typodupeerror

## Comment Re: WTF? (Score 2)321

No it doesn't.

Just because energy is on one side of the equation and mass on the other, that doesn't mean they are the same thing. There is an equivalence between them in the same that a certain amount of mass can be converted into a certain amount of energy (and vice versa), but it doesn't mean that they are the same.

And this equation is specific to the situation where the object with mass isn't moving (which is why E in this case is called the "rest mass energy"). More generally in special relativity, for an object that is moving and has a momentum p, the equation becomes:

E^2 = m^2c^4 + p^2c^2

## Comment Re: WTF? (Score 3, Informative)321

I suspect that you have missed the point entirely: silentcoder made the correct distinction between "mass" (an inherent property that depends on the number of atoms etc. in an object and that is independent of where the object is) and its "weight", which in physics terms means the force exerted by that object on something, which is the mass times the local acceleration.

Thus a person with a mass of 80kg standing on the Earth exerts a force due to gravity pulling them down onto the surface, i.e. 80 kg x 9.8 m/s2 = 784 Newtons. But for all sorts of obvious reasons, we just use the shorthand version to say that the person "weighs" 80 kg.

On the Moon, their mass would be the same, because they'd have the same number of atoms in their body. But they'd exert much less force on the surface, because the gravity on the Moon is only 1/6th of that on the Earth. So, they would weigh less. It's at that point that the shorthand way of talking about weight becomes useless.

Take the person and stick them infinitely far from any gravitating body and there would be no acceleration and thus no force, so the person would be weightless, but not massless (same number of atoms still).

Of course, in low Earth orbit, you're right in pointing out that the Earth's gravitational acceleration has not diminished much. However, while you're falling freely towards the surface of the Earth under that acceleration, the spacecraft you're in is falling out from underneath you at the same rate, so you don't exert a net force on it. Thus you're effectively weightless.

(If you're both falling freely towards the Earth, why don't you hit it at some point? Because you're flying sideways at such a high speed that the Earth's surface curves away from underneath you at just the same speed as you're falling towards it, so you never hit.)

But here's another thing. Under general relativity, gravity is much better thought of as a curvature of spacetime and it turns out that the motion of even massless objects (photons) is affected by that curvature (think Einstein, Eddington, etc.). Indeed, given a very strong gravitational field / very high spacetime curvature, e.g. around a black hole, photons can go into orbit. This is because while they don't have any mass, they do have energy.

So, in a more correct general relativistic setting, even your basic assertion that "to be able to orbit, you must have weight/mass" is wrong.

## Comment Re:Rather a very poor job. :-( (Score 1)100

I have to say, this is borderline pointless.

Firstly, most of the detail is of relatively uninteresting bits of snow, rock, and ice: there's no real motivation to zooming in and poking around, as there is in a similar multi-gigapixel panorama of a city, for example.

As someone said above, the real grandeur of the scene comes in taking in the wider view, at which point the whole hi-res aspect is totally moot.

Plus the mosaic making sucks. Really, right from the get-go, the repeated features in the foreground snowfield are utterly distracting, followed by blurring in many areas when you zoom in a little.

Admittedly, such things are extremely difficult to do well / right (I know from experience), but I've seen plenty of other panoramas which are far better in post-processing.

Ultimately, the question becomes: why bother? Oh, because it's "the most pixelliferous image ever taken". Sigh.

## Submission + - Rosetta team asks for help from amateur astronomers

schwit1 writes: Want to help the Rosetta science team study Comet 67P/C-G? If you are an amateur astronomer, you can!

They are asking amateurs with larger telescopes to observe the comet as it makes its close approach to the Sun and report their observations. The science team will compare their observations with the amateurs. This will teach us how to interpret future comet events when seen from afar.

## Submission + - Rosetta's "Ambition": science fact meets sci-fi

Trapezium Artist writes: The European Space Agency has released a cool short science-fiction film called "Ambition" to help build the excitement and engagement as the Rosetta mission is just weeks away from making the first ever attempt to land on a comet.

Made by Polish VFX and film house, Platige Image, directed by Oscar-nominated Tomek Baginski, and starring Aidan Gillen (Game of Thrones) and Aisling Franciosi (Jimmy's Hall), "Ambition" was filmed in Iceland and shows a Master and Apprentice transforming the desolate landscape into a giant solar system, in search of answers to fundamental questions about humankind.

The film revolves around key human traits of being adaptable, ambitious, and willing to take risks in reaching for the stars. In much the same way, Rosetta has become the first mission to rendezvous with a comet, to escort it, and on 12 November, to attempt a soft-landing on one, all in search of clues as to the origin of Earth's water and complex organic materials.

## Comment Re:In Orbit? (Score 1)54

Glad to see that you jumped in on this: good description.

Because the comet is so small, the gravity changes a lot with "altitude" from the surface. For a 2-km diameter sphere, say, then the difference in gravity between an altitude of 2-km and 6-km (i.e. between 4 and 8-km from the centre of the sphere) is a factor of 4. On the Earth, it barely changes at all between altitudes of 2 and 4-km, because this is a tiny change relative to the 6400-km radius of the Earth.

So, yes, at 100-km and 50-km, we'll be flying these hyperbolic arcs (slightly bent by the very weak gravity), using thrusters to "turn the corner" at the end of each leg. But at 30-km, we'll be on closed more-or-less circular orbits: I'm pretty sure that it is natural orbit though (and thus fairly long in duration), but not powered.

I do work on the project, albeit not on the flight dynamics side. One of our experts on this, Frank Budnik, did give a talk on this in the science session I moderated yesterday afternoon, starting at 11:28 into the recording of the live stream here:

## Comment Re:ATLAST (Score 2)46

Absolutely right: I was going to point out the same thing. It's many, many years away from any possible launch ...

For reference, the James Webb Space Telescope (or NGST as it was then) was beginning to be picked up as a serious prospect by NASA, ESA, and the Canadian Space Agency in the late 1990's. It's due for launch now in 2018.

(This is not meant as a criticism: I've been closely involved with JWST since 1998 and know how hard it has been in terms of technology, programmatics, and politics to get the good state it's in today, namely mostly built and now entering the comprehensive integration and test phase.)

So, very crudely, I'd say that something like ATLAST might be launched after 2035, if it gets picked up as the highest priority in the next US astronomy decadal survey.

## Comment Re:Surely ironic (Score 2)276

Good point; I did use the word "we" in a rather catch-all manner there, and I'd also agree that technologists are likely to have a much better record at predicting the future than journalists.

But I'd then turn the tables and say that it depends on the timescale implied by "future". On a ten-year horizon, I'd agree that technologists are likely to have a pretty good idea what's coming, in part because they're likely to be working themselves actively on new technologies and products for release on similar sorts of timescales.

But on a 100 or 50 or even 30 year horizon, as this article refers to? It seems clear to me that on some timescale, even technologists are unlikely to be that close, if only because they're probably called "futurologists" at that point, or "science fiction writers" :-)

On some timescale, almost everyone is going to be pretty much guessing ...

## Comment Re:Surely ironic (Score 3, Interesting)276

OK, now having read the linked article (oops), I do see that the author (Henry McCracken) realised that the cover painting had a humorous intent (not least that it was the April edition of BYTE), satirising the conservative opinion that future tech was likely to be an extension / miniaturisation of the then-prevalent PC paradigm.

Good to see I got it, though :-)

## Comment Surely ironic (Score 4, Insightful)276

C'mon, it's entirely obvious that that "PC on a watch" painting is a rather clever piece of irony or even satire, not a meaningful prediction of an actual future piece of technology.

That doesn't mean I disagree with the point of the discussion, namely that we're not that great at predicting the directions of future tech, but using this magazine cover as a direct illustration of that is, IMHO, rather disingenuous.

## Comment Re:What's been the hold up???? (Score 1)100

Actually, ESA built the Huygens lander which descended to the surface of Titan. It was carried there on the NASA-ASI Cassini orbiter after being launched by a NASA rocket, but Huygens was European-built, with instruments from Europe and the US.

Its the U-571 gambit: keep saying that things were achieved by the US independent of the truth of the matter, and pretty soon it becomes received knowledge.

"Conversion, fastidious Goddess, loves blood better than brick, and feasts most subtly on the human will." -- Virginia Woolf, "Mrs. Dalloway"

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