Primary Instruments: My eyes, various binoculars, and my Orion AstroView 120ST short tube refractor.
I also use my various other telescopes to observe satellites on occasion.
I really started getting interested in satellite watching some months after discovering Joe Orman's Naked Eye 100 list in July 2010. During my further research into Iridium flares and the International
Space Station passes I discovered some interesting websites that provide accurate predictions and data concerning the multitudes of satellites and "space junk" orbiting our home planet Earth. I
derive great pleasure from researching, anticipating, observing, photographing, and cataloging these various objects in space. True, they appear mostly as silent points of light passing
overhead in the darkness, but most of the items have an interesting background that can be researched and learned from.
Since Sputnik was first placed into orbit by the Soviet Union in 1957, many thousands of objects have wound up in earth orbit. Despite numerous objects re-entering and burning up in earth's atmosphere, thousands are still held captive and silently orbit the earth. The vast majority of these that reflect enough light to be easily observed are merely "space junk": rocket bodies, unserviceable satellites, debris, etc, and they serve no useful purpose. Hundreds of them are operational satellites performing a variety of functions ranging from
covert surveillance to earth resources to communications, among others.
Despite all these objects in orbit, collisions between objects are apparently rare. In 2009 an Iridium satellite and a Russian satellite "Kosmos 2251" collided, destroying both objects and creating a couple thousand of additional objects that are tracked by those in authority. What's left of the main
carcass of Iridium 33 is still visible as a "Tumbling Iridium". I haven't researched the Russian satellite to see if it is visible. (Note: as I type this on 27 December 2012 at 1545hrs MST, I've checked Heavens Above and Kosmos 2251 is making an (invisible) daylight pass directly over my location right now. According to the predictions,
the next VISIBLE pass won't be for another month, in late January 2013. I haven't seen this one yet so I'll have to mark my calendar!!)
For the purpose of this discussion, the term "satellite" refers to any of the satellites and space junk cataloged and tracked and subject to observation based on the predictions available online.
Types of satellite orbits:
Low Earth Orbit
Many of the objects easily visible to the naked eye and through binoculars are in Low Earth Orbit (LEO).
LEO is a geocentric orbit ranging in altitude from 160 kilometeres (100 statue miles) to 2,000 kilometres (1,200 mi) above mean sea level. At 100 miles, one revolution takes approximately 90 minutes, and the circular orbital speed is 8,000 metres per second (26,000 ft/s).
Medium Earth Orbit
MEO is a geocentric orbit with altitudes at apogee ranging between 2,000 kilometres (1,200 mi) and that of the geosynchronous orbit at 35,786 kilometres (22,236 mi).
Geosynchronus Orbit
Geocentric circular orbit with an altitude of 35,786 kilometres (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3,000 metres per second (9,800 ft/s).
High Earth Orbit
HEO is a geocentric orbit with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is the highly elliptical orbit, where altitude at perigee is less than 2,000 kilometres (1,200 mi).
There are many additional parameters and definitions associated with the various earth orbits. A good reference to consult is this article at: Wikipedia.
Orbital Inclination
Most satellites you can easily view are in either a polar orbit or an orbit where the satellite is moving roughly west to east,...e.g. with the rotation of the earth, know as a prograde orbit. Retro-grade orbits (against the rotation of the earth)
are much less common as they require more fuel to place them into orbit. However, many of the polar orbits are indeed more than 90-degrees and some are even into the 120-degree range.
A satellite is said to be in an "ascending" orbit when it's motion relative to the equator is south to north. Likewise, a "descending" orbit is north to south. For all the satellites you see moving through the sky, half of each orbit is considered
ascending and the other half descending. One day you may observe a satellite "ascending" while days later the same satellite may be observed "descending". It's bound to happen: you will likely see the same objects over and over again.
Precession. Due to the rotation of the earth, the orbital tracks of many satellites move to the west with each successive pass. Depending on the orbital inclination you may be able to spot a particular satellite on consecutive passes. I'm sure it applies
to numerous satellites and it also includes the International Space Station. It is also possible to see a satellite in the morning and then again in the evening. I've done this with the ISS.
Many satellites make about 14 or 15 earth orbits each day so they are apt to cross the sky above your location at least once per day...whether or not they will be visible is dependent upon timing.
An interesting discussion of precession can be found here.
Some categories of satellites:
Iridium Flares
Since about 1997 the Iridium satellite constellation has provided the very predictable "Iridium flares" where sunlight is reflected off one of three highly polished main mission antennae at a specific instance in time and viewable from only a specific location on earth.
Iridium flares are visible about every day of the year from numerous locations on earth. The opportunity to view a specific flare is highly dependent on the geometry between the observer, the satellite, and the sun. The apparent magnitude (brightness) of the flare also
depends on the geometry. For the most accurate results, the online prediction websites (Heavens Above, Cal Sky, etc) recommend you set up your viewing location with very specific latitude / longitude and elevation data.
International Space Station (ISS)
The ISS is viewable from most of the populated surface of the earth. Due to its orbital inclination of 51.6°, it is not visible above very high latitudes near the north & south poles.
During a favorable pass, the ISS is unmistakeable in the sky as it silently cruises overhead. The ISS is one satellite you can "easily" try to see nearly down to
the horizon if you have an unobstructed view. Real Time Satellite Tracking website (see figure below) offers predictions right down to below 1° above the horizon and I've managed to view it with binoculars at a bit above 2° above the horizon. Cal Sky predictions are similar while
Heavens Above predictions are typically limited to passes at least 10° above the horizon.
Example of Real Time Satellite Tracking predictions for ISS.
Tandem sightings are possible when various countries' modules maneuver into position for resupply docking and then during undocking.
Hubble Space Telescope (HST)
For North American observers, HST is visible only in the southern part of the sky, though in lower U.S. latitudes it can get about 50° above the southern horizon. While not particularly bright, the HST is often easily visible naked-eye and it occasionally will produce
an effect similar to a slow "tumbling" satellite. As seen from the USA, the HST typically appears to cross the sky from west to east in a nearly level line.
Rocket Boosters
Much of the easily-visible space junk in orbit consists of various components of rocket boosters/bodies. Some of these often approach magnitude of 1.0 or brighter and are easily seen.
NOSS Satellites
NOSS satellites typically travel in pairs with one trailing the other by just a few seconds. The pair are usually at or just below naked-eye visibility (depending on your sky conditions) and they are easily spotted
in the field-of-view of my 15x70 binoculars. For some reason, I get a kick out of spotting NOSS pairs.
Tiangong 1
Tiangong 1 is the Chinese space station module orbited in September 2011. During favorable passes it is an easy naked-eye target often reaching -1.n magnitude. Though I've not witnessed it yet, this is one
object where a tandem sighting is possible during a docking maneuver.
Cosmo Skymed Satellites
These Italian surveillance satellites are often predicted to "flare" similar to Iridium flares. Cal Sky predicts these flares on the same page as Iridium flares.
Tumbling Iridiums
Conveniently, Cal Sky has a separate page assigned to these. Tumbling Iridiums are out-of-service Iridiums that are spinning out of control and produce flashes at various intervals. Some are easily visible
naked eye...many of the satellites require binoculars to observe. Iridiums and Tumbling Iridiums was the first category of satellites where I managed to see the entire complement currently in orbit. Some of
the failed Iridium satellites were de-orbited prior to my interest.
X-37B
The U.S. government has sent the X-37B into orbit three times that we know of, most currently in December 2012. This mini "Space Shuttle Wannabe" is easily viewable during favorable, naked-eye passes.
Spotting Satellites
I often make a morning or evening out of spotting satellites. Usually I'll print out the Heavens Above predictions for the time period and note those satellites with favorable passes.
For me, a favorable pass is one that passes fairly close to a star that I'm familiar with. I'll further research each pass and estimate to the nearest second when the satellite is predicted to
pass by the star. I annotate the margins of the printouts with the minutes/seconds and the name of the target star. Armed with my mobile phone with an atomic time "app" I easily bag most of the satellites
I've researched. The observed magnitudes sometimes seem to vary considerably against the predictions from the websites. This is often a function of "phase angle" illumination - similar to geometry between
us, the moon and the sun. As a result, some of the dimmer predictions are often easier to see than expected and vice-versa. A good read on satellite observing is found here.
I should note that the internal hyperlinks in that document do not work...you'll have to scroll down the page to read the entire document.
Many of the satellites in the Heavens Above database have "?" for the magnitude and therefore do not appear on the "Daily Predictions for Visible Satellites" page. I sometimes try to see some of those.
Another handy feature of Heavens Above is that it can list satellites by their 5-digit "NORAD" identifier. Using this, you can start with the oldest known satellite up there and try to see each one in turn. Some of
these guys are pretty small and dim and require a telescope and a pass near an easily-identifiable star to have a good chance of spotting them.
I also use Jonathan's Space Report to research suitable target satellites. Several different files are available. One I like in particular is the satellite catalog.
It lists "everything" that has ever been tracked whether or not it is still in orbit.
