Have you seen a light in the sky, and are not sure what you have seen? Have you spotted/photographed what looked a bit like a satellite or satellite
flare and want to try and identify it?
If the answer is "yes", then you are in the right place.
The aim of this guide is to show ATS members and readers a few basic techniques, and what resources are available to view, track, and identify
To start off with, here is an FAQ that should answer some of the more common questions on the subject.
What is a satellite?
A satellite is defined as as anything that has been captured by the gravity of a larger object, and is in orbit around that object (Earth for the
purposes of this thread). A satellite can be anything from a tiny fleck of paint that has come from one of our space craft, to the Moon, which is a
The satellites that most people are familiar with are man-made, and have been launched into orbit for specific purposes, which can range from
communications, to studying our oceans/land/atmosphere, and military surveillance/spy satellites.
There is also another class of satellite that I touched on in the paragraph above the last paragraph. When rockets are launched into space, the
boosters used to help them get there are usually jettisoned, and can end up orbiting Earth. With more and more satellites being sent into orbit, and
missions being launched to other worlds, the amount of such debris/junk in orbit has steadily been growing and is starting pose a very real risk to
both manned space flight and other satellites in orbit.
The problem has also been made much worse in recent years by two events in particular; The
collision between two satellites (COSMOS 2251 and IRIDIUM 33) in orbit in 2009
especially by the Chinese A-SAT (anti-satellite) missile test in early 2007
scattered a huge cloud of debris in relatively high orbits, where much of it could take many decades to de-orbit, and poses a threat to many of our
satellites that are in LEO (Low Earth Orbit) as the debris makes it's way down through lower orbits.
What do satellites in orbit look like from the ground?
Most satellites that are visible to the naked eye will look like stars that move slowly but steadily across the sky in a straight line, usually about
the speed of a commercial jet aircraft at cruising altitude, but some will be a bit faster, and some crawl across the sky much more slowly. The ISS
for example is about average speed, and can take as long as 10+ minutes to cross the sky when it passes close or directly overhead.
There are also Geo-stationary satellites which are in orbits that keep them stationary in relation to the Earth, so as the name implies, from a
particular location on the ground they will always be found "sitting" in the same place in the sky, for night after night. Since they are spinning at
the same rate the Earth spins, the background stars will appear to move past them over the course of time, as Earth rotates.
THE BUSY-NESS OVER YOUR HEAD:
Click here for a larger version of the above
Earth orbit is crowded with nearly a thousand operating satellites and tens of thousands of spent rocket engines, splinters from satellite
collisions, and other space debris. Space is a busy place. This picture taken by expert satellite watcher Marco Langbroek frames some of the madding
crowd over Leiden, the Netherlands.
"This single image of a 10x14 degree-wide part of the geostationary belt was taken near midnight of June 18-19 and shows 30 satellites," says
Langbroek. "Each black box contains one or more (mostly) geostationary satellites plus a few rocket bodies: 23 commercial geostationary satellites,
one classified military geostationary satellite (Milstar 5), and 6 spent rocket boosters."
"The geostationary belt can be seen as a slanting line of objects diagonally over the larger image." he continues. "The geostationary belt (at
declination -7.4 degrees for the Netherlands) never comes high in the sky for my country (which is at 52 N). All the objects on the picture have an
elevation below 30 degrees. The image was taken from the center of Leiden--i.e. not an ideal dark sky. I did a slightly bad job in focussing, so the
image is slightly less sharp (especially near the edges) than it could have been with this fine lens. Still, an amazing number of objects recorded in
this small field of view!"
Source: spaceweather.com Archive (Monday, Jun. 25, 2012)
While some satellites are quite bright, the vast majority are relatively faint, and many are invisible to the naked eye, even when observing from
dark/un-light polluted skies.
Many of the brighter satellites tend to be constant in brightness as they traverse the sky, but this is not necessarily always the case. Sometimes, as
the the angle between observer/satellite and Sun changes, a part of the satellite (usually a panel or array) will find the right angle to catch the
light and increase the amount of light that is being reflected back to you dramatically.
Flares as they are called can occur without warning, and cause the satellite to increase by many hundreds of times in terms of brightness. Probably
the best known satellites for producing flares are the IRIDIUM satellites
, which frequently
flare to the same brightness as a quarter or half Moon.
An IRIDIUM satellite flare and Comet Holmes below and a bit right of the flare:
Whilst IRIDIUM flares are usually very predictable, tumbling or out of control satellites/debris can produce seemingly random flares, or there may be
a sequence in some cases.
It's not unusual that the satellite itself is too dim to see with the naked eye, and the first that you know of it is when you see a flash or a series
of them, rather than a single prolonged flare, though either is possible depending on the satellite and the way it is rotating. As some
satellites/debris rotate quite rapidly, short duration flares or flashes can be the result. In some cases a glint can last just a fraction of a
second, and resemble a "camera flash" going off in the distance.
Sometimes satellites that are visible will seemingly disappear, even if there does not appear to be any cloud to hide them. It can be that there is
some thin cloud that you have not noticed, but it can also be due to the satellite passing into Earth's shadow.
Satellites usually appear to be white in colour, but they can also be yellow, orange, or even red, which is usually because they are passing into, or
skirting Earth's shadow, and just like the sun rising or setting, the extra thickness of the atmosphere that the light has to pass through to get to
an observers eye scatters the blue light, making the Sun/satellite look red or orange.
Many of the above characteristics can mean that it's possible to confuse a satellite with other objects that can be seen in the sky such as aircraft
(you can't always hear the sound of the engine/engines if the aircraft is a long way off or at high altitude), or "sky lanterns", or even a slow
meteor. All of these things are often effectively point sources of light, and can be seen from deceptively great distances.
The only way to tell for sure in some cases is to track down and identify the satellite, if indeed a satellite was responsible, and I'll go through
how to do that in a bit.
Flares and misinterpreting brightness - Can satellites play tricks on our eyes?
When we see a light in the sky, our brains will be trying to make sense of what we are looking at, which includes trying to work out how far away the
object is. To do this the brain uses "visual cues
Under normal circumstances (on the ground, in daylight, at short-medium distances, and with objects that are familiar to us), this is usually quite
easy to do, but when we are looking at an unknown
object, of unknown
size, distance, and brightness in the night sky, and there are few
if any cues available, it can be very hard to make any sense of what we are seeing.
Under these circumstances our brains will try to make sense of what we are seeing by using any cues available. Unfortunately the only cues available
in the case of satellites (and many other objects in the sky) are the motion/direction, and the brightness of the object. The brightness of an object
can under normal circumstances be used to give us clues as to how far (or close) an object is. The brain will interpret a bright light as being close,
and a dim light as being further away, which is a natural logical assumption, and works quite well on the ground, with objects/lights we are familiar
However, an unidentified light in the sky is just that - "unidentified" and could be at any distance, and could be almost any brightness. In some
cases, satellites and other objects in the sky (meteors for example) can be surprisingly bright, which could
be misinterpreted as the object
being closer than it is, and just as a light of constant brightness disappearing into the distance looks like it's getting fainter, a light at a more
or less constant distance from an observer might appear to be receding into the distance although it's only getting dimmer.
So a satellite flare might easily be described as an object "powering up"(increasing it's brightness as the observer-satellite-Sun angle becomes more
favorable), and "shooting off into space" (when the flare has reached it's peak, and the observer-satellite-Sun angle becomes less favorable ie
See the thread How good are we at estimating the distance and altitude of UFOs?
more on the subject. There are many examples, and evidence from a related subject (meteors) that brightness plays a key role in how people pecieve and
identify unknown lights in the sky - often they will percieve bright meteors as being much closer than they actually are.
Do satellites always travel in a straight line across the sky?
Not always! Sometimes they can appear
to zig-zag or have a pendulum like motion as they move along in the sky.
to follow a path that looks like a sine wave:
In fact, all satellites do follow a straight (more or less) path in the sky, and this can be proved by photographing them, as I have done many times
in the past. Most recently during the The Perseid meteor shower
which peaked a couple
of weeks back, both me and my partner both saw satellites zig-zagging across the sky which passed through the field of view of our cameras, and they
all recorded as straight lines.
The reason for this odd motion is an illusion known as the autokinetic effect
, or it's equivalent
for moving objects. Once again, the lack of visual cues is probably the cause. It's actually quite common to observe this effect, especially with
fairly bright satellites in my experience.
Can a satellite make a sharp turn?
No it can't, but you might see something that appears
to be a satellite making a sharp turn or even a series of them.
For example if there were two satellites, and just as one moved into Earth's shadow (it would seem to disappear), the second happened to move out of
Earth's shadow, it might appear
as if one satellite had made an impossible turn.
In this diagram satellites A and B reach point X (on the boundary of Earth's shadow) at the same time - As A disappears, B appears.
There are certainly enough satellites up there for this chance occurrence to happen every so often.
This simulated image of the satellites in orbit exaggerates their size but gives you a bit of an idea how many are up there:
Do satellites ever travel in pairs or threes?
Yes! The best examples are the NOSS Double and Triple Satellite Formations
Can I see satellites for myself?
Yes! There are probably a few hundred satellites above you in the sky right now! If it's dark, and the sky is clear you may be able to see some, or
even many under the right conditions, which I'll go into a bit more later on.
Of course, if you live in a town or city, you won't see as many satellites as someone who observes from less light polluted or rural skies. Brighter
satellites and flares can easily be observed from light polluted skies though.
When are the best times to look for satellites?
When it's dark obviously, but it is also possible to see satellites in broad daylight if you are lucky or if you look for them carefully.
Before you do though, go to heavens-above
, enter in your observing location as precisely as you can, go back
to the home page and check the predictions for a few satellites. I would recommend checking for ISS passes and IRIDIUM flare predictions to begin with
if you have not tried before. TIP: Look for the more negative brigtness values - a -8 magnitude IRIDIUM flare will be much brighter and visible
(especially in light polluted skies) than a +1 magnitude flare.
How much you see will depend when you look. Certain times are better than others for observing satellites. The time of night is important, as is the
time of year (more so the further away from the equator you are) and your latitude. This is because how far the Sun dips below the horizon is
dependent on these factors, and in general, the best time to observe is when the Sun is just below the horizon.
For most people (at mid-latitudes) in the Northern hemisphere, during summer is a good time to observe since the Sun never dips very low below the
horizon at night, so satellites can generally be seen throughout the summer nights.
During winter however, due to Earth's tilt, the Sun gets very low below the horizon towards the middle of the night, meaning that much of the sky is
in Earth's shadow, and because of this not many satellites will be visible during winter nights, except close to sunset/sunrise.
In the Southern hemisphere, this situation is reversed, with winter nights being better for observing through the night. For observers on or near the
equator the situation throughout the year is similar to that for observers at mid-latitudes in the Northern hemisphere during winter - the Sun quickly
sinks very low below the horizon, so it's difficult to observe many satellites outside the few hours close to sunset/sunrise.
Tracking and identifying satellites and flares
Most people reading this will be observing satellites and flares visually
. In order to identify the satellites/flares that you see it will help
if you make some careful observations.
Firstly, it's important to note the position and direction of travel of the satellite in relation to the stars/constellations, so a good working
knowledge of the stars is very useful here. Take a star chart out with you, or get an app for your phone that will show you which stars you're looking
Secondly you also need to note the time when you see the satellite close to an identifiable star/constellation. Make sure you have a means of telling
the time with you, and keep it synchronized with an accurate source like your PC, which should be accurate to within a second if it's properly set up
and connected to the internet.
When you see a suspected satellite, try to note the position and time at two
positions along the track of the satellite in the sky. Later on
when you have finished observing, you can use this data to identify the satellite.
There are also applications available for modern mobile phones, iPads, etc, that will identify satellites when you point the device at the satellite
(scroll down for links to download these apps). I don't have a device like this, so I don't have any more to say about them, apart from they sound
useful, so give it a try if you have one.
With the correct equipment identifying a satellite will be much less hassle than doing it visually and making notes. If you photograph it, you'll have
a record of the track in relation to the stars, and if the camera is set up properly and synchronized, you'll also have an accurate record of the
To do this you'll need a DSLR
that is mounted on a tripod. A fast wide (ideally) to normal focal length
lens is also necessary, along with a cable/remote shutter release to trigger the exposure without disturbing the camera.
A setup suitable for the purpose can be bought surprisingly cheaply, if you buy used. You don't need the latest DSLR - older DSLRs will work just
fine. I use and recommend Canon DSLRs such as the EOS 20/30D (the bare minimum for this kind of job), and they can be bought from Ebay for as little
as around £100 (or $200) for a reasonable condition body. For the lens, a 50mm F1.8 will work and is cheap (£80/$160 new), but focal lengths between
20mm-35mm are preferred, as long as they are quite fast (F1.4-1.8 ideally). F2.8 is getting a bit slow, but you will still be able to photograph
Setting up your DSLR for satellite photography
Firstly make sure your battery is charged and/or keep a fully charged spare handy. Also synchronize your DSLRs clock to an accurate clock as I
Secondly, you need to be focused at infinity, which can be tricky as on some lenses, the lens will focus past infinity. I personally take a laptop out
with me, connect the camera to it, point it up at the stars, take a shot, and check focus by zooming into the image on the laptop, and adjusting the
focus manually as necessary. Auto-focus is useless in these situations. Newer cameras have a feature called "live view" with which you can zoom in to
check if stars are in focus.
At the same time make sure the lens aperture is set to wide-open (it's lowest
setting), and the correct ISO setting - I recommend ISO 800 on
older cameras, and 1600 on DSLRs that are less than 5-6 years old. The exposure length also has to be set, and this is determined once again by
checking my laptop screen. You want to capture some star detail, but at the same time keep the exposure short enough so that light-pollution does not
reduce the contrast of your image so much that spotting the trails that satellites leave behind is difficult. Many satellites only leave faint trails
that are easy to drown out if your exposure is too long.
The exposure length will depend on the conditions at the time of your observations. I like to keep exposures quite short - about 10-15 seconds is
ideal under most circumstances and with a relatively dark/un-light polluted rural sky, but if you are in a town/city, you'll probably want slightly
Make sure your DSLR is set to MANUAL ("M" on most cameras) mode
before you take any test shots. Also make sure the shooting mode is set to fire
the shutter continuously when the shutter release button is held down.
Once your lens is focused, and the DSLR set up, turn off your camera, cap the lens, and wait.
If you spot a satellite that you want to identify, turn the DSLR on, remove the lens cap carefully so as you don't change the focus, point the camera
where the satellite is, and lock down the remote cable with the "running lock" to trigger the first exposure, and subsequent exposures after it. As
far as I know, this trick only works with Canon DSLRs, which will continue to take exposures for as long as the "running lock" is engaged, or the
memory card fills up, or the battery runs out.
The result will be a series of exposures showing individual sections of the satellites track through the sky, which can, if you desire, be combined
into a single image using photo editing software.
Here's an example of two separate exposures combined, that shows the Moon and a flare from a satellite that I've identified as
Note the small gap that was caused by the time delay between subsequent exposures.
Identification of observed satellites
Now that you have some data, you can try and identify your suspected satellite.
To begin with, you'll need to know the coordinates of your observing location as precisely as possible. Using Google maps is usually the easiest way
to find this out. Once you know this, you can begin.
There are numerous ways to do this, and I'll start with the most basic:
As mentioned before, heavens-above
is a good place to start. Once you've entered your observing location
(height above sea level and timezone too), you can go back to the home page and click on various satellites to see if there were any visible passes at
your location and the time you were observing. Then, when you find a likely pass, click on the time to see the details and a star-map showing where
the satellite was in the sky.
is an on-line Java based satellite tracking app that you can also
On your own PC
If you still can't identify your suspected satellite, you may have to try doing it with satellite tracking software for your PC.
Probably the best software for this (that I have used anyway) is Visual SAT-Flare Tracker 3D
might have problems running this on newer operating systems, but it runs fine on XP for me.
To get to grips with Visual SAT-Flare Tracker 3D may take a few hours playing around with it (it took me the best part of a night), but it is a quite
powerful app with many useful features. Make sure you update the "tle" files (which contain the telemetry or "orbital elements" needed to accurately
simulate a satellites orbit) via the option available in the menu, and enter your location, then you can save your settings via the "save workspace
as" option that can also be found in the menu.
Then it's a question of finding the right area in the star-chart view, zooming in a bit, and setting the time to the time of your observation. You can
enable/disable various satellites if necessary to narrow down the possibilities, and make the area you are interested in less cluttered. Disable any
satellites that are doing in the wrong direction, or don't have a similar track to the one you are looking for. You should eventually get a close
match to your observations. If you don't make sure that you have the latest tle files, and if necessary find further tle files for satellites that are
not included in the "update" dialog. Most are as far as I can tell.
is a Windows PC planetarium application that can also be used to track satellites. See
for details. I don't use Stellarium, so can't say how good or bad it
is, or how to use it, but I'm sure the principals will be similar to Visual Sat FlareTracker3D.
For a list of further satellite tracking software, visit satobs.org
and scroll down to the "Predictions" section.
That is basically it. If you've tried all of the above, and still can't identify what you saw, then it probably was not a satellite that you saw.
Now there is no excuse for ATS members to misidentify satellites or satellite flares/flashes. You have been warned
If you have any comments, suggestions or questions please feel free to post them here or U2U me.
Related links and resources
Visual satellite observing guides
(lots of resources and info here)
(mailing list for visual satellite observers)
Archive of SeeSat-L
SeeSat-L Frequently Asked Questions (FAQ)
IBM PC/Windows based satellite tracking/prediction applications
Visual SAT-Flare Tracker 3D
Other tracking apps
Satellite tracking apps for iPhone, iPad, etc.
Ever Wonder What Satellites Are Flying Above Your Head? - The
Satellite Augmented Reality now in Android Market - AGI Blogs
A Beginner’s Guide to Photographing
The International Space Station (ISS)
Tips & Resources For Photographing Satellites
Sky and Telescope - Photographing Iridium
(Aimed at analog/film camera users but still relevant to DSLRs)
Astrophotography Techniques - Jerry Lodriguss
Lots of good info on photographing stars
here. The techniques/equipment used for photographing satellites (or their tracks in the sky) are similar to those used for photographing meteors.
Photographing Satellites - Taking Pictures of the ISS and Iridium
(Also aimed at analog/film camera users)
Depth cues and how we "see"
Perceptual distance and the constancy of size and stereoscopic depth
Depth Cues in the Human Visual System
edit on 28-8-2012 by
FireballStorm because: (no reason given)