To Mars or Bust - Radiation En Route
(Originally posted January 24, 2017 on Blogger)
The February edition of Scientific American is out, and within its glossy covers is an article about new studies that show cosmic radiation could be even more damaging to astronauts' brains than previously realized. The article is rather inauspiciously titled, "DEEP-SPACE DEAL BREAKER". (There is also a rather ad hoc article by Steinhardt & Loeb in that issue on cosmic inflation I'd like to respond to in another blog, but we'll save that for later!)
Two months ago I wrote about this very thing; the dangers of cosmic radiation to human life, with particular concern for exposure periods and exposure to interstellar HZE ions like Iron-26. For those interested in reading my blog on why I think there ought to be more serious ethical considerations to be genuinely mulled over before even beginning these grandiose, and oft hyperbolic, plans to send people to Mars, you can read that blog here:
That read goes against the current and on-going grain of public enthusiasm for sending people to, and colonizing the red planet. But I'm very comfortable in doing so as the science of why it isn't yet feasible is on my side. I even go so far as to suggest Venus as the far more logical target, and give reasons as to why.
Now I'm writing to further reinforce my claim that it is ethically vacuous to set arbitrary deadlines to send human beings to a planet that is far more inhospitable than the public realizes. Charles L. Limoli is a neuroscientist and radiation biologist at the University of California, Irvine, School of Medicine. In his article published in this February's edition of Scientific American, he makes no qualms that cosmic radiation may be more damaging to the human body, particularly the brain, than anyone previously realized.
Galactic cosmic rays (GCRs), as I explained in my November 22, 2016 blog entry, are cell-damaging 'bullets' shot from supernovae that can metaphorically rip through the bodies of astronauts who've ventured beyond the protective cradle of Earth's magnetosphere. These bullets aren't just randomly speeding through interstellar space, they "pervade the cosmos as a uniform field" (Limoli, C. 2017, 'Deep-Space Deal Breaker', Scientific American, February, p. 56). A frightening thought when you consider we have yet to develop a material dense enough, yet light enough to adequately shield spacecraft to protect against this unavoidable field of certain death (if not by post-mission cancer). Interstellar sources, as I wrote in my previous blog entry, "To Mars or Bust", aren't the only things to worry about; our own Sun turns our solar system into a shooting gallery filled with metaphorical bullets of ionized hydrogen. Though they have less mass, and varying energies, some can still penetrate the hulls of spacecraft, and of course, the bodies of astronauts.
When it comes to radiation, energy and the damage it can inflict on the human body are directly proportional. Limoli explains that when charged particles pass through the body, they strip electrons off atoms within the body leaving them in charged states (making them cations-or positively charged ions). However, in nature--and across the cosmos--it is the natural state for all things to find charge balance; that is, a neutral charge of zero. Atomically, this means having an equal number of protons (each with a +1 charge) as electrons (each with a -1 charge). If you have an atom with equal portions of both, it is neutral (zero charge), and very content with itself to put it anthropomorphically.
However, introduce interstellar GCRs or ionized hydrogen from our Sun, and electrons from those content little atoms can be stripped away, leaving them with a net positive charge and highly reactive. As such, the atom(s) affected, which are likely parts of molecules, can break the molecules they were a part of, which in turn damages things our bodies need to function. Limoli lists proteins, nucleic acids, and lipids among these vital things, and writes that there are many more.
The above electron-stripping bombardment by GCRs and ionized hydrogen result in free radicals inside the body. Free radicals are atoms or molecules that no longer have neutral charge, and when we have a situation where an atom (alone or as part of a molecule) that no longer has a neutral charge, we end up with highly-reactive particles that want to... no, NEED to find the electron, or electrons necessary to return it to its neutral state. In the human body, this more often than not means these free radicals will react with molecules that serve a life-supporting function, thereby changing them to do things that aren't in line with their original function; one example of this would be cancer cells whose main function is to reproduce themselves at the expense of performing vital functions we need to live.
Free radicals can break apart the sugar phosphate backbone of DNA, and you don't need to watch the introductory scene of the movie 'Prometheus' to know that disintegrating DNA is not the way to a healthy life!
In my blog entry from November (2016), I used units called Sieverts (Sv) when describing radiation exposure; how the shortest-possible trip to Mars could result in up to 0.78 Sv (or the equivalent of about 7,800 chest X-rays, which was the equivalent of getting a chest X-ray every 55 minutes for over 8 months), when the EPA average annual radiation dose in the United States is only 0.0062 Sv. Limoli uses the SI unit, the gray (Gy) in order to quantify the measure of radiation exposure on the body (in terms of unit of body mass). Given this, one gray is one joule per kilogram of body mass. In his line of research, scientists characterize radiation types by their linear energy transfer (LET).
LET is the amount of energy lost per unit distance traveled. Consider the fact that some charged particles pass through an astronaut's body. As they do this, they leave behind a trail of destruction. As explained above, this trail of destruction is the path of atoms and molecules left behind that have been stripped of some of their electrons, thus leaving them reactive and broken. When considering LET, high-LET radiation can be simply understood as being those particles that are able to leave wider paths of destruction relative to low-LET radiation because high-LET radiation has more energy to ionize more atoms. Of course, wider paths of destruction means more free radicals, more damaged DNA, and more potential for reassigned functions of previously life-supporting molecules (ie. cancer). The more damage done, the harder it is for the body to repair itself. There is a point at which cellular repair is a non starter; you're just dead.
This is why HZE ions like Iron-26 (see my Nov. blog) are so dangerous. No hull can withstand them, and certainly no human can either. The problem Limoli faced when doing this research was that having access to labs capable of reproducing high-LET radiation is limited. This of course should say something about the kind of energies we're talking about with regard to interplanetary exposure (as well as exposure on Mars' surface). Though Limoli was able to access NASA's Space Radiation Laboratory, his time was limited, but the results should be a wake-up call to Mars enthusiasts. As I wrote in my previously blog, I am absolutely a big fan of Mars, and sending robotic missions to the red planet in search of life, and evidence as to the geologic history of the planet. But I am not a fan of sending people there; if for no other reasons than moral ones. Not until this hurdle is hurdled.
What Limoli found was that exposure to high and even low-LET radiation levels of which mimic that which would be experienced by those venturing through space, causes damage to the brain; that part of the human body whose proper function is paramount to the success of any interplanetary mission.
Let's consider the microscopic parts of the brain that were notably affected, then look at the overall regions of the brain for perspective as to why these affected microscopic parts of the brain can be so devastating to anyone we send to Mars.
Within the brain are dendrites, which are branches in brain cells that receive electrical messages (excitatory & inhibitory) and propagate these messages, acting like critical mini-neural computers. As seen in the illustration to the right, dendrites can be visualized as being like branches on a tree. Considering their numbers, you can imagine the processing power they are capable of together (use your dendrites for this). There are also dendritic spines, which Limoli explains can be visualized as the leaves on the dendrite branches. These tiny fraction-of-a-micron-sized dendritic spines are critical in enabling learning and memory.
The damage done by radiation exposure similar to that one would be exposed to during interplanetary travel was to these dendrites and dendritic spines in the medial prefrontal cortex, which is associated with memory. Dendritic spines decreased in size and number, which means less processing power and poorer memory. Not good when you consider astronauts in-flight and on Mars' surface functions require critical decision-making skills, problem-solving skills, and of course a maintenance of sanity.
What is really concerning to me is that even low-LET radiation caused significant reductions in length, area, and branching of dendrites over the course of 10 to 30 days. The shortest possible length of time it would take to get to Mars is several times longer than this. Even dendritic spine density was reduced, and their reduction results in a proportional reduction in cognitive ability. We may send the "right stuff" to Mars, but there is a serious risk of them arriving as the wrong stuff.
If low-LET radiation can make a Bruce Willis out of a Buzz Aldrin, then imagine what high-LET radiation could do! No offense Bruce! You're smarter than me, that's why you have money and I'm broke! haha :/
Six-week exposure to 5 and 30 cGy (centigrays) reduced the discrimination index of laboratory mice by about 90%. This is a no-brainer (pun intended) when one considers the negative impact this would have with regard to vital mission activities. Limoli figured interplanetary dose rates would be about 0.48 mGy (miligrays) per day (this excludes HZE ions which can easily spike these numbers biocatastrophically), with on-planet dosages of about half that since the planet's mass would block radiation from beneath you (unlike the spacecraft). We have the technology to better protect our astronauts from low-LET radiation. Even if it means heavier hulls, we do have the rocket science down enough produce boosters capable of escape velocity (from Earth). But time and care should be taken in considering ways to protect our astronauts from high-LET radiation which is out there.
High-LET radiation will get through hulls and expose our astronauts to unreasonable levels of radiation that is highly-likely to negatively affect their abilities to perform critical mission duties, which puts them at serious risk. There could be a point where we not only need to protect our astronauts from the rigors of space, but also to protect them from themselves. Even current and developing drug and dietary counter-measures to cosmic radiation exposure will have little to no benefit when severe cellular damage from high-LET radiation is experienced, no matter how quickly after-the-fact countermeasures are administered.
There exists no evidence in humans that shows damaged dendritic complexity and spine density can repair themselves after cosmic exposure. Also, it has been long-known that most areas of the brain cannot generate new neurons easily, thus further inhibiting recovery, if recovery is an option afforded to astronauts returned from the red planet. This is serious stuff to consider before we send people to Mars. Stuff that should precede what are otherwise arbitrarily-derived deadlines to get people to Mars.
If anything, Venus makes more sense to me (if we must achieve interplanetary travel in our lifetimes). You can read about my arguments for Venus (50km up from its surface) in that November blog entry I wrote linked above.
Does all this mean we should give up on our dreams of setting foot on Mars? Absolutely not. What this all means is that we need to take a big moral step back and work on this elephant in the room before we blindly embark on history-making missions that could very well end up tragic when they don't have to be. The day will come when humans set foot on Mars, and the day will come when humans set foot on the moons of Jupiter and even Saturn (long after we're gone), but those future generations will never get anywhere if we don't resolve the problem of cosmic radiation exposure in interplanetary travel. Let's get them there safe, keep them safe while there, and get them home safe, before we start sending them into a cosmic battlefield without shield or sword.
As always, thanks for reading.