Lifespan of connective tissue cells

Not that I'm aware of. There isn't any blood flow to either tendons or cartilage as an adult, so the pathway for migration doesn't exist.

Tendons and cartilage are tissues composed of dead cells after their formation (the cartilage growth plates cease to exist in your teen and completely ossify, tendons I'm not sure on). Damage to tendons and cartilage is permanent, and can cause arthritis (when the cartilage is completely worn away and bone-on-bone contact occurs at joints).

Unlike living tissue, because tendon cells are dead, they are much, much easier to transplant - they do not produce any signaling molecules or have any surface proteins which would trigger an immune reaction for the vast majority of people. Shoulder surgeries often utilize this to the benefit of the patient, especially acromio-clavicular injuries.

If living cells are brought to the dead tissue, some repair is initiated, as I do recall a technique where surgeons tapped into the marrow of a patient's femur to stimulate cartilage reconstruction, but I also remember reading that the new cartilage formed wasn't the same hyaline cartilage produced by chondrocytes of the growth plate, so the repairs were temporary.

Artificially produced tendons do exist, and many are braided in order to foster interaction with host tissues (when you suffer from tendonitis, the tendon itself isn't repaired - the tissue around the tendon is repaired and strengthened). I'm not aware of any successful attempts to regrow tendons.

Attempts to grow cartilage in vitro have been successful, but re-introducing it into the body to produce a surface to articulate against has presented significant difficulties. Because we cannot (currently) reproduce biological articular surfaces, joint replacement often uses composites or surgical grade metals (both of which present problems of their own, but arthritis is not one as there's no living tissue to feel pain).

space time - Does matter accumulate just outside the event horizon of a black hole?

Yes, you are absolutely right, from OUR VIEWPOINT it does.

From Kip Thorne's book "Black Holes and Time Warps: Einstein's Outrageous Legacy."

“Like a rock dropped from a rooftop, the star’s surface falls downward (shrinks inward) slowly at first, then more and more rapidly. Had Newton’s laws of gravity been correct, this acceleration of the implosion would continue inexorably until the star, lacking any internal pressure, is crushed to a point at high speed. Not so according to Oppenheimer and Snyder’s relativistic formulas. Instead, as the star nears its critical circumference, its shrinkage slows to a crawl. The smaller the star gets, the more slowly it implodes, until it becomes frozen precisely at the critical circumference. No matter how long a time one waits, if one is at rest outside the star (that is, at rest in the static external reference frame) one will never be able to see the star implode through the critical circumference. That is the unequivocal message of Oppenheimer and Snyder’s formulas.”

“Is this freezing of the implosion caused by some unexpected, general relativistic force inside the star? No, not at all, Oppenheimer and Snyder realized. Rather, it is caused by gravitational time dilation (the slowing of the flow of time) near the critical circumference. Time on the imploding star’s surface, as seen by static external observers, must flow more and more slowly, when the star approaches the critical circumference, and correspondingly everything occurring on or inside the star including its implosion must appear to go into slow motion and then gradually freeze.”

“As peculiar as this might seem, even more peculiar was another prediction made by Oppenheimer and Snyder’s formulas: Although, as seen by static external observers, the implosion freezes at the critical circumference, it does not freeze at all as viewed by observers riding inward on the star’s surface. If the star weighs a few solar masses and begins about the size of the sun, then as observed from its own surface, it implodes to the critical circumference in about an hour’s time, and then keeps right on imploding past criticality and on in to smaller circumferences.”

“By looking at Oppenheimer and Snyder’s formulas from the viewpoint of an observer on the star’s surface, one can deduce the details of the implosion, even after the star sinks within its critical circumference; that is one can discover that the star gets crunched to infinite density and zero volume, and one can deduce the details of the spacetime curvature at the crunch.” P217-218

OK, so from our perspective all the matter will be clustered around the critical circumference and no further. That's fine, this shell in theory can exert all the forces required on the external universe such as gravitational attraction, magnetic field etc. The point like singularity which is in the indefinite future of the black hole, (from our point of view) indeed in the indefinite future of the universe itself could not exert such forces on this universe. This singularity is only "reached" as an observer rides in past the critical circumference and, through the process of time dilation, reaches the end of the universe.

This is obviously an area of active research and thinking. Some of the greatest minds on the planet are approaching this issue in different ways but so far have not reached a consensus but intriguingly a consensus appears to be beginning to emerge.

http://www.sciencealert.com/stephen-hawking-explains-how-our-existence-can-escape-a-black-hole

Stephen Hawking said at a conference in August 2015 that he believes that "information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon." His comment refers to the resolution of the "information paradox," a long-running physics debate in which Hawking eventually concedes that the material that falls into a black hole isn't destroyed, but rather becomes part of the black hole.

Read more at: http://phys.org/news/2015-06-surface-black-hole-firewalland-nature.html#jCp

In the mid-90s, American and Dutch physicists Leonard Susskind and Gerard 't Hooft also addressed the information paradox by proposing that when something gets sucked into a black hole, its information leaves behind a kind of two-dimensional holographic imprint on the event horizon, which is a sort of ‘bubble’ that contains a black hole through which everything must pass.

What occurs at the event horizon of a black hole is very hard to understand. What is clear, and what proceeds from General Relativity, is that from the viewpoint of an external observer in this universe, any infalling matter cannot proceed past the critical circumference. Most scientists then change the viewpoint to explain how, from the viewpoint of an infalling observer, they will proceed in a very short period of time to meet the singularity at the centre of the black hole.
This has given rise to the notion that there is a singularity at the centre of every black hole.

However this is is an illusion, as the time it will take to reach the singularity is essentially infinite to us in the external universe.

The fact that the matter cannot proceed past the critical circumference is perhaps not an “illusion” but very real. The matter must from OUR VIEWPOINT become a “shell” surrounding the critical circumference. It will never fall through the circumference while we remain in this universe. So to talk of a singularity inside a black hole is incorrect. It has not happened yet.

The path through the event horizon does lead to a singularity in each case, but it is indefinitely far in the future in all cases. If we are in this universe, no singularity has yet been formed. If it has not been formed yet, where is the mass?  The mass is exerting pull on this universe, correct?  Then it must be IN this universe.  From our point of view it must be just this side of the event horizon.

ASTONISHINGLY IT MAY BE POSSIBLE TO PROVE THIS. The recent announcement of gravitational waves detected on the merger of 2 black holes was accompanied by an unverified but potentially matching gamma ray burst from the same area of the sky. This is inexplicable from the conventional viewpoint which holds that all the matter would be compressed into a singularity and would be incapable of coming out again.

If 2 black holes merge and emit gamma rays… the above is certainly an explanation which is also consistent with General Relativity. The mass never quite made it through the event horizon (from our viewpoint) and was perturbed by the huge violence of the merger, some escaping. It may be a deep gravitational well, but a very powerful gamma ray should just be able to escape given the right kick (attraction by an even larger black hole approaching).

Further more refined observations of similar events, which are likely to be reasonably frequent, may provide more evidence. There is not likely to be any other credible explanation.

pharmacology - How do you design a drug to be delivered to the CNS?

In a nutshell: in order to pass through the blood-brain barrier (BBB) the substance has to mimick soee properties of the substances that are allowed to pass through.

There are different types of "transporters" -- integral proteins going through the cell membranes and accounting for the active transport of the substances they can actively bind to. Smaller molecules of drugs can use solute carrier transporter (SLC), whereas bigger molecules (oligo- and polymeres) would use the receptor-mediated transporter (RMT).

There is a nice article on this topic published by ScienceDaily.

Here is the quote:

One technology for enabling active transport of small molecule drugs
across the BBB involves targeting endogenous nutrient transporters.
These transporters are members of the solute carrier (SLC) transporter
superfamily. Transport of small molecules across the BBB by these
membrane proteins is known as carrier-mediated transport (CMT).

In order to design drugs that utilize CMT to cross the BBB,
researchers modify their chemical structures so that they resemble
nutrients that are transported across the BBB by specific SLCs. The
prototypical drug that uses this strategy (which was developed long
before mechanisms of CMT were known) is L-DOPA, the major current drug
for Parkinson's disease. L-DOPA is used to replace dopamine that is
lost due to degeneration of dopaminergic neurons in the substantia
nigra of the brain.

Another major system that is used in normal mammalian physiology to
enable needed molecules to cross the BBB is receptor-mediated
transport (RMT). The brain uses RMT to transport proteins, peptides,
and lipoproteins that are needed for brain function across the BBB.
Examples of biomolecules that are transported into the brain via RMT
include insulin, insulin-like growth factor (IGF), leptin,
transferrin, and low-density lipoprotein (LDL).

In RMT, molecules in the circulation may bind to specific receptors on
the luminal surface of brain capillaries (i.e., the surface that
interfaces with the bloodstream). Upon binding, the receptor-ligand
complex is internalized into the endothelial cell by a process called
receptor-mediated endocytosis. The ligand may then be transported
across the abluminal membrane of the endothelial cell (i.e., the
membrane that interfaces with brain tissue) into the brain. This whole
process is called receptor-mediated transcytosis.

galaxy - What's this red stuff in some astronomical photos, e.g. Centaurus A?

"Clean the image", how apt, because most of what you can see is caused by dust.

It is a bit difficult to comment specifically because I think you have chosen an example which is a combination of images and not all the images are in the visible part of the spectrum and so have had a "false colour" applied. In fact it is a combination of optical, X-ray and sub-mm images.

In general terms red is present in false colour astronomical images, usually used to representing the coolest or longest wavelength light in the image.

Red is also present in true-colour images of nebulae and is caused by light emitted due to transitions of atoms and ions between excited states separated by particular energies. Red light is predominantly due to Hydrogen alpha and ionised nitrogen emission.

The coolest stars are also red in true colour images, simply because their pseudo-blackbody radiation spectra peak in the red part of the spectrum for temperatures lower than 5000 K. The integrated light from old stellar populations (predominantly cool main sequence stars and red giants) would therefore have a reddish tinge - for example in elliptical galaxies.

As I said, it is hard to say in the case of this particular composite image, but the band across the centre of the galaxy is dust. It obscures (makes dark), the optical image, but warm dust glows in the sub-mm image. The structures coming out of the galaxy at right angles are jets from the active galactic nucleus traced in the X-ray.

human biology - Predictable microchimerism

Microchimerism, as defined by wiki, is the presence of a small number of cells in an organism that are genetically distinct, and originated from a separate organism. The example they give is an unborn child and the mother can 'swap' immune cells, and retain these for many years.

Your question about which parts of the genome are transferred is therefore not really valid, because the cells are transferred, rather than bits of genetic information. Therefore 'traits' are not transferred either, but distinct immune cell lineages can be transferred and persist - this can confer an immune advantage, so this 'trait' could be said to have transferred, but no genetic information has been swapped by the cells.

Although it does not say it on wiki, this presumably only applies to cells of the same species, but a different organism, otherwise infections would also count.

---

Update 20th July 2012

The New Scientist article you have linked to in the comments can be found online here.

It says that we may be more microchimeric than we imagined, because a recent study in Blood found that half of the mothers studied were positive for male T-cells (Dierselhuis, 2012). The authors note that it is remarkable that the immune cells from neither the parent or the offspring attack the other, and that this may explain the observation that siblings make better donors.

Another finding that mothers are found positive for cells from their own mothers, and even their own grandmothers, and that the cell numbers increase during pregnancy (due to clonal expansion of the inherited immune cells) reinforces the opinion that microchimerism is commonplace (Gammill, 2011).

So to reiterate, immune cells can be inherited from your mother, and she may also end up with leukocytes from her offspring, which can in turn be passed on. This is a very fascinating way in which the immune system seems even more complex and helpful.

Why did the moon abruptly change positions in the sky?

Compared to the planets the moon changes it's rise and set times very quickly. Using the calculator on this page it can be seen that if one were looking at the sky on May 1st, 2014 from Irvine, California the moon set at 10:08PM (which at 8PM would have had the moon most of the way across the sky as noted in the question.)

The moon rises and sets approximately one hour later each day. So, by May 14th the moon did not even rise until nearly 8PM - which also correlates with the observations noted above.

Contrasting that with the other context point made in the OP, using this page we can see that on May 1st Saturn rose at 8:07PM and on May 14th rose at 7:11PM.

So while the moon changes its rise a set times by almost an hour each day, the planets change much more slowly by comparison, almost an hour over two weeks.

If one weren't continually watching the moon's progression it would appear to abruptly change positions when comparing it to the planets' movements.

neuroscience - How reversible is DAT upregulation from long-term ADHD medication use?

First the boring (but most important part) looking at the study:

It is important to critique the study design FIRST to see what kind of information we can actually obtain from this study. This is a meta-analysis of cross-sectional studies (meaning there is no record of change over time) that include a variety of age ranges of ADHD patients (some on treatment, some not) who were compared with age, gender and IQ-matched non-ADHD people to measure DAT levels in the stratum measured with PET and SPECT.

What they found:

There were very heterogeneous results from all the studies, there was no statistically significant difference between DAT in ADHD and normal (note that they use the word "trends"). The only thing statistically significant that they found was that there was a wide spread of DAT in the ADHD population, and that exposure to treatment accounted for half of this spread (variance).

What can we say about this?

This is not a very conclusive study, but studies like this are often necessary to merit doing higher resource intensive studies. This study did not reach statistical significance in it's primary outcome, and on further search was able to only find a single outcome of significance. My summary of this would be: maybe this could represent a method of medication tolerance.

To answer your question
If hypothetically this was the cause and I had to make up a possible method of transcription control, I would say something similar to your answer. I would guess there is some transcription factor that is inhibited by dopamine in the cell. When there isn't dopamine (b/c it is being blocked from returning), it would somehow become active and start transcribing the gene for DAT.

Disclaimer
I am not a neurobiologist, this is my opinion after reading the article. Feel free to add to my comments

Reference:

Fusar-Poli, P., Rubia, K., Rossi, G., Sartori, G., & Balottin, U. (2012). Striatal dopamine transporter alterations in ADHD: pathophysiology or adaptation to psychostimulants? A meta-analysis. American Journal of Psychiatry, 169(3), 264-272.[Link]

Why is this video showing radio waves transmitted from a radio telescope?

I saw this video at Space.com and I noticed that at 00:05 the animation illustrates waves propagating from space into The Dish at Parks Observatory, but at 00:50 the direction of propagation is reversed! Would there be some reason for transmitting - for example to create an artificial "star" for wavefront correction analogous to adaptive optics at visible wavelengths, or could it be simply a mistake in the animation?

Here are some GIFs from screenshots. You can check the video at the indicated times.

circa 00:05

circa 00:50

star - Formulas for gravitatitional equilibrium

I am trying to calculate at which point gravitational equilibrium sets in for various bodys (planets stars neutron stars etc.) assuming they are perfect spheres. However the radius i get is not equal to what it should be according to wikipedia (i tried for the sun a planet and a neutron star, all of them were off by quite a bit)

below is the example of the neutron star radius i m trying to get

my result (factor) is 1 at:
Radius : 2228588 m
Density: 1.2768744892680034E11 kg / m^3

the wikipedia article about neutron stars says radius of a neutron star is about 12 km and the density about 10 times higher than mine but my result is 2228km which is quite a bit off, so i got a 2 part question:
a) am i calculating all forces i need?
and
b) are the formulas i am using correct?

i took them from wikipedia and / or university slides and i found multiple variations of almost all of them(most of which i actually didnt list below), so i m quite confused as to what is correct

i know that i m using iron as element and i should split it up and convert the protons to neutrons but that would only increase the degegeneracy pressure further resulting in an even bigger radius if i m not mistaken.
heres how i calculated it (everything behind a "!" is a comment that describes what it is):

!radiantion constant  in Wm^-2K-4

$sbolzmann = 5.67036713E-8$

!radiantion constant  in J / K

$kbolzmann = 1.3806485279E-23$

!gravity constant

$GRAVITY = 6.67408E-11$

! 1 mol

$mol = 6.02214085774E23$

! 1 u (atomic mass )in kg

$ u = 1.66053904020E-27$

! mass of 1 electron in kg

$ me = 9.1093835611E-31$

!hydrogen mass

$ mh = 1.00782503223 * u $

!neutron mass

$ mn = 1.00866491585 * u$

! speed of light, in m / s 

$ c = 299792458 $

! plank constant h in kg  *  m^2 / s    

$ hp = 6.62607004081E-34$

! reduced plank constant hr in kg  *  m^2 / s   

$ hpr = frac{hp }{ (PI * 2)}$

$ amount = 79378857878009573048911997815206 $

! iron with 26 protons 30 neutrons

$ element = 26Fe56$

$ elementmass = 55.934936$

$ atoms = amount$

$ neutrons = (56-26) * amount$

$ electrons = 26 * amount$

$ particlecount = atoms + electrons$

!4.4400513610370694E30 kg, about 2.3 Mass of the sun

$Mass = elementmass * amount $

$Radius = 2228588$

$Temperature = 9.45179584120983E-7$

$ fgravitation = frac{GRAVITY * (Mass^2)} { Radius }$

$volume = ((frac{4.0}{ 3.0}) * PI * (Radius^3))$

    !density in particles / m^3

$ particledensity = frac{ particlecount }{ volume}$

$a = 4.0 * frac{sbolzmann}{c}$

    !radiation pressure in  J / m^3

$ rpressure = frac{1.0 }{ 3.0} * a * (Temperature^4)$

    !gas pressure in J / m^3

$ gpressure = particledensity * kbolzmann * Temperature$

    !electron degeneracy pressure 

$ epressure = (frac{(PI^3) * (hpr^2)} { (15 * me)}) * ((frac{3 * electrons} { (volume * PI)})^(frac{5.0 }{ 3.0}))$

    ! electron degeneracy pressure formula 2

$ epressure2 = frac{((PI^2) * (hpr^2))} { (5 * me * (mh^ (frac{5.0 }{ 3.0})))} * ((frac{3.0 }{ PI})^ (frac{2.0 }{ 3.0})) * (( frac{(frac{Mass }{ volume})}{ 1})^ (frac{5.0}{ 3.0}))$

    !neutron degeneracy pressure

$ npressure = frac{(PI^3) * (hpr^2) }{ (15 * me))} * ((frac{3 * neutrons} {(volume * PI)})^(5.0 / 3.0))$

    !neutron degeneracy pressure formula 2

$npressure2 = frac{((3^(frac{10.0}{3.0})) * (hpr^ 2)) }{ (15 * (PI^(frac{1.0} { 3.0})) * (mn^ (frac{8.0} { 3.0})) * (radius^ 5))} * ((frac{Mass} { 4})^(frac{5.0} { 3.0}))$

$totalpressure = gpressure + rpressure + npressure + epressure$

$totalpressureforce = totalpressure * volume$

!if factor = 1 then the body is in equilibrium

$ factor = frac{totalpressureforce }{ fgravitation}$

Why can we see stars but not astronaut on the moon

An astronaut on the moon could only be seen by reflecting the Sun's light towards Earth. Stars on the other hand emit their own light.

To first order, the amount of flux incident upon the Moon from the Sun is the same as that at the Earth - about 1.4 kW/m$^{2}$.

Let us assume that an astronaut is perfectly reflective and that the relevant reflective area that we can see from the Earth is 1 m$^2$. NB: If the astronaut is not lit up by the Sun, then there is obviously no way that they can be seen.

Treating the astronaut on the Moon as an isotropic point source emitter of reflected light, we have a light source of power 1.4 kW at a distance of 400,000 km. The flux at the Earth is therefore $7times10^{-16}$ W m$^{-2}$.

How does that compare with starlight? Well, the total luminosity of the Sun is $3.8times10^{26}$ W. It has an absolute magnitude of 4.8. This means that if we put the Sun at a distance of about 20 pc, it would be about as faint as the faintest naked eye star in the sky. The flux received at the Earth from such a star would be $8times 10^{-11}$ W m$^{-2}$ and thus 100,000 times brighter than the astronaut.

No need to worry about the resolution of the eye, since both the star and the astronaut (at the distance of the moon) are unresolved points.

Also no need to go into the problems of contrast against the moon's bright surface (which you would need to consider if an astronaut's reflective area was 100,000 times bigger), the reflected light from the astronaut is just too faint to be seen at that distance.

What can the timing of human urination tell about the human's physical condition and circadian rhythms?

I've noticed a peculiar phenomenon. A subject drinks 400 ml of water, then observes time until the urge to urinate is felt. The time is 15 minutes. The subject releases water. 14 minutes later another urge to urinate is felt. The subject releases water again.

I'm particularly interested in what kinds of biological systems are involved in timing of such events.
Does the time depend on how full the subject's stomach is? Does caffeine and other diuretics play a part?
Is it time of day (circadian rhythm) sensitive?
Does that predict anything about the suppression/release of diuretic hormones?

What I'm trying to understand is if the timing between human urges to urinate after water consumption can be used to make predictions about the human biological clock and the state of various systems within the body (for example the digestive system).

I will be conducting this experiment at different times of the day. My hypothesis is that at night, when diuretic hormones are suppressed, the timing would be longer for the same amount of water consumed. This is based on my limited research in the area.
Update: I did perform the same experiment at night, the time was 75 minutes for the same amount of water. The experiment was performed at the end of one of sleep cycles, which makes me think that 75 minutes was the duration of the subsequent sleep cycle.

I appreciate your input on the subject, along with any keywords that can help me advance my research in this area.
Thank you!

plant physiology - Compare and contrast "Rubisco activity" and "assimilation rate" (is there a difference, and if so, what is it)?

Both are measures of carbon fixation rate.

RuBisCO activity specifically refers to the rate at which the enzyme RuBisCO fixes carbon to RuBP, and is measured by isolating the enzyme from tissue samples and, usually, using radiolabelled CO2 to measure how much carbon is fixed in a set amount of time in controlled conditions and with controlled supply of substrate.

Carbon assimilation rate is generally a whole-plant or whole-leaf variable, measured on a larger scale. It is usually measured by measuring leaf sugar carbon before and after a set period, again using radiolabelling. Conditions may or may not be controlled, and it can be measured in the field (unlike RuBisCO activity).

RuBisCO activity might differ from net fixation in the short term in plants which have an intermediate carbon fixation step, such as in CAM or C4 plants, if the net fixation measurement includes the intermediates - the pool of intermediate carbon-containing molecules have technically been fixed but not by RuBisCO. However, over longer time periods this effect will disappear, as ultimately all CO2 is fixed or re-fixed through RuBisCO.

Delta-v from Mercury surface to Venus surface

If you launch from the side of Mercury furthest from the Sun,
parallel to the horizon in the direction of Mercury's orbit at
13 km/s when the Mercury-Sun-Venus angle is 53 degrees, your
payload will reach Venus in 35 days, and its path would look
something like this (each black dot = 1 day). IMPORTANT CAVEATS BELOW IMAGE.

This doesn't really answer your question, since I'm not sure
13 km/s is the minimal required speed. I also made several
simplifying assumptions (perhaps too many), so the answer
above isn't exact.

Here's the heavily commented Mathematica script I wrote, which
also explains the simplifications I made.

(* cleaned up and well-commented version for SE *)

(*

I assume that Mercury's orbit is planar, and that Venus' orbit lies in
the same plane, or at least intersects it at the appropriate time.

This allows me to use a two-dimensional equation for acceleration due
to gravity, instead of a 3 dimensional one.

The equation (below) is the acceleration imparted to an object of mass
m1 at {x1,y1} by an object of mass m2 at {x2,y2}, given that the
gravitational constant is g.

Note that the mass of the object being accelerated (m1) is actually
irrelevant; however, I include it as a parameter for symmetry

*)

accel[{x1_,y1_},{x2_,y2_},m1_,m2_,g_]=g*m2/Norm[{x2-x1,y2-y1}]^3*{x2-x1,y2-y1}

(* 

The mass, semimajor axis, period, and radius of Mercury, in kg, m (not
km), and s

*)

mercsma = 57909050000;
mercper = 87.9691*86400
mercrad = 2439700
mercmass = 3.3011*10^23

(* solar mass, in kg *)

sunmass = 1.98855*10^30

(* gravitational constant of universe, in kg-m-s system *)

g = 6.6740*10^-11

(* 

Heliocentric, so Sun is always at origin. In theory, the positions of
the other planets (eg, Jupiter) could help boost your payload, so you
might be able to launch with a lower speed than I find below

*)

sun[t_] = {0,0}

(*

I assume Mercury's orbit is circular. Since the actual orbit is
elliptical, you could get a boost for your payload by launching it
when Mercury's distance from the Sun is increasing the fastest (in
other words, solar radial velocity is greatest)

I've chosen the x axis to be the line connecting the Sun to Mercury at time 0.

*)

merc[t_] = {Cos[t*2*Pi/mercper],Sin[t*2*Pi/mercper]}*mercsma

(*

I also ignore Venus' own gravity: you can do slightly better by noting
that Venus will pull the payload towards itself once the payload gets
close enough.

I do want to plot Venus' orbit, so I use the semi-major axis and
period values below.

Venus' starting angle (vsa below) was found by trial and error to make
sure Venus was at the right place when the payload crossed its orbit.

*)

vensma = 108208000000
venper = 224.701*86400;
vsa = 53*Degree;
ven[t_] = {Cos[t*2*Pi/venper+vsa],Sin[t*2*Pi/venper+vsa]}*vensma

(*

If we launch from side of Mercury furthest from the Sun, the payload's
starting position will be Mercury's position plus Mercury's radius in
the x direction

NOTE: This start position is completely arbitrary; you may get better
results by starting at different positions on Mercury's surface.

*)

s0 = {mercsma+mercrad,0}

(*

The initial velocity of the payload (with respect to the Sun) will be
Mercury's velocity + whatever velocity (delta v) we impart to the
payload.

Note that both the direction I choose for initial velocity (in the
same direction as Mercury's orbit) and the magnitude are
arbitrary. You may get better results by aiming the payload at a 45
degree angle or straight up or something.

NOTE: If I change 13000 to 12500 below, Mathematica will refuse to
solve the differential equation. This doesn't necessarily mean 13000
is a minimal velocity, but there is apparently some sort of important
change between 12500 m/s and 13000 m/s

*)

v0 = merc'[0] + {0,13000}

(*

Mathematica won't close-form integrate this problem, so we integrate
numerically, which requires a start time (0) and an end time (below).

I chose 35 days after confirming that's how long it takes the payload
to reach Venus.

*)

timelimit = 86400*35;

nds = NDSolve[{s[0]==s0, s'[0] == v0,
 s''[t] == accel[s[t],sun[t],1,sunmass,g] + accel[s[t],merc[t],1,mercmass,g]
},s,{t,0,timelimit}]

(* The use of [[1,1,2]] below is just Mathematica nesting weirdness *)

g= ParametricPlot[{nds[[1,1,2]][t],merc[t],ven[t]},{t,0,timelimit}, 
 Mesh -> timelimit/86400, AxesOrigin->{0,0}, PlotStyle -> {Blue,Red,Green},
 MeshStyle -> {Black}
]

the moon - Telescope buying guide for a beginner in India

If you are a beginning astronomer, there is not much point worrying about all the bells and whistles you can get with a good telescope.

Instead, the key points to look for are:

• light-gathering diameter - this gives an indication of how dim an object you will be able to see


• supports/stand/mount - a basic tripod will be fine if you want to see no further than Jupiter, but it will wobble too much for longer distance viewing, so a solid mount will be essential


• motor/computer drive - at high magnification, objects will pass the eyepiece very quickly, so you will want to look at tracking drives that can follow objects smoothly

But to be honest, you could start with a $100 telescope and really enjoy yourself learning the sky.

cosmology - How is it possible that the CMB approaches the earth from all directions?

The Big Bang happened everywhere and the recombination of electrons with nuclei - the thing that causes the CMB - also happened everywhere. So in every direction you look you can see the radiation from that recombination.

But that recombination also happened a long time ago and so, given light has a constant velocity, the places where we can see the recombination take place are also a long way away. And, as a result of the expansion of space-time, that radiation is very red-shifted, so instead of the recombination looking like it is happening at roughly 4000 degrees Kelvin, it looks like something happening at 3 degrees Kelvin.

That expansion is also what makes it look like the distant galaxies are all moving away from us and the further away the faster they seem to be moving.

navigation - How would an interstellar probe navigate the pull of gravity from stars and other large objects on the way?

Others have already clarified that the chance of encountering another massive body in interstellar space is astronomically small. However, there is still something to consider for course corrections, which is that a very small error in velocity early in your transfer orbit to Alpha Centauri can result in a huge error in your intercept with the system.

Think of it like somebody firing a single atom while in orbit around Earth, trying to set up the trajectory so that hundreds of years later, it precisely hits a microscopic target that was traveling in a different orbit. For extra fun, also try to take into consideration miniscule perturbing effects that cause the orbits to deviate from their idealized model over that period of time.

In practice, the above means that you do need some way to perform course corrections if you're going to intercept your target. As you say, 'real-time' remote control is not feasible, so what this means is that the probe would need some automated control systems to gather information and autonomously correct course along the way.

There's a trade-off in when you make the corrections. The earlier you make them, the less delta-V (and hence fuel) required. But, due to the mindbogglingly huge distances involved, the more precise/accurate you need to be. In practice, you probably schedule a number of short burns along the way, so that you keep the delta-V required for your 'late' adjustments to a minimum. How you schedule those adjustments is likely a matter of careful optimization based on estimated error magnitudes.

saturn - How to watch at best this week's planet alignement?

All these planets are naked eye objects, and except for Mercury, are easy to see. Venus is also part of this grouping, Meaning that all 5 planets known to ancient astronomers are on the same side of the Earth. (and the sun is on the other)

The planets are not exactly "aligned", just they are all visible in the sky, spread over a wide part of the south eastern sky, a little while before dawn.

Sky and Telescope has some nice guides to what you can expect. Mercury is always the hardest, as it is always in the twilight. Saturn is an easy sight, appearing as a slightly yellow star. Binoculars will show its moon, Titan.

Best time is around the first week of Feb about 45 min before dawn.

gravity - What's the difference between the apparent horizon and event horizon of a black hole?

The black hole region of a spacetime is defined as a region where nothing can escape to infinity and an event horizon at a given time is the boundary of a connected region of space which is part of the black hole region. As you're after a simple answer I won't give a formal definition of a black hole or an event horizon, but they can be found in Wald.

The problem though is to know whether anything in a region of space can escape to infinity depends on precise knowledge of the future, also not all spacetimes have a suitable notion of infinity. Yet clearly even when we don't know the entire history of the spacetime or when there isn't a suitable notion of infinity, we can still identify objects that are functionally equivalent to black holes. The apparent horizon if you like is the spatial boundary of what we might consider to functionally be a black hole.

As nothing can escape a black hole, even light directed away from it is pulled back towards the singularity so we know that a black hole can cause even light directed outwards, relative to a point in space, to move inwards to that point. The apparent horizon is the boundary between where outwardly directed light moves outwards and where it moves inwards.

In Schwarzschild spacetime, or more generally Kerr-Newman spacetime (in standard coordinates) the apparent horizon and the event horizon coincide. However, more generally, the location of the apparent horizon depends on how you 'slice' spacetime (it is observer-dependent if you like - unlike the event horizon). Apparent horizons needn't be associated with (formally-defined) black holes, however as long as the spacetime has certain properties, they will indeed be associated with black holes and will lie at or inside the event horizon.

Would a human body float in the dense atmosphere of Venus?

To survive high in the atmosphere of Venus, all you would nead to wear is suit that protects you from the sulfuric acid vapors in the air and a supply of breathable air. Assuming (for simplicity) that this gear has the same density as the human body (the air supply would probably much heavier), could you float on top of denser gas layers below you?

I believe the density of the gas would only have to be about as high as the density of water on the surface of the sea. And would that mean that when you pour out a bottle of water, the water would float in the air (given the temperature/pressure ratio is below the boiling point)?

Reclaimed bodies in planetary formation?

This question on Worldbuilding.SE got me thinking...

We currently think that many bodies that condensed during star/planet system formation, many bodies are ejected. I've heard it stated that there are more rogue planemos than planets.

Meanwhile, stars form in dense birth clusters. The medium has been enriched by dust from previous generations of stars. So, in general, shouldn't there be larger bodies enriching this medium as well, including as many pre-formed planets as stars being formed (average one per system) and a handful of planetary embryos?

Wouldn't these affect the dynamics of the subsequent formation? Given the current tale of how the condensation of larger objects interact with the disk, having some present initially would mean something, it would seem.

And if a pre-existing large body or brown dwarf is collected into the disk, wouldn't that affect the formation of the new system in drastic ways? And, given the number of systems studied now, wouldn't this be noticed as being a not-unreasonable part of a system's history?

bioinformatics - Online toolkit that provides functional similarity scores (in the form of a matrix) between two functional gene sets in the context of gene ontology

I think that you could try a similar approach to GSFS:

• use transduction in proteins (if you don't know star code, then you must use 3 strings for each gene)




• use a basic tool (a stand alone like UNIPROT tools) to identify the proteic domain type (chain alpha, ..)




• divide the genes by proteic domain type (pdt): which contains which pdt and the pdt order frequencies

Now, you could use DAVID or similar (try wconsensus: it's old, basic but very custom) to compare similar sequence and obtain your scores.