Figure 1: The MIR space station, the largest man-made object in orbit, is visible from all inhabited regions of the Earth.
Go outside on a clear moonless summer night, approximately fifty minutes after sunset, and scan the sky with your naked eye. As the sky gets darker and more and more stars appear you cannot fail to see one or more stars that are moving with respect to their surrounding stars. What you are seeing is probably one the the thousands of man-made satellites that are orbiting our small world.
On the 4th of October 1957 a new era started in mankind's history -- that of the Space Age. On that date, over forty years ago, the USSR launched SPUTNIK 1, the first man-made object to orbit the earth. Most of the world had expected the United States to be the first to achieve this milestone and little notice had been taken of the intentions of the USSR. To say that this caused major consternation in the western world is somewhat of an understatement. It was compounded a short time later when the female dog Laika was orbited in SPUTNIK 2. Some disbelievers stated it was a hoax but all one had to do was to go outside at a predicted time and sure enough there was a moving `star,' which was the orbiting rocket casing. Those of us young enough at the time will recall the excitement as each new space achievement took place. The first relay of television across the Atlantic, photographs of the far side of the moon, unmanned crash landings on the moon followed eventually by man actually landing on the moon, briefly exploring its surface and then returning to earth, and probes that were sent to the planets and returned photographs of strange worlds -- these are just a few of the dramatic events that took place.
Sadly, the exploration of space has lost its early impact on the general public and we all take for granted the television we receive via satellites, our international satellite telecommunications links, weather satellite pictures, navigation and numerous other benefits to mankind. It is only when something spectacular happens that one is reminded of what is going on about us.
It is not surprising therefore to find that most people know very little about space and even less about artificial satellites. Relatively few people have even seen an artificial satellite for the simple reason that not many people look skywards (except when they have seen a TV feature or film on UFOs and then suddenly become sky conscious). If you live in a city it is not surprising if you do not look skywards, since what is there to see other than a few paltry stars dimmed almost into insignificance by the light pollution of man? However, go out into the country, away from all bright lights, and see the stars as they should be seen. If you watch for several minutes, under suitable conditions which will be described shortly, you cannot fail to see several satellites and perhaps you might feel some of the wonder and excitement of the space age if you think a bit about what you are seeing, how it got there and its meaning.
In this article I will try to convey some of the excitement and thrills of satellite spotting. Perhaps this article may encourage you to explore further on your own and become one of the amateur satellite tracking fraternity. Amateurs played a major role in the early days of the space age, but today, with the advances in tracking technology, the amateur is no longer in demand. That is not to say there is no place for the amateur -- far from that, the sky is your limit in what you can do or hope to achieve. In a later article I will describe some of the things that amateur satellite trackers can do, but before you can run you have to learn to crawl and this article is aimed at helping you to crawl.
Many people think that satellite spotting or tracking requires expensive equipment. Depending on what you want to do you can start with nothing more than your naked eye. If you want to pursue tracking more seriously then a stop-watch and a pair of binoculars will be a big asset and will enable you to make valuable contributions. If you want to progress further, then a personal computer will be of great value since you can then make your own predictions. This is satellite tracking as opposed to satellite spotting.
Before going into the more advanced aspects of satellite spotting, some basic fundamentals have to be defined. For a satellite to be seen with the naked eye it must meet certain criteria, the most important of which are:
Satellites come in a wide variety of orbits. The type of orbit is chosen depending on the mission requirements of that particular spacecraft. Some satellites may be in circular orbits where the altitude above the earth is more or less constant over the entire orbit. In other cases one might have an elliptical orbit where the satellite might approach the earth to within a few hundred kilometres and then travel far out into space before returning earthwards. The point closest to earth is known as PERIGEE, whilst the furthest point is APOGEE. Most of the satellites described in this article have fairly circular orbits with the apogee height mostly under 1000 kilometres. Obviously the shape or size of the orbit determines the time the satellite takes to complete one orbit -- this is known as the orbital PERIOD and can be as low as 88 minutes (anything lower than that means the satellite is very close to falling out of orbit), or as high as several days in some cases. However, most satellites will have a period of about 100 minutes, or slightly less.
Another important parameter is the INCLINATION, the angle that the orbital plane makes with the earth's equator. Whilst a satellite orbits, the earth is spinning on its own axis so it should be easy to see that sometimes an observer on the earth's surface will see the satellite moving in a particular direction, but half an orbit later the satellite will be moving the opposite way. If the plane of the satellite's orbit does not pass near to your location then you cannot see the satellite since it will be below your horizon. If the inclination is equal to or greater than your latitude then you will see the satellite at some time or other, depending on where you are with respect to the plane of the orbit.
Whilst the satellite is whizzing around the earth, you are spinning along with the earth inside the orbital plane, so sometimes the satellite might be passing over in daylight, other times in the middle of the night. Earlier it was stated that one's latitude must be less than or equal to the satellite's inclination -- there is some leeway as the satellite is above the earth's surface and can be seen some distance away when at low elevations - (elevation is the angle above the horizon with 0 degrees being on the horizon and 90 degrees directly overhead). If the inclination is, say 28 degrees, and your latitude is also 28 degrees then the satellite can, under optimum conditions, pass straight overhead. If however you are at 35 degrees latitude south and the inclination is 28 degrees then it should be clear that the satellite, at its closest to you, will always pass north of you and that overhead passes are not possible. Roughly speaking, one must be at a latitude within about 15 degrees of the orbital inclination for visibility to be possible. When the inclination is greater than one's latitude, the orbital plane will intersect your location at some time or other. When the inclination is around 70 degrees or higher, then you will see the satellite travelling south to north or, if on the other side of the orbital track, moving north to south. It should also be apparent that for inclinations close to one's latitude you can only have the satellite travelling from west to east since all satellites (except for a few cases which we can ignore) follow a west-to-east path in the sky. This is because most launches make use of the earth's rotational velocity to reduce the launch energy requirements and launch with an eastward component.
To see an artificial satellite with the naked eye requires the satellite to be physically quite large or possess some other attribute that will draw the observer's attention. Besides the actual physical size one has to bear in mind the distance of the satellite from the observer since satellites orbit the earth at widely different heights. The brightness of a satellite is measured in the astronomical `magnitude' scale. An exceptionally bright satellite has a magnitude of +1, whilst a satellite that is barely visible to the naked eye will be about magnitude +5 or +6. A pair of good binoculars will enable one to see satellites down to about magnitude +8 under good conditions.
In order to spot a satellite with casual random looking it is necessary for the satellite to have an appreciable apparent speed at which it travels across the sky. This speed is a function of the satellite's distance from the observer, referred to as `range.' When the satellite is directly overhead it will usually be at its closest to the observer and have its greatest apparent speed. As the satellite's elevation decreases, it draws further away from the observer and appears to move slower. It also decreases in brightness on account of the increased range. By the time the satellite is near the horizon it will be moving much slower and may be so faint that one may not pick it up easily, so the best place to look is above an elevation of about 45 degrees. A satellite at an altitude of 300 kilometres has an angular velocity overhead which is twenty times faster than at the horizon.
Satellites higher than about 3000 kilometres move very slowly, even when directly overhead, and are not easily picked up at random unless the observer happens to be looking directly at the satellite when it passes close to a star. A phenomenon that will be noticed while looking for satellites is that eventually all the stars appear to be moving - this is a result of looking too long and straining the eyes. It is better to relax and not concentrate on a particular area for too long. Another phenomenon that will be noticed is that the observed satellite may appear to move across the sky in a jerky or slightly zig-zag motion -- this is caused by the eye not moving smoothly when changing the point of view.
Even if a satellite meets all the other visibility requirements it could still be difficult to observe. Like the moon, artificial satellites also go through phases. This produces a `phase angle,' which is the angle between the sun and satellite as seen by the observer on the earth's surface. Satellites are not normally observable at a phase angle near zero degrees (corresponding to new moon), and depending on the height of the satellite, the phase angle remains in the range 50 to 130 degrees with a mean value of around 90 degrees. At a phase angle of 50 degrees the satellite is almost 2 magnitudes fainter than its maximum possible brightness, whilst at 130 degrees phase angle it is only about 0.25 magnitudes fainter than its maximum brightness which occurs at a phase angle of 180 degrees. Clearly, a satellite cannot be seen at a phase angle of 180 degrees since it would then be on the opposite site of the earth to the sun and would consequently not be illuminated, but in eclipse.
As the satellite crosses the sky its phase angle changes and the brightness will be seen to vary quite noticeably. In addition to this change there are other factors which may cause the brightness to change rapidly. It has been found that most satellites have uncontrolled rotation about an off-centre (with respect to the body of the satellite) axis. The spin axis usually varies little during the course of an orbit, but it undergoes a slow precession over a length of time. The rate of spin or tumble decreases slowly under the action of magnetic and aerodynamic damping.
Satellites and spent rockets observed in the eastern part of the sky after sunset may appear steady, yet when in the western part of the sky they may start flashing, sometimes with invisible minima. This is due to specular reflection causing brightening when the angle of incidence from the sun to the satellite is equal to the angle of reflection from the satellite to the observer and the sun catches a shiny satellite surface as it spins or tumbles. In addition, external fixtures such as aerials and de-spin weights, solar panels, etc., can also cause bright flashes.
Some satellites exhibit very complex flash patterns which change quite noticeably over a period of time. Spherically-shaped satellites appear either steady or scintillate rather rapidly, which indicates the rotation of a sphere that has external protuberances. A remarkable similarity exists between the flash patterns of rocket casings in low orbits with similar rocket casings in higher orbits having a much more rapid tumble -- this is probably due to the damping effect of the earth's magnetic field. Rocket casings have a distinctive and more prominent appearance than satellites, usually flashing or tumbling very rapidly with flashes separated by between 0.5 and 2 seconds, while satellites exhibit a smaller range of brightnesses and are usually steady. Most satellites spotted casually are in fact rocket casings.
It frequently happens that a satellite is confused with aircraft navigation lights and even the most experienced observer occasionally gets caught. What must be remembered is that satellites appear whitish in colour with perhaps a tinge of yellow or orange, especially at low elevations, and that satellites do not change direction. Obviously, green and red lights are `out'! However, from time to time satellites may look a little different to what we are accustomed to. The following are some examples:
In order to observe a satellite visually the observing conditions must be suitable. Since one needs a relatively high contrast between the sky and the satellite it is rather pointless looking for the fainter satellites with a bright moon in the sky. As a rule, one looses about 4 to 5 magnitudes when trying to observe with a full moon in the sky. Generally, if the sky is covered in thin cirrus or smoke, one's chances are somewhat reduced and one can only expect to see the really bright satellites, so ideally one needs a dark sky away from city lights and pollution and little or no moonlight.
With all the above restrictions one might think it is pointless looking for satellites, but this is not the case. Most of us are not fortunate enough to have dark skies unless we take a long drive out into the country, so we have to cope with less than ideal conditions. This does not mean that one has to forget about seeing a satellite and I will now indicate to the potential observer how to optimize the chances of seeing a satellite.
Well over 20,000 objects have been placed in orbit about the earth, of which about half are still orbiting, the rest having re-entered the earth's atmosphere and burnt up. This still leaves more than 10,000 objects of which a good percentage are large enough to be seen with the naked eye. The slightest optical aid will increase this number considerably, so one stands a reasonable chance of being able to see several hundred satellites. In fact, under reasonably good conditions, the casual observer can expect to see several satellites per hour. On one occasion I was able to observe seven unpredicted satellites in the space of five minutes in different parts of the sky.
The requirement of the satellite having to be above the earth's shadow and also being solar illuminated should indicate to the potential observer that the best time to look is during the first two hours after twilight and the last two hours before dawn. As a rough guide the sun should be about 12 degrees below the horizon at start/end of the observing period.
Recalling the effects of phase on the brightness, if observing in the evening, the best direction is to face towards the east, i.e. with your back to where the sun has set, and looking from about halfway up from the eastern horizon to overhead. As the evening progresses the earth's shadow will creep up the sky from the eastern horizon and increase its elevation until at about local midnight, where it will be at its highest; thereafter the shadow starts lifting in the east as the sun approaches dawn. In the case of early morning observing the best place to look is high up in a westerly direction.
The height of the earth's shadow above the earth is a function of the position of the sun and the observer and it should be apparent that the earth's shadow will reach its lowest level when the sun appears highest in the sky -- that is, in mid-summer. In winter the earth's shadow extends further and in view of this, and the cold winter nights, the winter months are not the ideal time to look for satellites. In summer it is possible for most satellites to be visible at any time during the night in some part of the sky and in the southern hemisphere, for example, the sun will be in the south so the satellites will generally be clear of shadows towards a southern direction, whilst in winter the shadow will be least towards the northern part of the sky.
Having established the best time to look, the next question is in which direction to look. Satellites have been launched into virtually every conceivable type of orbit and on this basis one would expect satellites in any direction. However, the majority of satellites are in polar orbits, so they appear to be either travelling in a southerly to northerly direction or going the other way. There are also a few large satellites in low-inclination orbits and they have a westerly to easterly movement. There are a few that travel from east to west but they are probably too faint for the novice to worry about, although it is amusing to see a satellite `going the wrong way.'
If you want to observe the low-inclination satellites, then the best direction to look is towards the equator. As the inclination of the orbit approaches one's latitude the orbital plane will appear higher in the sky so that when the inclination is equal to the latitude the satellite passes directly overhead.
As the inclination increases, the satellite's motion assumes a greater north-south component. A satellite with an inclination near 90 degrees is called a polar orbiter since it passes over the earth's poles. It is quite possible to see the same polar orbiting satellite moving in opposite directions by viewing two passes about about half a day apart. On one pass it will move from (say) south to north, and then approximately ten hours later it will appear to move from north to south.
Unless the satellite is fairly high above the earth's surface, say about 800-1000 kilometres, it may be possible to observe only one pass in an observing spell of several hours. If, for example, the satellite was observed to be moving in a polar orbit and was spotted in the western sky it is unlikely that one will see the next pass, which will occur even more due west of the observer and probably thus below the horizon. If, however, a pass is observed low in the east in the early evening it is quite possible to see a second pass about 90 minutes later with the satellite now appearing in the western sky. Since the minimum orbital period is about 87 minutes this fixes the minimum time spacing between successive passes.
The length of time that a satellite may be observed on a pass or `transit' depends on whether the satellite is clear of the earth's shadow during the entire pass and on its orbiting altitude. If the satellite is only 200 kilometres above the earth it will traverse the sky from horizon to horizon in about seven minutes, whilst for a height of 500 kilometres this time increases to about eleven minutes. If the orbiting altitude is about 2000 kilometres then the satellite will take about 29 minutes to cross the sky, so if one assumes that the orbit is circular one can make a good guess as to the satellite's altitude by noting how long it takes to cross the sky.
The next article will deal with satellite tracking and what the amateur can do. It will also deal with predicting passes so one does not have to waste time looking for a `casual' satellite. A list of useful Internet WWW sites, where more information may be obtained, will also be supplied. Good spotting in the meantime!
