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Science and Astronomy Questions
Billy_MayesDate: Thursday, 18.07.2013, 14:46 | Message # 136
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Why aren't there any things naturally orbiting moons?




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EnkiDate: Thursday, 18.07.2013, 15:30 | Message # 137
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Quote (Billy_Mayes)
Why aren't there any things naturally orbiting moons?

It's naturally difficult when orbital bodies are reduced to such a scale, but theoretically possible for a moon around a gas giant.
They also want to put a very small satellite in orbit around our moon, which sounds like a bad idea to me, but it's Obama and the rest of the government, they have plenty of those.





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HarbingerDawnDate: Thursday, 18.07.2013, 15:33 | Message # 138
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Quote (Enki)
which sounds like a bad idea to me

Why?





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EnkiDate: Thursday, 18.07.2013, 16:23 | Message # 139
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Why?

It's a chunk of rock, which we're normally concerned about rocks smaller than 1 mm actually making it to the surface because of the damage objects can do at the speed these things can reach, but now we're going to bring a 40 km long one to our planet and try something as trivial as making it orbit stably around our moon.

I know it's quite possible they could pull it off, but NASA has made errors with equipment in space before and if they do here we could have a big rock flying into our atmosphere. I don't know how much of it would make it through, but whatever does could cause damage; maybe not to the whole planet or even a continent, but if it hurts or kills anyone, maybe a small town of people, our government is responsible and therefore, indirectly, us.

Maybe they could put safety precautions up to make sure this doesn't happen, but considering NASA is working on trying to make its own spacecraft so they won't burn up in the atmosphere, rather than worry about the safety of anyone that might happen to be in the path of a crash-landing satellite, no matter how small the chance may be, I don't think they'll particularly care here either, as long as its for the good of science. It simply won't affect them unless it falls right on top of their heads.

Don't get me wrong, I'm all for science and discovery, but man is not the expendable slave of science. Science is the inexpendable slave of man. When the government tested nuclear warheads, they did not test them on a civilian population, they did it in the middle of nowhere and when they used the nuclear warheads, they were to the immediate benefit of man; an entire war-front was eliminated and a global conflict ground to a halt. If NASA wants to benefit man by putting satellites around Earth and an asteroid around the moon, that's okay, and I realize that no matter what they put up there, it could fall on top of our heads. But if it is because of their refusal to worry about such a tragedy and they make a bunch of satellites that after ten years will be unable to charge their batteries and plunge to the surface with some new metal that doesn't burn up, or they fling an asteroid into orbit around the moon, it turns out it's only stable for sixty years and it will also plunge to the surface, and they didn't have a backup probe to launch and push it away, and these objects fall and kill people, then they have done a bad thing.

In the end, whoever these things hit will only be considered, by them, a necessary sacrifice for science, something no man has the right to do, no matter what his purpose. Yet still, they will say, "I had no way of knowing it would've fallen there, on that person, at that time!" They won't be held responsible. Why should they care, when it's more likely to hit anywhere on Earth than right where they work? Those reading this may say, well it won't fall on me, and it probably won't, but what about who it does fall on? Should we not care because it won't be us? Sure, maybe it's not that big of a problem yet, but you let them start pulling asteroids into orbit around Earth and the moon with no permanent electronic systems to guide them and then you'll have one.

I apologize for the essay length. smile





"If you arrive at a contradiction, check your premises. You will find that one of them is wrong." - Ayn Rand
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Edited by Enki - Thursday, 18.07.2013, 16:29
 
HarbingerDawnDate: Thursday, 18.07.2013, 16:58 | Message # 140
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we're normally concerned about rocks smaller than 1 mm actually making it to the surface because of the damage objects can do at the speed these things can reach

...what? Objects 1 mm in size can't make it anywhere near Earth's surface, they'd have to be MUCH larger. They don't pose any danger of damage at all until they get to over 10 meters in size.

And anyway, objects that do make it to Earth's surface are slowed to terminal velocity by the time they get here (only a few hundred MPH). The damage they cause is by the heat and pressure generated by their passage through the atmosphere as they slow down.

Quote (Enki)
now we're going to bring a 40 km long one to our planet and try something as trivial as making it orbit stably around our moon

Where have you been reading this? We're not going to try to move something 40 km long, that would be absurd! The plan NASA has for retrieving an asteroid aims at capturing one about 7 meters wide - too small to pose any danger to Earth even if it did go off course and hit us.

As for the rest of all that, all I can say is that you seem very misinformed about NASA's spaceflight safely procedures. What NASA does provides great benefit to the public - as you noted - but does NOT provide substantial risk. The greatest threat NASA poses to anyone is their employees driving cars on the road. You are FAR more likely to get hit by a car driven by a NASA employee - or even have a NASA plane crash in your neighborhood - that you ever are to get hit by any kind of falling satellite debris (which they typically prefer to intentionally burn up over the ocean, not let fall uncontrolled over who knows where, though those things do also happen from time to time).





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Edited by HarbingerDawn - Thursday, 18.07.2013, 22:39
 
apenpaapDate: Thursday, 18.07.2013, 16:59 | Message # 141
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Considering the Earth is a very small target and the Moon is very far away from it, you'd have to get incredibly (un)lucky to hit it instead of establishing orbit around the Moon. At worst, they'll hit the Moon, but the Earth? That'd take some serious misaiming skills.




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Edited by apenpaap - Thursday, 18.07.2013, 17:00
 
EnkiDate: Thursday, 18.07.2013, 19:58 | Message # 142
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Quote (HarbingerDawn)
...what? Objects 1 mm in size can't make it anywhere near Earth's surface, they'd have to be MUCH larger. They don't pose any danger of damage at all until they get to over 10 meters in size...Where have you been reading this? We're not going to try to move something 40 km long, that would be absurd! The plan NASA has for retrieving an asteroid aims at capturing one about 7 meters wide - too small to pose any danger to Earth even if it did go off course and hit us.

1mm asteroids don't pose a threat in space. The point was that if just 1 mm of an object survives the atmosphere, then the speed makes it dangerous to anything and anybody in the immediate area it lands. It's possible I got the number wrong, but if 10 m is their standard for damage, outside the atmosphere, which probably entails destruction of more than one building, or the injury or death of more than just one person, then 7 m is getting dangerously close.

Quote (HarbingerDawn)
As for the rest of all that, all I can say is that you seem very misinformed about NASA's spaceflight safely procedures. What NASA does provides great benefit to the public - as you noted - but does NOT provide substantial risk. The greatest threat NASA poses to anyone is their employees driving cars on the road. You are FAR more likely to get hit by a car driving by a NASA employee - or even have a NASA plane crash in your neighborhood - that you ever are to get hit by any kind of falling satellite debris (which they typically prefer to intentionally burn up over the ocean, not let fall uncontrolled over who knows where, though those things to also happen from time to time).


I was watching a program, NOVA I believe. They showed the crews that go out to retrieve fallen satellites and I thought they were about to tell us how they were studying the atmosphere's effects on certain metals so they can make the satellites burn up before they hit the ground, but it was the opposite. They were talking about studying the effect on metals so they can make the satellites come down in one piece and be reusable. My point is that if there is a chance it could kill somebody, we shouldn't be trying to increase the risk or ignoring it. I don't care how much taxpayer money we save. I don't want to be the taxpayer that is "sacrificed" for the greater good of science when those things come down and if I wouldn't want to be the collateral damage of an action, then I shouldn't endorse it.

Also, I realize driving is always a risk. But, we always do our best to minimize the risk of driving. Average people drive cars and its hard for the government to do anything too wild with our vehicles and our roads. On the other hand, average people don't drive satellites or raging asteroids in orbit around the moon. I wouldn't doubt if some people were barely lucid of anything going on in orbit around Earth, simply because they don't care enough or like to deny the lunar landings. This mentality of people, in my opinion, can have a negative effect on scientists' outlooks on people as they feel that because they're doing is so beneficial to them they ought to see it and yet some people appear not to see it, because some psychological condition forces them to ignore it, but that's getting into an area of philosophy.

But from what I see, their handling of satellites is dangerous to anyone at the wrong place at the wrong time. They ought to make satellites burn up more completely in the atmosphere, or guide them down into the ocean at the end of their lives if they really want to keep them so bad.

But you take the project of putting a big rock in the vicinity of Earth which is much more likely to cause damage than an artificial satellite, then they could have a mess, or the death of a couple people they just sweep under the carpet.

Quote (apenpaap)
Considering the Earth is a very small target and the Moon is very far away from it, you'd have to get incredibly (un)lucky to hit it instead of establishing orbit around the Moon. At worst, they'll hit the Moon, but the Earth? That'd take some serious misaiming skills.


The problem is that most asteroids miss Earth because they are going very fast, but they must slow it down to put it in orbit around the moon. Astronomers already question the stability of such an orbit and if it does come undone, the problem is that it is now going slower than Earth's escape velocity and of course Earth is a massive ball inflicting enough gravity to hold something as large as the moon, and will drag in whatever piddly object we put in orbit around it if the moon can't hold it in a stable orbit.

Again. I don't want to get rid of satellites or stop them from playing with gravity to advance man or else I loose the internet for one thing, but I don't want them to risk harming innocent people when they can do simple things to prevent it.





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HarbingerDawnDate: Thursday, 18.07.2013, 20:15 | Message # 143
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The point was that if just 1 mm of an object survives the atmosphere, then the speed makes it dangerous to anything and anybody in the immediate area it lands.

No, it doesn't, no more dangerous than dropping a grain of sand from an airplane. Because of terminal velocity.

Quote (Enki)
if 10 m is their standard for damage, outside the atmosphere, which probably entails destruction of more than one building, or the injury or death of more than just one person

No it doesn't... a 10 meter asteroid MIGHT break a few windows, or might cause no damage at all, depending on its speed, entry angle, and composition. 10 meters isn't "their standard", it's a number I pulled out for the sake of example which is around the lower limit of size for something to be able to cause any damage at all. For example, the Chelyabinsk meteor was over 17 meters wide, and it broke windows, damaged a few buildings, and killed no people. 7 would definitely cause no damage. There's no "dangerously close" involved, that's like saying that standing in water up to your chest is dangerously close to being in over your head. It's just not true.

Quote (Enki)
Astronomers already question the stability of such an orbit and if it does come undone, the problem is that it is now going slower than Earth's escape velocity and of course Earth is a massive ball inflicting enough gravity to hold something as large as the moon, and will drag in whatever piddly object we put in orbit around it if the moon can't hold it in a stable orbit.

Or it could crash into the Moon. But let's say it did come towards Earth. So what? It could not possibly hurt anything. That's part of the selection criteria for the asteroid: it can't be one that could cause damage if it hit Earth. There is 0 risk to anyone.

Quote (Enki)
I don't want them to risk harming innocent people when they can do simple things to prevent it.

They already DO. And as I already said, they risk harming innocent people when they drive to work in the morning, yet you're not advocating that they walk or take a bus. You seem to be pleading a special case here, that somehow the incredibly small risks involved in space operations are somehow so much more dire than other everyday risks, yet you provide no reasoning.





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Edited by HarbingerDawn - Thursday, 18.07.2013, 20:20
 
apenpaapDate: Thursday, 18.07.2013, 20:31 | Message # 144
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Orbital physics also mean that even if it broke free from Lunar orbit, it would have a very hard time going to Earth for the same reason it's very difficult for us to send a probe to Mercury or to very close Solar orbit: the Moon is moving around the Earth in its orbit, so to get closer to Earth you have to cancel out part of that speed. To hit Earth, you have to cancel out almost all that speed.

Even if it does hit Earth, the chances of anyone even noticing are minuscule. 70% of the planet is ocean, and cities only make up a tiny portion of the remaining land. And outside of cities, humans are spread over the land very thinly. As Harb said, NASA employees take far bigger gambles with other people's lives when they drive to work instead of taking a bike.





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SpaceEngineerDate: Friday, 19.07.2013, 14:05 | Message # 145
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Quote (Billy_Mayes)
Why aren't there any things naturally orbiting moons?

Because orbits of such things are unstable due to tidal effects. Firstly, moonlet (moon-of-the-moon) must orbit very close to its parent moon to have stable orbit, due to perturbations from a parent planet and other moons. Its typical orbital period is several hours. Next, parent moon is usually tidal locked to its parent planet - its rotation period is equal to orbital period, and typical value is few days to few months. So moonlet will revolve its parent moon faster than moon rotates around its axis, and tidal wave on the moon's surface, generated by moonlet, will brake its orbital motion. Moonlet's orbit will gradually shrink on timescale of millions years, until it or enters moon's Roche limit and being shredded in pieces, or crashed onto the moon's surface. This fate awaits the Martian moon Phobos.





 
HarbingerDawnDate: Friday, 19.07.2013, 14:56 | Message # 146
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This fate awaits the Martian moon Phobos.

And Neptune's moon Triton, which is being pulled in by the same mechanism due to its retrograde orbit smile





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Billy_MayesDate: Friday, 19.07.2013, 19:05 | Message # 147
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Ok, thanks for the answers, been wondering that for a long time.




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WatsisnameDate: Friday, 19.07.2013, 23:12 | Message # 148
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Basically just mirroring what Apenpaap and HarbingerDawn said already.

Quote (Enki)
The problem is that most asteroids miss Earth because they are going very fast, but they must slow it down to put it in orbit around the moon. Astronomers already question the stability of such an orbit and if it does come undone, the problem is that it is now going slower than Earth's escape velocity and of course Earth is a massive ball inflicting enough gravity to hold something as large as the moon, and will drag in whatever piddly object we put in orbit around it if the moon can't hold it in a stable orbit.


Orbital mechanics does not work this way.

First off, speed has nothing to do with impact probability. The only thing that matters is whether the trajectory intersects the planet or not.

Secondly there's no uncertainty over the stability of the planned orbit -- it is understood that there are stable orbits of the moon within certain ranges of altitude and inclination. But as Apenpaap noted, even if the asteroid should leave the intended orbit for whatever reason, it is not simply just going to fall down to Earth -- that would require a significant loss of orbital velocity to happen (Play some Kerbal Space Program, or Orbiter, and you can see for yourself!)

Much more likely would be that the asteroid simply crash into the moon instead. It could also conceivably go into a long chaotic orbit due to perturbations from both the Sun and Earth and end up escaping the Earth-moon system to become a sun-orbiting object once again. Or it could hit Earth, but if it did, its small size would mean the most probable outcome is a pretty light show in the sky without causing any damage or deaths.

Quote (Enki)
1mm asteroids don't pose a threat in space. The point was that if just 1 mm of an object survives the atmosphere, then the speed makes it dangerous to anything and anybody in the immediate area it lands.


What!? The reality is exactly the opposite of what you said! surprised I think you're forgetting the most important thing here -- the atmosphere.

A 1mm object actually IS a big deal in space because if it hits something, basically all of its kinetic energy 1/2*m*v2 goes into doing destructive things. At 1mm in size and traveling several thousand meters per second, that's a lot of energy going into a small area on the target, and can blow a hole through spacecraft hull or cause satellite failure. NASA has a whole research division that studies the effects of such hypervelocity impacts on materials for precisely this reason.

The atmosphere however makes it impossible for this to occur on Earth's surface, because it bleeds off the kinetic energy before the object can reach the ground. How much energy is lost depends on the initial speed, size, composition, and entry angle, but for a 1mm object at orbital or higher velocity the only possible outcome is complete incineration in the upper atmosphere (hence your typical meteors). For a 7m object like this asteroid, a few small fragments may reach the ground. But as Harbinger said, these fragments would only hit the ground at terminal velocity (a heck of a lot slower) and would do no damage. 1mm would go by utterly unnoticed, like a grain of sand being dropped from a building.

The worst-possible-case scenario would be a potentially fatal injury if a large (several centimeters) fragment happened to land on someone's head. But considering the population distribution on the Earth's surface, and the negligible risk of the asteroid missing/leaving its intended orbit and intersecting the Earth, and the fact that objects on the order of 5 to 10 meters across 'hit' the Earth once every few years on average without killing anyone, that's a risk I'm not even remotely concerned about. smile





 
Billy_MayesDate: Saturday, 20.07.2013, 22:05 | Message # 149
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Do galaxies orbit around a point, or do they just go around in the universe freely?




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Joey_PenguinDate: Saturday, 20.07.2013, 22:18 | Message # 150
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They're grouped into clusters that orbit around a common center of mass, like any star system.




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