temperature - How much heat emanates from the planets after formation?

The 2nd answer to this question by MBR got me thinking about this.

Is there some general guideline for residual heat a planet should emit, say 4.5 billion years after formation. I'm defining heat by the radiant energy that leaves the planet, not internal heat or surface temperature for rocky bodies and assuming little or no solar input.

Wikipedia lists an estimate of planet 9's temperature to be 47 degrees K based on remaining heat of formation, but I think that would depend pretty significantly on it's size as well as whether it had any recent impacts. I can't help but wonder if that's an accurate estimate.

Jupiter, as one example, emits about twice as much energy as it receives from the sun, and it's temperature is about 128 degrees Kelvin. Is it safe to assume that Jupiter's upper cloud temperature would still be ~ 100 Kelvin even if it was in deep space, but otherwise having the same formation and history.

Neptune is also a few degrees warmer than Uranus, so has got to be putting out a fair bit of internal heat as well.

Obviously there's a few factors at play - size, whether there was a recent large impact, abundance of radioactive elements, efficiency of circulation (solid layers are likely more efficient at trapping heat than circulating fluid layers).

I'm tempted to guess that there's too many unknowns on gas giant circulation, inner layers, composition and mass for there to be any good simple charts on this stuff, but I was curious if there was anything. I tried google without success.

orbit - Is there any point on earth where the moon stays below the horizon for an extended period of time?

Depends on the interpretation of your question...
The best places not to observe the moon are the north and south pole. On the north pole you will only be able to see objects above the celestial equator. As the moon orbits the Earth in one month its orbit is inclined from the celestial equator. This inclination is almost the same as the inclination of the ecliptic (path of the Sun) with the celestial equator. The ecliptic crosses the equator at two opposite points on the celestial sphere. This means that for about half its orbit, the Sun, and as the moon's orbit is near the ecliptic, also the Moon, will be above the ecliptic and therefore visible from the north pole.

That being said, the Moon does not follow the ecliptic precisely as the moon's orbit is inclined from the ecliptic by about 5°. The inclination of the ecliptic is 23°, so during very special circumstances the maximum altitude of the Moon above the horizon on the North pole will be 18° during one month. The duration that the Moon will be above the horizon on the north pole will be about 10 days (a guess) about 13.6 days (edit:see comments below).

So if we interpret your question as: Is there a place where the Moon will be below the horizon for a long period of time (> 1 month), then the answer is NO.

But if this happens near June, then the Sun will also be above the horizon at the north pole (for six months), and as the Moon will be close to the Sun (as it follows more or less the ecliptic), it will be very hard to see the Moon during that time. So if you specifically ask whether the Moon will not be visible for an extended period of time, then the answer is YES.

And there are of course also places with perpetual cloud cover ;-)

astrophysics - Does conservation of energy make black holes impossible?

For simplicity, let's consider a Schwarzschild black hole, so that the spacetime is spherically symmetric and static. In particular, the Schwarzschild time $t$ coordinate gives a direction in which the geometry 'stays the same' (a Killing vector field), and its inner product with the orbital four-velocity $u$ is conserved:
$$epsilon = -g(u,partial_t) = left(1-frac{2M}{r}right)frac{mathrm{d}t}{mathrm{d}tau}text{,}$$
where $tau$ is the proper time of the orbiting particle. One can think of this as the specific (per-mass) energy, including mass-energy: a particle escaping to infinity that becomes asymptotically at rest with respect to stationary observers would have $epsilon = 1$.

If we consider escape velocity for a particle at the event horizon, it has an escape velocity of the speed of light. The energy required to achieve this escape velocity would therefore be infinite.

The energy as measured by whom? Imagine a family of stationary observers everywhere surrounding the black hole, or at least along an infalling particle's trajectory. Those observers measure the speed and energy of the particle as it falls past them. As the particle nears the horizon, they will report speeds arbitrarily close to the speed of light and arbitrarily high energies.

But to a far-away stationary observer, those observers near the horizon are experiencing increasingly divergent gravitational time dilation, and the orbital energy of the particle stays constant. Flinging a particle of mass $m$ into the black hole will increase the mass of the black hole by $mepsilon$.

What's the critical flaw in this argument?

All energy gravitates, so I would say the main flaw is forgetting to add the gravitational potential energy of the particle to the mass of the black hole as well. Or better put, you should be concerned with the orbital energy, not just its mass+kinetic parts ('relativistic mass').

In this situation, you have a well-defined conserved orbital energy. If you insist on measuring the Lorentz factor according to close-by stationary observers, then yes, it diverges to $+infty$, but then you would have to admit a gravitational potential energy term that diverges to $-infty$, because their sum must resolve to be the orbital energy.

What is the longest observable wavelength of light using an optical telescope?

It depends on the telescope.

If the telescope is fully reflective (no transparent glass elements), it should be able to operate well-beyond visual wavelengths, with its transmission limited primarily on your mirror costing material.

http://www.thorlabs.com/images/TabImages/Concave_metallic_mirror_8_degree_AOI.gif

Any telescope with refractive (transparent glass) elements however, will quickly lose performance outside of the visual range due to transmission, but also from how the glass index changes with wavelength, which causes non-visual light to be defocused (an effect which increases the further you get from visual wavelengths).

Generally speaking, the defocused issue of refractive telescopes can be compensated if you have enough focus travel. You will see "chromatic aberration" becoming much more significant very quickly, as telescopes are only designed to correct this over visual wavelengths.

Ultimately, the limiting factor of refractive telescopes is the transmission of the glass in the lenses. Many glasses will transmit reasonably well from 400-2000nm, though some have worse performance and some have better.

epigenetics - Can rats pass on memories of a maze to their offspring?

The phenomenon you're talking about was a fad in the 60's, called 'interanimal memory transfer'. It started out when James McConnell performed a later-discredited experiment in which he found that if you chopped up flatworms which had been exposed to some stresses, and fed them to other unexposed flatworms, the unexposed worms became wary of the source of stress quicker after eating their dead companions. He jumped to the conclusion that a 'memory molecule' was being transferred, and that the cannibal worms gained the food worms' memories of the stress.

People then started looking to see if they could:

1. repeat the experiments


2. find the same phenomenon in other animals

In the first case, nobody could replicate the experiments in worms, but because McConnell was such a PR genius he managed to convince the public that his results were valid (see Rilling, 1996 for more on this).

In the second case, Frank et al. (1970) and others tried working with rats - I think this is the experiment you're talking about in the question. They found various interesting results including that if you trained rats to run through a maze by using particularly stressful negative reinforcement (like electrocution), then those rats' children would be able to learn the new maze much faster. However, Frank et al. didn't make the same mistake as McConnell - first of all they wondered if the parent rats might be leaving a scent trail. So they used duplicate mazes with the exact same design, putting the children into clean mazes. The children of adults who had already learned the maze continued to outperform the control rats - the explanation was not scent trails.

Next they wondered whether it might be that the second generation rats had been born with a higher wariness as a result of the stress their parents suffered; i.e. it could be a hormonal transfer from mother to child (e.g. cortisol, the stress hormone).

Frank et al. tested their hypothesis by torturing some rats for a while (rules about animal welfare were not strict in the 70's). They would lock some rats in a small jar and bash them about for a long time, then kill them, chop them up, and take out their livers. They fed the livers to other rats, and found that after eating the livers the other rats learned the maze much faster. They interpreted the results in what now seems a sensible light: the stressed rats were producing high concentrations of a stress-signalling molecule. When those rats either had children or were fed to other rats, they passed on high doses of the stress molecule. This raised the alterness and wariness of the receipient rats so that they were much quicker to learn which parts of the maze were dangerous.

There is no evidence that the child rats actually 'remembered' the maze - they still had to find their way around, but they were extremely wary of the electrocution plates and so avoided them, finding the safest way to the end. This is not a case of genetic memory.

the sun - Does the Sun impose back it's tidal forces onto the Earth (such as the Earth's to the Moon)?

Well, I wasn't sure if tidal forces between Earch and Sun were strong
enough to have any effect on the matter. Truth to be told, the article
barrycarter linked clarifies that tidal forces have neglible meaning
in comparison to the effect of Sun's nuclear fusion mass loss.

That's correct.

Some more details on this.

Tidal "tugging" is a two step process. The Moon's gravity raises a tide on the Earth and this tidal bulge, spins ahead of the moon and pulls the moon very slightly faster, which slowly pushes the Moon away from the earth. (Note, if the Earth rotated much more slowly, the effect would be the opposite, the orbiting moon would be ahead of the tidal bump and gradually slow down).

When the Moon was closer and the tidal bulge was a lot bigger and the corresponding push away was significantly faster. Still not fast, but quite a bit faster than the current 1.5 inches per year, perhaps several feet a year after formation if I was to guess. Even more for the brief period when the Earth had an entirely liquid-magma surface, but that was probably pretty temporary.

So, most of the distance the Moon moved away from the Earth after formation happened in the first billion years. The move away speed decreases with the distance, something roughly equal to the 4th power of the distance.

As you said, this exact same thing happens between the Earth and Sun, but the effect is even smaller, so small that it's not the primary effect in changes in the Earth's average distance from the sun.

Tides depend on gravitational field variation (not field strength), but on how much the field changes near side of object to far side. It also helps if the larger object that's being orbited has a liquid surface, oceans or molten or a large ocean under an icy surface. If the Earth didn't have oceans the Moon would moved away from the Earth much more slowly and it would be quite a bit closer to the earth than it is now.

Rocky surfaces still bulge a little so the effect still happens but it's smaller.

The sun's is plasma which is very fluid so the Earth and all the planets create a tidal bulge on the sun, but from the sun's point of view, all the planets are quite small, just tiny dots in the sky, so the tidal effect from the planets creates only a tiny tidal bulge on the sun.

Tidal effect can be roughly compared to the size of the object in the sky. for example, the Moon and Sun from the point of view of the Earth are about the same size in the sky, so they have similar tidal effects. The Moon's larger tides is due to the Moon having greater density than the Sun. More on Tides here.

(this doesn't apply to a tennis-ball in your hand, however, which can appear the same size as the moon or sun, because you have to consider the size from the center of the earth, not the size to your eye on the surface.)

The tidal force the sun receives from the Earth from 93 million miles away when the Earth is just a tiny dot, so effect and corresponding the tidal bulge is quite small, so the corresponding push is very small.

A curious sidebar, is that when the Sun goes Red Giant, the larger sun will have a greater distance variation to the Earth, which will probably be the closest planet at that point. The tidal bulge will be correspondingly larger and the Earth, quite a bit closer to the bulge, so when that happens, the tidal push the Earth gets will be measurably larger.

The tug from other planets, while, even smaller, can have an effect in moving planets too, more so when the planets are in orbital resonance, which, currently, no planets are. The solar system passing through a dust cloud can have a slowing down effect too. Needless to say, all the effects are quite small.

artificial satellite - How much time does the ISS take to repeat its orbit?

Objects in orbit pass over the surface above what's commonly called their ground track. For objects in low earth orbit, though they go around the earth every 1.5 or more hours, the Earth rotates beneath them so they don't trace a simple "circle" over the same points on land.

The orbit is around the Earth in a fixed plane, so we are likely (but not guaranteed) to see the satellite pass over 12 or 24 hours later.

biochemistry - Why is PEG important for efficient yeast transformation?

http://www.protocol-online.org/biology-forums/posts/26634.html

I found this link - it says 'we don't really know'. It says PEG binds DNA, I assume shielding the membrane from its negative charge and allowing internalization to happen.

I would guess that the amphipathic nature of PEG, being partly hydrophobic, also helps soften up the membrane. Interestingly, if you increase the PEG concentration beyond the limits, it decreases the efficiency of the procedure.

the sun - How much of the ambient radiation on Mars is arriving line of sight from the Sun?

Besides solar energetic particle (SEP) events, it's mostly (almost isotropic) galactic cosmic rays (GCR). GCRs may cause secondary radiation by interaction with the atmosphere or with rock. The smaller the solid angle of the visible sky, the smaller the mean galactic radiation dose. On Mars you get additional shielding by the atmosphere, hence dependence of the atmospheric pressure. More detail in this paper.
Shieldings from SEP events should be rich in protons, e.g. water or food, or you need to go deeper into underground, as soon as a SEP event is detected.
GCRs are partially shielded by the heliosphere. Hence solar maxima protect partially from GCRs, but SEP events get more likely during solar maxima.
Since GCRs are unpredictable, and SEP events can be detected, for solar maxima, protection strategies resultng in an overall reduced radiation dose can be elaborated.

pharmacology - What hydrolyses aspirin within the digestive tract and blood stream?

I have had some further thoughts after my previous question regarding the buccal delivery of medication. The active compound in aspirin (acetylsalicylic acid or systematically 2-Acetoxybenzoic acid) is salicylic acid (2-hydroxybenzoic acid).

I understand that the hydrolysis reaction occurs as follows within the stomach - therefore in the body it is an acid hydrolysis:

N.B. The H₂O is not shown in the diagram.

However having done this experimentally I know that in the lab to ensure a decent yield of salicylic acid I had to reflux the solution for several hours. Yet the onset of action of aspirin tablets is much faster than this. To me this suggests some enzymatic activity, but I have no idea which enzyme this is likely to be.

To follow on directly from my previous question, if the aspirin is absorbed directly into the bloodstream bypassing the stomach then what causes the hydrolysis in the blood stream and is it the same factor that increases the rate of hydrolysis in the stomach if the medication were to be taken orally?

heat - How hot must a star get before it is considered to be a star?

From a physics perspective

From a physics perspective an object is a star when it is undergoing nuclear fusion, generally of hydrogen atoms at its core, this is regardless of its temperature!

A star is not determined by its temperature, it is instead determined by it's internal processes.

This does mean that if Jupiter began nuclear fusion it would be considered a star, albeit a minuscule one.

In this case it is a yes/no distinction of if an object is a star.

From an observational point of view once something is classified as a star there are 7 groups it can fall in to determined by its features.

Sourced From: http://en.wikipedia.org/wiki/Star#Classification

Class Temperature
O: 33,000 K+
B: 10,500–30,000 K
A: 7,500–10,000 K
F: 6,000–7,200 K
G: 5,500–6,000 K
K: 4,000–5,250 K
M: 2,600–3,850 K

Note: Three more classifications L T and Y have been added to the colder end of this list, but I am unsure of the cut off points so omitted them.

But strangely they are not classified by temperature but by their spectrum, it just so happens that their spectrum correlates to their temperature! The temperature spoken of here is of the photosphere of the star (where the photons begin free streaming), not its core (where photons are created from ongoing fusion reactions).

Dwarf stars have their own classification system prefixed by the letter D though.

Quote from Wiki article:

White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.

Is it possible that Titan is a kuiper object captured by Saturn?

Titan is roughly ten times more massive than Pluto or Eris, the most massive known Kuiper Belt objects (KBOs). (Titan is in fact more massive than any other moon in the Solar System except Ganymede.) It would be a rather strange coincidence if the object that was by far the most massive of the KBOs was in orbit around Saturn, well interior to the Kuiper Belt.

Titan's orbit is prograde (it orbits in the same direction that Saturn rotates) and barely tilted with respect to Saturn's equator (less than half a degree), the rings, and most of the other moons. That strongly suggests it formed out of an accretion disk around Saturn, as the Galilean moons formed around Jupiter.

Titan also has an extremely dense atmosphere, unlike any of the KBOs.

So it's rather unlikely that Titan is a captured KBO.

(Neptun's moon Triton, by the way, is thought to be a captured KBO. But it is only slightly more massive than Eris or Pluto, and it has a peculiar retrograde orbit that is very difficult to explain if it formed around Neptune, but easier to explain if it was captured. And since Neptune is the furthest out of the (known) major planets, it's not that surprising that it could have captured a KBO.)

planet - Theoretically, can a solar system evolve around lagrange points

Yes, provided the star is big enough, the "Jupiter" is big enough and the planet is small enough.

The details are discussed herein: "Is there a ceiling for stable L4 or L5 masses?"

To summarise: The sun needs to be 25 times more massive than the giant planet, and the small planet needs to have negligible mass.

It is worth noting that the minor body could not be said to be dominating its orbit, and so would be considered a dwarf planet by the IAU.

senescence - Why isn't the p16-INK4a gene involved in apoptosis expressed in heart or liver tissues?

p16-INK4a is a part of a very important checkpoint mechanism. It's the "bad guy" in the context of aging because it induces senescence, and too much senescence leads to aging-related tissue degradation.

But senescence is important. It's one of the responses cells take when something goes wrong-- DNA damage, viral infection, telomere depletion, that sort of thing. Senescent cells have stopped proliferating. We have a word for cells that don't stop proliferating, and that word is "cancer". So, p16-INK4a is a major tumor suppressor. A universal p16-INK4a knockout would have a much harder time shutting down the proliferation of cells that had undergone DNA damage, and would therefore be much more prone to cancer. You'd have very young-looking tissues filled with tumors.

So, the headline question is why INK4a is not expressed in the heart or liver if it's so important. This is speculation on my part, but I think it's because those tissues are especially prone to being damaged by fibrosis, and a build-up of senescent cells would lead to increased fibrosis. Senescence is just one possible response to DNA damage, though. Another is apoptosis. If the senescence-induction pathways aren't active in heart and lung tissue, I'd expect the apoptosis-induction pathways to be pretty active.

orbit - Lack of objects between heliopause and Oort cloud?

There may be Sednoids there.

Sednoids are a hypothetical class of "inner Oort Cloud objects" named after their prototype, Sedna. Sedna's aphelion is ~936 AU, bringing it close to the inner boundary of the Oort Cloud. Sednoids may have aphelions ranging from about 100 AU to 1,000 AU.

The problem is, only two Sednoids have beet detected to date, 90377 Sedna and 2012 VP₁₁₃. Brown et al. 2004) suggested that ~500 may be detectable; surveys simply haven't tracked objects in that area.

Why are Sendnoids where they are? Three ideas have but put forth:

• A planet at ~70 AU scattered these objects into elliptical orbits.


• A close pass by a nearby star.


• Interactions with other stars in the Sun's original cluster.

These objects would fill in the space between the Kuiper Belt/scattered disc and the Oort Cloud itself.

A beginner project in plotting celestial bodies in 3D coordinate space

As a data visualization task, I'd like to plot some celestial bodies in 3D coordinate space. I assumed there would be a lot of databases where celestial bodies are recorded along with an XYZ coordinate relative to our sun. As I ponder more on it I begin to realize other features, for example how is the axis of alignment achieved and agreed upon?

Given the below data set, is it clear what system is in use and how I may interpret it in a way that gives me an XYZ I can begin plotting?

https://www.google.com/fusiontables/DataSource?docid=1TZ3eoWstpR8d0O2-eqoz3VFDsLwSSzNA5wX4LxE

genetics - effect of background selection on promoter regions compared to distant enhancers?

Has anyone looked at the effect of background selection on the levels of conservation of promoter regions compared to distant enhancers? Do promoter regions have a higher conservation due to background selection from nearby genes compared to more distant enhancer regions?

My attempts to google for this only gave me a 2004 Drosophila paper on a few genes survey:

http://mbe.oxfordjournals.org/content/21/2/374.full

I am wondering if this has been looked at before or if there are any other relevant bits of genetics one should consider first.

EDIT: a bit more of explanation into the hypothesis below, and a reference that could be useful:

Let's start supposing a region near a coding gene that can gain regulatory potential via a new point mutation or small translocation. This new regulatory potential needs to be positively selected after it appears, or it will gradually accumulate more mutations and disappear. But if the region is close enough to the coding gene, recombination rate between the region and the nearby gene body will be low, proportional to their distance. If the gene body is assumed under selective constraint, there will be a hitchhiking effect of the nearby regulatory region by the gene nearby. The background levels of mutation on the regulatory region are then going to be lower (lower background mutation) than in a region that is not near a gene under strong selective constraint. So one way of explaining why close promoter/enhancer regions are more conserved than distant enhancer regions would simply be due to linkage.

A second part of the hypothesis would be about the presence or absence of recombination hotspots in upstream regions of a coding region: if nearby promoter/enhancers are kept via linkage to the closest gene, unbiased measures of cross-over (e.g. PRDM9 experiments in mouse testis) should indicate if there is an increased linkage of functional genomic blocks, like enhancer/promoter/gene blocks (Hi-C, 4C, 3C data), whereas if recombination doesn't play a role, regulatory blocks wouldn't necessarily be kept in the same haplotype block.

ecology - When are population dynamics models useful?

I'll throw one more application into the pot. Population dynamics also forms the foundations of population genetics, population ecology, and more recently plays an important role in frameworks such as evolutionary game theory and eco-evolutionary dynamics.

Here the models are also used as a type of theoretical exercise or thought experiment (as a previous answer suggests). In the development of evolutionary theory we simply cannot observe the process over the timescales we require to test the hypotheses we make. Thus, the development of population models allows us to explore 'possible worlds', as Robert May once put it, to see what kinds of adaptations or population structures we would expect to see, given the assumptions we put in.

We are also seeing an increasing number of population models and dynamical models used in conjunction with experiments on microbes in the field of experimental evolution. Here we can observe evolution in real time, and many of the assumptions about well-mixedness and large population sizes that are often made in modelling populations are actually fairly accurate.

observation - Could science be lost if a phenomena is observed before predicted?

An observation could validate the predictions made by a previous theory. If something unpredicted is observed, then a new theory which is compatible with the observation should make predictions which later are confirmed by new observations.

But what if no such new predictions are possible to make? What is the scientific status of a theory which after the fact is only compatible with all data, but which cannot make any new predictions? If it would be considered weak, does this mean that discoveries through observation can actually destroy science?

Is this a valid argument for delaying the construction of better observatories until theories have matured given all existing data? Kind of comparable with the forward planetary protection argument which says that human spaceflight to Mars should wait until it is certain that there's no life there which could go extinct when it meets Earthly life.

geocentrism - How would the solar system look in a Geocentric model?

Only for Sun, Earth and Mars, but here's an interactive Flash that illustrates it graphically. Third link from the top "Part I, Equivalence of Hypotheses (Flash)" Note that the planets don't move as you move the mouse over the image, only the circles (the theory) change.
http://science.larouchepac.com/kepler/newastronomy/part1/MegaEquivalence.swf for the animation. More generally: http://science.larouchepac.com/kepler/newastronomy/
(And I don't know or care who that LaRouge Pacman is, some rich guy for some reason financed good explanations of what Kepler did, that's what I know about it and that's all very good)

If you move the mouse pointer to the upper left you get the heliocentric system. To the upper right you get the geocentric system. The point is to illustrate how Kepler proved that Ptolemy, Copernicus and Tycho all had the same mathematical model for the solar system. They just used different frames of reference to it, just choosing different fixpoints from where to view it.

Some dots in the illustration represent important points in that ancient model. Some of them are stand-ins for the foci of an ellipse in a model made out of perfect circles. And the Sun was actually not the center of the Copernican model, a point near the Sun was. According to Copernicus the Sun was just a decoration which randomly happened to hang around near the center. Kepler was the first to come up with the thought that maybe there is a relationship between the geometry and the physical existence of stuff. An idea later developed to the theory of gravity by Newton and Einstein, who just added to Kepler's initial formula for describing the movements of the planets.

Cluster of fast moving stars

It is highly likely that you might have observed an artificial satellite, most commonly of Iridium series which can flare as bright as -6 magnitude. You can have a check on it anytime. Heavens Above is one of the most trusted sources that can notify you depending on your location. Even NASA is having such service that can notify you about ISS passes. A dozen of mobile apps are there for assistance too.

If you think it was visible for a short time like meteors are, then you are free to report it here : AMS Fireball Report or here : IMO Fireball Report

Don't feel embarrassed if you might have mistaken a jet to that of a moving star. We all have faced that in our initial days of sky observation. Hope this helps

Approximate distance from Earth to Mars at a given moment with reasonable (+/-100km) accuracy?

Do not underestimate the astronomers! 100 km? That's absurd. Saturn('s center of mass) is located to within one mile's distance from the Sun. Mars is of course even much better located.

Here's a site I googled that seems to give current distance to Mars to within single kilometers. I don't know how that site makes up its numbers, but it is certainly possible to measure it like that. With mutual radio communication, there are really good opportunities for precis distance measuring. 135,834,832 km just now! Pretty close and that's why the ExoMars space probe was launched towards Mars a few days ago. And here is a table with the conunction distances just past and soon coming up.

solar system - Is there a photo that shows a iron meteor or asteroid in space in raw form having no layers of fusion crust?

In short, no.

The reason is that while finding meteorites on Earth is hard, finding them in space is a lot harder. A small asteroid, weighing only a few kg can't be spotted while it is still in space. The first we know of them is when they make their fiery descent.

On the other hand, there are some large asteroids that are metallic, such as 16 Psyche, and we have direct images of other large asteroids. Metallic asteroids are relatively rare, and have a higher albedo than stony asteroids, and much more than comets. However they are not shiny balls of metal. After a few billion years in space they absorb 80% of the light that falls on them.

human biology - Possible? When a pregnant woman suffers an organ damage, fetus would send stem cells to the damage organ to help repair it?

I am quite sure that there is this blood-placental barrier between the mother and the baby so that nothing (except a type of antibody) can pass through it.

But I remember reading somewhere that when a pregnant woman suffers an organ damage, fetus would send stem cells to the damage organ to help repair it.

Anything to support that?

astrophysics - Simulates Orbit - Astronomy

The equations of motion are just second order ordinary differential equations. They can be solved numerically by any of the usual methods, However, for two bodies an exact solution can be found, that solution was known to Kepler. To model the trajectory you need to know the orbital period and the eccentricty (e) of the orbit.

If you know the period of orbit of a body, then the "Mean anomaly (M)" is the angle time/period *2*pi radians (it increases uniformly from zero to 2pi in one orbital period.

The find the Eccentric anomaly ("E", the angle made by the body, the center of the elliptical orbit and the point of periapsis when the body is closest to the sun) you solve $M=E-esin(E)$ (it can be solved by newton's method quickly, though convergence is fastest for roughly circular orbits.

You then know the body is on a ellipse, with the sun at one focus and you have calculated the ray on which the body is found at a given time. This gives you the position of the body.

There is a rough implementation of this in a python gist

bioinformatics - Compressing structural information in PDB files

There seems to be a lot of redundancy in PDB files. These files can of course be compressed with general-purpose compression programs like gzip, but I can't help but imagine that these tools are overlooking a significant amount of redundancy in PDB files. Are there compressors that specifically target PDB files? If not, what are some aspects of PDB files that are ripe for compression?

Looking at a typical PDB file, some redundancies are immediately apparent. Other redundancies are less obvious. Consider this excerpt of two residues from 1MOB (myoglobin):

ATOM    332  N   LYS A  42      16.481  27.122 -10.033  1.00 11.15           N  
ATOM    333  CA  LYS A  42      15.926  28.134  -9.159  1.00  8.64           C  
ATOM    334  C   LYS A  42      16.970  29.081  -8.512  1.00 16.74           C  
ATOM    335  O   LYS A  42      16.687  30.075  -7.799  1.00 11.84           O  
ATOM    336  CB  LYS A  42      15.093  27.489  -8.043  1.00 18.03           C  
ATOM    337  CG  LYS A  42      13.731  26.888  -8.502  1.00 19.65           C  
ATOM    338  CD  LYS A  42      12.679  27.912  -8.953  1.00 17.94           C  
ATOM    339  CE  LYS A  42      11.438  27.406  -9.703  1.00 24.82           C  
ATOM    340  NZ  LYS A  42      10.474  28.567  -9.803  1.00 19.81           N  
ATOM    341  N   PHE A  43      18.218  28.599  -8.544  1.00 12.28           N  
ATOM    342  CA  PHE A  43      19.311  29.318  -7.919  1.00 11.81           C  
ATOM    343  C   PHE A  43      20.223  30.024  -8.949  1.00 10.95           C  
ATOM    344  O   PHE A  43      21.201  29.462  -9.450  1.00 10.08           O  
ATOM    345  CB  PHE A  43      20.138  28.301  -7.137  1.00  9.30           C  
ATOM    346  CG  PHE A  43      19.494  27.689  -5.877  1.00  9.53           C  
ATOM    347  CD1 PHE A  43      19.572  28.376  -4.679  1.00 12.01           C  
ATOM    348  CD2 PHE A  43      18.837  26.465  -5.923  1.00 10.54           C  
ATOM    349  CE1 PHE A  43      18.993  27.861  -3.536  1.00  9.59           C  
ATOM    350  CE2 PHE A  43      18.261  25.959  -4.775  1.00  8.62           C  
ATOM    351  CZ  PHE A  43      18.341  26.666  -3.597  1.00  7.89           C  

These two residues occupy 1,638 bytes as plain text; when compressed with gzip, they occupy 467 bytes. For reference, the format of ATOM records in PDB files is defined at wwpdb.org/documentation/format33/sect9.html#ATOM.

Almost all of the data in the above excerpt seems redundant. The first field (ATOM), second field (atom index, e.g. 332 in the first row), sixth field (residue index, e.g. 42), tenth field (occupancy, e.g. 1.00) and last field (element name, e.g. N) seem clearly extraneous. The fourth field (residue name) could be shortened from three characters to 1 character, or simply an integer. I'm not a data compression expert, but I imagine gzip picks up most of this redundancy.

Slightly less obviously, the atom names for each residue also seem unnecessary. To my understanding, the atomic composition of all residues' backbones will always be the same, and represented in PDB files as "N", "CA", "C", "O". The same for the atomic composition of the residues' respective sidechains: a lysine sidechain will always be "CB", "CG", "CD", "CE", "NZ" and a phenylalanine sidechain will always be "CB", "CG", "CD1", "CD2", "CE1", "CE2", "CZ".

A subtler redundancy, but one that might increase compressibility a lot, seems like it could be in the atomic coordinates themselves. For example, in the backbone, would it be possible to deduce each residue atom's X, Y and Z coordinates (12 data points: 4 atoms * 3 coordinates) given only their phi, psi and omega dihedral angles (3 data points)? Could applying dihedral angles to atoms within sidechains similarly remove the need to explicitly list the 3D coordinates there?

Could "temperature factor" (the second to last field in the excerpt) be losslessly removed, or compressed in some non-obvious way? What are some other possible optimizations that could be used to more efficiently compress PDB files? Are there any obvious grave performance implications of these various compression techniques on the speed of a hypothetical decompressor to convert back to the official PDB format? Have these questions been answered in the literature or an existing PDB-specific compression program?

Thanks in advance for any answers or feedback.

Edit:

Given that no PDB-specific file compressors seem to be available, I suppose my specific goal is to develop one. One potential application I see for this is in significantly decreasing fresh times-to-render in certain use cases of browser-based molecular visualization programs, e.g. Jmol, ChemDoodle Web Components or GLmol. Another application could be decreasing the time and size of data needed to download archives of PDB files like those described here.

This would of course require a way to efficiently decompress the packed PDB files, but this trade-off between decompression time and download time seems like it could be useful in at least some niche applications.

Edit 2:

In a comment, nico asks "How would compressing the file decrease render time?". Decreasing gzipped PDB file size (e.g. by half or more) and thus decreasing time needed to download the file would decrease the time between when the PDB file was requested from a remote server and when the structure was rendered by a molecular visualization program running on a client machine. Apologies if that use of "fresh time-to-render" in that context was unclear.

A lossless compression could also involve encoding the PDB file to an object (e.g. JSON) that is faster to parse for the visualization program, and decrease render times that way. Looking around further, if the application only required displaying the 3D structure and not also retaining data about specific atoms and residues, then using a binary mesh compression (e.g. webgl-loader) seems like it would probably decrease time-to-render even more.

senescence - Why are beta-galactosidase proteins overexpressed in senescent cells?

Looking at the articles referenced in the Wikipedia article, there's probably no direct physiological link between senescence and beta-galactosides. Lee et al (2006) and others before them have shown that the "hypothetical" protein is just regular lysosomal beta-galactosidase, which is present in higher concentrations in aging and stressed cells because the number of lysosomes increase under those conditions. There's a collection of references in the Discussion section of the Lee paper that delves deeper into the aging-lysosome connection.

I'm well out of my depth by this point, but if I were going to take a wild guess I'd suggest that lysosomal upregulation could be part of a stress response involving autophagy (i.e. the cell breaking down and recycling intracellular materials to meet critical demands), though I don't know the specific connection to aging. As far as beta-galactosides go: if you have any hanging around, you get to eat them, is all, I bet. I bet the references in Lee would tell you if I'm on track or not.

Do parallel universes exist? - Astronomy

The simple answer is (as with so much in astronomy):
We Don't Know

Parallel universes may or may not exist. There is no definitive way to prove that these universes do or don't exist.

A parallel universe is a separate existence to ours. The Theories that suggest that there may be parallel universes are classified as theories of multiverses. There are many theories of multiverse, all of which propose different ideas about what could exist beyond the limits of our universe. There are also theories that suggest that the multiverse doesn't exist, although the theories with most support are by far the multiverse theories.

For a nice reference in book form, see Steinhardt and Turok's "Endless Universe: Beyond the Big Bang". Also, see Max Tegmark's work on multiverses levels I-IV (Max Tegmark -> See his Scientific American article entitled Parallel Universes).

immunology - Is there a maximum amount of antibodies your body can keep?

Antibodies are simply proteins and like any other protein have a relatively short "life", so after clearing out an infection, they are not retained for long (most of them anyway). What the body keeps is memory cells which can produce a much more rapid response if they come in contact with the same pathogen again.

You could see it as a selective process: the body produces immune cells, one or more specific cells for nearly every potential antigen that might exist in the world (there are more steps involved of course but they are of little relevance here). Only those which are used at some point mature into memory cells.

Unfortunately, I just realised that I can't explain why the immune system goes through this selective process; producing cells equivalent to memory cells in the first place would make the immune response much stronger. One explanation I can think of for why the body produces naive cells first is that either a) their production or b) their maintenance is less costly.

The other explanation I could think of is that even immune cells in the periphery which have undergone the proofing mechanisms of immune cell development already, may not be perfect and target body cells every now and then - in that case it would be devastating if they would go all-out like a memory cell.

From my three explanations, only alternative b) would mean there could be a limitation to how many vaccinations our immune system can bear (if memory cells take more effort to maintain than naive cells, having too many of them may overstrain whatever systems maintain them). Otherwise I don't see any limiting factor for how many memory cells you can retain; except if their numbers become so large that your lymph nodes swell and that causes problems.

Edit note: I'm not aware of any research about that question and couldn't find anything either.

galaxy - How do I see more than just points in the sky?

I just bought a new telescope with these specifications:

• Aperture: 203 mm


• Focal Length: 1200 mm


• F/ratio: f/5.9

I usually use a 28 mm eyepiece, giving me about 42.5x magnification, but I also have a 20 mm and a 10 mm eyepiece.

I was trying to find Messier objects, but I only found the Orion Nebula, and even then it wasn't very impressive. I was wondering, what types of objects are see-able in my telescope? I read that Charles Messier found the Messier objects using a four-inch refractor, and I have an 8 inch reflecting telescope.

When I tried to find things like the pinwheel galaxy, or the crab nebula, I only saw points of light. So basically, how do I see things like nebula and galaxies?
For reference, I live in the midwestern United States.