Wikipedia:Reference desk/Archives/Science/2015 December 23

= December 23 =

Falcon 9 Flight 20 and why it is historical
I keep hearing news reports about how Falcon 9 Flight 20 is so ground breaking for landing back on the Earth and being reusable and such. But I thought that was already done with Jeff Bezos' Blue Origin company. So, what's the difference here that I'm not seeing? Why was the Falcon landing so much better? Dismas |(talk) 01:58, 23 December 2015 (UTC)
 * IIRC, the Blue Origin launch was not an orbital space launch. Falcon 9 was. Blue Origin was proof of concept, Falcon 9 was the real deal. --OuroborosCobra (talk) 02:00, 23 December 2015 (UTC)


 * Blue Origin's craft reached 100km altitude, then returned to a soft landing. Falcon 9 can reach low-earth-orbit - which is at least 160km altitude.


 * That doesn't sound like that big of a difference until you understand that Blue Origin's mission is to propel a lightweight re-entry vehicle "into space" and to come straight back down again...Falcon 9 can launch 13 tonnes of payload and add the necessary 8 km/second of horizontal speed to maintain a stable orbit. 13 tonnes is about half of the low-earth-orbit payload of the space shuttle at a tiny fraction of the cost and a tiny fraction of the turn-around time and without needing a runway, parachutes and all that stuff - which is an incredibly impressive feat.
 * SteveBaker (talk) 05:43, 23 December 2015 (UTC)

Thank you, both. Dismas |(talk) 18:40, 23 December 2015 (UTC)
 * Note that the first stage of the Falcon rocket that successfully landed was never in orbit - so it technically is a sub-orbital flight (that section goes up, does a flip, lands near where it launched). Blue Origin is a rocket with a completely sub-orbital flight (which has so far successfully landed the capsule but not the first stage). Rmhermen (talk) 23:09, 23 December 2015 (UTC)
 * You might not have heard the more recent news. Blue's flight back on 29 April 2015 landed the capsule and lost the booster, but just a month ago on 23 November 2015 they landed both capsule and booster successfully.  Video of the second flight is here.  That particular video was a little too slick for my taste.  I much preferred the production style used in the video of their April flight here.  (No CGI, less slick, feels more real but still with a strong emotional appeal.)  This raw video and audio from the April flight is also an interesting watch. -- ToE 01:10, 24 December 2015 (UTC)


 * Popular Mechanics's Two Quick Illustrations to See How Badly SpaceX Beat Blue Origin illustrates some differences by comparing trajectories and vehicle sizes. -- ToE 01:14, 24 December 2015 (UTC) Edit: Here is another vehicle size comparison image showing the 48 m F9 booster and the 16 m BO booster standing on either side of the 46 m Statue of Liberty.


 * By the way, can someone find us a decent hi-res free photo of the Blue Origin rocket? "Even more like a penis than the Falcon 9" ... :) Wnt (talk) 12:54, 26 December 2015 (UTC)

No large moons for gas giants and ice giants ?
In our solar system, none of the large planets have moons which are large, compared with themselves (although they may be "large" when their absolute size is compared with moons on the much smaller terrestrial planets and dwarf planets).

1) So, is this just random luck, or is there some inherent reason why large planets can't have moons which are a significant portion of their own size (which would probably make the moon also a gas or ice giant) ?

2) Would we able to detect this situation, if it existed, when we find exoplanets ? StuRat (talk) 16:50, 23 December 2015 (UTC)


 * I think that the real issue is that moons that are "large" compared with their primary are unusual. In our system, we basically are aware of one that belongs to a rocky planet, and that is explained by the giant impact hypothesis.  We previously thought that another planet, Pluto, had a large moon, Charon, but it has been downgraded to a dwarf planet, and there are other systems of dwarf planets that are essentially double dwarf planets.  So the real issue isn't about large moons of large planets, but about a large moon of a rocky planet.  Robert McClenon (talk) 17:03, 23 December 2015 (UTC)


 * Well, we have 4 large planets and 4 small planets (excluding Pluto). So, even there 1/4 small planets in our solar system has a large moon, compared to 0/4 large planets.  And if we include dwarf planets, some of those have large moons, too (not sure if we could detect them all).  So, it's an admittedly small sample size, but it's still enough to justify asking if there is a theoretical reason why large planets can't have relatively large moons. StuRat (talk) 17:12, 23 December 2015 (UTC)


 * The term would be double planet for planet pairs that are similar in size. It seems theoretically possible, and people have even discussed ways to look for them .  Searching for such objects is probably only just getting started.  This  suggests that gas giants paris might form in ~10% of planetary systems with gas giants.  Dragons flight (talk) 17:35, 23 December 2015 (UTC)


 * I believe the distinction between a "double-planet" and a "planet+moon" relationship is that the center of rotation of the two bodies is INSIDE one of the bodies if it's a planet+moon setup and BETWEEN the two bodies if it's a double-planet.  This means that density is an important factor in deciding which of those you have. SteveBaker (talk) 21:36, 23 December 2015 (UTC)


 * That would be the barycenter. I don't know if I agree with that def, though, as a small, distant planet co-orbiting with a black hole might then qualify, since the event horizon on the black hole is small, and the wide distance would move the barycenter beyond it. StuRat (talk) 22:10, 23 December 2015 (UTC)


 * Isn't Titan (moon) considered a large moon? Yes, it is smaller in relation to Saturn but it is larger than the Earth's moon - Unless I read something wrong. Just curious. Thanks for any replies. MarnetteD&#124;Talk 22:17, 23 December 2015 (UTC)


 * That's precisely why I included the note "(although they may be "large" when their absolute size is compared with moons on the much smaller terrestrial planets and dwarf planets)." I am only looking for moons which are large relative to their own planet. StuRat (talk) 05:54, 24 December 2015 (UTC)


 * Probably the same reason there is no planet close to the size of the sun. Binary is stable but if the center of mass of the three-body problem causes instabiltiy, it gets rectified.  It's likely a matter of time relative to size.  Two Jupiter size masses orbiting the sun as an orbiting pair would not be stable for a long period.  To use terms above, the barycenter dominates te 3-body problem for a long time and is effectively one mass.  When the barycenter isn't dominated by a single body, it deteriorates into a much less stable problem.  --DHeyward (talk) 07:05, 24 December 2015 (UTC)


 * The research paper I gave above says that orbiting binary planets at least 0.3 AU from a central sun-like star and at least 4 diameters apart from each other will be stable for longer than 10 billion years.  That's functionally stable for the life of star.  Dragons flight (talk) 12:24, 24 December 2015 (UTC)


 * If a planet was close to the size of the Sun it would be a star. Possibly a majority of stars are part of multiple star systems, so such a situation is not rare. My personal speculation is large planets like gas giants tend to pick up everything along their orbital path, which makes it difficult for two large planets to form near each other. Even smaller planets with nearby orbits seem to be prone to instability, as is thought to have happened with Earth and Theia. Following on from that, I wonder if interactions with other solar system bodies would destabilize things (which is thought to have been a factor in the giant impact hypothesis). Some scientists think the solar system may have had a fifth gas giant that got kicked out of the system early on due to perturbations from other planets. --71.119.131.184 (talk) 15:04, 24 December 2015 (UTC)


 * They are small as compared to their parent planets because they were formed (accreted) from the material that was left after the central planet formation. There were relatively little material left and the accretion process is also quite inefficient. Ruslik_ Zero 12:49, 24 December 2015 (UTC)


 * 4 diameters apart doesn't seem like much to ask. Are any moons in the solar system within 4 diameters of their planet ? StuRat (talk) 12:47, 24 December 2015 (UTC)


 * Io and the four inner moons of Jupiter, Dione and 3 other large and 14 other small moons of Saturn. Dragons flight (talk) 12:58, 24 December 2015 (UTC)

I'm not convinced that double planet is the correct term and our article may be misnamed. The term more usually used is binary planet, including in the two sources you offered as examples. The more usual meaning of double planet is simply an exosystem with two planets, of which several have been discovered. SpinningSpark 19:27, 24 December 2015 (UTC)


 * The barycenter of the Solar System lies outside Sol, given the mass of Jupiter. Yet Jupiter is not a star, nor a brown dwarf.  The barycenter of the Terra-Luna system lies 1000 miles below the surface of the Earth, so our planet and its moon are not a binary system.  But the Pluto-Charon system is. μηδείς (talk) 03:32, 25 December 2015 (UTC)
 * Well I don't know how to go about defining a binary system, there does not seem to be an official definition, but if you are trying to claim that double planet and binary planet are different things, then that is still at odds with our article which treats the terms as synonyms. SpinningSpark 16:55, 25 December 2015 (UTC)
 * I have no opinion on preferred terminology, I am just pointing out the interesting fact that the Pluto-Charon and the Sol-Jupiter systems orbit a barycenter between the two objects, and in this sense are unique in our star system, as far as i am aware. μηδείς (talk) 19:21, 25 December 2015 (UTC)

U.K. Rail
Watching a doc about railways in the uk, what's the difference between a mobile operations manager and section manager? They seem to do the same thing. — Preceding unsigned comment added by 2A02:C7D:B91D:CC00:603B:6322:7D0A:CFBA (talk) 17:26, 23 December 2015 (UTC)
 * A Section Manager is an administrative position (see this job description from Network Rail). They're responsible for, well, managing a section - for ensuring that all the work is done properly, and the contractors are competent and paid on time.  An MOM, on the other hand, would have been called a "foreman" in the old days - they're responsible for leading one of the small groups of people who actually do the work, and are expected to do some of it themselves.  See this forum thread. Tevildo (talk) 18:55, 23 December 2015 (UTC)
 * But on the doc it showed section managers doing maintenance work with their "gang". Are MOMs part of the section managers "gang" then? 2A02:C7D:B91D:CC00:603B:6322:7D0A:CFBA (talk) 22:40, 23 December 2015 (UTC)
 * Alao just found this . It seems to imply that MOMs respond to incidents more than do maintenance? I'm really confused now. 2A02:C7D:B91D:CC00:603B:6322:7D0A:CFBA (talk) 22:52, 23 December 2015 (UTC)
 * Yes, the essential part of the MOM's job title is "mobile". They have to be on-call and ready to respond to incidents requiring immediate attention, rather than being involved with routine planned maintenance.  The MOM will report to the Operations Manager (responsible for the day-to-day running of the railway), not the Section Manager.  The essential part of the Section Manager's job title is "manager".  They'll do quite a bit of on-site supervision of their gangs, and (especially if there are cameras present) may very well assist with the work, but they're not personally responsible for doing the work; rather, they're responsible for getting it done.  A Section Manager will earn considerably more than an MOM. Tevildo (talk) 23:24, 23 December 2015 (UTC)

I see. Thanks. That makes a bit more sense. Also the doc mentioned the many challenges the railway faces in the uk and how these cause delays, whether it's heavy rain, windy conditions, snow & ice. This is all understandable but how comes other countries can run their railways better with less problems and at a lower price too, Germany and Japan for example. Is it because our rail network is much older and wasn't designed to cope with today's demands? And why are our rail fares so expensive? Is this politics? 2A02:C7D:B91D:CC00:603B:6322:7D0A:CFBA (talk) 01:09, 24 December 2015 (UTC)
 * See History of rail transport in Great Britain 1948–1994 and Impact of the privatisation of British Rail for our relevant articles. I can also recommend Eleven Minutes Late by Matthew Engel. Tevildo (talk) 08:55, 24 December 2015 (UTC)

Also, wouldn't the section managers gang also do reactive work? 2A02:C7D:B91D:CC00:603B:6322:7D0A:CFBA (talk) 01:21, 24 December 2015 (UTC)
 * Yes, definitely, and a major part of the section manager's job will be to organize this work, and plan things so as to have adequate capacity for both routine and reactive maintenance. Tevildo (talk) 08:55, 24 December 2015 (UTC)

Birth control v. MTF hormone replacement therapy v. anti-aging hormone replacement therapy
A lot of these regimens basically involve an estrogen and a progestin (which can sometimes serve the dual purpose as an antiandrogen) but for a while I've been puzzled at why bioidentical hormones are only emphasized for transwomen, and the risks of birth control and anti-aging HRT often mentioned to transwomen are based off of studies that use ethinyl estradiol or Premarin. It kind of puzzles me why bioidentical hormones aren't marketed to ciswomen -- is it because ciswomen value "convenience" more than transwomen? I suppose bioidentical hormones can't be packaged in an IUD, so they would either have to be given as suppositories, sublingual tablets or injections instead. Yanping Nora Soong (talk) 20:41, 23 December 2015 (UTC)


 * I don't have a very good understanding of this topic, but seems a good start.  My feeling is that it is actually impossible to understand medical practice without looking deeply into what patents are held and successfully enforced by whom, because the soul of medicine is a mafia shakedown.  However, there are some more specific aspects like the endometrial overgrowth mentioned in the source above that should definitely give a person pause. Wnt (talk) 12:45, 26 December 2015 (UTC)