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The duration of crewed interstellar voyages is potentially limited by exposure to interstellar radiation.

radiation dose obtained in a non-relativistic space module moving in interstellar space would be, approximately, 70 rems/year https://arxiv.org/pdf/physics/0610030

The dose of radiation expected to cause death to 50 percent of an exposed population within 30 days ... is in the range from 400 to 450 rem https://www.nrc.gov/reading-rm/basic-ref/glossary/lethal-dose-ld.html

Although estimates of lethal chronic doses of radiation are based on scant evidence, the duration of interstellar voyages would appear to be limited to a decade or two unless radiation exposure can be reduced or radiation tolerance increased.

Increasing the spacecraft’s shielding mass reduces acceleration, prolonging a voyage of a given distance and therefore prolonging radiation exposure duration. This produces a trade-off where increasing shielding does not extend the maximum distance of a radiation-limited voyage.

The strategy of shortening the voyage duration by increasing the spacecraft to relativistic velocity suffers from a similar trade-off.

The most dangerous is interstellar gas, which acts as a flow nucleonic radiation bombarding a relativistic starship. Radiation flux is extremely high even for moderate relativistic velocities … The presence of a neutral component in interstellar gas excludes the implementation of magnetic shielding alone.

https://arxiv.org/pdf/physics/0610030

The strategy of increasing crew radiation tolerance by genetic engineering does not suffer these trade-offs.

Genetic engineering of humans is obviously a complex undertaking, on the same level of difficulty as developing interstellar propulsion. If the effective range of interstellar voyages is limited by both radiation sensitivity and delta-v, it makes sense to pursue solutions to both problems.

Humans are obviously not yet ready for interstellar voyages, but enhanced radiation resistance would be an advantage even within the solar system.

Tardigrades are small aquatic animals with remarkable resistance to radiation.

https://www.newscientist.com/article/2106468-worlds-hardiest-animal-has-evolved-radiation-shield-for-its-dna

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Tardigrades apparently owe a large part of their radiation tolerance to “Dsup”, a protein which protects DNA from a variety of damaging agents. The gene coding for Dusp was genetically engineered into cultured human kidney cells. This markedly reduced radiation-induced DNA damage.

Other studies https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6770827 suggest DNA repair mechanisms present in Tardigrades enhance the radiation resistance provided by Dusp.

Recent studies show that stress-related tardigrade genes may be transfected to human cells and provide increased tolerance to osmotic stress and ionizing radiation. With the recent sequencing of the tardigrade genome, more studies applying tardigrade omics to relevant aspects of human medicine are expected.

These findings hint at the possibility of genetically engineered space-faring humans with significantly improved radiation tolerance.

The most common malignancy induced by radiation is leukemia, particularly AML. If myeliod stem cells could be genetically engineered to be more radiation resistant (perhaps by the production of Dusp) there is the potential for reduced incidence of leukemia or, perhaps, more robust recovery from the treatment of leukemia.

The proposed genetic engineering could be performed in vitro, with the enhanced cells returned to the donor. Germ cells (reproductive cells) would not be modified in this process.

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    $\begingroup$ Well, you first! The problem is what else, in a very complex system, inserting and turning on that gene does. Is it conceivable that gene (or something else) might help? Sure. It is also conceivable that there will be subtle or dramatic impacts to other things. $\endgroup$
    – Jon Custer
    Commented Jul 1 at 13:13
  • $\begingroup$ I think we will need to build a generation ship, and that ship will be so huge that its hull will probably be able to provide adequate radiation shielding. But if we were to master genetic engineering, we would likely be much more successful at seeding other worlds with life from Earth - including humans. $\endgroup$
    – phil1008
    Commented Jul 1 at 19:21
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    $\begingroup$ Given that interstellar travel is impossible at the present time, nobody can say what limitations might apply if it ever becomes possible. $\endgroup$
    – John Doty
    Commented Jul 1 at 23:59
  • $\begingroup$ @JohnDoty ... people certainly thought about travelling to the Moon before it became possible. In fact, it would have never happened if it had not been thought about. $\endgroup$
    – Woody
    Commented Jul 2 at 0:10
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    $\begingroup$ @Woody The details of lunar travel needed working out in a context where they were practical. Early speculations didn't match the actual Moon landings very well. $\endgroup$
    – John Doty
    Commented Jul 2 at 1:00

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There is a seemingly valid assumtion here that we have three options

  1. Slow. Many years of radiation. Death.
  2. Fast. Low mass and thus low shields. Death.
  3. Reletavistic. Generating own problems. Death.

I would like to challenge option 1. The Earth is a heavily shielded (and slow) space object. We still receive daily radiation, but at a low enough level the body can deal wit that.

So heavily shielded, and slow will get us a long range. If we bring enough genetic variance (read: million-ish people) then we can go as far as the spaceship lasts.

Which is not infinite unless you stop to harvest fresh resources along the way. And dreadfully slow. And you need a huge vessel with plenty redundancy.

So I guess there is an answer; Long to infinity. But to damn slow.

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  • $\begingroup$ Nicely said, Hennes. But it doesn't address the possibility of increasing human radiation tolerance. SE tends to be hardware oriented. The solution to this interstellar conundrum may be wetware engineering. Your comment about a "million-ish people" implies that natural selection could provide a solution. Genetic engineering may get the same result, but sooner. $\endgroup$
    – Woody
    Commented Jul 9 at 15:05
  • $\begingroup$ The Starlost (what happens when you don't have sufficiently "plenty redundancy") $\endgroup$
    – uhoh
    Commented Jul 10 at 4:14
  • $\begingroup$ Re starlost. It has been decades since I read that. Re natural selection I was not trying to answer that, I was heading to a sufficient big ark ship which I realise is has several disadvantages. In this content speed. So if we do not modify humans the range stilll is long, but at a horrible cost in resources (cost to build the ship) and in time (due to slow). $\endgroup$
    – Hennes
    Commented Jul 10 at 10:13
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Self Shielding

I doubt radiation will be a limiting factor on an interstellar journey, because any conceivable interstellar ship will be massive.

Just as an order of magnitude look, let's think about food for 10 people on a 40 year trip.

Using 2 MREs at 600 grams each per person per day, we're looking at:

2 * 10 * 365 * 40 * 0.6 = 175,200kg or over 175 tonnes of food

If the living area of the ship is a 10x10x10 meter cube, you'd have about 300kg per square meter of shielding, just from stored food.

Now Expand That

So then we need to look into power sources, atmosphere control, water, waste management, recycling, spare parts, manufacturing capabilities, and dozens of other issues that will need to be solved.

Every single one of those is going to add to the mass budget.

And all of that is going to be shielding for the crew. Because equipment is very radiation tolerant, and people just aren't.

Going Fast

Increasing the speed actually makes this problem easier, because it means that the incoming radiation is going to be directional - it will primarily come from the front of the ship. Now you can design around putting storage up front and people in the back.

In the end, interstellar ships are going to be so huge, you can just use the rest of the ship as the shielding for the crew.

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  • $\begingroup$ I think the point that you make is valid, but some deep space radiation such as Fe56+ are very penetrating. The radiation levels inside the shield actually increases beyond a certain threshold thickness before eventually falling again dues to secondary and tertiary radiation showers from the impact of a single primary particle. $\endgroup$
    – Slarty
    Commented Jul 11 at 18:07
  • $\begingroup$ @Slarty - I hear you, but I don't think it's feasible to build a manned interstellar probe that is smaller than 100,000s of tonnes. With an aircraft carrier's worth of mass, you can do some pretty significant shielding. $\endgroup$
    – codeMonkey
    Commented Jul 11 at 20:12
  • $\begingroup$ Yes this is true, Although the whole proposition starts to unravel weight wise very quickly. An aircraft carriers worth of tonnage for the ship (which presumably would need to be spun to provide artificial gravity) is just the start. The tyranny of the rocket equation would mean that the propellant required to slow the ship down on arrival would outweigh the ship by a very large margin and an even vaster ;o) amount would be needed to accelerate the whole ship and all its deceleration propellant in the first place. Bottom line your argument is correct, but the whole idea is not practical. $\endgroup$
    – Slarty
    Commented Jul 11 at 22:22
  • $\begingroup$ Wouldn’t eating that food bombarded with radiation for years be an issue? But I would be surprised if people set off on such a long mission with all the food from the start, wouldn’t there rather be some way to create/grow the food on the way? $\endgroup$
    – jcaron
    Commented Jul 14 at 21:02
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    $\begingroup$ @jcaron - eating food bombarded with neutron radiation would be bad, but in this case, it would be gamma radiation, which should be fine. $\endgroup$
    – codeMonkey
    Commented Jul 15 at 13:25
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Radiation is just one of many almost impossibly difficult interdependent problems. The primary issue is one of the vast distance that must be covered. To overcome the distance requires traveling at high speed for a long time. Human occupants will take up a lot of mass and a lot of mass requires a vast amount of propellant. Trying to build massive radiation shielding for such a vessel only exacerbates the mass problem.

In a nutshell human interstellar voyages are not a practical proposition now and may never be. Radiation is just one more giant issue to deal with.

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