GeeKay

Yes, I'm looking forward to the advent of the James Webb Telescope and although I'm not superstitious, I'll be tempted to cross my fingers when launch day finally comes. Enthalpy, thank you for your comments about the transparency of the atmosphere. Intriguingly, the Bortle Scale - or someone referring to it - did claim an upper naked-eye limit of +7 magnitude, albeit under 'exceptionable' viewing circumstances. If so, then I guess this degree of clarity would also apply to space itself.

If glasses do not increase the resolving power of the eye, what's the situation with binoculars, telescopes etc? Or is there a trade-off here between resolution and field-width? I'm sorry if this seems a stupid question.

Re. the Rayleigh criterion. Yes, I begin to understand now. This raises another question, though: if the eye's resolving power is determined by the size of the pupil, does this explain how glasses (or other forms of external lenses) can overcome this problem?

I understand that there's a definite relationship between the size of an eye and the wavelengths it can receive. This being so, just how small can an eye be, while still able to receive light in the visual spectrum? NB. Here, I'm referring to what might be described as 'camera' (or animal) eyes, rather than the compound eyes of insects. Many thanks.

Just a suggestion: could the alleged meteorite once have been a moon of the Pitbull asteroid, but which was pulled away by Earth's gravity? I've no idea if Pitball ever came close enough to pass within Earth's Hill Sphere, but I do recall a close flyby made fairly recently (2013?) by another asteroid, which definitely had a moon, albeit a very small one.

Having recently chanced upon the Bortle Scale, I'm intrigued to know just how much increase there would be in stellar luminosity as observed from space - say, from the ISS. For the sake of argument, I'm discounting the effects of spacesuit visors etc. Many thanks. http://en.wikipedia.org/wiki/Bortle_scale

Yes, I do support the SETI programme - though I am aware of its inherent dangers (ref. Stephen Hawking). We don't know for sure if an intelligent species has to first get through an 'ethical' bottleneck in order to become a space-faring species. The SF novels by Larry Niven are instructive here. Unlikely though it might be in actuality, it's just possible to conceive of an alien species with 25th century space technology and the morality of The Beano (a British children's comic). Personally, I'm inclined towards the view aired by Frank Zappa, who when asked a related question, responded by saying that he considered the universe to be composed mostly of stupidity. It's the less 'stupid' parts we're discussing here, I guess.

So I gather then that a fairly largish object (an astronaut, say) could safely stand on that part of Phobos facing Mars and not be borne away by the Martian gravity field - or its tidal forces, I mean?

it would appear that any spacecraft being inserted into a fairly stable and fuel-efficient orbit round Phobos (and possibly Deimos too) requires a non-Keplerian orbit, such as are described in the link below. http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/42478/1/12-3014.pdf

Well, let's see: if one accepts that the mass of Mars is 639e21 kg and 1.0659e16 kg is that of Phobos, accepting too that the mean distance between the centres of these two bodies is about 12,7805 km, it turns out (or the calculations appear to indicate) that the gravitational attraction of Mars at this distance is 0.2094 Newtons. Meanwhile, an object orbiting Phobos at an altitude of (say) 31 km from the satellite's centre works out to a measily 0.00074 Newtons. If these figures are correct, this strongly suggests - to me at least - that no stable orbit is possible round Phobos. I did as a control apply the same set of calculations for the Earth and the Moon, and they checked out fine. Of course, the Moon is many times larger than Phobos, but then the distance between the Moon and Earth is in relative terms much greater than that between Mars and Phobos. Indeed, I understand that Phobos is in a dangerously low orbit, that it's not that far off its Roche Limit and that tidal stresses will eventually cause it to break up and form a ring round Mars and/or make a number of impressive dents in the Martian regolith. I hope I've not made too many assumptions - simple or otherwise - in my assessment of the situation thus far. But I do wonder if I've made a fundamental error somewhere along the line. Hence my query. Correction: a misplaced decimal point. The distance between the centres of Mars and Phobos is actually 12,780.5 km.

According to my (often suspect) calculations, it appears that no object is able to orbit Phobos, the inner moon of Mars, without being continually pulled in by Mars' own gravity field. Even the situation with the outer moon, Deimos, is a close call in this respect. I wonder, though, if I've got my maths right. This being more than possible, I would welcome any help or advice. Many thanks. PS. I am aware that maintaining a freefall orbit round either moon would be problematic in any case, given the varying gravitational fields resulting from their irregular dimensions. I'm ignoring that for now.

"I know Darwinian Natural Selection, but I don`t know how you are going to relate it with your explanation. Maybe you can clarify it?" Yes, that's relatively simple: although we Homo sapiens cannot make any direct comparisons, other than studying ourselves in this respect, it would appear that inquisitiveness is a normal component of intelligence. In this thread's context, inquisitiveness = a desire by intelligent extraterrestrials to make contact with other intelligent species. . . ourselves, for instance? Hope that clarifies things.

Nicholas, you should really consider reading up on Darwinian natural selection - assuming, of course, you haven't already? It is at present the only known mechanism that offers a thoroughgoing description of how living organisms thrive and evolve (and become extinct) over time. You won't go far wrong with born explainers like Richard Dawkins or Steve Jones. For starters just google their names. . . Hope this helps.

Yes, I should have qualified my final throwaway remark by rephrasing it as "living nature" instead of just "nature". One may still consider whether it is relevant to the above discussion. I did so myself, hence my tagging it with a question mark. Nevertheless, I still lean towards the adage "nothing ventured, nothing gained" serving as a convenient shorthand for the workings of Darwinian evolution. I could expand upon this point, but feel that in doing so would further increase the risk of my wandering away from the main themes contained in this thread and/or boring everyone rigid by stating the obvious, which is the sin of sins.

Having just reached the midway point of Jared Diamond's excellent (and endlessly thought-provoking) "Collapse: How Societies Choose to Fail or Survive", I too am struck by the fragility (in this instance) of humankind-based civilisations. Personally, I suspect that even if other high-tech civilisations do "currently" exist elsewhere in our Galaxy, we all may remain unknowable to each other, given the sheer immensity of cosmic distances, plus a stern reading of the Drake Equation. On the other hand, we Earthlings could be analogous to those Easter Islanders - that is, we are a relatively civilised species, doomed not only as a result of all the unhappy consequences that flow from our isolation, but also in part because our seclusion has rendered us insensitive to the existence of other (rather more highly) advanced civilisations, some of whom, unbeknownst to us, may have already established worthwhile - even trade-based? - communications with each other. . . though, I realise this is straying into wish-fulfilment territory. Also, it's worth keeping in mind that our projections about any advanced alien civilisations, and eventual ability to communicate with them, may be hamstrung by our present lack of understanding regarding the laws of physics. For all we know, we could be, relatively speaking, still at the Ptolemaic stage in our grasp of scientific knowledge; or as Arthur C Clarke once observed, we are endeavouring to make contact with super-advanced extraterrestrials with the modern equivalents of drumbeats or smoke signals. Still, I believe the attempt should be made, perilous though it might end up being for a whole variety of reasons. But then, nothing ventured, nothing gained. . . a fundamental law of nature?

I understand that many meteor trails we observe here on Earth occur considerably high up above the ground. If so, it would be a question of comparing the average air density of Earth's stratosphere with Mars' own rather more attenuated atmosphere. I guess too that Mars being a somewhat smaller 'target' for meteors would also affect the data, though I may be wrong here. That said, I'm inclined to agree with Moontanman that the meteorite strike rate would be a lot higher on Mars than on Earth - though still next to nothing compared to the intense peppering our airless Moon must receive on a daily basis!

I read somewhere recently that the atmospheric pressure on Mars is equivalent to that on Earth at an altitude of some 35 (miles/km?). This being the case, would the Martian atmosphere still be dense enough to burn up small incoming meteors? Many thanks.

Many thanks. According to the video, it seems that a typical solar flare extends out to about 'several dozen Earth diameters' by the time it crosses Earth orbit. This surprises me. I had always assumed these flares, and/or CME events, were messy, inherently chaotic and widely distributed affairs - like any unrestricted explosion, in fact. I hadn't expected them to be so pencil thin. It appears then that a spread of (say) just 400,000 km over a distance of 1 AU does make for an extremely narrow, almost laser-like beam. I don't pretend to understand the physics involved, but it might help to explain why any such impacts by these 'solar superstorms' (like the one that almost struck the Earth in 2012) are so fleetingly rare, after all.

As a result of recent events, one hears a great deal about solar flares - their attributes, and so forth. However, solar flares have one (I would surmise) fairly important feature, about which the internet appears to be struck dumb. It's this: assuming such flares expand ever further outwards - possibly conically? - once they leave the Sun's surface, just how spatially 'wide' are they by the time they reach Earth orbit? I guess this might vary, depending on the initial energy release etc. All I'm after is an average ballpark figure, if such a thing exists. Many thanks. PS. Happy Solstice Day!

Many thanks for explaining in some detail the cause of this effect. Enthalpy: your recollection of the runaway rotor still has me chuckling

A friend of mine once observed a puzzling phenomenon, which he has never been able to properly explain. This event took place long ago when he was doing an apprenticeship in electrical engineering at a naval dockyard and it concerned a 'runaway' effect that occurred when he was part of a team testing large industrial motors. The cause of this effect, as he recalled, was associated with 'back EMF' and it (or something akin to it) caused the motor to speed up uncontrollably. The runaway effect was so powerful that the instructors had to pull the plug on the motor and/or physically bring the armature to rest before the motor tore itself apart. My friend is a self-employed electrical engineer, yet he confesses that he is still unable to understand why the above effect took place. Therefore, he (and I) would very much welcome an explanation as to its likely cause, or the underlying principles that brought it about. Many thanks.

Yes, I understand now. Thank you for clearing this up for me

So the take-home message is this: any high-speed acceleration of stellar material, resulting from a supernova explosion (whether outwards or inwards towards the neutron core) is liable to produce gravitational waves - this occurring despite there being no overall change in the star's centre of gravity, for example? .

Can a supernova explosion (or implosion?) on its own create gravity waves? Or do you need to have (say) two stars 'merging' into each other for this effect to happen? In other words, is displacement necessary for the generation of gravity waves? Many thanks.

To the above contributors: many thanks for your helpful and insightful comments. I began this line of enquiry at the point where it seemed I'd run out of understanding about GR. I think I have a better grasp of it now. Heaven help my brain cells, though, should a quantum theory of gravity ever emerge!