August 31, 2011
Destination: Moon or Asteroid? Part I: Operational Considerations
Lockheed-Martin's Plymouth Rock mission concept
Part I: Operational Considerations
The current controversy over the direction of our national space program has many dimensions but most of the discourse has focused on the means (government vs. commercial launch vehicles) not the ends (destinations and activities). Near-Earth objects (NEO, i.e., asteroids) became the next destination for human exploration as an alternative to the Moon when the Augustine committee advocated a “flexible path” in their 2009 report. The reason for going to an asteroid instead of the Moon was that it costs too much money to develop a lunar lander whereas asteroids, having extremely low surface gravity, don’t require one. The administration embraced and supported this change in direction and since then, the agency has been studying possible NEO missions and how to conduct them.
On the surface, it might seem that NEO missions answer the requirements for future human destinations. NEOs are beyond low Earth orbit, they require long transit times and so simulate the duration of future Mars missions, and (wait for it)… we’ve never visited one with people. However, detailed consideration indicates that NEOs are not the best choice as our next destination in space. In this post and two additional ones to come, I will consider some of the operational, scientific and resource utilization issues that arise in planning NEO missions and exploration activities and compare them to the lunar alternative.
Most asteroids reside not near the Earth but in a zone between the orbits of Mars and Jupiter, the asteroid belt. The very strong gravity field of Jupiter will sometimes perturb the orbits of these rocky bodies and hurl them into the inner Solar System, where they usually hit the Sun or one of the inner planets. Between those two events, they orbit the Sun, sometimes coming close to the Earth. Such asteroids are called near-Earth objects and can be any of a variety of different types of asteroids. Typically, they are small, on the order of tens of meters to a few kilometers in size. As such, they do not have significant gravity fields of their own, so missions to them do not “land” on an alien world, but rather rendezvous and station-keep with it in deep space. Think “formation flying” with the International Space Station (ISS) without the option to dock.
The moniker “near Earth” is a relative descriptor. These objects orbit the Sun just as the Earth does and vary in distance to the Earth from a few million km to hundreds of millions of km, depending upon the time of year. Getting to one has nothing to do with getting to another, so multiple NEO destinations in one trip are unlikely. Because the distance to a NEO varies widely, we cannot just go to one whenever we choose – launch windows open at certain times of the year and because the NEO is in its own orbit, these windows occur infrequently and are of very short duration, usually a few days. Moreover, due to the distances between Earth and the NEO, radio communications will not be instantaneous, with varying time-lags of tens of seconds to several minutes between transmission and reception. Thus, the crew must be autonomous during operations.
Although there are several thousand NEOs, few of them are possible destinations for human missions. This is a consequence of two factors. First, space is very big and even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects. Second, many of these objects are unreachable, requiring too much velocity change (“delta-v”) from an Earth departure stage; this can be a result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (all orbits are elliptical). These factors result in reducing the field of possible destinations from thousands to a dozen or so at best. Moreover, the few NEOs that can be reached are all very small, from a few meters to perhaps a km or two in size. Not much exploratory area there, especially after a months-long trip in deep space.
That’s another consideration – transit time. Not only are there few targets, it takes months to reach one of them. Long transit time is sold as a benefit by asteroid advocates: because a trip to Mars will take months, a NEO mission will allow us to test out the systems for Mars missions. But such systems do not yet exist. On a human mission to a NEO, the crew is beyond help from Earth, except for radioed instructions and sympathy. A human NEO mission will have to be self-sufficient to a degree that does not now exist. Parts on the ISS fail all the time, but because it is only 400 km above the Earth, it is relatively straightforward to send replacement parts up on the next supply mission (unless your supply fleet is grounded, as currently it has been). On a NEO mission, a broken system must be both fixable and fixed by the crew. Even seemingly annoying malfunctions can become critical. As ISS astronaut Don Pettit puts it, “If your toilet breaks, you’re dead.”
Crew exposure is another consequence of long flight times, in this case to the radiation environment of interplanetary space. This hazard comes in two flavors – solar flares and galactic cosmic rays. Solar flares are massive eruptions of high-energy particles from the Sun, occurring at irregular intervals. We must carry some type of high-mass shielding to protect the crew from this deadly radiation. Because we cannot predict when a flare might occur, this massive solar “storm shelter” must be carried wherever we go in the Solar System (because Apollo missions were only a few days long, the crew simply accepted the risk of possible death from a solar flare). Cosmic rays are much less intense, but constant. The normal ones are relatively harmless, but high-energy versions (heavy nuclei from ancient supernovae) can cause serious tissue damage. Although crew can be partly shielded from this hazard, they are never totally protected from it. Astronauts in low Earth orbit are largely protected from radiation because they orbit beneath the van Allen radiation belts, which protect life on the Earth. On the Moon, we can use regolith to shield crew but for now, such mass is not available to astronauts traveling in deep space.
When the crew finally arrives at their destination, more difficulties await. Most NEOs spin very rapidly, with rotation periods on the order of a few hours at most. This means that the object is approachable only near its polar area. But because these rocks are irregularly shaped, rotation is not the smooth, regular spin of a planet, but more like that of a wobbling toy top. If material is disturbed on the surface, the rapid spin of the asteroid will launch the debris into space, creating a possible collision hazard to the human vehicle and crew. The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object and just as occurs in Earth orbit, each touch of the object (action) will result in a propulsive maneuver away from the surface (reaction).
We need to learn how to work quickly at the asteroid because we don’t have much time there. Loiter times near the asteroid for most opportunities are on the order of a few days. Why so short? Because the crew wants to be able to come home. Both NEO and Earth continue to orbit the Sun and we need to make sure that the Earth is in the right place when we arrive back at its orbit. So in effect, we will spend months traveling there, in a vehicle with the habitable volume of a large walk-in closet (OK, two walk-in closets maybe), a short time at the destination and then months for the trip home. Is it worth it? That will be the subject of my next post.
Destination: Moon or Asteroid?
Part II: Science Considerations
Part III: Resource Utilization Considerations
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Can you really pick a destination if the question of “why go?” has not been even properly discussed yet ?
You will get a bunch of different people coming into the discussion with a ton of different assumptions about that, without any hope of understanding .. which keeps being the case in space advocacy circles.
Comment by kert — August 31, 2011 @ 10:46 am
Can you really pick a destination if the question of “why go?” has not been even properly discussed yet ?
You’ve got to be kidding. That topic has been discussed ad nauseum for as long as I’ve been in the space business. I have repeatedly emphasized the “why” as I see it on this blog since I started writing it.
Comment by Paul D. Spudis — August 31, 2011 @ 11:00 am
Paul – This is the series that needs to be written. I look forward to your subsequent parts.
Partly in defense of your first commenter, I do see the “why” question as being lost in the conversation with policy makers and key stakeholders. Framing the why around one or more of several, clear rationales – demonstrating American exceptionalism, paving the way for commercial exploitation, for the science, for eventual settlement (to name a few) – is something that unfortunately needs revisiting over and over again. I see the rationale for why as being lost against fiscal pressures and the focus on the means that dominate the conversation today.
Comment by Joe Williams — August 31, 2011 @ 11:49 am
I do see the “why” question as being lost in the conversation with policy makers and key stakeholders. Framing the why around one or more of several, clear rationales – demonstrating American exceptionalism, paving the way for commercial exploitation, for the science, for eventual settlement (to name a few) – is something that unfortunately needs revisiting over and over again.
Well, we’ve certainly done that here on this blog.
In a nutshell, I believe that going to the Moon with people is part of our strategy to become a space faring species. In short, we need to go beyond LEO to maintain, secure, and extend our presence in space to support our technical civilization.
I outline the basic motivation here:
http://blogs.airspacemag.com/moon/2011/04/a-rationale-for-cislunar-space/
And here:
http://www.spudislunarresources.com/Rationale.htm
Comment by Paul D. Spudis — August 31, 2011 @ 12:28 pm
Polls continue to show that there is very little public support for a manned asteroid mission:
http://cosmiclog.msnbc.msn.com/_news/2011/08/30/7528922-space-agencies-set-two-courses
http://www.dailykos.com/story/2011/07/12/994065/-Developing-New-Crew-Launch-Vehicles-for-NASA?via=user
Comment by Marcel F. Williams — August 31, 2011 @ 12:32 pm
Marcel,
Beyond how well or poorly it polls, we must consider what we’ll get for the effort. I’ll start to address that in my next post.
Comment by Paul D. Spudis — August 31, 2011 @ 12:58 pm
“The reason for going to an asteroid instead of the Moon was that it costs too much money to develop a lunar lander whereas asteroids, having extremely low surface gravity, don’t require one.”
I hope in one of your future posts you address this point more directly than you did here. I don’t think you’ll have too much trouble convincing people that the Moon would be a more interesting and useful target than an asteroid if it cost the same. But if it costs more to go to the Moon is it enough better to justify the additional cost? Or why don’t we go to an asteroid first and then to the Moon when budgets allow it? You really don’t address those questions.
Comment by Bob Steinke — August 31, 2011 @ 3:07 pm
I hope in one of your future posts you address this point more directly than you did here.
I’ve addressed that previously, here:
http://blogs.airspacemag.com/moon/2010/12/can-we-afford-to-return-to-the-moon/
and in more detail, here:
http://www.spudislunarresources.com/Papers/Affordable_Lunar_Base.pdf
In brief, the Augustine committee is wrong about this. Moreover, they were provided with detailed evidence that they were wrong and chose to ignore it.
Comment by Paul D. Spudis — August 31, 2011 @ 3:27 pm
I look forward to this series. But the “why go” question has not, I believe, been answered in this blog for NEOs. I think that’s how that question came up. The “why” for a NEO may well be a harder sell than for the Moon, and I strongly suspect that the “whys” for one aren’t transferable to the other. The importance of having to “work quickly” at a NEO has to be based on exactly what work needs to be done there.
Comment by Henry Muslin — August 31, 2011 @ 4:03 pm
Although there are several thousand NEOs, few of them are possible destinations for human missions. This is a consequence of two factors. First, space is very big and even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects. Second, many of these objects are unreachable, requiring too much velocity change (“delta-v”) from an Earth departure stage; this can be a result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (all orbits are elliptical). These factors result in reducing the field of possible destinations from thousands to a dozen or so at best. Moreover, the few NEOs that can be reached are all very small, from a few meters to perhaps a km or two in size. Not much exploratory area there, especially after a months-long trip in deep space.
Certainly the delta v requirements to get to about a thousand NEO’s aren’t more demanding that lunar landings, so I must assume you consider the voyage times too long, what voyage time reduces the number of possible candidates to “a dozen or so at best”?
Comment by Andrew W — August 31, 2011 @ 4:08 pm
But the “why go” question has not, I believe, been answered in this blog for NEOs
And it won’t be here. I hope to show why human NEO missions are undesirable in comparison to human lunar return; my purpose is not to “sell” a human asteroid mission.
Comment by Paul D. Spudis — August 31, 2011 @ 4:13 pm
what voyage time reduces the number of possible candidates to “a dozen or so at best”?
This restriction comes from the NASA HEFT study, which assumed simple extensions of existing space systems and minimized mission times of 6 months or so (to mitigate de-conditioning and radiation exposure).
Comment by Paul D. Spudis — August 31, 2011 @ 4:15 pm
“I hope to show why human NEO missions are undesirable in comparison to human lunar return; my purpose is not to “sell” a human asteroid mission.”
No surprise there, and I think you’re right. For resource utilization, the Moon is a slam dunk. A visit to a NEO will be the only visit to that NEO for a long, long time. So all you do is see what’s there and wave goodbye. It’s not a place you go back to routinely to set up resource development, and it hardly seems clear that a NEO even offers the resources that the Moon does. The Moon provides something that a NEO does not.
For science, one could argue both ways. There is excellent science to do on NEOs, as there is for the Moon. But the Moon is amenable to telerobotic exploration from the Earth far more easily than a NEO is. If it were about science, and human space flight, a NEO might win. Not because the science is better, but because humans are more enabling for a NEO.
But it comes down to the purpose for human space flight. If that purpose is exploration, and exploration is … wait for it … defined as going places you haven’t yet gone, it’s hard to argue with a NEO. Many people do define exploration that way. Defined that way, exploration is very different than science or resource development. By that token, you should have three additional parts to this essay. Not two. Exploration, science, and resource development. The Moon might win the latter two, but NEOs are going to give you a run for your money on the first.
Comment by Henry Muslin — August 31, 2011 @ 5:22 pm
it comes down to the purpose for human space flight. If that purpose is exploration, and exploration is … wait for it … defined as going places you haven’t yet gone
I don’t define it that way:
http://blogs.airspacemag.com/moon/2010/01/have-we-forgotten-what-exploration-means/
Comment by Paul D. Spudis — August 31, 2011 @ 5:55 pm
“I don’t define it that way:
http://blogs.airspacemag.com/moon/2010/01/have-we-forgotten-what-exploration-means/”
What it seems that you think exploration means, if I interpret your piece correctly, is as travel that generates wealth.
I don’t think Edmund Hillary generated wealth.
For that matter, modern historians don’t really think that Lewis and Clark did either. Their work was ignored for almost half a century after they returned.
Amundsen may have been a failed explorer, in that he didn’t find gold at the South Pole. Nor Scott. Scott did spend a lot of wealth.
Alan Shepard? Neil Armstrong?
Let’s just say that not only have we possibly forgotten what exploration means, but that we quite possibly still haven’t reconciled what it means with regard to human space flight. Your writing about the meaning of exploration is, however, welcome discourse.
But I do look forward to your essays on human voyages to NEOs and the Moon.
Comment by Henry Muslin — August 31, 2011 @ 8:15 pm
“But the “why go” question has not, I believe, been answered in this blog for NEOs
And it won’t be here. I hope to show why human NEO missions are undesirable in comparison to human lunar return; my purpose is not to “sell” a human asteroid mission.”
I would say I know of no space rock that is worth going to at this time. Since we detected and tracked around 90% of space rocks 1 km in diameter or larger which are near earth. It’s unlikely a rock will be 1 km or larger in diameter which I might consider worth going to.
I think very small rocks could be interesting/valuable [though not in terms of manned trips to them].
In any case BEFORE I could be interested in a rock, what is needed is a rock with a trajectory. So I see going to a asteroid as merely general idea until a specific rock is being discussed.
Happily, we are in the process of identifing all these NEOs due to them being a potential threat, and perhaps in a decade or so one might be found which is suitable.
Comment by gbaikie — August 31, 2011 @ 9:07 pm
@ Paul
“Beyond how well or poorly it polls, we must consider what we’ll get for the effort. I’ll start to address that in my next post.”
I agree with you that space policy should not necessarily be driven by the polls, but advocates of the Obama space plan, including Mr. Augustine, frequently suggest that there is no interest in going to the Moon while offering a manned mission to an asteroid as a more interesting and cheaper alternative. But they are clearly wrong on both aspects of this issue.
If delta-v was all we had to worry about concerning interplanetary travel then traveling to Mars really wouldn’t be too much of a concern since the delta-v requirements from LEO to high Mars orbit to the surface of the Martian Moon Deimos is only about 6100 m/s compared to about 5900 m/s dv to reach the lunar surface.
IMO, protecting the human body and especially the human brain from the deleterious effects of several months or even years of exposure heavy nuclei is the most critical problem as far as interplanetary travel is concerned. And the same would be true for a visit to a NEO asteroid. This is going to require several meters of hydrogen, water, or polyethylene, requiring several hundred tonnes of mass to protect human passengers. So the 40 tonne space craft configuration shown above by Lockheed would probably require an additional 400 to 800 tonnes of mass shielding in order to safely accomplish the asteroid mission.
No such mass shielding would be required for a few days travel to the Moon.
Robotically transporting tiny asteroids (100 to 1000 tonnes in mass) to the Earth-Moon Lagrange points for study or utilization would make much more sense to me.
But any manned mission to an asteroid would be a titanically expensive stunt while establishing a permanent base on the Moon would not only teach us how to establish similar bases on the surface of Mars but would also be an investment in a new human populated industrial area that will eventually use lunar resources to dominate the multi-billion dollar a year satellite manufacturing, launching, and repair industry by the middle of this century.
Comment by Marcel F. Williams — August 31, 2011 @ 10:55 pm
The author stated – “The reason for going to an asteroid instead of the Moon was that it costs too much money to develop a lunar lander”
Using the link you provided to the Augustine report, on page 84 they outline what happens with the Constellation budget despite ending the Shuttle and the ISS as originally planned:
“The heavy-lift Ares V is not available until the late 2020s, allowing only orbital flights to the Moon. In addition, there are insufficient funds to develop the lunar lander and lunar surface systems until well into the 2030s, if ever. In short, this program operates within current FY 2010 budget constraints, but offers little or no apparent value.”
So the reason for ending Constellation had nothing to do with another destination, it was because the program could not reach the Moon without more money. Hence the reason why Congress agreed to cancel it, which in light of today’s decreasing budgets was definitely the right thing to do – we would have consumed NASA’s budget for decades without ever being able to go anywhere.
Comment by Coastal Ron — September 1, 2011 @ 1:08 am
The author said regarding an NEO trip:
“- When the crew finally arrives at their destination, more difficulties await.
-The lack of gravity means that “walking” on the surface of the asteroid is not possible
- We need to learn how to work quickly at the asteroid because we don’t have much time there.”
Aren’t these the same types of challenges that awaited us on the Moon? Apollo 11 only had 2h 37m of EVA time and as a whole were only on the lunar surface for less than a day. We will do at NEO’s what we did on the Moon, which is learn from each encounter. Besides, aren’t challenges what engineers and scientists live for?
The debate about which to do next is in some ways the short-term view of things, because if we are going to be a spacefaring nation or species, then we need to get competent at going everywhere.
But as of today, Moon return next, then NEO, or NEO and then return to the Moon – we can’t afford any of it right now as long as Congress wants a $38B rocket built first, so the whole debate is kind of a moot point.
Comment by Coastal Ron — September 1, 2011 @ 1:27 am
Isn’t it funny how the Mars zealots would sell out to the fear-mongering avoidance of ever having to construct a manned lander craft?! Have they ever stopped to consider just how many MANY orders of magnitude of complexity it would take, to construct & actually fly a manned Mars lander?! [Even if the proposed Mars lander was only to get the crew down one-way and one-time.] FLEXIBLE PATH IS PURE IDIOCY!!! You would never gain the experience of flying a manned lander down to a DUSTY planetary surface, ever, in modern times, under this “look but don’t touch” credo! The development of brand new 21st century crewed landing vehicles should,—to the contrary—be the name of the game, in NASA’s renewed quest to get astronauts to other worlds. A return to the Moon makes vastly more sense, if the overall intent is the attainment of true human space faring. One-time sensationalistic stunts just to be the first ones there, at a given destination, by contrast, makes totally NO SENSE. NOT ONE IOTA OF IT! People, please: let’s have the Moon revisited, ahead & way before we ever send people to hand-pick meteors upon a gigantic charcoal lug!
Comment by Chris Castro — September 1, 2011 @ 5:36 am
Dr. Spudis wrote:
“The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object and just as occurs in Earth orbit, each touch of the object (action) will result in a propulsive maneuver away from the surface (reaction).”
I worked on Moon and asteroid surface operations architectures for the original Space Exploration Initiative (SEI). This is indeed a problem. We looked at a number of scenarios including using an upgraded version of the Manned Maneuvering Unit (MMU) for travel/ positioning but the cost in terms of Nitrogen (the cold gas fuel) were high and navigation under the circumstances described was difficult. One guy even proposed a sort of ‘roll cage’ for the process, so the crew could sort of bounce off the asteroid surface when the navigation got too hard. Somewhere out there is some great Pat Rawlings art work of crew (in AX-5 configuration pressure suits) in the ‘roll cage’ exploring one the Martian moons.
The current thinking seems to be to use pitons (as used in Mountain climbing) to secure the crew to the asteroid surface (even using netting – secured with the pitons – so they can move around more freely). Similar concepts were reviewed in SEI.
A question for Dr. Spudis. I am not a geologist but the pictures I have seen of various asteroids make them look like little ‘mini-moons’. That is a good part of the surface appears to be covered with regolith. Would the regolith be substantial enough to withstand the required securing loads? If not how deep would the pitons have to be driven to be effective?
“We need to learn how to work quickly at the asteroid because we don’t have much time there. Loiter times near the asteroid for most opportunities are on the order of a few days.”
There are (in my opinion) only two ways to make Asteroid exploration/exploitation practical: (1) make the expedition large/elaborate enough to make the expedition very well equipped and able to stay with the Asteroid a much longer time, (2) Develop the capability to alter the Asteroid orbit to make it more accessible. Either of the options requires space faring capabilities well beyond those currently available. This brings us back to the moon. My first exposure to taking HSF seriously was the book “Islands in Space” by Cole/Cox, so I am anything but anti-asteroid. I do believe however that the path to developing those ‘space faring capabilities’ (that will eventually allow Asteroid exploration/exploitation) lies through the Moon with the use of Lunar resources.
Comment by Joe — September 1, 2011 @ 9:45 am
Joe,
the pictures I have seen of various asteroids make them look like little ‘mini-moons’. That is a good part of the surface appears to be covered with regolith. Would the regolith be substantial enough to withstand the required securing loads? If not how deep would the pitons have to be driven to be effective?
We don’t really know the answers to these questions with any degree of certainty. An asteroid like Itokawa looks to be a loosely bound agglomeration of fragmental debris. It may be that you could try to sink pitons into it for depths of several meters without hitting solid material. Other asteroids appear to be much more coherent. I would think that some type of hovering platform would be the best solution for close approach and interaction with an asteroid, but even that technique has issues with blast effects from thrusters.
Comment by Paul D. Spudis — September 1, 2011 @ 10:08 am
Comment by Joe – September 1, 2011 @ 9:45 am
“(1) make the expedition large/elaborate enough to make the expedition very well equipped and able to stay with the Asteroid a much longer time… Either of the options requires space faring capabilities well beyond those currently available. ”
I don’t think it makes sense to go to an asteroid in an MPCV-type vehicle (ala LM’s Plymouth Rock proposal), since that vehicle doesn’t get us any closer to how we should be traveling in deep space. Instead we should be focusing on using true spaceships like NASA’s proposed Nautilus-X, which are meant for testing out technology and techniques for long journeys into space.
NASA’s estimate for developing Nautilus-X is $7B (apply your usual b.s. factor here), and doesn’t require the co-development of an HLV, which seems to be the downfall of all beyond-LEO missions these days. And to Joe’s point, we should not be sending just one ship out, but maybe a couple – a main ship and emergency backup (like an inflatable on a tanker for fast return). With our current budget situation, this would not happen by 2025.
But this really gets back to the “Why?”. For the Moon in the 60′s, it was political. If the current goal is to reach Mars, then more focus should be on the technology and techniques we need to reach Mars. If the goal is to just spread out into space, then the Moon could be the next step. However the average taxpayer could care less, so until some sort of National Imperative becomes apparent we shouldn’t expect any consistent clarity of purpose from our political leaders (unless it’s pork spending).
Comment by Coastal Ron — September 1, 2011 @ 11:18 am
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Comment by Paul D. Spudis — September 1, 2011 @ 10:08 am
“We don’t really know the answers to these questions with any degree of certainty. An asteroid like Itokawa looks to be a loosely bound agglomeration of fragmental debris. It may be that you could try to sink pitons into it for depths of several meters without hitting solid material. Other asteroids appear to be much more coherent. I would think that some type of hovering platform would be the best solution for close approach and interaction with an asteroid, but even that technique has issues with blast effects from thrusters.”
The more detail you go into, the more complicated its gets. Nice. Seriously, that is to be expected. However it does seem to bring additional factors into any selection process for which asteroid(s) to be visited. Now in addition to mass and orbital mechanics you would need to consider Asteroid composition. Obviously the “much more coherent” Asteroids would be easier to deal with, but are they the most interesting in terms of science and/or resources?
I look forward to your next articles on the subject.
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@Coastal Ron
The Nautilus-X appears to be just another fantasy ship. Where’s the hundreds of tonnes of mass shielding for the Nautilus-X in order to avoid damage to the human brain from heavy nuclei?
Comment by Marcel F. Williams — September 4, 2011 @ 10:13 pm
Comment by Marcel F. Williams – September 4, 2011 @ 10:13 pm
“The Nautilus-X appears to be just another fantasy ship.”
The Nautilus-X has been proposed by NASA’s Johnson Space Center, so it looks pretty realistic.
“Where’s the hundreds of tonnes of mass shielding for the Nautilus-X in order to avoid damage to the human brain from heavy nuclei?”
You have stated the “hundreds of tonnes of mass shielding” figure before, but you’ve never stated where this requirement comes from.
Comment by Coastal Ron — September 8, 2011 @ 1:59 am
The “requirement” for hundreds of tons of mass shielding is purely conjectural. There have been about a dozen astronauts and cosmonauts who have spent multiple tours on ISS of about 6 months each, who have received about the same cosmic ray dose that an astronaut would receive on a Mars round trip. They don’t seem to have suffered any ill effects from such exposure. The TOTAL radiation dose from a 2 1/2 year Mars round trip would be about 50 REMs, which is not enough to cause radiation sickness even if received all at once.
Comment by Dick Morris — September 10, 2011 @ 6:00 pm
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