Ask An Astronomer

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Re: Ask An Astronomer

Postby Lord Chrusher » 04 Feb 2012, 18:13

Henrietta wrote:
Master Gunner wrote:I'm looking for a new background image for my laptop. What insanely large image do you think would make for a cool background? (Other than the Eta Carinae Nebula, that I still use on my desktop).

The Astronomy Picture of the Day archives run back to 1995; you should be able to find something you like.

ETA: I'm currently using this one.


The European Southern Observatory has a lot big images. I got the images at the start of the thread there.

Going to the start of the APOD archives is trip back in time to when the

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Lord Hosk wrote:What about the possibility of He-3 mining on the dark side of the moon which could yield significant quantities. While metals are far to expensive to bring back I have heard that as He-3 is extremely rare on earth as nuclear technology advances it takes the cost to over $2000 per L which make it viable.


Fusion power is upper there with Moon bases on being twenty years in the future for the last sixty years. While using helium-3 does have the advantage of producing far fewer neutrons than using using deuterium and/or tritium as fuel it does require a much higher fusion temperature.

Even though helium-3 is much more common on the Moon than on the Earth it is still quite rare. You would have to process over a hundred million tonnes of Moon rocks to get a tonne of helium-3; to supply all the United States' electrical needs would require twenty tonnes a year. To supply a significant fraction of the world's electricity would require a ten billion tonnes of Moon rocks to be mined and processed, a quantity on par with the amount of coal produced globally each year.

As a low neutron fusion fuel boron-11 produces even fewer neutrons and is much more common on Earth (boron-11 makes up eighty percent of naturally occurring boron; we all ready produce over a million tonnes a year of boron trioxide) boron-11 is likely a better fuel than helium-3.

Having said all that helium-3 is about the only thing that there is more of on the Moon than on Earth and if its price was high enough it would be worth going to the Moon to get it. However if it was that expensive we would most likely use some other, cheaper form of power.
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Re: Ask An Astronomer

Postby Lord Chrusher » 04 Feb 2012, 18:26

Brad wrote:Placing a heavy gas in a dradle-shaped container and rapidly spinning it to create a gradient of atmospheric density and firing a laser into said gradient produces bent (smoothly defracted) light, but not a gravitational effect as a black hole does.

That doesn't seem fair. Is that fair? I want my anti-gravity moon shoes.

A real question - space elevators seem like a pretty swell idea. Space station took a lot of shuttle runs to get that stuff up there - why didn't the elevator happen? Is it another physics thing that doesn't work like it says on the box or costs silly?


It isn't fair. Gravity is much, much weaker than electromagnetism. Gravity is about a trillion trillion trillion times weaker than electromagnetism.

As ecks and Dutch guy have said we currently have no way of making a space elevator.
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Re: Ask An Astronomer

Postby Brad » 04 Feb 2012, 18:34

What's up with dyson spheres? Wouldn't that kill all life?
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Re: Ask An Astronomer

Postby Master Gunner » 04 Feb 2012, 18:42

So using Moon-derived helium-3 for terrestrial power needs is out of the question. One suggestion I've heard in the past, however, was using the moon as a "refueling station" for inter-planetary ships. Would that be feasible, or would the energy needed to get fuel (and water) from the Earth to the Moon likely be less than the energy needed to process the requisite moonrock? I admit that this is a bit out of your area of expertise, though.
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Re: Ask An Astronomer

Postby EikoandMog » 04 Feb 2012, 18:50

Brad wrote:What's up with dyson spheres? Wouldn't that kill all life?

I think I can field this one.

Short answer, all life in the solar system eventually dies.

Long answer. The idea behind a Dyson Sphere is to encapsulate a star for permanent solar energy. Basically, no radiation will ever escape that star again as long as the construction of the dyson sphere remains sound. Things start getting cold in the solar system. Really, really cold. As in "break out the Kelvin, Celsius won't work anymore." Things freeze, plants can't make their food anymore, other things don't get their food and yeah, things just die.
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Re: Ask An Astronomer

Postby Master Gunner » 04 Feb 2012, 18:58

However, Dyson Spheres as represented in fiction would take up all the matter in the solar system (and then some) anyways, and all life is generally relocated to the inside anyways (preventing heat buildup is often overlooked, but presumably not an insurmountable problem next to building the damn thing in the first place). However, in reality, what Dyson proposed was a massive grid of solar power satellites to capture as much of the sun's energy as possible to fulfill the eventual power requirements of a civilization. As it would not be a solid shell, enough life on planets would likely not be seriously effected.
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Re: Ask An Astronomer

Postby EikoandMog » 04 Feb 2012, 19:04

I just assumed the fiction sort and went from there, really.
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Re: Ask An Astronomer

Postby Lord Chrusher » 04 Feb 2012, 20:17

The outside of a Dyson sphere would still be as bright as the star - if you want the temperature inside the sphere to remain constant you have to do something with stars power. However the sphere would be much, much redder than the star. For the Sun and sphere the same radius as the Earth's orbit the outside of the sphere would have to be 393 K (120 C). Unlike the Sun which has a surface temperature of about 5770 K and emits most of its energy around a peak of 0.5 micrometres, such a Dyson sphere would emit most of its light around a peak of 7 micrometres.
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Re: Ask An Astronomer

Postby EikoandMog » 05 Feb 2012, 04:31

Well, I just learned a lot about what a Dyson sphere would entail. Great info, Chrusher.
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Re: Ask An Astronomer

Postby plummeting_sloth » 06 Feb 2012, 20:46

Master Gunner wrote:So using Moon-derived helium-3 for terrestrial power needs is out of the question. One suggestion I've heard in the past, however, was using the moon as a "refueling station" for inter-planetary ships. Would that be feasible, or would the energy needed to get fuel (and water) from the Earth to the Moon likely be less than the energy needed to process the requisite moonrock? I admit that this is a bit out of your area of expertise, though.


Well, this is part of the reason there was excitement when the blew up a piece of the moon and found water ice there. With enough prep, a mining center (particularly a robotic one) would essentially be self-sufficient.
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Re: Ask An Astronomer

Postby allonewordnospaces » 06 Feb 2012, 21:57

Is there any merit in doing another mission like Voyager now, with modern equipment? Would the possible return be worth the investment?
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Re: Ask An Astronomer

Postby Lord Hosk » 06 Feb 2012, 22:37

Why does Captain Picard keep saying "earth miles" when talking about distances on Titan or the measurements of Saturn's rings?
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Re: Ask An Astronomer

Postby the amativeness » 07 Feb 2012, 01:50

Lord Hosk wrote:Why does Captain Picard keep saying "earth miles" when talking about distances on Titan or the measurements of Saturn's rings?


Probably because space travel is usually made with references to sea travel (Admiral/Captain/Fleet etc.). Distance is usually measured in "nautical miles" which are slightly different than "land miles". Kinda weird saying "land" in space, though.
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Re: Ask An Astronomer

Postby Lord Hosk » 07 Feb 2012, 02:42

By Captain Picard I mean Sir Patric Stewart was narrating a program on the science channel.
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Re: Ask An Astronomer

Postby Lord Chrusher » 07 Feb 2012, 04:04

The definition of the a nautical mile was tied to the Earth's diameter. One nautical mile was originally defined as a minute of arc along any line of latitude or a minute of arc along the equator. Now it is defined as exactly 1852 metres.

I do not know why Sir Patrick Stewart would use the term land miles.

allonewordnospaces wrote:Is there any merit in doing another mission like Voyager now, with modern equipment? Would the possible return be worth the investment?


Not really. The Voyager missions just flew by the planets, Jupiter and Saturn in the case of Voyager 1 and Jupiter, Saturn, Uranus and Neptune in the the case of Voyager 2. To get more detail and to fully explore the moon systems of the giant planets you want to go into orbit of the planet. It is not efficient to build a robotic probe the goes to one planet and stays there for years than to carry the fuel required to leave orbit and move onto another planet. Galileo (to Jupiter), Juno (recently launched, also to Jupiter) and Cassini (to Saturn) are this style of mission. However a couple spacecraft have flown by on their way to other objects - Ulysses to the Sun (it is on a highly inclined orbit over the poles of the Sun), Cassini to Saturn and New Horizons to Pluto - to pick up speed from Jupiter's strong gravity.

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Speaking of Voyager, was reading about when Voyager 1 reached the heliopause the other day. How big is the Solar System by your reckoning? Has Voyager 1 left the Solar System yet?


I think you are asking which is more impressive: Voyager 1 still sending data over thirty four years after it was launched or the Spirit mission lasting 2269 days when it was supposed to last 90.

I think the Spirit is the more impressive of the two. The Voyager spacecraft where designed to operate for more than a decade - Voyager 2 was meant to get to Neptune, a trip that took it over 12 years. As well the studying the outer Solar System was one of the goals of the mission and several of the instruments where designed to provide useful data at these great distances.

Spirit ended up lasting almost 25 times longer than planned. Although space is a harsh environment, the Voyager space craft did not have avoid getting stuck, nor survive changing seasons. Spirit used solar panels that where at the mercy of the Martian elements (dust storms) while the Voyager space craft used radioisotope thermoelectric generators which provide a slowly decaying source of heat and electricity for decades (they utilise the heat from the decay of plutonium-238, which has a half life of 87.7 years, to generate electricity and to keep the spacecraft warm).

The solar system like many astronomical objects does not have a clearly defined edge. For example since stars are plasma, the surface is defined as where the star becomes opaque to visible light. The size of star clusters and galaxies is typically given in terms of their half light radius, the radius which in half of their light is emitted.

I am going to put a lower limit 1000 astronomical units (1 astronomical unit = radius of Earth's orbit = 149.6 million kilometres) as 90377 Sedna, a minor planet, at its furthest from the Sun is about 1000 astronomical units.
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Re: Ask An Astronomer

Postby Lord Chrusher » 17 Feb 2012, 04:53

A pretty picture:
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Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Saturn eclipsing the Cassini spacecraft. The pale blue dot above and to the left of the brightest rings is Earth.


Also one of the best articles I have read in the popular press about cosmology.
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Re: Ask An Astronomer

Postby Lord Chrusher » 05 Mar 2012, 05:36

I should get back to answering questions but I am quite busy with paper writing.

This, however, is awesome.

Cities look orange due to the sodium lighting commonly used for street lights. The green and red colour of aurora is cause by oxygen emission.
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Re: Ask An Astronomer

Postby Geoff_B » 05 Mar 2012, 05:42

Ooooh... pretty...
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Re: Ask An Astronomer

Postby Master Gunner » 05 Mar 2012, 10:02

Very pretty.
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Re: Ask An Astronomer

Postby Lord Hosk » 07 Mar 2012, 01:04

:shock: :cry: (sigh) thanks
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Re: Ask An Astronomer

Postby plummeting_sloth » 07 Mar 2012, 11:04

I have such a strong expectation of a video like that to conclude with "Now I rule the sky, mortals! Gaze up and dread!"... then the missiles start falling.
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Re: Ask An Astronomer

Postby Elomin Sha » 16 Mar 2012, 16:28

I have a dumb question for you when I was reading my Atlas of the Universe last night.

Pluto isn't considered a planet any more. If a ball of ice cannot be called a planet why are the gas giants classified as planets?
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Re: Ask An Astronomer

Postby Lord Chrusher » 16 Mar 2012, 20:26

Planets are not defined by what they are made out of.

A planet is something that:

Is not a star (sets an upper limit on mass)

Orbits a star (moons are not planets)

Its shape is determined by self gravity (its shape is nearly spherical; sets a lower limit on mass)

Has cleared its orbit (it is easily the most massive object in its neighbourhood)

Is not an artificial satellite
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Re: Ask An Astronomer

Postby Elomin Sha » 17 Mar 2012, 02:10

That clears it up, thanks.
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Re: Ask An Astronomer

Postby Dutch guy » 17 Mar 2012, 09:24

And pluto fails that because of it's shape and not clearing its orbit right?
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