The new NASA Administrator Jim Bridenstine has announced that he wants to make sure there is never another day when humans are not present in space.   "In fact," he said, "we want lots of humans in space."  But is the present version of humanity really up to the job?

For humans, space is a very hostile environment.  There's no air to breathe, and "no one can hear you scream".  There's also no gravity, the lack of which over a few months will cause your muscles to degenerate and your bones to lose mass.  Further, outside the Earth's protective geomagnetic field and atmosphere, your body will be irradiated by much more ionizing radiation, which will damage or kill the cells of your body, will produce dangerous mutations in your future children, and will increase your chances of developing cancer in a decade or so.

Charles Stross in his novels in the Saturn's Children universe and particularly in his short story "Bit Rot", has envisioned a race of radiation-resistant android servants that had been engineered by humanity to better withstand the hostile environment of space, including working in vacuum without a spacesuit and enduring long periods of zero gravity without degenerating.  According to Stross' scenario, sometime in the twenty-third century humanity will go extinct and our former humanoid servants will take over jobs for which they are better equipped: exploring space and populating the stars of our galaxy.  Reengineered humanoids might be a nice solution to the man-in-space problems described above, but we must work with the humans that are presently available.  This raises the question of whether there are technological work-arounds for the space-related frailties of humans.

First, let's consider the problem of human degeneration in low gravity.  Despite what you may have been led to believe by science fiction, there's no plausible way of creating a local gravitational field except with a planetary mass.  It takes a lot of mass to bend space enough to make a gee-level field, and there does not seem to be any field-based alternative.

Fortunately, Einstein's equivalence principle leads us to another alternative: an accelerated reference frame is equivalent to a gravitational field.  The acceleration can either be linear acceleration, as in a spaceship accelerating forward, or angular acceleration as in the rotating space station in the film 2001.  With our present technology, gee-level linear acceleration can't be maintained for very long without running out of fuel, but rotation, once started, requires no fuel to continue.

Therefore, the human need for gravity can be accommodated by providing rotating vehicles and habitats in space to produce centripetal acceleration.  The pseudo-gravitational force provided in a rotating habitat is proportional to R, the distance to the axis of rotation, and is inversely proportional to the square of  f, the frequency (or angular velocity) of rotation of the habitat.  For example, the wheel-like space station in the film 2001 had a distance from the habitable region to the rotation axis of about R = 160 m.  To produce 1 gee of  artificial gravity, that habitat would have to rotate at an angular speed of  f = 2.36 revolutions per minute, a bit more than twice the rotation rate of the second hand of a wall clock.

The problem with such a rotating habitat is that centrifugal gravity is accompanied by the Coriolis force, a velocity-dependent "sideways" force that is proportional to the "vector cross product" of the speed v of an object in the local rotating environment and the rotational speed f, where the direction of f is along the axis of rotation.  One of my early Alternate View columns for Analog (AV-18 in the February-1987 issue) goes into considerable detail about Coriolis effects in a rotating space station.  Here let us just say that the Coriolis force will produce stomach-wrenching annoyances that tilt the floor when you turn or nod your head and make thrown objects veer off in unexpected directions.  In a small habitat or ship where R is small and f  must be large, it would be troublesome, disorienting, and difficult to adapt to.  The Coriolis effects could be made almost unnoticeable, however, by making the habitat's R sufficiently large.

The moral here is that long-term off-planet human existence in space will probably require a considerable investment in rotating ships and habitats.

The problem of space radiation does not have a well-known techno-fix.  The Sun, particularly at times of solar flares, spews out floods of fast electrons and protons that make the Northern Lights on Earth and represent radiation hazard for space travelers.  Moreover, galactic and extra-galactic cosmic rays include a population of very energetic highly-charged atomic nuclei, frequently iron nuclei, that create a radiation shower in shielding.   These bare nuclei ionize so strongly that, if they encounter the DNA of a cell, will almost always break both bonds of the double helix, making natural DNA repair effectively impossible.

The standard international unit of radiation dosage is the sievert (or Sv), defined as one joule of ionizing radiation energy deposited in one kilogram of mass.  A sievert represents a seriously large exposure to radiation.  A dose of about 4.5 Sv is enough to kill about half of a population of humans in 30 days.  More typical exposures are measured in millisieverts (or mSv), one thousandth of a sievert.  For example, the average human living on the Earth's surface will receive a yearly dose of about 3.6 mSv, a CAT-scan delivers a dose of about 8.5 mSv, a Department of Energy radiation worker is allowed a yearly dose of 20 mSv, and a Mars colonist would receive a yearly dose of about 234 mSv on the surface of that planet.  A 234 mSv dose is not lethal, but it greatly increases the likelihood of mutations in children produced by colonists and the incidence of cancer in later life.  To put it another way, unshielded life on Mars will deliver a dose of ionizing radiation that is 65 times larger than that of the average Earth resident and 12 times larger than that allowed for a DOE radiation worker.  That is enough for a great deal of concern about the health of Mars colonists.

The cells of a living organism damaged by radiation take three paths: cell repair, cell senescence, and cell death.  For very large doses the dominant effect is cell death, bringing with it the symptoms of acute radiation poisoning: immediate hair loss and low blood pressure, nausea and bloody vomiting in 10 minutes, bloody diarrhea and fever in 1 hour, headache in 2 hours, and ultimately death.  For milder exposures the outcome depends on the character of the radiation.  Electrons, muons, x-rays, and gamma rays tend to produce single breaks in DNA strands that, if they don't pile up, can be fixed by the ever-present internal cell repair mechanisms.   Protons and heavier nuclei have high ionization densities that produce more damaging double DNA breaks.  The radiation damage might be in a "junk" DNA region where it would have little effect, but some of the double-break radiation damage will inevitably render the cell non-functional.  Then the cell will either die or go senescent.

In the average human intestinal cells die and are replaced every 10 days, skin cells are replaced every month, red blood cells are replaced every 4 months, and liver cells are replaced every year.  In case of mild radiation exposure, cell death is preferable to cell senescence, as long as it does not add too much to the normal rate of cell replacement.

When cells do go senescent, there is a problem.  They shut down their normal functions and express the protein p16, thereby warning cell-reproduction machinery not to cause this cell to divide and reproduce.  However, because of their internal malfunctioning they also become "zombie cells", sending out harmful chemical messages to their cellular neighbors that create inflammation and disrupt operation.

In a recent test with mice, a quantity of fat cells were withdrawn from test-subject mice and externally exposed to x-ray radiation until the fat-cell population became about 80% senescent.  Then the treated cells were re-injected into the test subjects.  The effect of the presence of senescent cells was compared with a control group that had the same treatment without the radiation exposure and induced senesce.  It was found that when as little as 0.1% of the mouse fat cells were made senescent, this produced observable degradation of the motor-activity and fitness of the test subjects.  The conclusion was that even a small fraction of senescent cells present in living organisms degrades health and fitness.

At present the only remedy for space radiation exposure that has been seriously considered is the use of shielding, which requires lots of mass to be transported into space.  It's envisioned, for example, that if a Mars mission carries with it a large quantity of water for consumption and propulsion reaction mass, the crew must be housed behind water-filled walls to reduce space radiation exposure, and there may also be smaller extra shielded regions to which the crew can retreat in the event of a major solar flare.  Such shielding requirements greatly complicate manned-mission design.

Therefore, I have a suggestion for an alternate and complementary way of dealing with the problem of space radiation.  In my Alternate View column "Can We Cure Aging?", published in the May-June 2018 issue of Analog, I described a radical new DNA-based technique developed by the Seattle-based startup Oisin Biotechnologies for reducing the effects of human aging and for treating cancer.  Oisin has sequenced a plasmid DNA-ring that, when deposited inside a cell wall by a bubble-like liposome, detects whether the cell has become senescent and is expressing the protein p16, and if so triggers a suicide gene that causes the senescent cell to neatly disassemble itself and go away.  Oisin has also developed an alternate plasmid that detects the expression of protein p53, which is a signal that a cell has become malignant and cancerous.

Ionizing space radiation is normally not so intense as to produce massive cell death in exposed humans in space.  Rather, cell damage accumulates over a period of time, with the rate of damage accumulation much larger than in environments on Earth.  That accumulated damage mainly takes the form of senescent and pre-cancerous cells.  With the added body-burden of senescent cells, the space-traveling humans will have reduced fitness, premature aging, and a much greater cancer risk.

The good news is that by applying the Oisin treatment, damage from space radiation at moderate exposure levels producing senescent and pre-malignant cells can fixed.  The damaged cells will be swept away, to be replaced by healthy ones, potentially restoring space traveling humans to optimum fitness and providing the ability to better withstand the effects of space radiation.

I should also mention a slight downside to the Oisin treatment.  It has been discovered that senescent cells play an valuable role in the healing of wounds, by sending out chemical signals that promote the wound closure and healing.   Therefore, an astronaut in a hypothetical situation in which he is both wounded and exposed to radiation should be treated for the wounds first and for the radiation exposure only after the wounds have healed.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

Space Radiation: The NASA Space Radiation ebook: https://www.nasa.gov/sites/default/files/atoms/files/nasa_space_radiation_ebook_0.pdf

Senescent Cell Clearance in Transgenetic Mice:
"Clearance of p16^Ink4a-positive senescent cells delays ageing-associated disorders", Darren J. Baker, et al, Nature 479, 232-236 (10 November 2011); see also
"Ageing: Old cells under attack", Daniel S. Peeper, Nature 479, 186-187 (10 November 2011)

Oisin Biotechnologies web site:
            http://oisinbio.com

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