At which altitudes do visible satellites orbit?

I wonder if one can e.g. say that maybe all satellites that are
visible to the unarmed eye orbit in altitudes below 1000 km?

The vast majority visible without binoculars are in low orbits.

Or could one see e.g. a GPS or a geostationary satellite
without having to rely on to a telescope? Probably not.

GPS are faint and require a telescope. A couple of times a
year around the equinoxes, a few operational geosats can be
observed for a few minutes per evening without magnification.
Quite a few more are easy to see with handheld binoculars.

Also, there are a few non-operational geosynchronous objects,
most of them drifting a few degrees to the west from night
to night, which are tumbling and flash brightly enough to
be seen without magnification.

Actually, I am even amazed that it is (is it?) possible to
see a satellite that is 600 km away with only my bare eyes!

I kind of felt the same way when I started observing -- to
see something a few meters in size at such ranges, it was an
unexpected phenomenon for me. But it's certainly a fact.

How big does a satellite (or its solar panels) have to be to be visible
with bare eyes from earth (for different altitudes, e.g. 800 km)?

This depends on the shape and orientation of the object. A white
sphere and a cube of the same size covered with dark solar cells
would be very different in terms of ease of observability. The
main mission antennas of Iridium satellites, about the size of a
typical door of a house, can make incredibly bright reflections
-- many times brighter than Venus -- at ranges of more than 1200
km from the observer.

How far away is the furthest away satellite that is still
visible with bare eyes? How big is it? Which one is it?

One satellite, called ETS 6 or Kiku 6, which is in a failed
orbit, has been seen without binoculars flashing at a range
of about 40,000 km from the observer. Quite a few at 36,000
km can be seen at least occasionally. They are almost all
big communication satellites, basically a box with two
"wings" of solar panels. There's quite a bit of information
about them on the internet.

most satellites out there belong to one of roughly three groups:

- Altitudes 600 - 1000 km (many ...)

There are quite a few between 1000 and 1600 km -- Globalstars
for example, and a number called "Cosmos".

- Altitudes 20.000 km (GPS)
- Altitudes 36.000 km (geostationary (many...))
(- other satellite orbits, e.g. like chandra, XMM, far out and/or
very excentric)

One other type of operational orbit is very eccentric but not as
high as Chandra and XMM. These are Molniyas and others in the
same type of orbit, with a low perigee in the far southern
hemisphere and a very high apogee (40,000 km) above the northern
hemisphere.

Here are a few very useful web sites:

Visual Satellite Observers Home Page --
http://www.satobs.org/

Visual Satellite Observing FAQ (HTML) --
http://home.att.net/~sue.worden/SeeSat-FAQ/

Visual Satellite Observing FAQ (text) --
http://users2.ev1.net/~mmccants/faq/

Jeff Hunt's Introduction to Visual Satellite Observing --
http://home.att.net/~janjeff/satintro.htm

Ed Cannon - - Austin, Texas, USA
http://wnt.cc.utexas.edu/~ecannon/starting.htm

I wonder if one can e.g. say that maybe all satellites that are
visible to the unarmed eye orbit in altitudes below 1000 km?

The vast majority visible without binoculars are in low orbits.

Or could one see e.g. a GPS or a geostationary satellite
without having to rely on to a telescope? Probably not.

GPS are faint and require a telescope. A couple of times a
year around the equinoxes, a few operational geosats can be
observed for a few minutes per evening without magnification.
Quite a few more are easy to see with handheld binoculars.

Also, there are a few non-operational geosynchronous objects,
most of them drifting a few degrees to the west from night
to night, which are tumbling and flash brightly enough to
be seen without magnification.

Actually, I am even amazed that it is (is it?) possible to
see a satellite that is 600 km away with only my bare eyes!

I kind of felt the same way when I started observing -- to
see something a few meters in size at such ranges, it was an
unexpected phenomenon for me. But it's certainly a fact.

How big does a satellite (or its solar panels) have to be to be visible
with bare eyes from earth (for different altitudes, e.g. 800 km)?

This depends on the shape and orientation of the object. A white
sphere and a cube of the same size covered with dark solar cells
would be very different in terms of ease of observability. The
main mission antennas of Iridium satellites, about the size of a
typical door of a house, can make incredibly bright reflections
-- many times brighter than Venus -- at ranges of more than 1200
km from the observer.

How far away is the furthest away satellite that is still
visible with bare eyes? How big is it? Which one is it?

One satellite, called ETS 6 or Kiku 6, which is in a failed
orbit, has been seen without binoculars flashing at a range
of about 40,000 km from the observer. Quite a few at 36,000
km can be seen at least occasionally. They are almost all
big communication satellites, basically a box with two
"wings" of solar panels. There's quite a bit of information
about them on the internet.

most satellites out there belong to one of roughly three groups:

- Altitudes 600 - 1000 km (many ...)

There are quite a few between 1000 and 1600 km -- Globalstars
for example, and a number called "Cosmos".

- Altitudes 20.000 km (GPS)
- Altitudes 36.000 km (geostationary (many...))
(- other satellite orbits, e.g. like chandra, XMM, far out and/or
very excentric)

One other type of operational orbit is very eccentric but not as
high as Chandra and XMM. These are Molniyas and others in the
same type of orbit, with a low perigee in the far southern
hemisphere and a very high apogee (40,000 km) above the northern
hemisphere.

Here are a few very useful web sites:

Visual Satellite Observers Home Page --
http://www.satobs.org/

Visual Satellite Observing FAQ (HTML) --
http://home.att.net/~sue.worden/SeeSat-FAQ/

Visual Satellite Observing FAQ (text) --
http://users2.ev1.net/~mmccants/faq/

Jeff Hunt's Introduction to Visual Satellite Observing --
http://home.att.net/~janjeff/satintro.htm

Ed Cannon - - Austin, Texas, USA
http://wnt.cc.utexas.edu/~ecannon/starting.htm

Ed Cannon wrote:

Here are a few very useful web sites:

Visual Satellite Observers Home Page --
http://www.satobs.org/

Visual Satellite Observing FAQ (HTML) --
http://home.att.net/~sue.worden/SeeSat-FAQ/

Visual Satellite Observing FAQ (text) --
http://users2.ev1.net/~mmccants/faq/

Jeff Hunt's Introduction to Visual Satellite Observing --
http://home.att.net/~janjeff/satintro.htm

Ed Cannon - - Austin, Texas, USA
http://wnt.cc.utexas.edu/~ecannon/starting.htm

Plus

Heavens-Above

http://www.heavens-above.com

and

http://www.donbarry.org/

--Bill Thompson

Ed Cannon wrote:

Here are a few very useful web sites:

Visual Satellite Observers Home Page --
http://www.satobs.org/

Visual Satellite Observing FAQ (HTML) --
http://home.att.net/~sue.worden/SeeSat-FAQ/

Visual Satellite Observing FAQ (text) --
http://users2.ev1.net/~mmccants/faq/

Jeff Hunt's Introduction to Visual Satellite Observing --
http://home.att.net/~janjeff/satintro.htm

Ed Cannon - - Austin, Texas, USA
http://wnt.cc.utexas.edu/~ecannon/starting.htm

Plus

Heavens-Above

http://www.heavens-above.com

and

http://www.donbarry.org/

--Bill Thompson

Fred wrote:


Hi,
I'm pretty new to this topic - so I wonder if one can e.g. say that
maybe all satellites that are visible to the unarmed eye orbit in
altitudes below 1000 km?



How big does a satellite (or its solar panels) have to be to be visible
with bare eyes from earth (for different altitudes, e.g. 800 km)?

It's not (directly) the size, it's the amount of light it reflects
contrasted with the blackness of space.

How far away is the furthest away satellite that is still visible with
bare eyes? How big is it? Which one is it?

The moon. :-) If you mean artificial, I don't know.


I have the impression (from looking at J-Track at
http://liftoff.msfc.nasa.gov/realtim.../JTrack3D.html)
that most satellites out there belong to one of roughly three groups:

- Altitudes 600 - 1000 km (many ...)
- Altitudes 20.000 km (GPS)
- Altitudes 36.000 km (geostationary (many...))
(- other satellite orbits, e.g. like chandra, XMM, far out and/or very
excentric)

A lot of meteorological (and other?) satellites are at about 800-850 km, in
the range necessary for sun-sychronous polar orbits (every ascending or
descending Equator crossing is at the same local time). For example, all
the NOAA polar orbiters and the DMSP satellites.

This page has a neat view of the distribution of satellites (and a whole
lot more).
http://www.astro.princeton.edu/~mjuric/universe/




--
Clay Blankenship Change 'Z' to 's' to reply
Monterey, CA

Fred wrote:


Hi,
I'm pretty new to this topic - so I wonder if one can e.g. say that
maybe all satellites that are visible to the unarmed eye orbit in
altitudes below 1000 km?



How big does a satellite (or its solar panels) have to be to be visible
with bare eyes from earth (for different altitudes, e.g. 800 km)?

It's not (directly) the size, it's the amount of light it reflects
contrasted with the blackness of space.

How far away is the furthest away satellite that is still visible with
bare eyes? How big is it? Which one is it?

The moon. :-) If you mean artificial, I don't know.


I have the impression (from looking at J-Track at
http://liftoff.msfc.nasa.gov/realtim.../JTrack3D.html)
that most satellites out there belong to one of roughly three groups:

- Altitudes 600 - 1000 km (many ...)
- Altitudes 20.000 km (GPS)
- Altitudes 36.000 km (geostationary (many...))
(- other satellite orbits, e.g. like chandra, XMM, far out and/or very
excentric)

A lot of meteorological (and other?) satellites are at about 800-850 km, in
the range necessary for sun-sychronous polar orbits (every ascending or
descending Equator crossing is at the same local time). For example, all
the NOAA polar orbiters and the DMSP satellites.

This page has a neat view of the distribution of satellites (and a whole
lot more).
http://www.astro.princeton.edu/~mjuric/universe/




--
Clay Blankenship Change 'Z' to 's' to reply
Monterey, CA

Clay Blankenship wrote in message ...
Fred wrote:


Hi,
I'm pretty new to this topic - so I wonder if one can e.g. say that
maybe all satellites that are visible to the unarmed eye orbit in
altitudes below 1000 km?



How big does a satellite (or its solar panels) have to be to be visible
with bare eyes from earth (for different altitudes, e.g. 800 km)?

It's not (directly) the size, it's the amount of light it reflects
contrasted with the blackness of space.

How far away is the furthest away satellite that is still visible with
bare eyes? How big is it? Which one is it?

The moon. :-) If you mean artificial, I don't know.


I have the impression (from looking at J-Track at
http://liftoff.msfc.nasa.gov/realtim.../JTrack3D.html)
that most satellites out there belong to one of roughly three groups:

- Altitudes 600 - 1000 km (many ...)
- Altitudes 20.000 km (GPS)
- Altitudes 36.000 km (geostationary (many...))
(- other satellite orbits, e.g. like chandra, XMM, far out and/or very
excentric)

A lot of meteorological (and other?) satellites are at about 800-850 km, in
the range necessary for sun-sychronous polar orbits (every ascending or
descending Equator crossing is at the same local time). For example, all
the NOAA polar orbiters and the DMSP satellites.

This page has a neat view of the distribution of satellites (and a whole
lot more).
http://www.astro.princeton.edu/~mjuric/universe/

That Princeton diagram shows Vanguard 1 in a higher orbit than Chandra
- which is totally untrue.

Clay Blankenship wrote in message ...
Fred wrote:


Hi,
I'm pretty new to this topic - so I wonder if one can e.g. say that
maybe all satellites that are visible to the unarmed eye orbit in
altitudes below 1000 km?



How big does a satellite (or its solar panels) have to be to be visible
with bare eyes from earth (for different altitudes, e.g. 800 km)?

It's not (directly) the size, it's the amount of light it reflects
contrasted with the blackness of space.

How far away is the furthest away satellite that is still visible with
bare eyes? How big is it? Which one is it?

The moon. :-) If you mean artificial, I don't know.


I have the impression (from looking at J-Track at
http://liftoff.msfc.nasa.gov/realtim.../JTrack3D.html)
that most satellites out there belong to one of roughly three groups:

- Altitudes 600 - 1000 km (many ...)
- Altitudes 20.000 km (GPS)
- Altitudes 36.000 km (geostationary (many...))
(- other satellite orbits, e.g. like chandra, XMM, far out and/or very
excentric)

A lot of meteorological (and other?) satellites are at about 800-850 km, in
the range necessary for sun-sychronous polar orbits (every ascending or
descending Equator crossing is at the same local time). For example, all
the NOAA polar orbiters and the DMSP satellites.

This page has a neat view of the distribution of satellites (and a whole
lot more).
http://www.astro.princeton.edu/~mjuric/universe/

That Princeton diagram shows Vanguard 1 in a higher orbit than Chandra
- which is totally untrue.

"Clay Blankenship" wrote in message
...
Fred wrote:
A lot of meteorological (and other?) satellites are at about 800-850 km,
in
the range necessary for sun-sychronous polar orbits (every ascending or
descending Equator crossing is at the same local time). For example, all
the NOAA polar orbiters and the DMSP satellites.

The orbital planes of sun-synchronous orbits precess at the rate of +0.9856
deg/d, negating the -0.9856 deg/d precession due
to Earth's orbit about the sun.

The rate of precession of an orbital plane is a strong function of orbital
inclination, semi-major axis and eccentricity, in that order. As a result,
sun-synchronous orbits can be achieved over a wide range of altitudes.
Generally, the greater the altitude, the greater the inclination required to
be sun-synchronous. This affords satellite designers considerable
flexibility to tailor orbits to specific missions.

Here are some examples of the diversity among sun-synchronous orbits:

Early U.S. spy satellites required fairly low orbits to achieve adequate
resolution. For example, KH 9-1 typically orbited below 300 km, inclined at
96.4 deg. In this example, it's altitude was just 155 X 273 km:

1 05297U 71056A 71209.61717514 .01079902 00000-0 11491-3 0 700
2 05297 96.3698 280.3821 0089129 171.5008 188.7764 16.23473167 6907

Japan's IGS 1A and 1B spy satellites are in a 491 X 493 km orbit, inclined
at 97.4 deg: Their altitude was chosen because it results in a four day
repeating ground track, and they manoeuvre periodically to maintain that
precise altitude. Here are recent elements of 1A:

1 27698U 03009A 03355.82789570 .00000000 00000-0 00000-0 0 09
2 27698 97.3973 65.5455 0002000 118.8856 241.1142 15.25965018 08

U.S. spysat USA 129's orbit is 273 X 1046 km, inclined 97.8 deg. Its
resolution at perigee is believed to be about 10 cm; at apogee, about 40 cm.
This was its orbit following its recent re-boost manoeuv

1 24680U 96072A 03364.48263571 .00022153 00000-0 24238-3 0 04
2 24680 97.7982 63.7841 0549873 82.5150 283.8156 14.71918203 09

ESA's Spot 5 resources satellite's orbit is 829 X 830 km, inclined 98.7 km.
As Clay pointed out, this is a popular sun-synch orbit. Here are Spot 5's
recent elements:

1 27421U 02021A 03365.77780731 .00000115 00000-0 74306-4 0 9900
2 27421 98.6931 77.2986 0001086 73.9868 286.1415 14.20044241 86114

The old U.S. ESSA 8 weather satellite is in a nearly sun-synch orbit, 1418 X
1469 km orbit, inclined at 101.9 deg:

1 03615U 68114A 03363.73767168 -.00000029 00000-0 10000-3 0 3524
2 03615 101.8616 53.9709 0032141 205.7927 154.1544 12.56164060606687

The Azur radiation research satellite was launched into a nearly sun-synch
orbit, 394 X 3151 km, inclined at 103 deg:

1 04221U 69312.13202672 .00000050 00000-0 64262-5 0 17
2 04221 102.9711 126.6582 1692769 162.0110 204.9165 11.80786751 15

Ted Molczan

"Clay Blankenship" wrote in message
...
Fred wrote:
A lot of meteorological (and other?) satellites are at about 800-850 km,
in
the range necessary for sun-sychronous polar orbits (every ascending or
descending Equator crossing is at the same local time). For example, all
the NOAA polar orbiters and the DMSP satellites.

The orbital planes of sun-synchronous orbits precess at the rate of +0.9856
deg/d, negating the -0.9856 deg/d precession due
to Earth's orbit about the sun.

The rate of precession of an orbital plane is a strong function of orbital
inclination, semi-major axis and eccentricity, in that order. As a result,
sun-synchronous orbits can be achieved over a wide range of altitudes.
Generally, the greater the altitude, the greater the inclination required to
be sun-synchronous. This affords satellite designers considerable
flexibility to tailor orbits to specific missions.

Here are some examples of the diversity among sun-synchronous orbits:

Early U.S. spy satellites required fairly low orbits to achieve adequate
resolution. For example, KH 9-1 typically orbited below 300 km, inclined at
96.4 deg. In this example, it's altitude was just 155 X 273 km:

1 05297U 71056A 71209.61717514 .01079902 00000-0 11491-3 0 700
2 05297 96.3698 280.3821 0089129 171.5008 188.7764 16.23473167 6907

Japan's IGS 1A and 1B spy satellites are in a 491 X 493 km orbit, inclined
at 97.4 deg: Their altitude was chosen because it results in a four day
repeating ground track, and they manoeuvre periodically to maintain that
precise altitude. Here are recent elements of 1A:

1 27698U 03009A 03355.82789570 .00000000 00000-0 00000-0 0 09
2 27698 97.3973 65.5455 0002000 118.8856 241.1142 15.25965018 08

U.S. spysat USA 129's orbit is 273 X 1046 km, inclined 97.8 deg. Its
resolution at perigee is believed to be about 10 cm; at apogee, about 40 cm.
This was its orbit following its recent re-boost manoeuv

1 24680U 96072A 03364.48263571 .00022153 00000-0 24238-3 0 04
2 24680 97.7982 63.7841 0549873 82.5150 283.8156 14.71918203 09

ESA's Spot 5 resources satellite's orbit is 829 X 830 km, inclined 98.7 km.
As Clay pointed out, this is a popular sun-synch orbit. Here are Spot 5's
recent elements:

1 27421U 02021A 03365.77780731 .00000115 00000-0 74306-4 0 9900
2 27421 98.6931 77.2986 0001086 73.9868 286.1415 14.20044241 86114

The old U.S. ESSA 8 weather satellite is in a nearly sun-synch orbit, 1418 X
1469 km orbit, inclined at 101.9 deg:

1 03615U 68114A 03363.73767168 -.00000029 00000-0 10000-3 0 3524
2 03615 101.8616 53.9709 0032141 205.7927 154.1544 12.56164060606687

The Azur radiation research satellite was launched into a nearly sun-synch
orbit, 394 X 3151 km, inclined at 103 deg:

1 04221U 69312.13202672 .00000050 00000-0 64262-5 0 17
2 04221 102.9711 126.6582 1692769 162.0110 204.9165 11.80786751 15

Ted Molczan