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Mars trips may involve less radiation exposure than previously thought:
There's no question that the first human mission to Mars will be extremely dangerous. Some studies have suggested that the radiation levels would exceed the maximum career dose for a given astronaut, greatly increasing the risk of cancer and other illnesses. It might not be quite so bad as it sounds, though. Newly presented ESA ExoMars orbiter data indicates that astronauts would receive "at least" 60 percent of their maximum recommended career radiation exposure on a round trip to Mars that takes six months both ways. That's still several times what ISS crew members receive, but it's relatively gentle compared to what some had feared.
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Mars Trips May Involve Less Radiation Exposure Than Previously Thought
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(Score: 5, Interesting) by takyon on Thursday September 20 2018, @07:23AM (20 children)
https://www.nasa.gov/sites/default/files/files/1_NAC_HEO_SMD_Committee_Mars_Radiation_Intro_2015April7_Final_TAGGED.pdf [nasa.gov]
NASA's maximum acceptable cancer risk is 3%:
How many astronauts would be fine with a 5-10% increased cancer risk? Probably all of them.
That could be cut to 3 months [reddit.com], or possibly 1.5 months [nextbigfuture.com]. That will go a long way towards reducing the radiation risk.
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(Score: 2) by Runaway1956 on Thursday September 20 2018, @09:36AM (12 children)
Thanks for the good link - https://www.nextbigfuture.com/2018/06/spacex-bfr-used-for-massive-space-development-orbital-lunar-and-mars-colonization.html [nextbigfuture.com]
As with most forms of travel on earth, time spent traveling is a function of fuel consumption. The faster you want to go, the more fuel you need. If you build an efficient engine, then give it a small fuel tank, you'll go a ways, then the engine dies. Give it more fuel, it will go further. If speed is your goal, then you stop worrying about efficiency. If you don't want to spend a lot of time at gas stations (or equivalent) then you give it big fuel tanks. And, if you're driving off into the wilderness to explore, you ensure that your fuel tanks are full, and that you're carrying all the "extra" fuel possible before you leave that final fuel station.
It has never been much of a secret that there are quicker ways to get around in space. The objections have always centered around cost and availability of fuel.
Here, on the ground, we have fleets of tank trucks running up and down the highways, delivering fuel to your local gas stations. It's about time someone thought of building such a fleet in space, to deliver fuel to a well positioned "gas station".
Now, they might want to go one step further. Use a pair of detachable engines to get their honging huge spaceship moving from the "gas station", and when they've expended most of their fuel, they detach, and come back to the station. Then, the spaceship to Mars has a good deal of excess fuel, effectively turning that ship into a tanker, carrying fuel for Mars and/or any stations over Mars.
(Score: 2) by c0lo on Thursday September 20 2018, @09:54AM (8 children)
Not such a big advantage as you may think.
You see, the big difference between car travel and space travel: when you shutdown your engine during a car travel, the car stops quickly; when you shutdown the engine in space travel, you don't have enough friction to stop, you'll just continue by inertia and under the influence of the gravity of whatever big masses are around.
To actually stop at a 'space fuel station' you'll have to match the both the fuel station's position and velocity. Which, unlike you stop to a car petrol station, will cost you fuel
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by Runaway1956 on Thursday September 20 2018, @10:25AM (2 children)
Still, an advantage in the long run. When, eventually, there is regular traffic, the interplanet traffic need never dive into our gravity well. Those ships making the trip to Mars will never need to come all the way down to LEO, thus saving tons of fuel by not having to leave LEO.
And, eventually, I expect that fuel won't have to be sent up from earth. The asteroids and the outer planets and moons have plenty of fuel sources. Sure, they are further away than earth, but it's all "downhill" from there to earth. Cheap fuel could become a reality almost immediately after the first space mining company kicks off their operations. (Not quite immediately - the first mining operations will be looking for more spectacular materials, like precious metals and engineering metals. Imagine the headlines if someone stumbles over a little cache of diamonds on an asteroid!!)
Even further down the road, our high orbit station will have sister stations, probably over Mars, maybe in the asteroid belt, and almost certainly near the moons of the larger planets.
(Score: 2) by c0lo on Thursday September 20 2018, @11:16AM (1 child)
Yes, for sure, it will be a significant saving in comparison with lifting that fuel from Earth surface.
Not such a big advantage for the rest... the brake into Sun's gravity's well will cost the same energy as the escape from Sun's gravity well.
True, between the asteroid belt and somewhere close to Earth you'll have Mars to do a gravity assist braking, but you could do only once for a reasonable transit time - will save you some energy, but not a significant percentage.
(Remember that solar probe [soylentnews.org] they sent "down" recently? No less that 7 Venus gravity assist braking needed to slow down [wikipedia.org] over 7 years duration)
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by bzipitidoo on Thursday September 20 2018, @03:01PM
The plan I heard years ago, from Zubrin, was to first send an unmanned fuel generator to the Martian surface, several months ahead of the manned space ship. The manned ship carries enough fuel for a one way trip.
As to refueling en route, it seems the refueling ship should be the one that does almost all the work of matching position and velocity.
Anyway, I still think colonizing Mars is one of those seductive ideas that a cold, rational cost benefit analysis will show is so not worth doing in the immediate future. It's like the flying car that we've been dreaming about since the 1940s. Sounds great, but it's still not practical. A big problem with flying cars is they're heavy and simply take too much energy. A helicopter can get about 7 miles per gallon. They also take a lot more space than a car. Then there's the problem of piloting them, and not getting yourself and your passengers killed in a crash. Takes a lot of training to handle a flying craft safely. Maybe next century on flying cars and Mars.
(Score: 2) by fyngyrz on Thursday September 20 2018, @04:13PM (4 children)
Try looking at it the other way around: do it like military tankers do for fighter planes. The tanker matches velocity with the fighter. Likewise, the space tanker would match velocity with the mars-bound craft.
What this does is eliminates the need for the mars-bound craft to carry and accelerate extra fuel; that's done by the tanker, a special-purpose craft that doesn't have to carry much of anything except fuel. In turn, that means the mars-bound craft can carry more stuff that is relevant to its task - passengers, supplies, etc.
Another thing: fuel for takeoff and fuel for landing can be supplied at the trip ends, rather than being carried to the ultimate orbital source and destination by the craft making the whole journey.
Another: tug/pusher roles. Ships dedicated to basically nothing but engines could temporarily attach themselves to the whole-trip craft and help it accelerate and decelerate for extended periods, all near their fuel sources. This is (essentially) what multiples stages on to-orbit rockets do. No reason it can't be used in space, with the extra bonus that recovery would be a lot easier.
There are lots of good reasons to set up such a system, and most (perhaps all) will make the craft that make the actual trips do their jobs better.
Yet another thing: some acceleration could be provided at the starting point in space by (relatively) fixed position acceleration devices, basically something that could add a few gees for a limited amount of time. As such a thing would be essentially stationary (in orbit), refueling it wouldn't pose the same degree of challenge.
Might even be possible to implement the reverse at the destination orbital terminus: a "catcher" that can apply several gees of deceleration for a short period of time. Every little bit helps. Again, such systems would be (relatively) fixed in orbit, and could be refueled without much ado.
This is all predicated on the idea that there will be ample fuel, but reducing the costs of accelerating it for the whole-trip craft is still a good idea.
(Score: 2) by Immerman on Friday September 21 2018, @12:10AM (3 children)
Midflight fuel tankers still have the problem that you're accelerating all that fuel up to speed regardless - so why not just include it with the original ship and avoid accelerating the additional mass of a second ship up to the same speed? (not to mention presumably then stopping the tanker and returning to the depot). If you're looking to make the trip dramatically faster than a Hohmann transfer orbit, then you're probably only going to have lost a small percentage of speed by the time you pass the fuel depot, so the deltaV advantage will be minimal.
Not to mention you have to get that fuel out to the depot in the first place, from its origins at Earth or Mars (or at least their moons). The asteroid belt is even further away, so you can't make it in place unless you first move an entire asteroid into place, which is even more fuel intensive. Though I suppose that's not terribly relevant if speed on a particular flight is your primary concern. You'd need a whole solar-orbit ring of such depots though, unless you're willing to severely limit your launch windows to the times when all three destinations are in alignment
Certainly you want orbital fuel depots at either end - in fact you don't necessarily want to land in the same ship that you fly between planets in - the design criteria for the two tasks are rather different. Planetary shuttles for loading/unloading, and then more space-optimized ships between planets (nuclear powered ion drives would be *far* faster than chemical rockets between planets, but useless, dangerous mass during landing and takeoff.)
"Pusher ships" near the ends make sense as well, especially for conventional rockets - as you say, every little bit helps, and the rockets can return to service the next one easily enough.
Some sort of "stationary" launch platform makes sense as well. A giant "spinning cable" launcher in orbit would likely be the simplest solution - you could spin it up to speed over a long period (ion drives?) then dock with the center and slide out along the cable for a substantial boost. Heck, launch the "tugboats" as well for even more boost - At that point even they would likely be going considerably faster than needed make the whole journey as well, albeit more slowly than the "second stage" ship they gave an initial boost to. So you might be better off letting them make the journey and do the same job at the other end, rather than burning all that course-reversing fuel for a quicker turnaround.
(Score: 2) by fyngyrz on Friday September 21 2018, @07:21AM (2 children)
Not midflight. Early stage leaving, and late stage arrival. As to why, because if you try to carry it on the whole-trip vessel, then you also have to accelerate, and decelerate, the extra tankage, piping, etc. The tanker can do that on a short leg, and at both ends. The whole-trip vessel has more important stuff to carry than fuel, and the more of it you can dedicate to that stuff, the better. Passengers, supplies, etc. Remember: the postulated circumstance means plenty of fuel is readily available, so the trick is to figure out how to use it most efficiently for the whole-trip vessel, not in the general case.
No. None of those. Those are terrible sources. The best source is cometary bodies.
No. You get fuel moving, slowly and efficiently, from the source (which is WAY past the asteroid belt) and once you have a stream of source material coming in, that's when you get to use it efficiently.
(Score: 2) by Immerman on Friday September 21 2018, @02:00PM (1 child)
Either way you have to accelerate the fuel to the same speed as the main rocket anyway - so why accelerate it separately? What's the advantage? Unlike on Earth, there's no cost associated with distance traveled - it doesn't matter if the fuel travels a thousand miles or ten million - it takes exactly the same amount of fuel to reach speed X.
I'm picturing your scenario: tanker accelerates away from from planetary neighborhood slowly, ship accelerates away more quickly, eventually overtaking tanker as they match speed, then fuel is transferred. Is that about right?
The problem is, in that scenario the tanker has spent no less fuel getting the "transfer fuel" up to speed than the original rocket would have - and it actually spent *more*, because it accelerated the fuel, plus a whole second rocket, plus enough extra fuel to get the second rocket back home (or stopped at the far end). You would have spent less fuel getting to the same scenario by simply giving the original ship much larger tanks - the square-cube law means one bigger tank weighs less than two smaller tanks with the same capacity, and thus gets more total acceleration from the same amount of fuel.
Booster rockets might make sense because they're extra engines and thrust getting the main ship up to speed, which then depart so that the main ship doesn't have to continue to accelerating the extra engines. I'm not even certain of that though - usually boosters are used in scenarios where thrust is important or you don't want to build a bigger ship - once you're outside a gravity well thrust no longer matters much - only absolute delta-V. Doesn't make much difference whether it takes you two hours or two days to get up to interplanetary coasting speed, unless you're going so fast that an extra few days is really worth shaving off the travel time.
Of course that all changes if you're trying to get on and off a planet with the same rocket you use to travel between them - but if you're talking about having all this in-space infrastructure, then why would you do that? You don't need a rocket that can provide (and survive!) massive thrust for interplanetary travels - the large engines and strong skeleton are just wasted mass you have to accelerate. You want one that can burn its fuel as efficiently as possible, wringing every last erg of potential thrust out of it to minimize the losses to the rocket equation
>No. You get fuel moving, slowly and efficiently, from the source (which is WAY past the asteroid belt) and once you have a stream of source material coming in, that's when you get to use it efficiently.
Unfortunately there is no "slowly and efficiently" in space - Hohmann transfer orbits are as good as it gets unless you can use gravitational slingshots. But for that you need to swing past a planet with a significant speed difference. If you're trying to bring a trans-Neptunian object close enough to Neptune for a slingshot you have to accelerate it (slow it down) enough to fall all the way in to reach Neptune first. If you swing past just right, with twice Neptune's speed, you can dump almost all your orbital speed and fall almost straight toward the sun - but then you need to slow down with just as much delta-V as it would take to push the thing out to Neptune's orbit in the first place. You might be able to dump most of that speed with another slingshot around Jupiter or Mars, but the planets have to be in the right alignment to make that possible - which means you can't do it frequently - once every few years at best, possibly only once every few decades.
(Score: 2) by fyngyrz on Friday September 21 2018, @06:45PM
Yes.
No. That's a not a problem at all. Fuel is not scarce. The resource to be conserved is the amount of mass carried in the whole-trip vessel.
Think about fighter jets. Why don't they carry all the fuel required at takeoff to go as far as one might want and fight as long as one might want?
Answer: Because they need to be able to accelerate, and decelerate, well beyond what they could if they were carrying all that mass.
So: tankers. Problem a considerable way towards being solved.
Long-haul spacecraft: same issue. The more mass on the vessel, the more it takes to accelerate it and decelerate it. This matters at both ends, because it means that speeding it up is harder, and so is slowing it down.
It's not the tanker we're concerned about. That can make a traversal at the long-trip transit rate and service multiple craft in the process, thus meaning that the long-trip vessel isn't carrying anything it doesn't absolutely have to carry. Less mass means less cost to accelerate and decelerate what actually needs to get to the destination.
Sure there is. Why do you think comets fall into the inner solar system? All it takes is a nudge. If you want something to end up at a specific point x, it'll need to be a very careful nudge, but it'll get there. Eventually. And eventually is fine.
(Score: 2) by Immerman on Thursday September 20 2018, @04:56PM (2 children)
Not quite. Fuel depots in high Earth/Mars orbit would make sense, because they're at the starting point of your journey, and you don't want to have to waste fuel capacity climbing any further our of the planet's gravity well than necessary.
Unfortunately you have the rocket equation working against you to offer diminishing returns - every extra kg of fuel you load is an extra kg of mass you need to accelerate, and so your maximum achievable speed plateaus.
There's also very little range limit on rockets, because it doesn't take any fuel to maintain speed - you get up to speed, and then coast until you've almost reached your destination, then burn the rest of your fuel to slow down. Unless you're using Hohmann transfer orbits for efficiency, in which case you accelerate just enough to eventually reach the destination orbit, and then accelerate again to match speed. But that takes much too long for transporting people further than the Moon.
"Gas stations" along the way are basically pointless. You'd get up to speed at Earth, then coast, then have to use all the "stopping at the end" gas to stop at the "gas station" to refill, then get up to speed again (basically the same speed you were going before you stopped, because you've got the same amount of gas, and hadn't lost much speed climbing the gravity well between planets) then stop again at the destination. Grand total, you've actually taken slightly longer to get to Mars because you stopped for completely useless gas. The only way it makes sense to refuel is if you're already stopping anyway for other reasons, or if you *don't* stop, and instead the "gas station" matches speed with you. Which of course burns an enormous amount of fuel, but could be done.
There's only a few ways I've heard of to improve the situation.
- The first, and most appealing, is ditching chemical rockets for powerful nuclear-powered ion thrusters. You get orders of magnitude more thrust per pound of reaction mass that way, practically eliminating the limits of the rocket equation when flying around the inner system - you accelerate halfway to Mars, then turn around and decelerate the other half of the way. An relatively weak ion thruster can thus get you to Mars far faster than a far more powerful chemical rocket.
Of course that probably means your interplanetary ships never land - ion drives aren't nearly powerful enough to get off the surface, and you don't want your chemical rockets hauling all the the "useless" mass of the nuclear reactor and ion drives in and out of a gravity well every trip. Instead you use chemical rockets as shuttles to carry crew and cargo to the interplanetary vessel.
- You could build some sort of spinning "launch catapult" at the ends, so you can accelerate massively without using rockets. That could be done, but requires building extremely strong orbital structures probably hundreds of miles long. Well within current material science, but *really* expensive until we start mining asteroids.
- You forget about speed entirely, and instead move small asteroid "cyclers" into resonant orbits that approach each planet every few orbits. Hollow it out and you've got a nice thick radiation shield so that you don't have to worry about exposure during the trip. Make it into a zero-G cruise ship to keep folks entertained for the many months of the trip, and maybe return to being a mining outpost for the couple of years before it comes into alignment for a "quick" trip again.
(Score: 2) by Runaway1956 on Thursday September 20 2018, @10:33PM
Scroll up to fyngyrz post. He's on the right track with the fuel matching velocity with the ship, rather than the ship matching velocity with the fuel. Let me add "drones" to the equation. Neither the ship nor the gas station need change speed, direction, or anything at all. A simple message, "On or about Aug 5 2041 we'll be passing the vicinity of your station, outward bound to ___________. We would like to contract for xxxx gallons of ___________ fuel to fill our tanks."
Sometime near Aug 5, the ship is met by one or more drones, each of which transfers fuel to the ship.
I mean, it can't be considered efficient for either ship or gas station to accelerate to match orbit with the other, but drones can deliver exactly what is needed, in the proper orbit, at the proper time, no more, no less.
(Score: 2) by toddestan on Friday September 21 2018, @03:16AM
Another way to do it is to use solar (light) sails, and then use powerful lasers to push the craft around. This has the advantage of the craft not having to carry fuel to propel the craft, and not having to carry the fuel to propel the fuel that propels the craft, and not having to carry the fuel that propels the fuel that propels fuel that propels the craft, and so on. The main energy source would be a stationary power plant which can be as large and bulky as it needs to be. If you want to go really fast (as in, a significant portion of the speed of light) this really is one of your only choices, because even if you can convert fuel (mass) to energy to acceleration with 100% efficiency, you still need a truly massive ship if you want to carry all your own fuel.
One disadvantage of this system is that you need two powerful lasers to push the craft around, one to accelerate it up to speed from where you started, and a second to slow it down at your destination. Which presents a problem if you're going someplace no one has gone before. Though you could deploy an unmanned laser station to your destination before the manned flight, using conventional rockets to slow it down (you could still use your big-ass laser to accelerate it).
(Score: 2) by Immerman on Thursday September 20 2018, @02:45PM (6 children)
Just to clarify your statement
Your link says ≤ 3% REID (Risk of Exposure Induced Death) , 95% C.I.
I'm pretty sure that's a 3% risk of dying from radiation exposure (with a 95% confidence interval?), which includes a lot more than cancer - two slides down they mention acute radiation syndrome, degenerative tissue effects, and cardiovascular and central nervous system risks.
NOT a 3% increase in cancer risk (which generally indicates [baseline risk]*103%)
Pretty sure you're right about there being plenty of volunteers even if they new 10% of them would die from radiation-exposure though. As I recall there were plenty of volunteers for the Apollo mission when they thought it might have to be a one-way mission. A 10% chance of dying at some point because of your trip is a lot better odds than a 100% chance of dying in a can on the moon within a couple of weeks.
Besides, the baseline risk of developing cancer at some point in your life is apparently already almost 40%, though I couldn't find information on what the baseline risk of being killed by cancer is. Though the timeline is important - that document doesn't seem to put any timeline on their risk assessment. Is that lifetime risk? Within a year? Rather different implications.
(Score: 2) by takyon on Thursday September 20 2018, @02:59PM (1 child)
I think it is just cancer based on the wording I quoted. Plus those two risks didn't seem to be described as fatal, just undesirable.
If you are right, then it is even safer than I thought.
It's not like the spacecraft provide no shielding whatsoever. And with the mass limit and huge payload fairing of SpaceX's BFS, you could line the "walls" with additional shielding. Even extra layers of plastic [wikipedia.org] can provide shielding.
Solar flares could be a big danger, but the astronauts could receive advance warning and seal themselves up in bags, compartments, or spacesuits temporarily.
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 2) by Immerman on Thursday September 20 2018, @05:10PM
Hmm, you might be right - except about the safer part. It is after all risk of DEATH either way - only the mechanism is in question.
And unfortunately the spacecraft provide pretty close to no shielding. They have to, if they want to avoid killing the passengers. The problem is that you have two kinds of radiation to deal with - low energy and solar wind stuff that's dangerous as-is, and can only be somewhat shielded against without thick, heavy shielding. And high-energy cosmic rays that are (relatively) harmless as-is, but will turn any sort of heavy shielding into deadly radioactive particle cascades that can only be blocked by *extremely* thick shielding on the order of several pounds per square inch.
If you're talking about a "stationary" space station, especially one built from materials harvested in orbit, then extremely thick shielding is no problem. For a ship that has to accelerate though, flimsy "stop the easy stuff" shielding really is about the best you can do - presumably with some sort of heavily shielded "solar flare shelter" that you try to avoid spending too much time close to under other circumstances, since it's going to be constantly spawning those radioactive particle cascades.
(Score: 0) by Anonymous Coward on Thursday September 20 2018, @03:00PM (3 children)
You may be making an incorrect distinction here, cancer is now thought to be many diseases rather than a single disease. Radiation syndrome, degenerating tissue/organs, heart attacks, and alzheimers/parkinsons/etc are may all be different manifestations/symptoms of cancer.
(Score: 2) by Immerman on Thursday September 20 2018, @04:12PM (2 children)
*may* be. As in, also may not be. My distinction stands until further evidence proves otherwise.
Killing and injuring a bunch of random cells with radiation can be pretty damaging all on it's own, they don't need to start self-replicating uncontrollably for it to be a problem.
(Score: 0) by Anonymous Coward on Thursday September 20 2018, @06:16PM (1 child)
Its going to require more cell divisions than otherwise would have been necessary to repair the damage. More cell division -> cancer
(Score: 2) by Immerman on Thursday September 20 2018, @07:02PM
No, more cell division increases the risk of cancer - but is not itself cancer. Most cell lines die from other problems long before they become cancerous.