Question on how to distinguish between asteroid and man-made satellite

[CitronBleu]

Hello all and thank you for reading!

I have a question, and perhaps your skills and expertise in the matter at hand may illuminate my ignorance.

I have lately been reading about the phenomenon of near-Earth asteroids and I am curious as to how one may make the distinction between a near-Earth asteroid passing in the sky and a man-made satellite passing in the sky.

Are there more NEAs, or artificial satellites in the skies above our heads?

How frequently can NEAs be observed from Earth with a telescope, binoculars, or with one's own eyes?

Do they look similar? and is it possible to confuse both types of objects?

Thank you!

CitronBleu

[iamjulian]

NEAs are plentiful, but not many visible with the average telescope at any given time. If you see something pass through the eyepiece it will almost certainly be a satellite. NEAs will move faster than say main belt asteroids, but they will still be much slower than satellites. Satellites are stupidly close to the Earth compared to most NEA "close" approaches.

That's not to say there are never any. There is a famous one called Apophis (spelling?) which will come quite close in something like 2029. It will still be further out than the satellites, but it is much bigger than a satellite, so will be naked eye (about mag 3 or 4). If you are interested in tracking NEAs then you need to be thinking about astrophotography rather than visual. Your chances of "seeing" them are few and far between.

Hope that helps.

[CitronBleu]

NEAs are plentiful, but not many visible with the average telescope at any given time. If you see something pass through the eyepiece it will almost certainly be a satellite. NEAs will move faster than say main belt asteroids, but they will still be much slower than satellites. Satellites are stupidly close to the Earth compared to most NEA "close" approaches.

That's not to say there are never any. There is a famous one called Apophis (spelling?) which will come quite close in something like 2029. It will still be further out than the satellites, but it is much bigger than a satellite, so will be naked eye (about mag 3 or 4). If you are interested in tracking NEAs then you need to be thinking about astrophotography rather than visual. Your chances of "seeing" them are few and far between.

Hope that helps.

Hello iamjulian and thank you for the information. That was very helpful.

I have another question and maybe you can also help me understand this.

I am curious as to how we can observe artificial satellites from earth. Many of these satellites are the size of a washing machine, some are larger. They are between 300 and 32,000 km away from us. How is it possible to see such small objects at such a distance?

We see them because they are brightly lit.

Do Satellites have electrical lights shining down toward us? and if so how do the satellites power these lights? Or is the light we see off these objects a reflection of the sunlight? But aren't these objects in the Earth's umbra (shadow)? Then how can they be so bright?

Thanks!

CitronBleu

[Shibby]

Hello CitronBleu,

I think I can answer a few of those questions ...

Yes, satellites appear very small at those distances, but we can still see them because they are bright. Think of a star - they are so far away (light years!) that we observe them as single points of light. However, we can still see them because they are incredibly bright. Also, satellites are indeed lit by the sun. Because they are quite far from the surface of the Earth, they are often not within Earth's umbra. Think of the moon - we can always see it at night, lit up by the Sun. Only occasionally does it pass into Earth's shadow; this is known as a lunar eclipse.

Hope this helps!

[iamjulian]

Hello,

Yes, they are relatively small, but they are reflective and light travels long distances with ease. If satellites are in the Earth's shadow, you cannot see them, at least, not very easily. The ones you can see, are visible because they are reflecting enough sunlight to make them visible. Have you ever seen the ISS (International Space Station)? It is very bright, not least because it is big and because it has bright solar panels. Sometimes when you are watching it, it suddenly fades and disappears. It is simply moving into the Earth's shadow and not reflecting as much light. Just because the sun has set from your location, if you move a hundred miles to the West it may still be visible. Because you can see a satellite, does not mean it is directly above your location.

An object does not need to be large to be visible, indeed a few years ago some astronaughts dropped a tool box while they were up mending the ISS. The tool box floated off and because it was on the same trajectory as the ISS, it was possible to view it. Maybe not naked eye, but certainly small telescope. Heavens Above is a great website to help you know which satellites you can see and when.

[CitronBleu]

Hello, Yes, they [man-made satellites] are relatively small, but they are reflective

Would you happen to know what is the general reflection coefficient for light off the mylar foil, or any of the material, that covers the outer surface of satellites? Put another way how much of the light bounced off the satellite will get to us, according to the average size of an orbiting satellite and it's distance to the viewer?

Perhaps this could be used to determine, scientifically, how such a small object can be detected with one's own eyes?

Because they [man-made satellites] are quite far from the surface of the Earth, they are often not within Earth's umbra [so cannot be seen].

If satellites are in the Earth's shadow, you cannot see them, at least, not very easily.

Hello Shibby and thank you for taking the time to answer my question.

The length of Earth's umbra is determined to be from 1.36 to 1.40 million km long, using a simple equation available online.

Here is a diagram I found of Earth's umbra, to scale, to about twice the distance to the Moon taken from informational site Wikipedia:

I understand that at high latitudes a viewer within Earth's umbra could technically observe an object in the sky while it (the object) orbits outside of the umbra, and directly reflects the Sun's light to the viewer.

However how about if the viewer is located at the Equator?

Wouldn't the umbra extend to a minimum of Earth's radius, 6,371 km? (If you look at the above diagram and place yourself right on Earth's equator and look in any direction, the umbra will totally cover one Earth radius distance within your sight)

How can an artificial satellite be detected from so far, considering its size, often no larger than a dishwasher?

Have you ever seen the ISS (International Space Station)?

No.

[CitronBleu]

Shibby I am sorry I reread your quote and the proper comment in brackets should be: So can be seen.

[iamjulian]

Sorry, I have no idea how reflective satellites are in quantifiable values. The more light they reflect, the brighter they will be. The bigger they are, the brighter they will be. The higher they are, the dimmer they will be. Beyond that you'd have to ask someone else.

This link may be useful:

http://www.satobs.org/observe.html

[CitronBleu]

I wrote about an observer looking up at the sky from the Equator. What I should have written was an observer looking up at the sky from a point located on the ecliptic plane, since Earth is tilted.

Sorry, I have no idea how reflective satellites are in quantifiable values. The more light they reflect, the brighter they will be. The bigger they are, the brighter they will be. The higher they are, the dimmer they will be. Beyond that you'd have to ask someone else.

This link may be useful:

http://www.satobs.org/observe.html

Iamjulian, what are the main characteristics of asteroids and artificial satellites in the sky which one may uses to separate them apart? I've looked at several videos on youtube of passing man-made satellites and of near-Earth asteroids and they both look similar in brightness and velocity.

[acey]

Predicting the apparent magnitude of an artificial satellite due to reflected sunlight is a complicated problem, and is dealt with in these papers:

http://adsabs.harvard.edu/abs/1963SSRv....2..250V

http://www.nicap.org/mantell/mmccants_wsm.pdf

[iamjulian]

Can you link to some NEA videos that you were looking at. My (limited!) understanding is that if you look up and see a naked eye, binocular or small telescope object, moving at the speed of a satellite, then 99.99% of the time it will be a satellite. NEOs are rarely large and near enough to be visible to the eye. I suspect the videos on youtube are either actually satellites, or stacked images taken with a camera, or relatively rare NEOs.

But I may be wrong

[CitronBleu]

Predicting the apparent magnitude of an artificial satellite due to reflected

is a complicated problem, and is dealt with in these papers:

Thanks for the links acey! I am going through them.

Can you link to some NEA videos that you were looking at. My (limited!) understanding is that if you look up and see a naked eye, binocular or small telescope object, moving at the speed of a satellite, then 99.99% of the time it will be a satellite. NEOs are rarely large and near enough to be visible to the eye. I suspect the videos on youtube are either actually satellites, or stacked images taken with a camera, or relatively rare NEOs.

[CitronBleu]

http://www.youtube.com/watch?v=Suv3PseT9iU

[CitronBleu]

Here is a very obviously low-velocity NEA:

Asteroid 85,000 Km from Earth:

[iamjulian]

Thanks, but I don't follow you when you say they appear to be the same velocity. None of those videos show NEOs going anything like the speed of your average satellite. The first one looks like a satellite until you see the time spinning past at the bottom - meaning the frame rate has been speeded up.

Maybe some satellites do appear to travel that slowly, but I don't think many of the easily visible ones do. Whenever one passes through my field of view at the telescope it covers one degree in perhaps three seconds.

Is this any help to your understanding?

[Shibby]

Yeah Julian is correct - satellites typically appear to take a few minutes to cross the entire sky. Those videos are time-lapse movies covering several hours and we do not know the FOV.

I'm not sure I entirely understand your question about observing from the equator. The location of the observer makes no difference to whether the satellite is illuminated by the Sun or not.

[CitronBleu]

I don't follow you when you say they appear to be the same velocity. None of those videos show NEOs going anything like the speed of your average satellite

Hello iamjulian.

You are right, these particular asteroids do appear to move slower than man-made satellites. I am wondering if pehaps some of the objects we see in the sky are NEAs but are not identified as such. Some of these asteroids could be unidentified and close to Earth and appear to be moving faster, which would make them appear to be moving at the same velocity as satellites.

Whenever [a satellite] passes through my field of view at the telescope, it covers one degree in perhaps three seconds.

Which is very quick. I am curious as to what you see in the telescope... do you see a point of light, features? Characteristics that could unambigiously identify the object observed as a man-made satellite and not as an asteroid?

I'm not sure I entirely understand your question about observing from the equator. The location of the observer makes no difference to whether the satellite is illuminated by the Sun or not.

Edit: the observer is located on a point intersecting the ecliptic plane (not the Equator, since the Earth is tilted, and we are dealing with Earth's shadow).

You are right. The location of the observer makes no difference as to whether the satellite is illuminated by the Sun or not.

However the North or South latitude of the observer increases or decreases the umbra coverage between the observer and the satellite. Put another way if the observer is looking straight up, in the direction of the Ecliptic plane's North (the Sun's North), his distance with the edge of the shadow created by the Sun's light projected on Earth will vary. The higher latitude, the smaller the distance; and conversely, the lower latitude, the larger the distance.

But this is all skewed a bit because the Earth is tilted!, so right now in this season I am imagining that the observer is located on the Sun's Equator, which is around the Tropic of Capricorn during the night time.

I'll try to find a diagram to explain.

If we assume the observer is viewing the sky in the middle of the night, and the Sun is directly opposite him, then the umbra would at the very least equal the distance of one earth radius.

Something that could help me understand how we can observe the light reflected off a satellite is to hear from anyone who tracks satellites and who is living on or around the Tropic of capricorn. If the observer is viewing the sky in the middle of the night (with the Sun directly opposite his field of view):

The satellite, for it to be illuminated by the Sun, would have to be at least 6,371 Km farther away from the observer. (Do we detect satellites this far away with the naked eye?); or

The satellite would be dead-center in Earth's umbra, so could not be illuminated by the Sun's light.

Is anyone reading this living around the Tropic of Capricorn, and is tracking satellites? Can you observe man-made satellites in the middle of the night (say between 00:00 and 03:00)?

[CitronBleu]

Whenever one passes through my field of view at the telescope it covers one degree in perhaps three seconds.

iamjulian let me ask my question another way:

What is the usual average estimated distance between you and the satellites you observe in the sky?

[acey]

Can you observe man-made satellites in the middle of the night (say between 00:00 and 03:00)?

Not if the sun is too far below your horizon.

http://www.heavens-above.com/ShowFAQ.aspx?lat=54.57206&lng=-1.58203&loc=Unspecified&alt=0&tz=CET&FAQID=501

Lots more satellite info here:

http://www.heavens-above.com/?lat=54.57206&lng=-1.58203&loc=Unspecified&alt=0&tz=CET

[iamjulian]

I see numerous every night because there are so many of them, but I don't look for them intentionally. I believe they range in orbit from a few tens of miles to a few thousand miles. Through the telescope they appear as points of light, no detail, and they always make me jump when they race through the otherwise tranquil field of view.