spacecraft are reticent about this show stopper, because they want to fly missions, like all of us, but NASA flight surgeons who have the final say and have to sign off on human missions, know that the risk is real. They know that state-of-the art technologies do not allow us, and it is futile to put our brave crew who are chomping at the bit to go, because they know exactly what will happen to them. They can even predict when their bodies will start to fall apart during transit, the point at which they will exceed the doses that humans can withstand without harm! Radiation doctors and professionals know that crew will perish during transit to Mars, and that we do not yet have the technology to protect them against GCR or anomalously large solar particle radiation storms, especially the dangerous, energetic secondary particle radiation that can cause a range of effects, from immediate crew impairment to slow and painful death.

We know full well the effects for radiation sickness and how systems shut down in death from our long and varied terrestrial experience with nuclear weapons development mishaps as well as nuclear reactor accidents like Chernobyl, and more recently from the crew who were exposed to deadly radiation from Fukushima reactor collapse and containment.

NASA has an active radiation monitoring and countermeasures program. For those wishing to dig a bit deeper, a quick look through their Man Systems Integration Standards(MSIS) will reveal enough gory and precise details of how the human physiology reacts to radiation and when the body starts to fail. Space radiation is the issue that pulled the plug on the daring Inspiration Mars mission, that recently proposed to put a crew on a free return trajectory on a flyby mission to Mars and back. The buck stopped at the NASA Astronaut Office, at the flight surgeon's desk, to be precise.

Experiments are underway on the international space station to ascertain what doses humans can handle. But once outside the Van Allen belts, the radiation environment is much more severe, as seen from the recent Curiosity rover that carries an active radiation monitor. It is clear we need better radiation protection for long expeditions, especially during transit, and we also need better data from deep space missions using biological samples(not crew !), yet to be manifest. We also know that radiation exposure during transit has a different pattern than on an extraterrestrial surface that blocks much of the GCR, due to the sheer mass of the planet. Again, the Moon, lacking a magnetic field blanket, offers the best site in our proximity to gauge the risk of long term solar particle radiation exposure as well as GCR, and the effects of deep isolation on crew, and is the ideal location to hone measures to combat these crucial issues.

All is not lost, though. We know that we can shield from this deadly radiation if our transit vehicles have thick enough shields of water. We also know that the tons of food and consumables for the expeditions as well as the large propellant tanks of Hydrogen could be configured as radiation shields around the crew compartment on these months long transits. Some engineers even think that water tanks would be the compact way the carry the propulsion reactants that could be manufactured as needed, enroute, both for outbound and inbound legs of such a long expedition. But once we get to Mars surface, how to survive the solar particle radiation that is quite high even there, on the surface? Unfortunately, we do not have an answer to this lethal issue yet !

To add to all the controversy to the exploration and settlement of Mars, is the issue of contamination and quarantine. Some scientists believe human activity on extraterrestrial bodies will endanger potential life forms that may exist there. And the search for life on Mars has only just begun. It may be decades before we know if there are life forms there or not. Until then, human activity may have to wait, if we are to follow their advice. There has not been much debate about this issue with regard to the "magnificent desolation" that is the surface of our Moon.

But all this begs the question: do we have to wait for technologies to develop, or are there worthwhile missions to do and gain invaluable experience while we get all these "good to have" technologies certified and commissioned for a Mars expedition ?

To be specific, are there space missions that can speed up technology evolution and inspire the public simultaneously while helping to fire up our STEM education and groom the next generation of explorers and engineers; missions that could also use space activity as sheer inspiration for our youth? The lure of the international space station seems to have run out of steam, at least among the public and the media. Space tourism may have some answers, perhaps ? And the US president's plan to send crew off to some unnamed asteroid for a nebulous and uninspiring mission seems to have weak and uninspiring support in Congress.

The Moon, on the other hand, offers all the excitement, now, as opposed to the next decade or the one after. A highly literally visible neighboring celestial neighbor is just three days away, and a dozen of our best and bravest have left their footprints there, not to mention their roving vehicles, some half century ago. NASA orbiting missions right now are providing the sharpest resolution imagery of the Moon as well as all the data, including radiation, for crew to quickly transit cislunar space and arrive at the destination. There are several nations at work right now, planning the next lunar missions. China has already landed a rover on the Moon. India was instrumental in locating water ice at the poles, that along with constant sunlight and mild surface temperatures all year round in the polar regions, could provide a stable setting for astronauts to learn to live and evolve systems for permanent settlements anywhere in the solar system.

And just for those scientists aiming for the next few decades of Nobel prizes, some of the finest scientific discoveries of great and immediate import to our species anywhere on the solar system is waiting for us on the Moon. The Moon, while we struggle to quilt solar activity information of the past few thousand years together, holds an unperturbed record of solar activity over the last few billion years, almost back to the genesis of our solar system and the formation of the Earth-Moon system, and this precious, nay priceless repository of information could tell us more about solar behavior over geologic time than any other body in our solar system. This data could form the framework for the puzzle as we build reliable climate change models and shape our policies. To be explicit, Mars exploration cannot tell us that.

Madhu Thangavelu
- Madhu Thangavelu conducts the ASTE527 graduate Space Exploration Architectures Concept Synthesis Studio in the Department of Astronautical Engineering within the Viterbi School of Engineering, and he is also a graduate thesis adviser in the School of Architecture at USC. He holds degrees in both engineering and architecture and has contributed extensively to concepts in space architecture, especially dealing with extraterrestrial development. He is the author or co- author of over 50 technical papers in space architecture, lunar base design and human factors, and co-author of the book The Moon: Resources, Future Development and Settlement(1999) published by John Wiley and Sons and second edition by Springer/Praxis in 2007. He is the invited author of the chapter "Living on the Moon" in the Encyclopedia of Aerospace Engineering, a major reference work published by John Wiley and Sons in 2010 and the on-line second edition updated in 2012. He is a member of the USC team that won the NASA NIAC Phase I award in 2011 and Phase II award in 2012. He is a former AIAA officer, having served as Vice Chair for Education in the Los Angeles section.