Activity for HDE 226868
| Type | On... | Excerpt | Status | Date |
|---|---|---|---|---|
| Answer | — |
A: What is the highest pressure humans can live in in comfort? The change in pressure below the ocean is approximately $$\frac{dp}{dz}=\frac{1\text{ atm}}{10\text{ m}}$$ where $z$ is measured from the surface of the water downwards. We also have to add in the one atmosphere of pressure from the atmosphere, meaning that the pressure is really $$p(z)=1+\frac{1}{10... (more) |
— | almost 8 years ago |
| Answer | — |
A: Is an interstellar clock orbitally possible? Your main problem is that the planets will comprise a chaotic system. This is why the stability of the Solar System is so difficult (possibly impossible) to determine. Our best models are valid for perhaps $\sim10^8$ years - at best (see Laskar et al. (2004)). This is the Lyapunov time, over which or... (more) |
— | almost 8 years ago |
| Answer | — |
A: Could a carbon/silicon/oxygen planet exist? Models of such planets exist, courtesy of Unterborn et al. (2014). They discuss planets of similar composition to yours - looking at the value of the mass fraction given by $(\text{Mg}+2\text{Si}+\text{Fe}+2\text{C})/\text{O}$. A model planet like yours that would orbit 94 Ceti (HD 19994) would have ... (more) |
— | almost 8 years ago |
| Answer | — |
A: What would it be like to live on a rapidly rotating planet? When will the planet break apart? A solid body will break up if the centrifugal force at a point is equal to the gravitational force. At Earth's equator, the equation becomes $$\frac{GM{\oplus}}{Re^2}=\omega^2Re$$ where $M{\oplus}$ is the mass of Earth, $Re$ is its equatorial radius, and $\omega$ is... (more) |
— | almost 8 years ago |
| Answer | — |
A: Ugh, I'm stuck in an orbital spaceport. But why? I came up with some basic ideas before posting the question: Space weather Radiation problems can severely damage a spaceship's instruments, making navigation and flight severely dangerous or impossible. Additionally, impacts from micrometeroids/debris can damage the ship, even if protections like ... (more) |
— | almost 8 years ago |
| Question | — |
Ugh, I'm stuck in an orbital spaceport. But why? This is, for the record, the same universe as in Is space piracy orbitally practical? and How can I prevent Kessler Syndrome among space stations?. In my world, circa 2100, space stations orbiting Earth, Venus and Mars provide transit hubs for the inner Solar System. Small shuttles take people to Lo... (more) |
— | almost 8 years ago |
| Answer | — |
A: How long would human immunity last in space? I'd suggest that they may lose at least partial immunity to a decent extent within a decade or two of leaving Earth. According to Smith et al. (2014) (explained for us laypeople here), the number of disease outbreaks has been rapidly increasing over the last few decades: Specifically, there's bee... (more) |
— | almost 8 years ago |
| Answer | — |
A: What could make a star green? Give it a circumstellar cloud of oxygen. Some planetary nebulae, such as NGC 6826, appear green because of ionized oxygen. Image in the public domain. Yes, this is a true-color image. I see no reason why you couldn't surround the star with an extremely dense cloud of hydrogen, containing a relat... (more) |
— | almost 8 years ago |
| Answer | — |
A: How fast can heat be carried away from a small source? Let's imagine that the hottest part of the apparatus is the barrel, so to speak, which is cylindrical with length $l$, radius $r$ and temperature $Tb$. We can surround it with a fluid of temperature $Tf$. Now, the barrel has a heat transfer coefficient of $h$. The change in heat energy of the barrel ... (more) |
— | almost 8 years ago |
| Answer | — |
A: How would Newton's Laws and the rest of physics be changed on a non-inertial frame? They could still tell the difference. A non-inertial frame involves acceleration of some sort, i.e. non-uniform velocity. Such a frame can certainly have uniform speed, but the direction of motion of an object in the frame will be constantly changing. For instance, an observer on a merry-go-round is... (more) |
— | almost 8 years ago |
| Question | — |
How can I prevent Kessler Syndrome among space stations? In a world I'm building - the same one as in Is space piracy orbitally practical? - there are many space stations in low Earth orbit, used as orbital spaceports, each about ten times the size of the ISS. Small shuttles bring in passengers and crew from Earth, and larger craft, docked at the stations,... (more) |
— | almost 8 years ago |
| Answer | — |
A: Natural subterranean cave formations on Mars Apparently, this was investigated by NASA in the early 2000s in what became known as The Caves of Mars Project. Its goal was to find possible places for humans to live over extended periods of time, safe from the weather and radiation. Boston et al. (2004) wrote a summary of the results: There is ... (more) |
— | almost 8 years ago |
| Answer | — |
A: How does one calculate the tidal heating of a satellite? Wikipedia gives the formula for the tidal heating $\dot{E}$ as $$\dot{E}=-\text{Im}(k2)\frac{21}{2}\frac{R^5n^5e^2}{G}\tag{1}$$ where $R$ is the radius of the satellite, $n$ is something weird called its mean orbital motion, and $e$ is the eccentricity of its orbit. I actually don't like this represe... (more) |
— | almost 8 years ago |
| Answer | — |
A: Would Earth's extraterrestrial colonies have a higher average intelligence? Note: This answer assumes that those going to Mars have a high IQ (which I believe the OP intended), and that IQ is indeed a fairly good measure of intelligence. Yes. I think the question of the most important factor, though, boils down to nature versus nurture: Will these individuals be smarter ... (more) |
— | almost 8 years ago |
| Answer | — |
A: Accuracy of timekeeping based on the age of Earth Try radiometric dating. Accuracy: $\pm0.11\%$. Radiometric dating - see also the excellent USGS page - uses the decay of radioactive isotopes of elements to determine the age of a sample. It works as follows: Assume that you have a sample of $N0$ atoms of element $A$ at time $t=0$, and that no ato... (more) |
— | about 8 years ago |
| Answer | — |
A: Gravity on a hollow non-enclosed world In general, the problem with hollow-Earth setups is that objects in hydrostatic equilibrium cannot be hollow - and planets must be in hydrostatic equilibrium. Planets form through collisions of smaller pieces of rock and dust, and eventually accrete enough matter to become substantially large. There ... (more) |
— | about 8 years ago |
| Question | — |
Is space piracy orbitally practical? I'm considering a story set about fifty years (2065) in the future. It takes a rather optimistic view; space exploration has led to colonies on the Moon and Mars, as well as several space stations orbiting Venus for scientific purposes. There are currently several exploratory missions planned or in p... (more) |
— | about 8 years ago |
| Answer | — |
A: Is it worth sending a manned mission to a black hole? What could be learned? There are so many things you could study by looking at a black hole. There are lots of open or partially unsolved problems that surround them: Does Hawking radiation exist? While it has been predicted theoretically, direct evidence is lacking - well, nonexistent. Stellar-mas... (more) |
— | about 8 years ago |
| Answer | — |
A: How small in diameter a planet can be while retaining most of Earth's properties? Surface gravity Surface gravity is really the most important quantity when it comes to determining many of your planet's properties. It can be used to constrain atmospheric composition, planetary mass and radius, composition, and more. A planet with mass $M$ and radius $R$ has a surface gravity of ... (more) |
— | about 8 years ago |
| Answer | — |
A: How can I explain space travel being accepted and supported in a typical fantasy setting? Make it a ruse - an offering to the gods. I'm reminded, strangely, of the Minotaur. Every seven or nine years (accounts differ), seven boys and seven girls were sent from Athens to Crete in tribute to King Minos, and were promptly devoured by the beast, until Theseus defeated it. From the perspecti... (more) |
— | about 8 years ago |
| Answer | — |
A: How to explain lack of flatland? A recent Snowball Earth period might help. Heavy glaciation can cause enormous changes to the terrain. There are no longer any glaciers where I live, but there once were, during a recent ice age. Their legacy includes Glacial erratics - boulders dropped in the middle of nowhere Glacial valleys (sh... (more) |
— | about 8 years ago |
| Answer | — |
A: Is there a problem with interpreting dark matter as hidden dimensions? This has been proposed, believe it or not. Gravity is the weakest of the four fundamental forces, a mystery which has been dubbed the hierarchy problem. Several solutions have been floating around; one is that there are large extra dimensions through which gravity propagates. Gravity behaves accordin... (more) |
— | about 8 years ago |
| Answer | — |
A: What direction is my asteroid coming from? To properly determine the orbital path of the asteroid, you'd need to do a numerical simulation (the three-body problem here likely not having an analytical solution). There are certainly $n$-body tools in place, although you would have to modify them a little bit to use them (you could also build yo... (more) |
— | about 8 years ago |
| Answer | — |
A: Would ripping the core from a Sun-like star cause it to explode? Let's think about why a supernova happens in a massive star. You probably know that after a star develops an iron core, further nuclear fusion is not possible on a large scale. Yes, you can produce heavier elements via neutron capture, which indeed happens during supernovae (via the r-process) and in... (more) |
— | about 8 years ago |
| Answer | — |
A: Where on a tidally locked planet with a 25 °C maximum is the 0 °C isotherm? This is just a basic answer, but I used Samuel et al. (2014), who in turn cited the climate model of Léger et al. (2011) for a tidally-locked planet with no atmosphere. They give the formula for surface temperature as $$Ts=\left(\frac{\epsilon5}{\epsilon2}\right)^{\frac{1}{4}}\left(\frac{R}{a}\right... (more) |
— | about 8 years ago |
| Answer | — |
A: Creating realistic world - star cluster? Let's determine some basic parameters of the cluster. You say you want the stars to be 1-2 light-years apart. This is a little tricky because star clusters don't necessarily have uniform densities. Globular clusters, for instance, have radial density profiles decreasing from some core density at $R=... (more) |
— | about 8 years ago |
| Answer | — |
A: Large Thick but Hollow Planet with small inner planet Sefa's answer is correct. Due to Newton's shell theorem, the force on any object inside (and due to) a spherically symmetric object of uniform mass density of exactly zero. The exact integration (see e.g. Wikipedia or these notes) is messy, but it indeed relies on the density of the shell, $\rho$. N... (more) |
— | about 8 years ago |
| Answer | — |
A: Is atmospheric composition based on water content? In a young planet, oxygen can be produced from water vapor via photodissociation, which occurs along a pathway like $$\text{H}2\text{O}+h\nu\to\text{H}+\text{OH}$$ $$\text{H}+\text{OH}+h\nu\to2\text{H}2+\text{O}$$ $$\text{O}+\text{O}\to\text{O}2$$ In The Atmosphere and Ocean: A Physical Introduction,... (more) |
— | about 8 years ago |
| Answer | — |
A: How can I Determine the Caloric Intake of a Species? We can use something called Kleiber's law (which I originally found here). It states that a power law describes the relationship between metabolic rate $q$ and mass of an animal $M$ (in kilograms): $$q\propto M^{3/4}$$ You can then use this to determine the number of calories an animal consumes over ... (more) |
— | about 8 years ago |
| Answer | — |
A: How to "set" the lightsail? First, let's look at the different types of trajectories a solar sail can take. They differ mainly based on something called the lightness number, $\beta$, which depends on the composition and structure of the sail. $\beta$ can be used to determine the type of trajectory the solar sail will follow: $... (more) |
— | about 8 years ago |
| Answer | — |
A: Could complex life evolve after planetary catastrophe in just 120 million years? It seems like you've generated a chthonian planet, although typically, they're theorized to form when normal solar winds from a star slowly strip away the atmosphere, not the release of a star's outer shells as it evolves to become a white dwarf. For instance, Hébrard et al. (2003) estimated that HD... (more) |
— | over 8 years ago |
| Answer | — |
A: How many plants does it take to breathe? 13 square meters BIOS-3, a sealed, underground compound designed to mimic a spacecraft, managed to generate oxygen for humans using algae. Its specifics are outlined in an article by Salisbury et al.: A crew of up to three people 315 cubic meters of living space (14 m by 9 m by 2.5 m) 3 phytotrons... (more) |
— | over 8 years ago |
| Answer | — |
A: Could a solar system with large amounts of dust and debris exist? Give it a protoplanetary disk: Image in the public domain. Protoplanetary disks are circumstellar disks that form early on in the life of a planetary system, from the original protoplanetary nebula around the star. They can survive for over 10-20 million years (see Mamajek et al. (2009)), meaning... (more) |
— | over 8 years ago |
| Answer | — |
A: Is homicidal smiley gas possible? My chief reference here is Appendino et al. (2009), though this possibility was first brought to my attention by this National Geographic article. The Joker's real-world substance of choice? Oenanthe crocata, a type of water dropwort, later infamously dubbed the "sardonic herb". Ancient Sardinians a... (more) |
— | over 8 years ago |
| Answer | — |
A: Orbital period of a tidally-locked Earth-like planet around a red dwarf I like to use Kasting et al. (1993) when talking about better climate models of planets around main sequence stars, as it evaluates habitable zones in while taking into account atmospheric effects. Anyway, the authors address your specific question, calculating the tidal locking radius $r{\tau}$ afte... (more) |
— | over 8 years ago |
| Answer | — |
A: An animal that puts its prey to sleep Check out the cone snail. Cone snails like to stupefy their victims (see also Safavi-Hemami et al. (2014)) by either Using an extremely fast "harpoon" with venom to stab their prey. Releasing a small cloud of dense insulin to daze their prey, which is used for slower-moving animals. The cloud may ... (more) |
— | over 8 years ago |
| Answer | — |
A: Are space stations like Sevastopol (Alien Isolation) realistic? The major problem with building a space station like a city is that you can't really make it anything like Earth: O'Neill cylinders are big enough that they can have their own weather systems. The only way you can do this in a city-like space station is to create a giant dome encasing the entire th... (more) |
— | over 8 years ago |
| Answer | — |
A: About how many planets are plausible for this binary-star system? Let's start with the easiest of your questions: Are any multi-planetary binary-star systems known? There are, in fact, quite a few: 55 Cancri: Five planets orbit 55 Cancri A (a G-type yellow dwarf). Four are gas giants, three of which orbit inside 1 AU, while the fifth is a super-Earth orbiti... (more) |
— | over 8 years ago |
| Answer | — |
A: How Close Are These Two Planets? Using Kepler's third law, I get $$a=\sqrt[3]{\frac{P^2G\times(M+M)}{4\pi^2}}=5.32\times10^4\text{ kilometers}$$ where $a$ is the semi-major axis, $P$ is the time it takes the planets to orbit each other, $G$ is the universal gravitational constant, and $M$ is the mass of one of the planets - one Eart... (more) |
— | over 8 years ago |
| Question | — |
Are tropical fjords possible? I'm currently working on two connected continents for the world I'm building. They've created a convergent boundary where they meet, which has given rise to an east-west mountain range, in addition to a north-south range on the northern continent. There's also a deep bay or sea that will eventually c... (more) |
— | over 8 years ago |
| Answer | — |
A: Can a disease be airborne but not be contagious by other means? Let's imagine that the organism causing the disease (a virus) has two states, depending on whether or not it's inside its host. State #1: Active In this state, the organism is inside the host. It's feeding on nutrients it finds and subsequently reproduces, sending its offspring or replicas througho... (more) |
— | over 8 years ago |
| Answer | — |
A: Can there be a moon within a planetary ring? Ever heard of shepherd moons? Shepherd moons are moons (typically not very massive) that orbit in the middle of planetary rings, creating gaps in the material. They also keep the ring material where it is, rather than letting it dissipate: Image courtesy of Wikipedia user The Viewer under the Crea... (more) |
— | over 8 years ago |
| Answer | — |
A: Hydrogen sulfide replacing water? Yes, it's quite possible for hydrogen sulfide ($\text{H}2\text{S}$) to replace water as the solvent for life. It already plays a major role in chemosynthesis in hydrothermal vents. The basic reaction is $$12\text{H}2\text{S}+6\text{CO}2\to\text{C}6\text{H}{12}\text{O}6+6\text{H}2\text{O}+12\text{S}$$... (more) |
— | over 8 years ago |
| Answer | — |
A: Ultraviolet bioluminescence With regards to the real-world, the NOAA says that some deep-sea crabs have low-level UV sensitivity in certain pigments. This would seem to indicate that creatures there may emit UV light - which is exactly what you're looking for - potentially as a form of communication1: In addition, data obta... (more) |
— | over 8 years ago |
| Answer | — |
A: Can a planet have 10x Earth mass, 4x Earth gravity and support intelligent life? I'm going to work off of celtschk's excellent answer, which correctly comes up with a radius of $\sim1.6 R{\oplus}$ and a density of $\sim2.5\rho{\oplus}$, where ${\oplus}$ denotes Earth. If we look at the mass-radius curves of Mocquet et al. (2014), we see that the planet lies very close to the line... (more) |
— | over 8 years ago |
| Question | — |
How can I move a planet? There are a lot of answers on Worldbuilding about how to destroy planets - just as an example - and they seem to be the method of choice for many people intent on destroying the world. My objection to this is that this is extremely difficult; moving a massive, orbiting planet isn't like picking up a ... (more) |
— | over 8 years ago |
| Answer | — |
A: VY Canis Majoris planetary system This is actually pretty simple. We just have to calculate the stellar flux, which is the luminosity of the star divided by the surface area of a shell with the radius of the planet's orbit. In other words: $$F=\frac{L}{4\pi r^2}$$ Now, let's denote the Sun's luminosity as $L{\odot}$, the current radi... (more) |
— | over 8 years ago |
| Answer | — |
A: Getting Day-Night Cycle and Seasons for my Dyson Outies First, let's start by determining the mass of the black hole. Using Newton's law of universal gravitation - which I think we can safely use, at a distance of 1 AU - we get $$g=\frac{G(M{\text{black hole}}+M{\text{Dyson sphere}})}{r^2}\to M{\text{black hole}}=\frac{gr^2}{G}-M{\text{Dyson sphere}}\sim... (more) |
— | over 8 years ago |
| Answer | — |
A: Can this unique planet be colonized? Here are some jumbled thoughts I had: There's no way the planet can have a magnetic field strong enough to levitate rocks. In my answer to Can there be planets with extremely strong magnetic fields?, I calculated that for Earth to have a surface magnetic field as strong as a kitchen magnet, it would... (more) |
— | over 8 years ago |
| Answer | — |
A: Could an impact crater make a world habitable? Yes! And even without the tidal heating. O'Brien et al. (2005) modeled impact craters on Titan using finite-difference methods for various impact scenarios, including a wide range of sizes and temperature profiles. Here are some of their findings: A 15 km diameter crater in water ice with a dept... (more) |
— | over 8 years ago |