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Question about Satellites

Badger

Member of the Peanut Gallery
Badger
Joined
Jul 28, 2002
Messages
3,435
The other night, an hour or so after dusk, I was outside, looking at the sky. In the space of about 5 minutes I'd seen 7 satellites.

The thing that got me wondering was that 5 of them were travelling south to north, one was travelling north to south, and only one was travelling west to east.

I was wondering why I saw so many travelling in a north/south direction. I think that due to the timing, more light was reflected from material than later at night, so I was seeing a lot of low orbit "junk". I also think that this "junk" is meant to orbit low and in a north/south direction to minimize collisions with regular satellites, but am not sure.

Can anyone shed any more informed light on this for me?

Thanks in advance!
 
Badger said:
The other night, an hour or so after dusk, I was outside, looking at the sky. In the space of about 5 minutes I'd seen 7 satellites.

The thing that got me wondering was that 5 of them were travelling south to north, one was travelling north to south, and only one was travelling west to east.

I was wondering why I saw so many travelling in a north/south direction. I think that due to the timing, more light was reflected from material than later at night, so I was seeing a lot of low orbit "junk". I also think that this "junk" is meant to orbit low and in a north/south direction to minimize collisions with regular satellites, but am not sure.

Can anyone shed any more informed light on this for me?
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Well, shortly after dusk is the best time to see satellites. The sky is dark, but the satellites are not yet in the Earth's shadow.

Some satellites are put in orbits that take them northerly or southerly. Some satellites are put in "polar orbits," which caused them to fly over the poles. These satellites could be used for any number of purposes. They are not necessarily space junk.
 
To cover only part of the earth you put your satelite in geo-stationary orbit. To cover a band around the equator you put it in orbit west-east.
To cover the whole earth you have to have it in a polar orbit.
 
So are there more satellites in polar orbit than other orbits?
 
Badger said:
So are there more satellites in polar orbit than other orbits?
Click to expand...

I haven't found any data on the number of satellites in different orbits. But these two links have great info on satellites and orbits:

NASA
Space Today

I'm not sure if polar orbit means only those orbits that cross right over the poles, or if there is a margin of some degrees to either side. But I'd still guess there are more satellites in 'non-polar' orbits. If you don't require complete coverage of the Earth, and for many purposes the Arctic and Antarctic just aren't interested, you can cover most of it in a high inclination orbit.
 
The most popular orbits are probably geostationary, given their use in communications for the majority of the planet. It's only places very far north that can't use those.

But in general the orbit depends on the mission. So there are plenty of satellites that are in other orbits. And it is also true that when a satellite's mission is over it will probably be put into a different orbit when puts it out of the way.
 
I have a question that might be a nobrainer to most. Why are satellites so visible when many of them are so small? I understand that meteorites are often extremely small yet they look so huge coming through the atmosphere. And, oh yeah, please answer in basic English please!
 
Denise said:
I have a question that might be a nobrainer to most. Why are satellites so visible when many of them are so small? I understand that meteorites are often extremely small yet they look so huge coming through the atmosphere. And, oh yeah, please answer in basic English please!
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Well, satellites and meteors (not to be confused with meteorites, which are the objects that actually hit the Earth) are easy to see for different reasons. Meteors actually glow when they burn up in the atmosphere. They are small, but they move so fast (relative to the Earth) that they give off a lot of light when they burn up.

Satellites don't give off their own light (they reflect light from the sun), but they are bigger than meteors. Also, some satellites have reflective or white surfaces that cause them to reflect more light. Not all satellites are designed to be reflective, however, so not all satellites are easily visible.
 
Denise said:
I have a question that might be a nobrainer to most. Why are satellites so visible when many of them are so small? I understand that meteorites are often extremely small yet they look so huge coming through the atmosphere. And, oh yeah, please answer in basic English please!
Click to expand...
Satellites are more likely than natural objects to have big shiny metal surfaces. If you ever fly over a city near sunset, you will see flashes from various windows as they happen to align properly with you and the sun, planar reflectors.

You generally don't actually see the meteor itself, you see the trail it makes through the atmosphere, which is much bigger.
 
Thanks for the clarification on meteors and meteorites. What I'm asking, I guess, is how we can see it from such a distance away. If I am a mile away from someone at night, on the ground, and they light their lighter, I would probably not be able to see it. Or would I, I could be wrong here- often am. How many miles from the surface of the Earth are satellites and how come we can see them but not a person a mile away on Earth. Sounds dumb, but that's my revised question.
 
Yup, you're wrong. You could see someone's lighter from a mile away, easy.

Heck, we can see lights on airliners, flying 7 miles up at night, with no problem, or ships lights on the horizon (5 miles or so) for that matter.

As a side note, did you ever see a satellite that changed brightness? Went from dim to bright and back at regular intervals? That's a piece of space debris that's tumbling, and thus changing the amount of light it reflects to the ground as it does so. I think those are neat as I immediatly imagine a panel or bit of rocket or something gently tumbling through the silence and blackness of space.
 
Denise said:
Thanks for the clarification on meteors and meteorites. What I'm asking, I guess, is how we can see it from such a distance away. If I am a mile away from someone at night, on the ground, and they light their lighter, I would probably not be able to see it. Or would I, I could be wrong here- often am. How many miles from the surface of the Earth are satellites and how come we can see them but not a person a mile away on Earth. Sounds dumb, but that's my revised question.
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There are several things.

Satellites are reasonably large objects. I think the Hubble is about the size of a bus (I'm going by memory here). They are usually made to be highly reflective in order to minimize heating. In space, the sun appears brighter than it would be at noon at the equator, and that reflects off the metal surface of the satellite.

The density of the atmosphere becomes significantly less as you go up in altitude. The denser the air, the more it absorbs and scatters light. There's also dust in the air, again more at lower altitudes. So when you look at something on the horizon, you are looking through miles of dense, dirty air, whereas when you look overhead, the interference from the atmosphere is significantly reduced. Sunsets, for example.

The satellites you see moving quickly across the sky are in fairly low orbits. Some spy satellites are in orbits so low they are practically skimming the atmosphere, although these are probably designed for low visibility. You're probably seeing satellites that are a couple hundred miles "up."

Finally, unless you are in a completely dark area, like out in the middle of a desert, and its a new moon in the middle of the night, there's going to be some light reflected off the surface of the earth. Even a little bit of light can make it hard to make out a terrestrial object. A satellite, on the other hand, is against a black sky with only the stars to compete, and the human brain is pretty good at picking out moving objects. (Actually, light pollution from cities is a serious problem for astronomers.)

Even though geosynchronous orbits are popular, geosynchonous orbits must be at a precise distance from the earth, and its rather far away (around 26,000 miles if I recall correctly). You're not going to see those with the naked eye.
 
Badger said:
Yup, you're wrong. You could see someone's lighter from a mile away, easy.

Heck, we can see lights on airliners, flying 7 miles up at night, with no problem, or ships lights on the horizon (5 miles or so) for that matter.

As a side note, did you ever see a satellite that changed brightness? Went from dim to bright and back at regular intervals? That's a piece of space debris that's tumbling, and thus changing the amount of light it reflects to the ground as it does so. I think those are neat as I immediatly imagine a panel or bit of rocket or something gently tumbling through the silence and blackness of space.
Click to expand...

Well, now I have had another duh moment! I did see a satellite that got really really bright and looked like it was going to fall on us, then went back to looking normal. Someone said it was an irridium flare and someone else said it probably wasn't. It was my first post to this forum!
 
Zombified said:

There are several things.

Satellites are reasonably large objects. I think the Hubble is about the size of a bus (I'm going by memory here). They are usually made to be highly reflective in order to minimize heating. In space, the sun appears brighter than it would be at noon at the equator, and that reflects off the metal surface of the satellite.

The density of the atmosphere becomes significantly less as you go up in altitude. The denser the air, the more it absorbs and scatters light. There's also dust in the air, again more at lower altitudes. So when you look at something on the horizon, you are looking through miles of dense, dirty air, whereas when you look overhead, the interference from the atmosphere is significantly reduced. Sunsets, for example.

The satellites you see moving quickly across the sky are in fairly low orbits. Some spy satellites are in orbits so low they are practically skimming the atmosphere, although these are probably designed for low visibility. You're probably seeing satellites that are a couple hundred miles "up."

Finally, unless you are in a completely dark area, like out in the middle of a desert, and its a new moon in the middle of the night, there's going to be some light reflected off the surface of the earth. Even a little bit of light can make it hard to make out a terrestrial object. A satellite, on the other hand, is against a black sky with only the stars to compete, and the human brain is pretty good at picking out moving objects. (Actually, light pollution from cities is a serious problem for astronomers.)

Even though geosynchronous orbits are popular, geosynchonous orbits must be at a precise distance from the earth, and its rather far away (around 26,000 miles if I recall correctly). You're not going to see those with the naked eye.
Click to expand...

Thanks! I understood the whole thing and that's what I was looking for!
 
All I can add to this discusion is that the reflection power of glass is enormous. Sunlight reflected from houses and buildings in a Los Andes ski resort near Santiago is amazingly powerful, even from a distance of 100 km. One afternoon while driving to the city, I was stunned to see these "fires" in the middle of the mountains. For a moment I thought I was witnessing real UFOs :D

If we consider that satellites circle the earth at an altittude of a few hundred kilometers (a typical shuttle orbit altitude is 400 km), then it's something to be expected that we see them as dots of light, given the right lighting circumstances.
 
http://liftoff.msfc.nasa.gov/RealTime/JTrack/

For all your at-home satellite-tracking needs. PLots current locations of many, many, many satellites from orbital data.

I haven't used this myself in years, but i seem to recall last time I looked at it there were quite a few amateurs offering additional plugin data for other satellites, too -- although I'm not 100% positive that was this program.

Edited to add: oooooh, a new 3-d version!!

Edited to add: For kicks, check out the 3-d version, and set the timing to udate every 1/4 second. I'm amazed how fast some of these things move... but if that's not enough, change the timing to x1000 instead of realtime. Then click and drag in the window to move the view around so you can see everything -- it looks to me like the majority of these things are geosynchronous, around the equator.

One more thing -- click on ground trace for the satellite you have selected to get an idea of how it moves in relation to the Earth.

I have the JRE installed, from sun. You can get the newest version at java.sun.com, and it may help you run this program faster if you're having problems. Microsoft's implementation of java is sorely lacking.

-Chris
 
So I played with J-Track 3d a bit and I came across the COBE R/B, which I tried to look up info for. The JTrack page tells me if it ends in R/B it's just a rocket booster for the satellite witha similar name. So I looked up COBE, which is the Cosmic Background Explorer, launched in 1989.

1989! Is NASA's database outdated, or is it's rocket booster still up there orbiting?

I'm noticing lots more now, the earliest I see is 1982.

Now I'm basically naieve about this kind of physics -- but it seems to me that these rocket boosters have a lot of kinetic energy to be in orbit for all that time. Couldn't more of that energy somehow be imparted to the satellite they are launching? How much energy is being wasted here -- and is it just unavoidable?

Someone who knows something about the subject, please educate me!
 
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