Of the thousands of man-made objects orbiting the Earth, some are large enough and low enough to be seen with the unaided eye. They are generally visible shortly after sunset or before sunrise, moving across the sky for up to several minutes. This section will explain how they are visible and how to use predictions to observe satellite passes.
How are they visible?
There are now hundreds of operational satellites orbiting the Earth, serving a wide range of purposes: relaying communications; taking pictures for weather prediction, environmental monitoring, or military intelligence; conducting research on the space environment and of space beyond our planet. Satellites we can see might be as small as a trash can or a small car, but the International Space Station (ISS) has solar panel arrays almost as long as a football field. In addition there are thousands of pieces of "space junk": satellites no longer working, rocket boosters used to launch them, and smaller pieces of debris--even tools lost by astronauts!
Larger satellites, whether they still work or not, may be seen by the sunlight they reflect. Satellites that we can see are in low Earth orbit, from 150 km to 800 km (100 to 500 miles) above the Earth's surface and take 1-1/2 to 2 hours to orbit the Earth--this includes the ISS and the space shuttles when in space. For comparison, the Earth is 12,700 km (6,400 miles) in diameter, and the Moon is 350,000 km (240,000 miles) from the Earth.
The diagram below shows when we can see these satellites. Any visible satellite is visible because it reflects sunlight. Satellites generally can't be seen during the daytime or in the middle of the night. In daytime, the sky is too bright. In the middle of the night, those satellites close enough to the Earth to see are in the Earth's shadow, so there is no sunlight to be reflected. But for a few hours after sunset or a few hours before sunrise, the sky is dark but we can often see satellites outside the Earth's shadow, reflecting sunlight to us.
Where do we look?
Whether and where you can see a given satellite is determined by the satellite's path as well as your specific location. There are many web sites which will give you predictions for viewing satellites once you specify where you are. One such site is Heavens Above. You can select your city from its database, or you can input your latitude and longitude. For Richardson, Texas, this is 32.948° latitude (positive for north) and -96.729° longitude (negative for west).
Such sites will give predicted passes such as this for the Hubble Space Telescope (HST) from Richardson:
----Start------- --Max altitude-- ---Ends---------
Date mag time alt az time alt az time alt az
2 June 2.7 22:12:04 10 SW 22:15:10 29 S 22:15:10 29 S
A visible pass starts either when the satellite is high enough above the horizon to see or when it leaves the Earth's shadow and is lit by sunlight. Its location in the sky may be specified by altitude and azimuth, as shown in the diagram below: altitude (same as elevation) here means height above the horizon in degrees, and azimuth is the compass direction. An altitude of 90 degrees is directly overhead; 45 degrees is halfway from the horizon to overhead, etc.
For the prediction given above, the HST would first be visible at 10:12 PM (or 22:12 in 24-hour format) about 10 degrees above the horizon--about the width of your outstretched hand at arms length. It will move from there to a maximum altitude of 29 degrees at 10:15 PM. At that time, you should see it by looking south (azimuth) then up 29 degrees from the horizon (one-third of the distance from the southern horizon to the point directly overhead). This is also the end of the visible pass, because at this point the HST will enter the Earth's shadow.
Its predicted brightness is given by the magnitude. The magnitude scale used by astronomers is logarithmic--every change of 5 magnitudes is a change of a factor of 100 in brightness. It is also "backwards"--a greater magnitude means a brighter object. The dimmest stars visible with the unaided eye far from city lights are magnitude 5; the brightest stars in the night sky are magnitude 0 to -1.5; Venus and Jupiter at their brightest are about magnitude -3 to -4; and the full moon is -12.7. The HST here is predicted to be magnitude 2.7, which might be bright enough to see despite the city lights in Richardson.
Predictions for viewing satellites aren't perfect. Sometimes the satellites' operators may change its orbit slightly, affecting the predictions. Satellites in very low orbits are affected by drag from the traces of the Earth's uppermost atmosphere. Still, predictions made a few days or less in advance are usually good--demonstrating much understanding of physics, math, and computer programming by those who calculate the predictions!
Once you've seen some satellites, you'll recognize them easily--moving slower than airplanes (which are brighter and usually have flashing lights anyway) and much slower than shooting stars, which are gone within a second or two (shooting stars are meteors, small bits of dust, vaporizing when they hit the atmosphere, by the way). This slow motion is misleading: satellites are orbiting the Earth at several km (or miles) per second, but you're watching them from a few hundred km (or miles) away. Some satellites will slowly brighten and dim, in a cycle over several seconds. This is an indication that the satellite is slowly spinning. Others may briefly brighten for a short time, as sunlight catches a reflective surface like a solar panel. (See the Iridium flares below for another example.) If you match a satellite you see to a prediction giving you the satellite name, you can look up what kind of satellite it is from Heavens Above or Encyclopedia Astronautica, among other places.
What are Iridium flares or flashes?
One specific set of satellites is of special interest to viewers. The Iridium satellites are communications satellites with shiny dish antennas which can give flashes of reflected sunlight. If you're in the right place, the flash of light--lasting just a few seconds--will be brighter than Venus. Predicting what you can see is very dependent on your location--just a few kilometers makes a noticable difference.
Web sites such as Heavens Above give predictions for viewing Iridium flashes; for these, it is best to specify your latitude and longitude to within 0.01 degrees (some web sites can help you find this for the part of town where you are observing from). Also, you should calibrate your watch with a time standard (an atomic clock standard, or on line via Heavens Above) since the flashes last only a few seconds. These predictions depend not just on the satellite's orbit but also the satellites orientation: if the direction the shiny antenna is pointing in changes just slightly, the prediction will be off. It's good to start looking for a Iridium flare a few minutes early, and continue a few minutes late if you haven't seen it yet. Sometimes a predicted flash won't happen at all, but usually they do.
How do satellites stay in orbit?
This question often comes up when watching satellites. The orbits of satellites around the Earth, the orbits of planets and moons and comets, the arc of a tossed ball, the straight down plunge of a dropped book--all of these represent freefall motion under the influence of gravity.
Consider an object thrown horizontally sideways: it curves down until it hits the ground, going further if it is thrown faster. Thrown sufficiently fast, it will take so long to curve down that the Earth's surface will curve away that fast. The object will then miss the surface, continuing to curve around the Earth in an orbit. In fact, Isaac Newton described how a satellite could orbit the Earth like this in 1687! His thought experiment (see figure below) pretended you could fire a cannon sideways from the top of a mountain. If the cannon could be fired fast enough, the cannonball, although continuously falling towards the center of the Earth, would curve around the Earth and hit you from behind after completing a full orbit. Of course, the mountain would have to be ridiculously tall, so that the atmosphere would not slow down the cannonball.
An object is in freefall if only gravity is acting on it--no air resistance, no support from the ground, etc. It might be falling straight down, or if it has sideways motion it will follow a curved path (like a tossed ball or an orbiting satellite). A person in freefall experiences weightlessness. The observer is falling, so there is a gravitational force on them, but there is no apparent gravity in their frame of reference. Some amusement park rides allow you to briefly experience freefall: riders are dropped from a tower and are briefly in freefall before being safely slowed to a stop.
A space shuttle orbiter orbiting the Earth is in freefall, constantly falling towards the center of the Earth, but always missing. Astronauts aboard in this state of freefall feel weightless, just like the amusement park riders. (NASA now calls this "microgravity" instead of "zero gravity" to acknowledge the imperceptible but still measurable tidal forces across the orbiter.) But keep in mind the Earth is exerting a force on the orbiter and the astronauts, forces about 88% of what they experience while on the Earth's surface.
An object in orbit will continue to orbit, unless it runs into something or a force acts on it to take it out of orbit. The space shuttle requires a great deal of rocket fuel plus strap-on rocket boosters to reach orbit--that is, to reach an altitude above the Earth's atmosphere and a high enough sideways velocity to "miss" the Earth as it falls. Once in orbit, the shuttle continues in orbit without needing to fire any rockets: it is continuously falling towards the Earth, but due to its sideways velocity of about 8 km (5 miles) per second, its path curves around the round Earth. To return to Earth, the shuttle must fire rockets to slow it down, so that its path enters the upper part of the Earth's atmosphere. The drag from even very thin air is enough to slow the shuttle more, until it finally reenters the atmosphere before making a landing.