Find information on several thausands of satellites: their launch dates and functionality; track the satellites, see the earth as seen from the spacecraft, and get all kind of positional coordinates you could (n)ever think of. CalSky predicts highly accurate transit times, close encounters with the sun or moon - and even the times of satellites crossing the disk of the sun or moon; the Iridium flares, even the telescopic flares from reflected moonlight.
The months with the long days and the short nights are well suited to watch for artificial satellites. In the end of the fifties, the emersion of a satellite was still something sensational and many tried to find the stars created by humans in the night sky. Today, only few people can claim having consciously seen a satellite. In this introduction we aim to show you, how amazingly simple it is to see and track the spacecrafts, while they draw their course across the sky. Afterwards you can exploit the extensive calculations CalSky provides for this task. In this satellite section of CalSky you can also learn of the missions of thausands of satellites, find spy satellites, and geostationary satellites (which cannot be spotted with the naked eye).
Since the autumn 1957 the number of satellites is constantly increasing. Today the motion of about 13'500 artificial bodies in an earth orbit are well-known. Those satellites observable with the naked eye circle the earth closely above the atmosphere in about 200 to 800 kilometers altitude. There, the satellites fall in 90 minutes around the whole earth. Some satellites specialized for telecommunications and for scientific research circle in higher altitude. Such satellites, and the still more higher circling television satellites can only be spotted with amateur telescopes.
Today everyone can receive the signals of television satellites with a satellite-dish. They circle in an orbit with a radius of approximately 42'000 km once per day around the earth. Since this lasts equal long as the earth requires for a full turn around its own axis, these satellites seem to stand steadily in the sky. Thanks to this circumstance, we do not not have to continuously aligned our satellite-dishes to the television satellites. If we had to, the receiving of TV-programms would be importantly more expensive and satellite receivers not as common. Satellites, which apparently stand still in the sky, are called geostationary satellites. Beside the television satellites, the meteorological satellites METEOSAT are other examples of geostationary satellites. They take the satellite images, which we see in the daily weather news. If you telephone oversea, then it is probable that your conversation is conducted by a geostationary satellite. Thus, your words travel a distance of 80'000 km, which corresponds to 20% of the distance earth - moon. Hence, you may hear your echo after a delay of 0.25 second.
Also, more lower flying satellites help in daily life. There are mobile telephones, with which the conversation is conducted via satellites in a low earth orbit (LEO), rather than via antennas on the ground. Iridium Satellite LLC is one of these companies, which operates such a satellite network. There are also other companies that offer similar services.
The determination of the geographical position at any location used to be complex and complicated. Today everyone can buy a receiver for the signals of the navigation satellites (global positioning system) for a low price. This receivers permit to exactly read off at any time and place on earth the geographical longitude, latitude and elevation after only a few seconds. Today, the american NAVSTAR system is the most common GPS network and is financed and maintained my the US army. Since knowing the position is very important for warfare, the satellites used to transmit an artificially worsened signal quality in order to reduce the use to enemies. In April 2000 the Bill Clinton decided to switch off this 'feature' of seleced availability (SA). Since May, the accuracy to expect from GPS is in the order of 15m for handheld receivers (i.e. from Garmin).
Professional receivers can aquire an accuracy in the order of centimeters after using an integration time of hours or even days. GPS is also used in navigation systems in airplanes and automobiles. But also the scientific surveying of the earth itself is conducted by GPS.
If you are skygazing up to the star-studded night sky it will occur that you suddenly notice a star, which draws speedy and uniformly across the sky. Surprised, you will follow its way from the north to south. Suddenly, its light will become rapidly dimmer. Shortly after this event, it will become invisible, long time before the satellite reaches your southern horizon. In this example, you observed a satellite on a polar orbit. Polar orbit means, that the satellite circles the earth on a track, which leads it from the north pole to the south pole and back again.
In May, June and July the sun goes only little under the north horizon (for an observer in the northern hemisphere). This also means that the umbra of the earth does not rise very steeply from the north to south. Even at deep night, satellites encircling the earth in an altitude high enough will have bright daylight. We noticed our satellite, when it was still illuminated by the sun and thus the moving 'star' became visible. If the satellite moves on direction south, the sun will also set. Since the movement is rather fast, sunset and twilight will only be of short duration; we see the light go out in only a few seconds (figure 2).
Depending upon physical dimensions of the satellites, they appear differently bright when sunlit. Very large satellites, like the International Space Station ISS, or the Space Shuttle STS appear as bright points of light. However, small satellites can only be detected using telescopes. Between this extremes there are many graduations.
A while ago we had the example of a satellite flying from north to south. Hence it was on a polar orbit. This orbit is used advantageous for earth remote sensing satellites, since the whole earth can be seen. Examples of this class are LANDSAT, NOAA (high resolution weather satellites), ERS, RadarSat, Lacrosse (radar imaging satellites, see the calculations from CalSky for when you are in their radar beams!) The international space station ISS moves approximately from southwest to northeast or from northwest to southeast. Likewise the space shuttle - if it is visible at higher latitudes at all (Cape Canaveral is at latitude of 27° - to save energy the inclination of the orbit is often unchange. hence the space shuttle is visible only at latitudes lower than about 30°). However you will not observe satellites often, which move from eastly directions. (There is a small group of satellites that orbit the earth retrograde - e.g. the Israeli photoreconnaisance satellites Ofek, they will fly from east to west.)
This is due to the fact that the earth rotates from the west to the east. At the equator one moves at 1600 kilometers per hour or 6% of the satellite velocity. If one starts the satellite with the turn of the earth, then the rocket must achieve only 94% of the satellite velocity. The turn of the earth supplies the missing 6%. If one wants - for whatever reasons - to circle the satellite against the sense of rotation of earth, then the rocket must first brake the velocity of 1600 km/h, and accelerate afterwards to the full rate of an earth orbit. This requires additional expensive fuel, without any use of it. Thus it manifests by itself, why nobody so far sent a satellite on an orbit running from the east to the west.
Particularly spectacular are the so-called flares of Iridium. Flare means a light phenomenon with rapidly rising and fading brightness flanks, and Iridium is the bankruptcy gone enterprise, which offered satellite telephones. These satellite telephones transmitted the conversation to and from one of many satellites (about 70) circling in 800 km above the ground (figure 3). The speciality of this satellites are the three radio transmitting antennas, which are as plain and reflective as mirrors (the smaller planes in the lower part of the satellite body in figure 3). They reflect the sunlight to earth, and project a light spot of more than 100 km width on the ground. This spot is moving at the rate of the satellite of about 30'000 km/h. If you happen to be in or close to the center line of this light spot, you are verbatim in the limelight:
Some of the satellites of the Iridium constellation are out of control and move in an instable turning. Hence it happens by accident, that the mirroring antennas are in the right geometry with you and still produce a great show. Actually, this presentation is even more striking, since it consists of a periodic series of dazzling flashes. Inbetween, you can track the satellite with binoculars. CalSky predicts the opportunities of the shows of the tumbling Iridums. Each of the tumling Iridiums has its own characteristic flashing pattern - some with a periode of only 1 second, where others flash only every 40 seconds.
Still you can witness such Iridium flares. CalSky calculates the most favourable flares for your site. The appearance is strongly depending on the observation place. If you observe several kilometers away from a locality stored in the database, you should re-adjust the coordinates which you read out from a map and enter them my clicking on the earth icon in the site box CalSky gives you the azimuth (counted from north 0°, to the east 90°, south 180° and to the west 270°), and the elevation above the mathematical horizon (0° is the horizon, 90° the zenith, i.e. directly overhead). The maximum brightness is outputted in astronomical magnitudes. The smaller or even more negative the value, the brighter is its appearance. CalSky is also able to draw the predicted flare into a star map. However, the brightest flares will appear during twilight.
Additionally, do not forget the conventional satellites. To observe the manned Space Shuttle or the ISS moving on their ways across the sky, possibly shorty before or after docking with another orbiting object, maybe with the knowledge of an astronaut doing an EVA (extravehicule activity), are very striking experiences.
With such bright satellites the dependancy on the observing site is no longer critical (as compared with predicting Iridium flares). It suffices to select a close city in our data base. If you want to see a specific satellite, it will do to know the point in time and the direction in which to look to. However, if you want to learn which satellite crossed your ever best long time exposed astro photo (figure 5), you must input the coordinates of the observation place as exactly as possibly.
CalSky is automatically updating its data bases several times a day. Several thausands of satellites and pieces of debris are at your disposition, and wait to be drawn with its apparent tracks in a star chart.
Have you ever seen the space shuttle? We will provide you the required information for when and where to look.
Original text with kind permission of Dr. R. Brodbeck