Posts by cray74 1081 publicly visible posts • joined 29 Nov 2011
How are the uranium tubes going to be vaporized in the explosion? RTGs are not just built to survive rocket explosions, they've demonstrably survived the experience. They've also survived re-entry, just as designed. And those were older, less durable* RTGs from the 1960s and early 1970s, not the improved armored vaults dangled off space probes today.
*Where "less durable" is used generously; RTG's were armored beasts in the 1960s, too. The Nimbus B-1's SNAP-19 RTG was recovered and the fuel reused. Apollo 13's RTG landed in the Pacific without leaking.
I like the desiccation idea. Lowering the water content of biofuels removes a significant heat-absorbing component and allows better combustion efficiency.
I suspect you could take the raisin approach, leaving them out in the sun, but that'd probably just cheer up the hippies to be desiccated by solar energy. Using the waste heat from dry cask short-term nuclear waste storage for the drying process has a more karmically appropriate ring to it.
"I'm sure the content will be fabulously misguided and will provide bored sailors with endless frustration as they read Moby Dick, The Old Man and the Sea or 20,000 Leagues Under the Sea."
I was about to protest that other articles on NeRD mentioned that the US Navy has a huge library of tens of thousands of eBooks. However, while Googling some references to show how diverse NeRD's content would be, I discovered that NeRD is coming preloaded with a library of 300 books.
I could handle a preloaded ereader with 300 books if I had a chance to periodically update / swap from a large DoD library or even the Gutenberg Project. Even if I had to get an officer's stamp of approval and go to an approved, security-monitored exchange point. But when you're stuck with 300 factory-installed ebooks for all time?
I now agree that the content will be fabulously misguided and frustration-inducing.
The recent scifi novel, "The Martian" by Andy Weir does address poo, and is an amusing read besides. Actually, the first part of this "cast away" story is rather like this Register article: how to survive with very limited resources and dealing with the resulting monotonous diet. (Which consists of potatoes from poo-fertilized Martian soils.) It also contains useful tips on surviving in isolation when your primary form of entertainment is a collection of 1970s music and sitcoms.
"If it's that big, and spinning that fast, can I safely assume that aliens get flicked off if they go near the equator?"
Alas, no. Even in Hal Clement's "Mission of Gravity," its super-fast giant world couldn't tear itself apart or flick aliens off. Gravity only lowered to about 3Gs at the equator and was pretty epic near the poles. [Edit: actually, it's pretty close.]
The problem for centripetal alien-flinging planets is that on the scale of planets centripetal forces get fairly weak. The equation is Acceleration = Velocity x Velocity / Radius. You want that acceleration to match or exceed gravity for aliens to start flinging off the planet. For a planet with 1.65x Jupiter's radius (about 120,000,000 meters, rounding up), you need very high velocities to equal gravity.
A quick approximation of relative gravities is: relative mass / (relative radius x relative radius. Beta Pictoris b (BPb hereafter, or maybe "Beepee") is 10x the mass of Jupiter and 1.65x the radius, so it should have about: 10 / (1.65 x 1.65) = 3.67x Jupiter's gravity. That's nice to know, but not handy for comparing to centripetal acceleration, which the prior equation presents in meters per second (or traditional El Reg units, and I'm less familiar with those than a Lockheed engineer is with metric units). Jupiter's surface gravity is 2.53x that of Earth, so Beepee has 3.673 x 2.53 = 9.3G's at its gaseous "surface," give or take some rounding errors. A final conversion to m/s/s gives 91m/s/s.
Alrighty then, back to spinning in circles. Beepee is spinning at 100,000km/hr, which, now that I think about it, translates to 27,777 meters per second or more than twice Earth's escape velocity. On a smaller planet, aliens would be flinging off the equator and make amusing strings of craters on any moons that got in the way. Very Kerbal.
But Beepee is not a lesser planet. It has a surface gravity of 9.3Gs. The equatorial centripetal acceleration is (27,777m/s x 27,777 m/s) / 120,000,000m = 6.43G. 9.3G - 6.43G still leaves almost 3Gs at the equator.
Wow, that does get close, close where someone should doublecheck my number crunching. Earth's spin effects only lowers perceived gravity by a small percent at the equator so I didn't anticipate such a drop on Beepee. But, no, aliens cannot flee Beepee's gravitational might simply by vacationing at the equator. There's still an unhealthy margin of remaining gravity there.
Gas giants also collect rocky material in the same fashion as terrestrial planets, so they're subject to the same bombardment by planetismals. Were your model to work, Loyal Commenter, gas giants would end up with the same randomized spin.
Rather, what appears to happen is something like this:
Step 1: interstellar cloud of dust gets upset (due to a passing star, a nearby supernova, losing money on the races, etc.)
Step 2: This disturbance causes some spot in the cloud to get more mass than the rest, meaning more gravity, and a runaway collapse sets in - definitely a physical collapse, possibly emotional.
Step 3: Most of the cloud collapses into a proto-star (example: Sol has 99.9% of the system's mass) with most direction of the cloud's motion heading toward the proto-star. However, some vague currents and drift leftover from the cloud manifest as a common spin direction. As seen in Sol and the debris disk around a number of other stars, the stars, disks, and planets all tend to move in the same direction on roughly the same plane. In the case of Sol, the planets, asteroids, and sun also show a common direction of spin with a few caveats.
Step 4: To get those caveats, outside forces are involved. Small bodies like asteroids tend to have their spins disturbed by, yes, collisions. There's also the YORP effect, and the wiki article on that discusses the spins of asteroid depending on size. Mercury is close enough to Sol for most primordial spin to have been lost to tidal effects. Venus might've had a normal planetary spin until drag and tides in its thick atmosphere slowed it (according to one group of modern theories). Earth probably had a reasonable spin like most planets until it got beaned by Theia, which left it with a very high spin until Luna's tides damped that. Uranus probably started with a typical spin until - per 2011 theory - it took a succession of large impacts.
Step 5: Profit!
Summary: You were correct to note the influence of impacts on spin, but that's not the only means of altering planetary spin and it does apply to gas (ice) giants like Uranus, too.
Beta Pictoris B is about 10 times the mass of Jupiter (current estimates range from 4 to 11x Jupiter's mass). It has about 1.65x Jupiter's radius, so about 4.5x Jupiter's volume.
As a rule of thumb, few objects with masses between Jupiter and small red dwarfs (100x Jupiter's mass) get much larger in diameter than Jupiter unless very high temperatures make them fluffier. With increasing mass comes increasing density, which creates enough gravity to compress the planets / brown dwarfs / red dwarfs in that mass range. This produces oddities like red dwarf stars that have "surface" gravities many times higher than Sol's gravity and brown dwarfs that are ten times more massive than Jupiter but scarcely larger in diameter.
By the time you're done layering on requirements for insulation, vacuum operation, accessibility by space suit gloves, compatibility with existing mounting points on external station structures, backup power supplies, redundant components, and cooling and heating, computers less powerful than your smartphone tend to grow to the size of microwave ovens.
"Are they so unsure about ending in the right place? "
Not really. They're worried about it staying under control as it falls, like the last Falcon 9 (September '13) landing mishap. The falling first stage went into a roll beyond the ability of its thrusters to control and tumbled to the water. Hitting the bullseye was less of a concern, since SpaceX's aim was pretty good by that point.
"Otherwise, why not do it on the ground in the middle of the desert?"
Because this rocket launch is to service the International Space Station under contract from NASA, and thus this Falcon 9 needs to launch from Florida to keep SpaceX's customer happy. Other than some metaphorical moral deserts in Miami and Orlando, Florida doesn't really have a desert for rocket testing. However, Florida does have a lot of test range infrastructure and Snark Infested Waters that have been receiving test rockets for decades. Beyond keeping its paying customer happy, SpaceX is piggybacking this launch to try out landing hardware and protocols, but that's purely secondary. Get stuff to the ISS first, play Harrier second.
Further, SpaceX is paying attention to rocket design history. The first bits of rocket hardware rarely work as planned even after hundreds of tests in the lab. Rather than trying to get all the widgets and gizmos to work right on just the second try, SpaceX is only testing a limited number of features (thus minimizing variables) on this flight. The flight in September failed for a reason. SpaceX'd like to make sure that reason is solved and try out a few new things. Actually getting everything to work for landing is going a bit too far, and they've got a Grasshopper to test reusable flight (in a desert, in fact).
"considering the only thing they plan to do is to hover gently above the ocean before sinking it."
Considering that hasn't been accomplished previously, that's not an "only thing." The last time SpaceX tried that the rocket went into an uncontrolled roll and crashed. (Also into the ocean, since the paying customer wanted a launch from California into a polar orbit.) And despite that, SpaceX was very happy with the September flight. It gave them mountains of information about real world aerodynamics of rocket stages falling arse-first from space and important lessons about residual fuel behavior. (The crash was caused by fuel 'whirlpooling' in the fuel tanks.)
It might not seem like much to have a rocket land vertically on telescoping legs, but that's actually an achievement involving a huge amount of engineering addressing thousands of problems in a handful of test flights. That's quite different than testing a new car airplane design, where you can fly or drive the vehicle thousands of times before beginning to sell them. You're condensing a lot of mistakes into a few flights, and crashes are to be expected.
"But with all that mass, how does it not immediately collapse into a black hole?"
For the same reason the Milky Way doesn't collapse into a black hole: it's all in orbit. Black holes form when a lot of fairly stationary mass (e.g., a star) finds a reason to implode (like a cessation of fusion heat that was keeping it inflated.) The magical moment happens when a sufficient portion of the star's mass gets inside a certain radius.
If everything stays in orbit (or gets blasted away by a neighboring O-type star that can't contain its hot gaseous emissions), then you don't get a black hole.
Even if Neptune & Co were in the Goldilocks zone, it wouldn't be for long, and it'd be a Goldilocks zone inhabitated by bears.
Stars like that are unstable on human time scales. Today they're a yellow star 1300 times Sol's diameter, tomorrow a core hiccup makes them an even-larger red giant, and the next day they cool and contract a bit. (Where "day" might mean "decade" or "century.")
Further, low density stars like this are producing epic solar winds that carry away entire solar masses of their own fumes. A terrestrial planet in the Goldilocks zone will have its atmosphere sand-blasted away very quickly (where "quickly" means "decades" or "millennia," give or take a few orders of magnitude.)
Airships have the potential to accomplish tasks helicopters and airplanes cannot. They can't fly on station for weeks, unlike airships, and while helicopters can serve as skycranes the potential cargo capacity of airships is much higher.
This first (re)start airship isn't going to exploit the full capability of airship technology, especially since it depends on aerodynamic lift. But there's some interesting possibilities for cargo aircraft that can carry 50 or 100 tons into BFE where there are no roads, airports, or waterways, and then lower the payload into a rough field. The same capability would be handy for delivering over-size loads into urban areas, too.
"So it's not punishment you want, it's plain simple public revenge? ... Tell you what why bother with all that justice bullshit either, eh? Just instigate several teams of covert death-squads ..."
That's a slippery slope fallacy. That Shane 4 suggested quick punishment (or revenge) for a self-admitted and convicted killer does not directly lead to eliminating the preceding trial, due process, careful consideration of evidence, and appeals.
@DougS: Oxygen's needed at about 1kg per day per adult. A crew of 10 on a 2-year round trip to Mars would only need 7300kg (no margin) without recycling, just CO2 scrubbing. And before you recycle oxygen from CO2, keep in mind that the human metabolism as produces water (lots of hydrogen in food to react with inhaled oxygen). Mir and ISS both used water electrolysis to recover oxygen from water, while carbon dioxide is mostly scrubbed and dumped overboard.
@BlueGreen: you have touched on some of the differences and challenges for sending astronauts to Mars versus nuclear robo-tanks, but those are the tip of the iceberg. A nuclear submarine has most of the features you mentioned, but it's also not nearly as expensive as a manned Mars mission. The big difference is getting all those systems (or at least their backups) to work flawlessly for several years at absolute minimum masses. "When failure's not an option, success gets really expensive."
This mission was very successful on a lot of points: launch, navigation to the moon, motor restarts in space, soft landing, deploying the rover, getting the instruments working, communication with home, etc. Those are the hard parts of getting to the moon (or Mars, and elsewhere) that have historically fed a lot of US and Rooskie space probes to the Great Galactic Ghoul.
If you look at a timeline of Russian moon landings, it was like they were blindly spraying a probe machine gun into the sky until they hit the moon. Blew up on the launch pad. Escape stage failed, stuck in orbit. Bad navigation sends probe into deep space. Escape stage failed, stuck in orbit (again). Motors fail to fire for landing. Smacked into a lunar mountain. The US had its own litany of failures getting to the moon, or even getting rockets off the launch pad.
Venusian and Martian exploration is littered with tales of failure, from the difficulty the Soviets faced just getting camera covers off (Venus landers) to the US's trouble with English-metric conversions (leading to the Mars Climate Orbiter's unplanned lithobraking maneuver). We're still at about a 50% success rate with Mars missions.
And China's first lunar landing mission got off the launch pad, navigated to the moon, soft landed, and started working. Yeah, it failed before it was supposed to, but Jade Rabbit also bypassed all the trouble points that repeatedly wrecked so many other space probes. That's plenty of success, and one huge learning experience.
The damage to the wheels makes me wonder, "Why not steel?" But I already know the answer: with NASA's prior rover experience, aluminum wheels seemed like a sure bet. I would've stuck to proven wheel materials, too.
Still, in the future I'd give steels another look. There are some nice precipitation-hardened stainless steels popular in the aerospace industry (like 15-5, 17-4, 17-7, etc.) that have great strength (much better than aluminum and, yes, titanium alloys, which are over-rated); good hardness (again, well beyond Al and Ti alloys); great wear resistance (something Al and Ti aren't known for); good toughness (though I'd have to check to compare to Al and Ti alloys); and they retain those properties to cryogenic temperatures well below anything seen on Mars. There's also a lot of industry experience in shaping those stainless steels.
Actually, since corrosion probably isn't much of an issue on Mars, I'd also want to poke into some of the lower strength, higher toughness maraging and Aermet steels. Stainless steels are nice, but they compromise their properties some to stay shiny. Beasts like maraging and Aermet alloys don't bother staying pretty for the public.
Because of the point loading issue Dave 126 brought up, you'd want the wheels to be thick (about the same as the current aluminum wheels) so there'd be a weight penalty. But a weight penalty in a mission-critical item like your wheels isn't necessarily a vice.
There are a number of transparent armor architectures. Those used in vehicles tend to be alternating layers of glass (or SiAlON, or sapphire) and polymer. The glass is a hard surface that blunts and fragments a projectile, while the polymer provides a means of soaking the bullet's energy (primarily through delamination between the glass and polymer - it takes energy to peel the layers apart).
Plain polycarbonate and other polymers tend to be too soft to stop rifle-caliber and larger bullets, for all their fantastic strength. Hence, hard surfaces are often added. This applies to body armor, too. While armor vests meant to halt pistol bullets rarely include hard surfaces, it's easier (and thinner) to add a ceramic or steel plate if you want any chance of stopping rifle bullets. The hard plate blunts and fragments the bullet, while the Kevlar (or equivalent) polymer vest behind it acts like a catchers mitt for the remainder.
I have seen a pure polymer armor transparency that could stop a .44 Magnum (insert Dirty Harry references). A former boss kept it, and the embedded bullet, on his desk. However, it was about 5 to 6 centimeters thick. And a .44M pistol round isn't nearly as challenging to stop as a rifle bullet.
As I recall, after the experience with the Pathfinder rover, some sort of solar panel cleaning system was considered. However, it was decided that the weight of a cleaning system could be better used for bigger solar panels (which would provide a passive mechanism for extending the solar panels' lives.) The experience of Spirit and Opportunity will definitely lead to some more thought on future solar-powered Martian rovers.
Considering the number of soft landing probes that the USA and USSR slammed into the moon or had malfunction in some unexpected way, China's first-time success in a lunar soft landing and getting a rover cleanly deployed is impressive. That's just a lot of untested engineering and variable to go wrong. It'd be a fairy tale ending to have Jade Rabbit achieve everything the designers and be the next methusaleh rover after Opportunity, but more realistic to figure something will go wrong on the first try.
Hopefully the Chinese will have enough telemetry to learn from the problem and avoid it with the next rover. Engineering mysteries are annoying.
"But on a phone can they make it shatter resistant."
Probably not so much. You can do something like putting a plastic backing on the sapphire (like car windshields) to give it some tensile strength on the back side (the direction in which an impact is likely to flex the sapphire sheet), or at least hold the sapphire together when it cracks.
But sapphire's virtue is hardness, not toughness, and it's nothing a ceramic you can easily chemically modify. Unlike glass - glasses are chemical mutts and you can tweak their composition all day long, the way Corning does with Gorilla Glass.
Your best hope to improve the toughness of sapphire is to see if you can heat treat it until some residual compressive stresses are left in the inner and outer faces of the sapphire cover, which would suppress crack formation a bit. But I'm not familiar with sapphire tempering treatments. (Again, unlike many glasses.)
The study isn't necessarily to confirm this happens, since I think there's enough news stories of idiots walking into fountains and traffic while texting to confirm it.
Rather, this is exactly the sort of study a health insurance company would love to have.
"In order to qualify for our lowest premiums, you need to go to a clinic for a series of blood and urine tests to confirm that you don't use alcohol or other drugs; are not pregnant (or planning to have a kid, rug rats are expensive to us, so we'll give you a further discount if you get sterilized); and don't have cancer.
"Further, you'll need to install this app on your phone, which utilizes the phone's accelerometer to determine if you're walking and texting, which is known to increase the rate of traumatic injuries over the normal population by 11.2%. Medical bills incurred from texting and walking will result in an increase in premiums."
Lawyers could also exploit this information for plaintiffs and the defense.
"These heartless corporations were only concerned for their shareholders when they put an addictive texting device in my client's hands despite scientific evidence that texting and walking leads to an 11.2% increase in traumatic injuries. They are solely responsible for him walking into traffic because of the distraction of texting..."
"The plaintiff's claims that Google, Verizon, and Samsung are responsible for him walking into traffic while texting are baseless, as it is well-established in scientific studies available to the public that one should not text while walking. Any reasonable adult should know to walk and text responsibly."
"In these days of austerity ans in many places (including the UK and USA) abject poverty, why is so much money still being spent on space exploration?"
Anyone who poses this question has never compared a government's space spending with its social welfare spending. A couple of examples:
The US spends proportionally (and absolutely) the most money on space exploration, yet the federal government spends about 120x more on social services than space exploration. In 2012, its federal budget had $17.7 billion for NASA versus $2,041.9 billion for the budgetary sum of: the Social Security Administration; Department of Health & Human Services; Department of Education; Department of Veterans Affairs; Department of Housing; and the Department of Labor. (This does not include US state government spending on social services, or space spending by various military branches, which are relatively small compared to the above budgets.)
The UK spent $379.5 million in 2010 on the UK Space Agency, plus $300 million for the ESA. In comparison, its 2010 social services budget (the sum of pensions, health care, education, and welfare) was $540 billion, so the ratio of social services to space spending was 795:1.
One of the implications from these numbers is that when governments are spending $X bajillion dollars on welfare and there's still poverty, increasing anti-poverty budgets by 0.1% to 1% is probably not going to miraculously fix matters.
On the other hand, there's still utility in exploration and scientific research. Even if a rock-tasting robot on Mars doesn't yield any life-changing data, the engineering that went into it usually finds a way into improving our lives. The World Wide Web was developed to help physics research centers like CERN share data more easily; I don't know if CERN's research in the early 1990s revolutionized the world (examples welcome, of course), but the WWW has had a huge impact.
It's interesting how definitions of "failure" has changed over aircraft generations. Compare the Comet and the DC-10 to the 787. The DC-10 liked to blow out a cargo door and cut hydraulic lines. The Comet liked to disintegrate from fatigue failures. The 787 has a replaceable battery problem. The number of deaths from design flaws in modern airliners is a lot lower than 20th Century airliners, especially the airliners developed in the 1950s to 1970s.
The body count paid by those earlier generations of aircraft means that today the media has to turn a smoking battery into a moment of drama (because the wars around the world got boring) rather than explosive decompression or turbine disk failures sending hundreds of people screaming to their deaths.
The 787 might get tarred and feathered like the Comet, but only because standards have risen so much.
I was thinking sort of the same sort of thing. I grew up with 9 planets and a few thousand minor objects in the solar system. It was thrilling and revolutionary to finally get a close-up of a comet with the Halley Armada in 1986, and equally exciting to finally get a close-up of an asteroid with Galileo's Gaspra flyby. The Voyager flights to Uranus and Neptune also put pictures to specks of light.
Now, in the last decade, discovery rates of solar system minor objects has been in the tens of thousands per year, though one planet disappeared due to a paperwork shuffle when Pluto gained a bunch of cousins. Exoplanets were rumors and always disproven before confirming some planetary corpses around a neutron star in 1992 and a brown dwarf in 1989, and then just a few more exoplanets were confirmed in the later 1990s. Suddenly: about 1000 confirmed exoplanets in the last decade, plus direct imaging.
20 years ago, I figured I'd never see an exoplanet in a life zone in my lifetime. Now, it doesn't sound too far-fetched to read about scientists directly analyzing the atmospheric composition, temperature, and gravity of exoplanets.
Sure, but transparency is a relatively easy quality to provide once you get beyond problems of corrosion (which tends to impair transparency) and long-term data stability. There are plenty of transparent oxides. Like, window glass, and many other glasses. Fused quartz. Sapphire. SiAlON. Most oxide minerals have transparent forms when they're pure and single crystal.
I guess diamond is a pretty stable transparent, durable material, too, though you'll want to avoid excessive heat or fire. The oxide forms of carbon are rather less useful than diamond for long term data storage. ;)
Interesting to see silicon nitride described as having "high fracture resistance." I suppose if you're comparing it to other ceramics and glasses, it is fairly tough, but silicon nitride is a brittle material compared to most metallic alloys.
I'd like to see accelerated corrosion test results for this material (humid atmosphere, not salt fog). Tungsten isn't in the same category as platinum or most stainless steels for corrosion resistance. Silicon nitride is fairly inert but when you're aiming for a billion years in an oxidizing environment, nitrides will yield to oxygen. You might be better off starting with an oxide matrix (though they're admittedly more brittle than even silicon nitride.)
"I don't know off hand what isotope its RTG is using"
Plutonium-238. It's been a long time since NASA tried any other RTG or radioscopic heater material. Its standard RTGs and heating elements are entirely built around Pu238.
"but I'm going to guess their design lifetime is based on wear and tear and mechanical failure, not the power supply."
Yep. The Spirit and Opportunity Rovers suffered increasing mechanical failures in their arms and drive trains. Spirit eventually got stuck in soft sand at a non-optimal angle for solar charging and froze up over the winter, which is arguably a power supply failure, but by that point it had two dead wheels.
"The problem with that is there is NO such thing as "the usual battery" when it comes to powering a whole house for say 16 hours at a time" ... "Current tech is ... too bulky (again, the lead-acid situation). "
Lead-acid batteries have a long history of working with home solar and wind systems (and they're popular on sail boats, too). And while lead acid batteries heavy, they're not bulky for the kilowatt-hours they store. A battery box about the size of a standard server rack could easily store 30 to 40kWh of lead-acid batteries, and house the switching gear for a whole-home UPS / solar charging system. The following link is a typical selection of lead-acid batteries for solar systems. You can Google up plenty of other lead-acid batteries for renewable home power storage with searches like: solar lead acid; solar deep cycle battery; solar battery box; etc. There are lots of vendors out there.
http://www.sunwize.com/documents/sunwize_solarready-nomod_8-08.pdf
A 26kWh battery box, as offered by Sunwize, will deliver 1.625kW continuously over 16 hours. Homes are typically approximated as using 1kW on average, so that's enough to cover 16 hours of darkness. (Despite the name, I'm sure Sunwize's switch and charging system will work fine with home wind turbines, too, so you don't necessarily stop battery charging when it gets dark and stormy.) Just make sure your foundations are up to the task because the lead-acid rack will put nearly a ton on a small 58" x 15" footprint.
"So why make an exploration machine with wheels if they are fundamentally less useful over uneven land?"
Wheels are highly reliable compared to tracks, and highly reliable and very proven compared to robotic legs. They might not have the all-terrain capability of tracks and legs, but obstacles are avoidable whereas hardware failures are harder to fix on Mars. With advanced imaging of the target zone (e.g., Mars Reconnaissance Orbiter), you can avoid the largest obstacles to wheels.
"L2 is stable like a pin balanced on its point is stable; ie. not very much."
L2 is a pretty broad pin. The annual delta-V consumption to maintain an L2 halo orbit is 30 - 100m/s, which compares well with stationkeeping in geostationary orbit (50-55m/s), low Earth orbit (25m/s for a 400km orbit like the ISS), or low lunar orbit (0 to 400m/s). There are a lot of gravitational influences out there to make any orbit imperfectly stable, and sometimes other effects (e.g., low Earth orbit's atmospheric drag.)
"I'm also curious why L2?"
As a sensitive observatory, Gaia is going to L2 to get away from Earth's heat, light, radio noise, and view obstruction.
Heat: Gaia is depending on passively cooling down quite a bit (hence that sunshield parasol), and being in low Earth orbit would bring a lot of infrared heat from multiple angles (since Earth is a big disk to objects in LEO, not a point source). While an astronaut wouldn't feel toasty from Earth's radiating heat, it's enough to make passive storage of mild cryogenic liquids (e.g., liquid oxygen) difficult, or to passive cool an astronomical satellite's instruments. Moving to L2 reduces the cooling problem to the sun, which is a point source easily hidden behind a modest sunshield. (Even heating from zodiacal light can be a problem for REALLY sensitive instruments or passive liquid hydrogen storage, though I don't think Gaia cares.)
Light: glare off Earth is another bother for optical satellites like Gaia. Moving to L2 reduces Earth to (almost) a point source.
Obstruction and orbital problems: As noted above, Earth is a big object to the perspective of a satellite in LEO, blocking out more than a third of the sky. Short orbital periods means satellites are constantly having their views altered, and tidal effects can stabilize (or tumble) satellites in undesired ways. Gaia might find it annoying to be trying to aim at Ceti Alpha 6 when Earth is like, "LOL! Nope, time for gravient gradient stabilization!"
Noise and other disturbances: There's a lot of radio chatter and time spent dodging orbital debris in low Earth orbit.
Higher Earth orbits present radiation issues from the Van Allen Belts or, in the case of geostationary orbits, a lot of radio chatter and crowded traffic lanes. Geostationary is cluttered to about its limit before communication satellites cause radio cross-talk and interference problems.
So: L2. There's no traffic to speak of, no radio interference, no brutish Earth blocking views, no debris to dodge, much simpler tidal / gravity issues, and vast, empty vistas for Gaia to work with.
Since Gaia's at L2, it'll need fuel-consuming course corrections to remain in a halo orbit there. Gaia also uses nitrogen for aiming rather than reaction wheels, which have an infamous habit of wearing out at awkward times. The amount of propellant and nitrogen is limited. How those limits compare to 5-year periods, I don't know.
The US abandoned the idea of nuking the moon in the 1950s. Today, there's far too much dollar value in space hardware in cislunar space to be scrambled by the x-ray pulse of a vacuum nuclear detonation to consider it, unless a Texas-sized alien spaceship needs to be disposed of by Jeff Goldblum and Will Smith.
No, it means the four arms stand out for having younger, high-mass stars. I think. As noted in the article, the longer-lived, smaller stars have time to smear out of the arms and spread into a disk. The so-called arms and gaps of a galaxy are distinctive for the presence and absence (respectively) of gas and short-lived stars. I think.
There's a point where spacecraft just wear out and are no longer usable, and are no longer safely controllable. (And there's a weird fear that Earth germs might contaminate Saturn's moons. It'd stink to get a sampling probe to, say, Enceladus and find thriving microbes...only to realize they're E. Coli from some Cassini technician's unwashed hands.) As you suggested, Lincard1000, Cassini is being parked for a few years in an equatorial orbit around Saturn, but after that the last bit of information is gathered, it's being dumped into Saturn to avoid an accidental collision with Saturn's moons.
I like the nuclear idea, but the energy usage for travel to and from Titan doesn't necessarily impact the energy obtained from Titanian methane. Small payloads of workers and machinery to start the extraction process wouldn't use as much energy as could be extracted from Titanian methane. Meanwhile, launching that methane back to Earth can be done with energy sources not involving the methane or even much in the way of other terrestrial energy sources. For example, launching methane off Titan might be done with an electric mass driver (using a few tons of terrestrial uranium), while the subsequent flight and navigation to Earth could be done with solar sails and gravitational assists, if you're patient. Those energy sources don't really draw on terrestrial energy sources much.
The problems for exporting Titanian methane to Earth are, in my opinion, two-fold:
First, it's a lot of work and expensive equipment to obtain it. Second, there are cheaper ways of making more methane on Earth, which isn't short of carbon or hydrogen. Even if it's energy-intensive to combine, say, terrestrial coal and water or carbon dioxide and water into methane, it's still probably cheaper than going all the way to Titan for methane. You'll probably get richer building a solar power satellite or nuclear reactor to make methane on Earth than to get it from Titan.
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