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The ABC News article President Trump prompts state of confusion over space policy with tweet covers POTUS' tweet and NASA director Bridenstine's clarification tweet a few hours later, as well as cites this evidence of the "intertwination" of the Moon (or at least the Gateway) and crewed missions to Mars by quoting NASA's page Moon to Mars Overview:

Exploration of the Moon and Mars is intertwined. The Moon provides an opportunity to test new tools, instruments and equipment that could be used on Mars, including human habitats, life support systems, and technologies and practices that could help us build self-sustaining outposts away from Earth. Living on the Gateway for months at a time will also allow researchers to understand how the human body responds in a true deep space environment before committing to the years-long journey to Mars. (emphasis added)

I assume that "how the human body responds" excludes the overtly psychological issues of spending months in a tin can, and focuses on more biological and medical issues, though these can never viewed as completely separate.

Question: What are the aspects of "a true deep space environment" available on the Gateway that have not been thoroughly explored by decades of habitation by hundreds of astronauts in all of the crewed stays on nine space stations already?

Is this simply an excuse to radiate human subjects without exactly saying so?

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    $\begingroup$ +1 for "excuse to irradiate" $\endgroup$ – Bit Chaser Jun 10 '19 at 0:32
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Your last sentence hits the major point. Other than Apollo, no humans have been outside of the Van Allen radiation belts. All of the astronauts on the nine space stations have been largely spared the radiation that they would be subject to for the months-long trip to Mars and back. The Apollo astronauts did get such an exposure, but their missions lasted days, not months. We simply don't have good evidence of what such a long exposure has on the human body, and what will be effective (while still being cost-conscious) to prevent it.

However, I don't feel that doing such a long-term study in lunar orbit is such a good idea. It would be cheaper, safer, and more realistic to a Mars Mission to put a spacecraft in a highly-elliptical Earth orbit, with an orbital period of several days. That way, if something develops that requires the mission to be aborted, the orbit naturally gets them in the proximity of Earth in just a few days. (Sort of like Apollo's "free return" trajectory.)

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  • $\begingroup$ The Gateway will indeed be in a high Earth orbit and not a lunar orbit. See What is a near rectilinear halo orbit? and also Why is a near rectilinear halo orbit proposed for LOP-G (formerly known as Deep Space Gateway?) Think of it as a high Earth orbit in 1:1 resonance with the Moon. It just doesn't ever get close to the Earth. $\endgroup$ – uhoh Jun 10 '19 at 0:01
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    $\begingroup$ @uhoh - It's just as erroneous to think of the near rectilinear halo orbit as an Earth orbit as it is to think of it as a lunar orbit. That orbit would not exist if either the Earth or the Moon didn't exist. $\endgroup$ – David Hammen Jun 10 '19 at 1:27
  • $\begingroup$ @DavidHammen here's the way I view that. If the Moon disappeared, the Gateway would continue in orbit around the Earth. It would still energetically bound to Earth (I'm pretty sure). However if the Earth disappeared the Gateway would drift away from the Moon and have nothing further to do with it. So I disagree that it's "just as erroneous". How about 1/81 as erroneous? Wouldn't you say that SOHO and DSCOVR are in heliocentric orbits that are in 1:1 resonance with Earth for example? $\endgroup$ – uhoh Jun 10 '19 at 1:33
  • $\begingroup$ @DavidHammen the orbits of Io, Europa, and Ganymede around Jupiter don't depend on each other to exist, nor do the orbits of Hyperion and Titan around Saturn. In these lopsided scenarios I think it's okay to call them real orbits around the primary in resonance with the much lower mass secondary, and I think the Gateway's halo orbit fits that definition. But I'm not sure, so I've just asked Is the Gateway's halo orbit bound to Earth? Would the Gateway remain in Earth orbit of the Moon disappeared? $\endgroup$ – uhoh Jun 10 '19 at 2:01
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    $\begingroup$ But "highly elliptycal" orbit will be crossing Van Allen belts. I doubt it's an equivalent of beyond-Earth-orbit space. The radiation in VAB is higher than in far space (excluding GCR radiation, that would be nearly the same I think). $\endgroup$ – Heopps Jun 10 '19 at 9:07

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