Follow Slashdot blog updates by subscribing to our blog RSS feed


Forgot your password?

Comment Re:Are and storms that fierce on Mars? (Score 1) 49

Clarification on radiation shielding: you generally don't use just a hydrogen rich layering, there may be metallic layers as well (such as the craft's outer skin, tankage, etc). But most of the high energy solar and GCR is charged particles, mainly protons. The lower end of the energy range will almost entirely impact whatever shielding you use, creating a small shower of secondaries. Some high energy particles will impact, some will pass right through. Those that pass through will most likely pass through everything, and those that do impact crew will mostly impart only a tiny fraction of their energy to them. Those that impact the shielding create an ever-growing shower of secondaries; where the secondaries aren't sufficiently blocked poses more of a risk to the crew than had the particle not impacted anything at all on the way in. Primaries at over 10MeV or so are particularly prone to kicking off secondaries, and once you get into hundreds of MeV spallation starts becoming a significant component.

All of this together means that the most important particles to block are the secondaries, in that they're more numerous, less likely to cause negative side effects by blocking them, etc. Heavier secondaries like alphas are easy to block, while it's unrealistic to block a significant fraction of high energy gammas on something as light as a spacecraft. This leaves the neutron secondaries as your prime target for elimination, which can generally be captured if moderated down first, but otherwise pose a risk to the crew. The lighter the element and the higher the cross section, the better the moderator; also, the lighter the element, the more you can carry on a spacecraft. Hydrogen fits all three bills well. Once moderated down, then the capture cross section becomes key. Hydrogen can manage thermal neutron capture over a sufficient distance, but far better is something like boron. In fact, metals can sometimes be counterproductive, especially on the inner side of the shielding. They increase the risk of spallation, bremmstrahlung, and your neutron captures are much more likely to produce short half life isotopes which will then undergo beta- decay.

There's no need for an unusual amount of metal in the shielding (over what would be needed to build the craft itself), and no need to make it a faraday cage. EM radiation and charged particles are very different beasts.

Comment Re:Are and storms that fierce on Mars? (Score 1) 49

It is apart from solanine. Potato starch is indigestible raw. It passes all the way into the intestines intact, where it then begins to ferment under the influence of anaerobic bacteria. This yields significantly less caloric energy as well as indigestion and bloating.

Anyway, Weir wouldn't have had to worry about potatoes greening (solanine) because he had at least 2-3 orders of magnitude too little light to actually grow potatoes, thinks that the entire part of the plant above the soil is the "fruiting body", and thinks that potato mounding involves completely burying the plant and planting new potatoes directly on top of it. Not to mention the perchlorates, ethylene gas, or the 50 other things that would have actually killed his potatoes if grown as described. (Note to anyone who's ever owned a winter greenhouse or done significant indoor plant growing: expect to repeatedly hit your head against the wall if you read The Martian).

Oh, and try not to think too much about his plan of having humidity condense on the habitat and rain back down as a method for watering the plants (sensitive life-critical electrical systems and condensation: best friends 4everz!). It's bad enough when it happens in your apartment... I remember the day when my light fixture fell to the floor and broke because it had filled up with water and become too heavy to support itself - sure explained the reason why the breaker to that room kept throwing! At least in the movie they seem to have added a grow tent, judging from the trailer (haven't seen the movie yet). Although grow tents bring their own problems... and most clear plastic sheeting is polyethylene, which is a pain to bond.

Comment Re:Are and storms that fierce on Mars? (Score 1) 49

At least the story is internally consistent: because the Hab is radiation-proof, radio waves don't go through it

Yet another Weir misunderstanding, confusing all forms of radiation as if they're the same thing. If you want to block radio waves with as little mass as possible, you use metals. If you want to block streams of charged particles with as little mass as possible (the actual goal), you use hydrogen-rich materials, ideally with a borated inner liner. Weir has a history of misunderstanding radiation and confusing all types as if they're the same thing - check out his rant about how horrificly dangerous the radiation from an RTG is ;) Speaking of that...

I've also seen reviewers complaining that Mark Watney oversells the dangers of the radiation inside an RTG. In the book at least he is joking around a lot and using imprecise terms such as "box full of radiation"

He's not "joking around", the rant is like a page and a half long, describing it as vastly more dangerous than Pu-239, with a long line of superlatives for how to describe its incredible "danger". He talks about how it gets glowing hot with radiation and extends that logic to meaning that said radiation would be a lethal threat to his protagonist should the case crack. Which is of course absurd. Alpha doesn't even penetrate the outer layer of dead skin - alpha emitters are only dangerous if ingested or inhaled, and there's no realistic way to do that with an RTG, they're designed to even withstand unshielded reentry without burning up (and have done so - ex. Apollo 13). He'd be at far more risk of burning his suit - they're designed to operate at temperatures of 1000-1100C on the inner core and can still be very hot on the cooling fins (which, by the way, are often very large - on Curiosity, they're the giant angled section in the right near the guy in this picture. That's just to dissipate the heat used to produce a mere 125 watts of electrical power.)

My explanation of how the Hab is radiation-proof: a superconducting magnetic shield.

Microwave communications are based on photons, aka chargeless particles, aka no Lorentz force, aka no deflection.

Only protects against charged particles though...wouldn't stop gamma rays. How common is random gamma radiation on the surface of Mars?

Nor neutrons. Nor very high energy particles, such as in GCR, according to studies of realistically-implementable systems. But lower energy charged particles comprise the lion's share of the radiation exposure. Also, a lot of the neutrons and gamma that one would be exposed to with conventional shielding are secondaries.

Comment Re:Are and storms that fierce on Mars? (Score 1) 49

There were far more major glaring errors than that. I managed to read about a quarter of the book, needing something to bang my head into on almost every page. No, I don't want to turn this thread into yet another "rip on the terrible 'science' in The Martian" thread, so I'm not going to start yet another "list" like I've done the last times the book came up on Slashdot.

Honestly, with how much he screwed up the science in general, I doubt Weir's "I did it for artistic license" excuse about the dust storm. It comes across as a post-facto to explain away one of his screwups that was getting the most complaints.

Comment Re:Decline of Soda?, Two words.. (Score 1) 316

Shudders why? Unless you have phenylketonuria, it's not relevant, and if you do, then it's just one entry in a long list of things you should probably avoid. Aspartame decomposes in the digestive system to aspartic acid, phenylalanine, and methanol. The amount of methanol is comparable to that in wines and fruit juices, the aspartic acid is far lower than is found in most dietary sources, and the phenylalanine is comparable to common dietary sources and less than many phenylalanine-rich dietary sources.

Comment Re:Better to drink from a leaking garbage bag (Score 3, Interesting) 316

Weird to see people complaining about sugar but switching to fruit juice, though. Many if not most fruit juices have a higher sugar concentration than coke.

Now, that's only from the sugar perspective. Caffeine has its good and bad sides, so if one wants to cut down, there's that. Phosphoric acid may or may not have a negative effect on bone density (lower bone density is associated with soda consumption but there's dispute over whether it's the phosphoric acid or just the aforementioned caffeine). Fruit juices have vitamins and minerals that most colas won't. But really, the biggest health issue with colas is the sugar, and one may actually increase their sugar intake by switching to juice.

Comment Re:Airstrikes on population centers (Score 1) 314

You must be referring to the groups that exist only in US propaganda fantasy land.

PKK, FSA, al-Nusra, Ahrar ash-Sham, and tons of other militias are opposed to and regularly fight Daesh. They control large swaths of Syria, and have recently been making major progress in the northwest, taking over Idleb - which was almost certainly the trigger for Russia to step up its game, as they're nearing Latakia.

Check a map.

Comment Re:Airstrikes on population centers (Score 1) 314

It's not that simple of an arrangement. The Kurds are indeed in the north, mainly the northeast. Assad's strongholds are in/around Damascus and among the Alawi populations on the coast (that is to say, west of the Alawiyin mountains), although he also controls many scattered pockets elsewhere, even ones touching Kurdish territory. The FSA and Al-Nusra control large chunks from the western Turkish border down to Idleb, just on the east side of the mountains, as well as many pockets elsewhere. As for Daesh.... they're bloody everywhere. Their territory is shaped like a porous sponge, following rivers and roads. They reach up to part of the Turkish border in the north, east into large chunks of Iraq, south into the southeastern deserts, southwest to towns near Damascus, and west to the FSA / al-Nusra areas. Pretty much everywhere in the country borders them... except where the Russians are. Latakia is only under threat from the FSA, al-Nusra, and related allied militias. And that's who they're bombing.

Comment Re:Airstrikes on population centers (Score 1) 314

ISIS, ISIS, ISIL, Islamic State... these are all "respectful" terms. They want to be referred to as the "Islamic State", as their goal is to reestablish a new caliphate.

Daesh is an acronym of their Islamic name. Acronyms are rarely used in Arabic, which has led to confusion and anger on Daesh's part. It removes the "Islamic State" part that's so important to them. And it sounds similar to a word meaning "one who crushes underfoot". Daesh threatens to kill anyone caught using that term for them, which to me is reason enough alone to use it. It's also what the local opposition to them calls them, not wanting to dignify them as a legitimate caliphate.

Comment Re:Ignore the "humans almost went extinct" bit (Score 1) 48

From the article:

Such tsunamis may not have the same long-distance range as those that originate from underwater earthquakes, such as the 2004 tsunami in southeast Asia that travelled thousands of kilometres from where the seafloor ruptured.

The article does not say that a volcano in Indonesia caused a tsunami in West Africa. Please read it.

Comment Re:Airstrikes on population centers (Score 5, Insightful) 314

What do you mean "both sides"? There's several dozen different major militias, which really if anything fall into three "sides": Assad, Daesh (what you call ISIS), and a loose, sometimes self-sniping (but decreasingly so) alliance of kurds, secular arabs (the nominal FSA), and islamists. All three sides oppose each other.

Russia supports Assad, the party recognized by the UN and human rights groups as responsible for the lion's share of the war deaths and over 10k tortured to death in its intelligence centers. However, it's doing this not by opposing the opposition uniformly, but by heavily focusing on non-Daesh entities. If successful, this would leave a conflict between Assad and Daesh, wherein the west would basically be forced to accept Assad. Iran and Hezbollah are Russia's copatriots in this.

The US and the Gulf states support the non-Daesh forces. The US strongly supports the FSA, would support the Kurds if not for how it would cost them Turkey's support, and is willing to overlook the islamists so long as they continue along their path of denouncing anti-western activity. The Gulf states by contrast have largely been supporting the Islamist militias - Saudi Arabia in particular focusing on Ahrar ash-Sham, while Qatar seems to be in bed with al-Nusra.

Israel wants Assad and Daesh gone, and seems content at sniping at either of them within the Golan Heights, but doesn't seem to want to take a larger, riskier role.

The strategies used by the US and the Gulf states are similar in regards to Daesh: A continuous but restrained bombing campaign. Both the US and the Gulf states take part in this. The arming strategies have somewhat differed, however, and not simply in regards to what groups are the beneficiaries. The US has been very hesitant to deploy weapons to Syria, waiting three years starting and not giving anything heavier than a TOW. The strongest focus has been on coordinating small numbers of FSA members to operate as effective US ground spotters against Daesh. It's not gone very well. Providing intelligence has proven more useful, and the weaponry, although limited, has allowed for more effective operations in certain fronts, such as Idlib. The Gulf states however have focused more on money and arms to their groups, and started it early. The early successes of the islamist militias while the FSA was flailing led to many waves of desertion, turning it from the largest opposition group to at its lowpoint nearly a running joke.

Turkey has proven willing to support taking on Daesh although uses the opportunity to snipe at the Kurds. Turkey's policy of chasing back Syrian planes who even approach their border has created an effective narrow no-fly zone in Syria's north, which militias on the ground have taken advantage of. With Russia's involvement now, however, it's questionable whether Syria will be able to continue that policy, out of fear of hitting Russian jets.

Everyone has their own endgames in mind.

In Russia's and Iran's, the conflict turns into "Assad vs. Daesh", the west reluctantly agrees to accept Assad, wipes out Daesh, and their only Mediterranean ally remains in power. They know he'll probably undertake some serious purges over the next several years while trying to wipe out any vestiges of opposition remaining. Their media will happily not report it.

In the US's and Israel's preferred scenarios, the secular/kurdish/islamist coalition wipes out both Assad and Daesh, with their help on the latter. Each ends up with regions under their control. The goal would be a Lebanon-style power sharing agreement. A more realistic expectation would be a Libya-style post-dictator power vacuum with random sniping militias. Those who support this view that as a vastly better improvement than the current situation or an Assad re-conquest.

In the Gulf states view, they really could care less whether the post-Assad, post-Daesh environment would be a Lebanon-style arrangement or simply another dictator, this time not allied with Iran against them. They'd be quite happy with an Islamist government, so long as it's domestic-focused and has no territorial ambitions.

Turkey will be happy with any situation that doesn't put a force hostile to them in charge and which can disarm or otherwise check the Kurdish militias in the north.

Despite all of the different militias and parties, the conflict really isn't as complicated as it looks. You have Daesh which wants a new multinational caliphate, you have Assad who wants total control over all of Syria, and you have everyone else who doesn't want either Daesh or Assad and are willing to put up with each other to do so. There's only really one irony, and that's how the US and al-Qaeda are basically allies in this. While there's some talk of al-Nusra breaking with Zawahiri (a move that may cost them volunteers but gain them more financial and arms support), as of yet it hasn't happened. al-Nusra is working alongside other islamist militias, the FSA and Kurds, and as of recently the level of infighting has been rather low (though neither the FSA nor Kurds trust them). The weird thing is how much they've been "behaving" themselves (at least by the standards of "totally unregulated militia in a bloody civil war") - there've been abductions and suicide bombings, but also some things totally out of character for al-Qaeda, like apologies when their soldiers kill civilians and nuns complimenting their respectfulness. They don't even impose sharia in all of the towns they control, allowing for the example the Christian towns to continue to govern themselves, which is another weird one. And supposedly this is all on the orders of Zawahiri too. Strange times...

The life of a repo man is always intense.