189 posts • joined Sunday 7th December 2008 13:40 GMT
Do reliable, cheap UV lasers exist? I thought anything much beyond green was a bit of a pain to get working.
Re: the "gravity" of the situation
First sentence should probably read "the orbital period-distance equation", not "Kepler's laws" (it's technically the 3rd law).
Re: the "gravity" of the situation
Planet mass is almost entirely negligible in almost all applications of Kepler's laws, definitely so in the solar system (the barycentre of the SS is within the Sun itself, so assuming that the Sun is stationary is a pretty good approximation).
Seeing as the Earth's mass is 1 part in 10^6 of the Sun's, the Sun contains 99.86% of the entire mass in the solar system, and the vast majority of that remainder is in Jupiter (which orbits pretty happily), slurping up a few extra asteroids will have an approximate effect of "nothing".
~mico - it's actually about 0.05%, so assuming you guessed, you weren't far off!
The last I read, Sgr A*'s radius was definitely less than 1AU (distance from here to the sun), with a mass of 4E6 times the sun's.
Re: couldnt the astronoughts
They'd be astronuts to even consider doing such a thing!
Not only designed years ago, but also using specially-hardened cosmic-ray-resistant skycrane-landing-capable hardware. Space is a nasty place - fewer dust bunnies, though.
Here's the software guy's explanation of it all: http://www.youtube.com/watch?v=BArApRIjdTI&feature=player_detailpage#t=1987s
What's the chance of your birthday being the 5th of March?
That's the chance that your birthday is the same as mine.
(I hope you're trolling...)
@AC: It'll be interesting to see how many posts in we get before that!
Re: Is there an end point?
Well, we've not reached it yet - and that's about the best answer you can get, most likely!
Re: Having been rear-ended by people texting ...
Also don't see how this avoids the texting problem anyway - you don't need signal to be reading/writing a text.
Re: Which Star ?
No idea where you got that information from! The generally accepted system is:
Kepler 36 is the system. In a binary (or more) star system, the stars would generally be A and B - in a single-star system, it is acceptable to drop the A (so Kepler 36 = Kepler 36A). Objects orbitting around Kepler 36A are labelled with lowercase letters, with the central object (the star) being designated Kepler 36Aa = Kepler 36a. The planets are then Kepler 36Ab and Kepler 36Ac, but again the A can be dropped as there is no ambiguity.
An alternative example would be the 16 Cygni system, consisting of 3 stars (16 Cyg A, B and C), with the 16 Cyg B system containing two components - 16 Cyg Ba, the star, abd 16 Cyg Bb, a planet.
Hessman et al's paper is a good reference for this: http://arxiv.org/pdf/1012.0707v1.pdf
Re: The pictures don't do it justice
Neptune is 10x wider than the moon, but this planet is 5x further away - so it would only be double the width of the moon in the sky, which actually seems to be what that image shows!
Re: How do they do that?
And a few notes now I've posted and re-read that:
The Mass/Radius of the star comes from its colour - bluer stars are more massive, larger, and hotter; red stars are smaller and cooler. We've spent a lot of time modelling these! Look up the "Main Sequence" if you want to know a bit about that.
We're not entirely sure on planet compositions for things that aren't represented in the solar system - this is why we're not too sure what a Super-Earth is actually like. When do you stop being a ball of rock and become an ice/gas giant? The theoretical chappies have been having great fun with all that.
Spectroscopy isn't so good for getting actual planet sizes - you can get the mass ratio of the star:planet easily, and if you guess the star's mass, you can get the planet's one too. But as mentioned above, if something is 3 times the mass of the earth, do we model it as a ball of rock or a ball of gassy ice? Ideally you want to combine information from both spectroscopy and transits ("photometry").
Re: How do they do that?
No spectroscopy here (most likely, anyway) - stars have an annoying habit of being so bright that they drown out any light from the planet itself. Instead, you look at how the light from the star dims as the planet passes in front. You can guesstimate the mass of the star quite well, and from that and the orbital period, you can get the star-planet distance. You then know a bigger planet will block more light at a given distance than a tiny one, so you can work out the radius of the planet by the amount of starlight blocked, the radius of the star itself, and how far apart they are. You then use fairly handwavey arguments and say that if it's Earth radius, it's like Earth, and that if it's Neptune radius, it's like Neptune!
That is, of course, much simpler than how it actually is!
Once the Planetary Scientists get their go at a funding round, all the Astronomers will throw a similar fit over whatever potential project gets chosen instead. Almost every budget (whether it be ESA, NASA or someone else) will pick one over the other, and the next budget will give the other lot their go - I've heard some fairly bitter comments about missions that got cancelled "because it was the planetary scientists turn"...
Re: It's kV not kv, BTW
Ah, someone got there before I did. Glad I'm not the only one who knows their units!
Re: Oh FFS
Which works great, until you find a house where you can't get decent satellite coverage - the rather large hill to the south of my home (and the other 34 houses on the road) would make it pretty difficult for me to see any geostationary satellite without a rather large tower for the antenna. Oddly enough, I don't believe that the council would be too happy with someone attempting to stick up 20m poles with antennae on top in a conservation area...
Eurgh, percentages? Horrible things. Decimals make much more sense.
Re: other bright things
Pleiades are still up high in the early evening - Cassiopeia, Orion, Canis Major, Ursa Major, etc. are all easily visible right now.
As always, I shall plug the excellent (and free) Stellarium - the best bit of kit I've found for "what's that bright thing up there?", "what can I see tonight?" and "where is wotsit going to be?", with all sorts of advanced options whilst remaining blindingly simple to use: http://www.stellarium.org/
Re: Re: How do they confirm the planet?
Having glanced through a paper on this (http://arxiv.org/abs/1105.3544), it seems that they grab a series of images - the lensing varies with time (it brightens the background star), so they can plot the light received versus time and compare with models of what a lensing event should look like. The paper takes some time to discuss how they tried to cut out other events that look similar (supernovae, cataclysmic variables).
For completion, the paper the article is about is available at http://arxiv.org/abs/1201.2687 (not all their references are about gravitational microlensing - Bihain et al 2009 seems to have chosen to look in the infrared to find suitably cool objects)
Re: Re: 'Idle' computer power isn't so idle these days
There is (was?) a climate modelling project out there - but as with all these things, it was designed to give a client a chunk of modelling data ("model 1830-2030 using these initial parameters") - and it didn't care if you took a month to do it. The MET want to run single iterations of their model, but in time for the evening's forecast, using what I'd imagine is "a lot" of data, which can't be split up into areas (easily, anyway) - you can't just model North America on one client and Europe on the other.
I would like to disassociate myself from these boffins.
Using E = 1/2 * m * v^2, and rearranging for v = sqrt (2E / m), a 1 ton car would be doing 594mph, and a London routemaster (using dogged's mass) would do 224mph.
Also, dogged's maths is off - multiplying mass by 7.7x reduces speed by 2.77x, for the same energy.
(Aimed at all the posts below too)
The data's all there with filesizes - it's the first link in the article. It does indeed add up to a shave under 500gig.
"Size" isn't everything
Are you talking mass or radius here? In terms of mass, above 13 M(Jupiter) you have a brown dwarf. Radius also doesn't increase with mass, much, at that size - you just squash the gas down more (Saturn is half the mass of Jupiter, but only 1/7th smaller in radius). Brown dwarfs at the lower end aren't significantly higher in radius than Jupiter.
If you had a planet 25 times the mass of Jupiter, it's definately a Brown Dwarf. If you have one 25 times the radius, you're larger than the sun.
"Hudgins didn't elaborate on exactly what he meant by that well-examined patch of sky being a bit larger than your fist, but we can only assume that he's referring to a fist being held at arm's length – and if that's the case, the sky being "positively loaded" with exoplanets may even be a bit of an understatement."
Your fist at arm's length is about 10 degrees wide. Let's say for argument's sake it's 10 degrees high, as well - so your fist covers 100 square degrees (so it is pretty damn close to Kepler's FOV, actually). There are 41,253 square degrees of sky - so Kepler studies approximately 1/410 of the sky.
In that region, it studies 145,000 stars. Let's say 2300 planets is about 2000 stars, accounting for a few multi-planet systems - so of those 145,000 stars, 1 in 73 have had a potential planet come between us and the star 3 times in 3 years (I believe it's 3 transits to be a candidate, anyway - that might be "confirmed", though). Just think of all the planets whose orbits do not come between us and their star (anything more than a few degrees either way); that are far enough out to orbit less than once every year (every planet outside Earth in our solar system); and are too small for Kepler to see (<Earth-ish size).
"Understatement" may be an understatement.
"The microlensing process can tell boffins the mass of the planet, but unfortunately, it can't give the scientists any idea of what that world is made of. Just because a world is within the habitable zone, doesn't necessarily mean it will have the life-giving composition of our own planet."
Not quite sure how you linked "mass" to "habitable zone" there - the mass of a planet isn't necessarily linked to where it orbits (depends what theory you feel like using, and exosystems have thrown out some really odd distributions). In fact, the mass of the planet is a pretty damn good indicator of composition - if it's the mass of Jupiter, it's probably not made of rock.
Is it just me who thinks those photos make it seem like it's really rather dark in there?
Geodesy - a pretty well established science, and you've been able to show the earth's gravity varies by position for a long time.
As for doing it by satellite, GOCE's been in the news a lot, surprised you didn't hear: http://en.wikipedia.org/wiki/GOCE
That's half the issue here - going from meteorite to rocky planet, there's very little difference other than "size" (and for gas giants, it's just the fact they're massive enough to hold onto Hydrogen/Helium/Volatiles).
Size does determine whether or not you can differentiate into a distinct structure (Core/Mantle/Crust) or are just a pile of assorted rubble. The thing is we don't quite know at what size that happens - hence why Vesta being less rubble-y and more structure-y is a bit of a surprise!
Yeah, that part's pretty hard to imagine - Plasma's a bit warmer than that.
The issue with "other forms of life" is one of chemistry, really. You need to base it around some fairly common element - so nothing heavier than Iron (the element at which nuclear fusion gives up), and the lighter the better.
You want something which is able to bond to as much as possible - Carbon and Silicon are the two good examples there. You then need to work your way up from that:
Hydrogen will almost certainly be involved, as it's so damn common
Nitrogen isn't too useful as it's pretty unreactive chemically in its standard (N2) form (note that 80% of the atmosphere is Nitrogen, and it does bugger all most of the time)
You then need to consider energy requirements (photosynthesis, geothermal, or "eating stuff" are the three I can think of), and so on...
Asimov (I think, anyway) wrote a short story on that - a set of astronauts are sent out to the moon, and upon getting round to the dark side, find it's made of wooden scaffolding and paper (one of them having spent a fair bit of the mission saying "wouldn't it be weird if it was all just made to fool us..."). Upon said crew member getting a little "worked up", they are amazed to find ground crew opening up the hatch - discovering it was all a psychological experiment, they never actually left the Earth, and they weren't meant to last long enough to get round the back of the moon...
$5m in 2013?
That's some funny predicting you've got going there. Third it 2010-2011, third it 2011-2012, then... halve it 2012-2013?
"The space truck is the size of a small SUV"
We can launch something that size and (hopefully) land it on another planet? Oh science, you are wonderful.
Phobos-Grunt was intended to land on Phobos and then launch back to Earth again - so it needs to carry enough fuel to wander all the way back to Earth again. Curiosity is only going one way, so needs half as much fuel (for argument's sake, I doubt the planned routes to and from Phobos are identical) - so NASA can afford to go on the slightly more "expensive" route by making their fuel tanks a little bit bigger.
This wiki article gives a fairly good overview of how much "Delta-v" is used for certain transfers: http://en.wikipedia.org/wiki/Delta-v_budget
Interestingly, according to that page, orbit-keeping is fairly cheap (25-100m/s a year) versus the minimum 12,400m/s to Phobos and back. Assuming a couple of percent spare fuel on board, it might actually be possible to wait til the next window, on a fuel basis alone.
Thing is you need to build a particle accelerator over in Italy as well then - and those are neither small nor cheap.
In addition, the neutrino pulses can only be created when you whack a "bunch" of protons from the ring into the neutrino-makery-thingy - the beam isn't constant, so you'd have to wait until a bunch is in the right place on the ring, and you'd have some even more fun errors taking into account the time for the system to react to the incoming signal and the time for the bunch to reach the target to be neutrino-ised.
Also, not quite sure what this would achieve anyway, other than showing that the effect happens both ways, rather than in only one!
Active != Adaptive
"employs piezoelectric actuators to warp the shape of a telescope mirror"
Changing the telescope mirror is Active Optics, which works against wind, gravity (a 10m telescope bends a bit as you turn it around), etc.
Adaptive Optics works with an entirely different (much smaller) mirror. It's a bit hard kicking around a few tons of mirror accurately on millisecond timescales. You can even fit Adaptive Optics to refracting telescopes ("ones wot got lenses"), whereas it's a bit harder to fit actuators on a lens (tends to get in the way of the light, apparently).
I did snigger somewhat
"It is easier to reach Mars than to visit even the base of the Earth's thin crust"
Try telling the Russians that...
@Too Long An Interval
Actually, you can get some pretty speedy astronomy these days - pulsars are measured down to periods of 1.4ms, and sampling theory states you'll need to get a snap less than every 0.7ms for that.
You are right to say that you'd get nowhere near 60Hz for something as faint as city lights, though.
Isn't that what they all say?
The stars orbit eachother at about .2AU, the planet is out at .7AU. Page 11 gives a diagram, 13 has numbers: http://arxiv.org/ftp/arxiv/papers/1109/1109.3432.pdf
It should be noted that they predict that the orbits will change over time - they mention that the planet will stop eclipsing the stars at some point in the future, before returning to eclipse again after a couple of decades.
Plane of motion is an appropriately boffiny term that comes to mind.
Or as my lecturers would say...
Oh Bugger, A Fucking Goat Kicked Me