medical - How important is the size of an astronaut?

In the early days of crewed space exploration, size and weight of crew was an issue. The original batch of Soviet cosmonauts selected were restricted to 1.75m height and 72kg mass, for example, and US astronauts to 1.80m and 82kg.

As launchers got bigger and spacecraft got more complex (and roomier), these restrictions became less important because the crew represented a progressively smaller share of the mass budget.

For example, in the Mercury program, the astronaut's mass was ~5.8% of the spacecraft mass; in Gemini, the crew was ~4.3%; in Apollo earth-orbit configurations, ~1.2%.

At some point in that progression, it makes sense to start relaxing the astronaut height/mass restrictions. China's space program simply hasn't advanced far enough for them to do so yet.

As for inherent advantages of a larger crew member, all other things being equal, larger people in good health are generally stronger, which can be important during certain EVA operations. In most other criteria a smaller crew member would probably be very slightly preferable primarily due to resource consumption rates.

cosmology - What does Stephen Hawking mean by 'an infinite universe'?

A relativistic universe which is expanding faster than light (like ours) is effectively infinite for all practical purposes.

Also due to its relativistic nature and faster than light expansion, even if you assume it's not infinite at some given moment, it still doesn't have any edges or borders - for you. You're in some random place in it, the maximum speed of any possible interaction is speed of light, and the universe is expanding faster than light - then anything that happens at some (real or imaginary) "edge" is outside your realm of existence. It is effectively infinite - for you.

Give it enough time and it would grow as big as you want. Give it asymptotically infinite time and it will grow asymptotically infinite.

As to what the "edge" might be, see Eternal Inflation. This is a model in modern cosmology where local bubbles like ours have stopped inflating (they still expand, but not at the tremendous rate of the initial inflationary phase); however, inflation continues forever outside the bubbles and at the edges. Therefore the bubbles keep growing indefinitely at faster than light speeds. Because speed of light is a limit for any interaction inside the bubbles, for any internal observer each bubble is effectively infinite for any practical purpose.

Be aware that there is no proof that this is actually the case, but this is a model that fits well what we now observe.

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EDIT: TBH, I'm not even sure this is a question for StackExchange. It's very open-ended, and we don't really have the conclusive answers. All we can say for now is that the universe appears to be infinite for all practical purposes, but we can't know for sure. So often scientists like Hawking just simplify the language and refer to the universe as "infinite" without any of the qualifiers that would be required in a strict context.

I don't think there's any single, final answer here.

orbit - Is there online data on asteroid axial tilts?

From this article, http://hubblesite.org/pubinfo/pdf/2010/33/pdf.pdf, here's the rotation axis for 4 Vesta: RA=307.5°±3.1°, Dec=43.1°±1.2°. Update, per TildalWave's comment: RA=305.8°±3.1°, Dec=41.4°±1.5°

Rotational motion of Ceres.

An old article (1986): Magnusson, P., Distribution of spin axes and senses of rotation for 20 large asteroids, Icarus 68:1 1-39 http://www.sciencedirect.com/science/article/pii/0019103586900722

planet - Spheres in space

I know that gravity will turn a mass into a sphere, which is why
planets and stars are that shape. But then we have asteroids and small
moons which are not spherical. Such as a couple of Pluto's moons. Is
there a size at which they will become spheres? Is it dependent on
their composition?

Google searches provide a wide variety of answers to this and while the other question is answered, it's only answered briefly, so I thought I'd give this a try.

Mike Brown's Planets was the best link I could find on the subject, and I can't swear by his correctness, but his numbers are in the range you find in other articles.

While we can't see most of the objects in the Kuiper belt well enough
to determine whether they are round or not, we can estimate how big an
object has to be before it becomes round and therefore how many
objects in the Kuiper belt are likely round. In the asteroid belt
Ceres, with a diameter of 900 km, is the only object large enough to
be round, so somewhere around 900 km is a good cutoff for rocky bodies
like asteroids. Kuiper belt objects have a lot of ice in their
interiors, though. Ice is not as hard as rock, so it less easily
withstands the force of gravity, and it takes less force to make an
ice ball round.

The best estimate for how big an icy body needs to be to become round
comes from looking at icy satellites of the giant planets. The
smallest body that is generally round is Saturn's satellite Mimas,
which has a diameter of about 400 km. Several satellites which have
diameters around 200 km are not round. So somewhere between 200 and
400 km an icy body becomes round. Objects with more ice will become
round at smaller sizes while those with less rock might be bigger. We
will take 400 km as a reasonable lower limit and assume that anything
larger than 400 km in the Kuiper belt is round, and thus a dwarf
planet. We might be a bit off in one direction or another, but 400 km
seems like a good estimate.

Other estimates I've read have 600 KM diameter for rocky bodies and Ceres is ice and rock, not really a rocky body, so I think there's some uncertainty in there.

Does it need a certain mass?

I personally don't like talking about planets by their mass cause it gets rather unwieldy in size. Take Ceres, mentioned above. It's 8.958 × 10^20 kg. A 400 KM diameter ice world would have a mass of, figure a water-ice average density, about 3 x 10^19 and 200 KM diameter, 1/8th of that, about 3.75 x 10^18. The minimum mass is somewhere in those ranges, but I think radius and composition are easier to work with.

Is it only because of the heat of collisions melted the rocky planets
that they became this shape while they were semi liquid?

I can only give an intuitive answer here, but collisions that liquefy a planet are rare and, especially with smaller planets, would be more likely to blow the planet to bits as soon as melt it. Take the giant impact on Mars. Article Here and Here. The 2nd article says that the object that hit Mars was thought to be traveling at about "6 to 10 kilometers per second." and size "roughly 1,600 to 2,700 kilometers across" - so, bigger than Ceres, Smaller than the Moon and this may have melted half of Mars' surface but it didn't melt all of it cause it left a measurably difference in Mars crust one side to the other. If a collision of that enormous size didn't melt Mars all the way around - I'd wager that such melting is pretty rare.

2nd point, as a general principal, the Solar system objects orbit in the same direction and the further out you get from the sun, the slower the orbital speed (though the more likely you get eccentric orbits). The speed of collisions in space are roughly the vector addition of the 2 objects orbital speeds, which are often somewhat in line with each other, plus the escape velocity of the more massive object (or a bit more than that if both objects are massive), so once objects get to be a pretty good size, about the size where they become spheres, collisions that would melt the object grow less and less likely, and if it is large enough to do that, you should also expect a fair bit of debris from impact. My guess, based on just barely enough knowledge to think about this stuff, is that planetoids and large enough to maybe get spherical asteroids are unlikely to form into spheres due to melting from heat from collisions because due to the relatively weak gravity of smaller objects like that (Ceres has an escape velocity of 0.51 km/s, about 1140 MPH), an impact to generate enough heat to melt it would blow half the dwarf planet away, give or take.

Now, if you have a very hot planet that's very very close to the sun and at near liquid temperature anyway or if you have a coalescing and a multitude of successive impacts and lots of heat collecting in the object, then, sure. It's certainly possible but my guess is that a planetoid or asteroid forming into a sphere due to melting it's rare.

That's a layman's answer.

the moon - Day and night temperature on an earthlike planet with longer rotational period

I'm trying to understand the climatic effects of the far future scenario of an Earth-like planet with a reduced rotational speed caused by tidal locking with the moon (day-night period of 28 days, one hemisphere always facing the moon, the other never facing the moon).

I understand that observations of the moon's surface temperature have shown day temperatures of 120°C and freezing cold nights of -230°C.

How would the day and night temperatures of tidal locked Earth vary?

I am also interested in climatic effects caused by the reduced tides, coriolis force, increased evaporation, but this may reasonably be outside the scope of this question and it's answers.

Can a supernova make a new star?

This idea has been around for decades, so I'm not sure who first came up with it. Here's a reasonably sourced article on the involvement of supernova in solar system formation:Exploding Star May Have Sparked Formation of Our Solar System

The shock wave from an exploding star likely helped trigger the formation of our solar system, according to a new 3D computer model, researchers say.
The solar system is thought to have coalesced from a giant rotating cloud of gas and dust known as the solar nebula about 4.6 billion years ago. For decades, scientists have suspected a star explosion called a supernova helped trigger our solar system's formation. In particular, the shock wave from the explosion is thought to have compressed parts of the nebula, causing these regions to collapse.
According to this theory, the shock wave would have injected material from the exploding star into the solar nebula.

So yes, material from supernova can end up triggering the formation of new suns, and material from supernova do end up mixed in with the nebular material that makes the new stars.

the sun - Is it possible to move a planet out of its orbit? At least a lighter planet?

yes it is possible by very few different ways.

https://www.uwgb.edu/dutchs/pseudosc/flipaxis.htm

Nothing acting solely from on or within the Earth could change its orbit or seriously alter its rotation.
One way to move an object is to throw mass in the opposite direction, the way jets or rockets do.
If we think really big and imagine blasting a chunk out of the Earth as big as North America and 100 miles thick so that its final speed, after escape, with respect to the Earth is 25,000 miles an hour, we will have expelled only 1/500 of the total mass of the Earth. The Earth would move in the opposite direction 1/500 as fast or 50 miles an hour. The speed of the Earth in its orbit is about 67,000 miles an hour. We will not change the orbit of the Earth very much--if we apply the impulse to speed up the earth in its orbit we would put the Earth into a new orbit with its most distant point about 70,000 miles further from the Sun than now--and the Earth's distance from the Sun varies now by three million miles over the course of a year! Exactly the same arguments apply to changing the orbit of the Earth through the impact of a large asteroid. The largest asteroid, Ceres, about 600 miles in diameter, is only about as massive as our hypothetical chunk of Earth above. Changing the orbit of a planet is a tall order. An impact big enough to have even a tiny effect on the Earth's orbit or rotation would almost certainly destroy all life on Earth as well.

http://usatoday30.usatoday.com/news/science/astro/2001-02-15-orbit.htm

hope this two links provides enough satisfactory explanation to you. if not do say so i will make further search to clear you the answer.

Can you see man-made lights on the dark side of the Earth from the surface of the moon with the naked eye?

It's highly doubtful you could see any normal light source on the surface of the earth.
Using $$text{brightness} = frac{text{luminosity}}{4 pi times text{distance}^2}$$
(with brightness in watts, and luminosity in watts per square meter.
and distance to moon of $3.84 times 10^8$ meters.)

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Try a hypothetical light source 100 megawatts output, all visible light, no heat.

$$text{brightness} = frac{100 times 10^6}{4 pi times 1.474 times 10^{17}}$$

$$text{brightness} = 5.4 times 10^{-11} text{ watts per square meter}$$ at the lunar surface.

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That's pretty dim. By, contrast sunlight at earth's surface runs about 1300 watts per square meter.
In reality it'd take about a gigawatt to produce 100 megawatts of light in the visible range.
That's about what it takes to power a city of a million homes.
Cities also bounce most of the light they do produce off the ground, which'll have an albedo of somewhere around 0.3. So with ordinary city lights it'll take over 3 gigawatts to reach $5.4 times 10^{-11}$ watts per square meter on the lunar surface.

You might fare better with a big laser. The Apache Point Observatory Lunar Laser-ranging Operation picks up multi-photon signals from the Apollo retroreflectors using only a 1 gigawatt laser and a 3.5 meter telescope. As the article states, the laser beam only expands to 9.3 miles in diameter on the way to the moon, so you might see it wink at you.

At 10 parsecs, the sun has a magnitude of 4.83. It'd be visible on an average night. That magnitude corresponds to a brightness of 3X10e-10 watts per square meter, about 5.6 fold brighter than our hypothetical earth based light source. That puts our light at magnitude 6.5 to 7. Naked eye visibility runs to about 6.0

fundamental astronomy - US observations relevant in UK?

If they mention times of night then you have to figure out to what extent the time zone (BST/GMT vs the US time zones) makes any difference, but it shouldn't make more than an hour difference either way for an object that is moving sidereally with the rest of the night sky.

The main difference would be that the USA is at lower latitudes than the UK. Therefore there could be instances where an object with a small or southerly declination is better seen from the USA than the UK. Conversely, more of the sky is circumpolar from the UK.

Finally, if you are trying to see a specific event at a given time (UT), then of course it may well make a difference. The Sun could be up in the UK and down in the USA, or vice-versa.

space time - traveling past the speed of light

According to the theory of relativity (If I understand correctly), nothing is supposed to be able to travel faster then the speed of light. I believe, to my limited knowledge and grasp of the concept, that quantum theorie speaks against, i.e. extend, the theory of relativity, hence my question:

Suppose we find (or imagine) a place in the universe where there is no obstruction for this thought experiment to take place. We take a spherical object and attach a spire at both sides so you get an object shaped like this: -O-

We start turning the object so the ends of the spire travel in an orbital mather according to the spherical object. What if the sphere starts to orbit near the spead of light, wouldn't the ends of the spires travel faster than the speed of light?

Sorry if this is the wrong place for this question.

the sun - What is the air pressure in the heliosphere (Sun's atmosphere)?

I am going to assume that what you mean here is "what is the pressure" in the solar wind? There is no "air"!

The solar wind is pretty complex, consisting of a "fast wind" observed predominantly at high solar latitudes and a slower more variable wind a low latitudes. Both components essentially consist of an expanding stream of protons, electrons plus a small fraction of helium nuclei.

I think all the information you want is contained in this paper by Ebert et al. (2009), but I will attempt a broad-brush summary.

The fast wind has a typical proton number density of $n_p=3$ cm$^{-3}$, a temperature of $T=2times10^{5}$ K and a speed of $V=700$ km/s. Note that "temperature" is a slippery concept here since the velocity distribution of the particles is non-Maxwellian. However, ignoring this and assuming the protons are matched by numbers of electrons, then the "thermal pressure" $P_{th} = (n_p + n_e)kT simeq 10^{-11}$ Pa. However, this is negligible compared with the dynamic pressure $P_d = rho V^2$, where $rho$ is the mass density given roughly by $rho = n_p m_p$. Thus $P_d simeq 2times 10^{-9}$ Pa (i.e. about $2 times 10^{-14}$ bar).

The "slow wind" is quite variable, but mean values could be $n_p = 6$ cm$^{-3}$, $T=8times 10^{4}$ K and $V=400$ km/s. Here again, the thermal pressure $P_{th} simeq 10^{-11}$ Pa is dwarfed by the dynamic pressure $P_d = 1.6 times 10^{-9}$ Pa.

The thermal pressure component falls as distance from the Sun cubed, ie. as $P_{th} propto R^{-3}$. This is because for mass conservation, then a sherically expanding wind must have a density that falls as $R^{-2}$, but it is experimentally found that the proton temperature also falls as $simeq R^{-1}$.

The dynamic pressure does fall almost as $R^{-2}$. This is because there is very little radius dependence for the proton velocity (i.e. the protons do not slow down, right out to the heliopause - as measured for instance by Voyager 2).

exoplanet - What are the analysis steps in taking raw data from Kepler to a planetary system determination

I wish to get a concise list of the analysis steps required to take raw light data from a Kepler data set of a star through the steps needed to get to an analytical determination of the existence of a planetary system. I would also like any software analysis tools that would be suggested for each data analysis step. A good reference specifically outlining these steps would also be helpful.

I would first like to replicate the complete set of steps needed to corroborate the existence of a well excepted planetary system. After that I would feel confident that I could move on to analysis of some Kepler KOI data sets that NASA has published or even some data from a non-KOI entity. I am an amateur and I realize that astronomical knowledge needs to be applied at each data step and that this might not be an automated process but the steps of this analysis would open up new areas of study for me.

I have tried to assemble the basic data analysis tools from various astronomical python sites and other light transit tool sites and get a little familiar with them but I am not clear as to the actual steps as to how these programs feed to each other. I have looked at the NASA Kepler pipeline to see the steps but it is not clear to me what specific software they apply. I have read several papers confirming exoplanetary discoveries but from them I can't get a complete picture of the data analysis steps and software employed.

It is disappointing that the actual code listings are not presented in the papers or in a separate reference doc. In the past (50 years ago) , it was required to publish code listings in reports and references to the code listings in any papers. But, this discussion is probably another subject.

Tom Kosvic

orbit - Earth's gravitational pull on ISS

Your question presumes that the ISS is beyond Earth's gravity, that it has escaped earth's gravitational pull. This is not correct. All objects with mass in the universe affect all other things with mass in the universe, the effect just gets weaker with distance. So the ISS is feeling the effect of gravity from Earth significantly more than the moon is.

The reason the ISS doesn't just fall to the earth, either directly or gradually spiralling towards the earth, is that it is travelling fast enough around the earth that it is continually "missing" earth. It is sometimes described as 'falling' constantly around earth.

If I am to be properly correct though, the reality is that ISS is in fact falling towards the earth, getting closer and closer to Earth all the time. It needs occasional boosts to push it further back out in it's orbit.

Just to blow your mind a little bit: The ISS is pulling on the Earth with the same force that the Earth is pulling on ISS.

neutron star - Pulsars with accreting disk in binary system

Consider a binary system with a neutron star and a companion star. It is fair to assume that the two stars have had tidal interaction, such that their angular momentum is aligned. This is just saying that both stars rotate in the same direction as their orbital motion.

Now say that the companion is filling its Roche Lobe and steadily transferring mass into the gravitational potential of the neutron star.
At this point there are two things we need to realize

1. The transferred matter has velocity and angular momentum which is set by the motion of the companion star.


2. The matter is being transferred in a rotating system, so from the neutron star's reference frame it is subject to the Coriolis effect.

Then, as the matter stream moves down the potential well, it will fall behind the neutron star (Coriolis effect), only to loop around and settle into an orbit (angular momentum). The disk then rotates in the same direction as the orbital motion, making it prograde with with neutron star rotation.

Note that this explanation is rather simple and by no means comprehensive. There may very well be peculiar situations in which a retrograde disk forms. I know that at least for black hole binaries there are some systems suggested to be retrograde.

For more details on mass transfer you can look at this file.

galaxy - Are there heavenly bodies between galaxies?

Certainly "empty" space between galaxies has some things in it. Space has properties in and of itself, such as dark energy and virtual particles appearing in pairs and then disappearing, but ignoring the virtual and the not very well understood, empty space is full of photons, more specifically, Cosmic Background Radiation and all kinds of wavelengths of light from galaxies as well as cosmic rays and a boatload of neutrinos.

Most of the matter that came out of the big bang was gravitationally drawn into galaxies, but likely not all of it, so there likely is some primordial matter between galaxies too, just not very much of it. (precisely how much is a bit over my pay grade).

Also, as Conrad Turner points out, 3 or more body gravitational interactions can from time to time, kick a planet or star out of the galaxy but gaining sufficient velocity to escape the dark matter halo completely is probably quite rare, outside of galaxy on galaxy collisions, but rogue stars/planets outside of galaxies probably happens from time to time.

the moon - Is a spotting scope or binoculars a better choice for astronomy?

I'm interested in observing the moon and planets, and maybe some nebulae and star clusters.

I have a pair of binoculars:

• Nikon OceanPro 7x50 binoculars (around $300)


• Celestron 20x80 binoculars (around $100)

I'm looking at a step up, to 100mm. Specifically, I'm trying to choose between 25x100 binoculars (either a Celestron or an Orion) and a 100mm spotting scope.

The only reason I'm considering a spotting scope is that it has a magnification of 22-66x, compared to the binoculars' 25x. But I want to check if such high magnifications really work for astronomy.

Let's say I'm observing the full moon. What's the highest usable magnification, without resulting in too dim an image?

An Amazon review says that:

I have found that while the readability improves from 22x to about 38x
the resolution actually decreases after that and I never use it from
40-66x, there is no point. Maybe it would have been better to have
optimised it for 22-40x.

If the scope is limited to 40x in daytime use, I expect it to be worse at night, even observing the full moon.

I'm considering buying the scope only for the increased magnification over the binoculars, but if I end up using the scope only at 22x, it defeats the point of buying the scope in the first place. Should I stick with the binoculars?