Post by: Airthumbs on 17/01/2013 02:31:37
Post by: CliffordK on 17/01/2013 03:43:39
If you think of an isosceles triangle,
3,474 km (diameter of the moon) on short side.
1,737 km (radius of the moon)
384,399 km (distance to the moon)
SOH CAH TOA
Ok, I need a "right triangle" for my trig functions, but I can use half an isosceles triangle, ant thus use the radius.
Tan(½θ) = opposite/adjacent = 1,737/384,399
½θ = arctan(1,737/384,399) = 0.0045 radians.
θ = 0.0090 radians
It should be about the same as using the sin function as the hypotenuse and adjacent side are similar in length.
Sin(½θ) = opposite/hypotenuse = 1,737/384,399
½θ = arcsin(1,737/384,399) = 0.0045 radians.
θ = 0.0090 radians
Now, work it out the opposite direction.
Say you want to calculate size at 100km (about the max for a balloon).
tan(½θ) = opposite/100km.
opposite = radius of balloon = 100*tan(0.0045)
And you get about a 0.45 km radius, or about 0.9 km diameter
========================================
Oops.
I think I did this a bit complicated.
One should be able to do the same with merely fractions.
Consider
= 
= 
= 
x Ok, so for 100km, one gets,
x And at 100km altitude, one gets 0.9km diameter
At 200km altitude, one gets 1.8km diameter
It is always great to do my calculations 3 different ways, and come up with the same answer [:)]
Your balloon would have to be quite large if you chose to put it into a geostationary orbit.
diameter =
x = 323 km diameter. Fortunately, pressure is low enough that it wouldn't take much gas to fill the balloon, provided adequate elasticity of the material, and resilience to heat, cold, and micro-meteors.Post by: Airthumbs on 19/01/2013 16:26:55
Post by: CliffordK on 20/01/2013 11:36:01
Thanks fantastic CliffordK. I think that at about 1km in Diameter the balloon would have roughly 125 square kilometres of material.
I think you have an error in your surface area calculation. Either that, or you need to find a new tailor.
Surface area of a sphere: A=4πr2
If D = 1km.
r = 0.5km.
A = 4π(0.5)2 = π = 3.14 km2
Post by: David Cooper on 20/01/2013 21:06:42
Post by: CliffordK on 20/01/2013 23:03:31
The amount of shade created by 10x per day eclipses for 1 minute would be equivalent to 1x per day for 10 minutes.
One should be able to maintain a lighter than air object in a very low orbit. Would it truly be orbital between 50 and 100km? However, it might tend to drift from the desired ecliptic. One would probably have to get it up to at least the 200km altitude to be able to easily maintain the ecliptic orbit.
Post by: CliffordK on 20/01/2013 23:26:20
The L1 (http://en.wikipedia.org/wiki/Lagrangian_point#L1) is about 1.5 Million KM from the Earth, which is about 5x the distance to the moon. The advantage is that rather than providing some shade for a few minutes a day, it would provide partial shade 100% of the time.
I'm having troubles comparing the two.
If the Earth's circumference is about 40,000 km, then a 1km object in LEO would fill about 1/40,000th of the sky. Except, that doesn't really capture the partial shade given by the object. Also, note, I think, the farther from Earth, the wider of a path, N/S, that is covered by the eclipse.
So, a 2km object 200km from Earth might provide more shade than a 1km object, 100km from Earth, for any observer that wasn't at the Ecliptic.
Post by: CliffordK on 21/01/2013 01:48:02
I've been thinking about this some more.
Your satellite, in a low earth ecliptic orbit will actually be somewhere between the Earth and the sun for somewhere between 1/3 and 1/2 of the time, which is pretty good.
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However, for an object with a 1 km2 cross section, the area of a complete shadow will be somewhat less than 1 km2. It may, in fact, only create a complete shadow at a single point along the ecliptic. A partial shadow would only be created for less than a dozen miles or so in all directions.
So much for the pretty sights in England. Assuming an ecliptic orbit, one would never see it North of the Tropic of Cancer, or South of the Tropic of Capricorn.
The Earth has a cross section of about πr2 = 3.14 x (6,378 km)2 = 128 million km2
So, your little 1 km2 satellite, while it may be, between the Earth and sun for nearly half the time, it is only blocking about
of the sunlight.You'd need a bunch of them to make much of a difference.
Post by: Airthumbs on 24/01/2013 04:15:36
Getting a bit desperate I know but desperate times call for desperate measures!
I was actually trying to calculate the weight of the material required to create a false moon and was thinking it is actually possible. :) However I have serious doubts as to my other plan above!
Post by: CliffordK on 24/01/2013 08:07:47
Interesting, a fake asteroid entering low Earth Orbit.
Astronomers would be quick to determine the size and distance, unless you had their cooperation to say it was bigger than it really was.
I suppose making it look big would be for the WOW effect.
In Low Earth Orbit, It would probably only be visible for a few hundred miles each direction of its orbital path. Put it up a few thousand miles, and it would be visible by many more people. But, to make it appear as large as the moon, say at 1000km, it would have to be much larger.
Of course, you could just shoot it out on a solar orbit that would collide with Earth in a year or so in the future. And, since there would be little mass, it would burn up in the upper atmosphere with little or no damage at the surface.
Post by: Airthumbs on 25/01/2013 17:08:26
Post by: yor_on on 25/01/2013 17:15:19
At least initially. Then you have a problem..
And as Clifford points out, a lot of astronomers, mathematicians and physicists benevolent silence.
Post by: Airthumbs on 25/01/2013 17:21:23
I would say it need a he* of a lot of 'lighter air' :)
At least initially. Then you have a problem..
And as Clifford points out, a lot of astronomers, mathematicians and physicists benevolent silence.
I understand, yor_on, how about just doing it for fun...... people fly kites for fun, maybe scientists should fly artificial moons :) Space is so serious!
Post by: Airthumbs on 25/01/2013 20:45:38
Post by: Pmb on 25/01/2013 22:41:33
I'm up for any suggestions that might get this project off the ground :)Im curious as to how you're going to avoid it getting hit by all the space junk in orbit?
Post by: Airthumbs on 25/01/2013 22:47:52
Im curious as to how you're going to avoid it getting hit by all the space junk in orbit?
What a fantastic idea PMB, if you could fill it with some kind or aero gel you could mop up space debris with it.
Post by: Bill S on 25/01/2013 23:33:51
Post by: Airthumbs on 25/01/2013 23:38:50
And if we keep this thread going long enough there will be plenty of hot air about to lift the thing off the ground. :)
From what I understand hot air will only get it so far, Helium would be better but that still won't reach orbit. However if it's hot air that's needed and possibly quite a lot of methane too then it's a start, at least it's off the ground :)