======================================================================== Visual Satellite Observing F A Q Chapter-06 What Can I Expect To See When I Look At Satellites? ======================================================================== ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + This FAQ chapter is "under construction". Some of the + + sections may be unwritten as yet. Other sections may + + contain out-of-date, unreviewed, or "starter" material. + + Yet other sections may be works in progress, partially + + written and reviewed. + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + In this chapter, the following sections are considered + + to be completed (written and reviewed): + + None + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ---- 6.0 What Can I Expect to See When I Look at Satellites? Typically, satellites look like slowly moving stars. But there are many exceptions to this behavior. Most satellites will have a changing brightness during a pass. Usually this is a slow change of brightness due to changing illumination conditions, but there are also many satellites for which the brightness changes are more extreme (from visible to invisible in a few seconds), spectactular (bright glints off solar panels or antennae), regular (tumbling rockets) or unexpected (entry into the shadow of the Earth). The brightness of satellites is as diverse as that of stars. Just as with stars, the brightness of satellites depends on many things. For satellites, factors such as distance, size, phase with respect to the sun (not unlike our natural Moon), and surface properties are of importance. The brightest of satellites (such as Space Station Mir and the Space Shuttles, which can be as bright as Venus, the brightest planet) are among the fastest moving satellites, because they are in relatively low orbits and, hence, are closer to the observer. At the other extreme of visibility and apparent speed, it is possible to detect geostationary satellites, although a telescope is usually needed to spot these dim and (apparently) stationary objects. Above all, satellites often have a few surprises up their sleeve. From one pass to another, their brightness can be orders of magnitude different, their rotational status can have drastically changed, they may suddenly appear with a companion (a supply ship or refuse kicked out), and so on. Even during one pass, a satellite can suddenly appear or disappear due to the interplay with the Earth's shadow cone. And, most magnificent (and rare) of surprises they hold in reserve for the innocent observer, they can choose to reenter into the Earth's atmosphere right when you're looking at them. This phenomenon never fails to deeply impress the fortunate observers who are lucky enough to witness a reentry. To a certain extent, it is this wide range of appearances and the prospect of surprises that underlies the attraction of watching satellites to many observers. ---- 6.1 Factors Influencing Brightness The satellite's brightness is expressed in magnitudes (an inverse logarithmic scale which follows the response of the eye), used in astronomy to indicate the brightness of stars, planets, and other heavenly bodies. Smaller and negative magnitudes indicate greater brightness. For example, the brightest stars visible are around magnitude 0 to -1. The planet Jupiter is about magnitude -2.5, and Venus can become as bright as magnitude -4. ---- 6.1.1 Range And Altitude For a satellite of given size, the farther the satellite is from the observer, the dimmer the satellite will appear. The distance from the observer to the satellite is called the "Range" of the satellite. It is usually expressed in kilometers or miles. Generally, the larger the range, the fainter the satellite will be. The range depends on the orbital altitude of the satellite and on its elevation above the observer's horizon. A satellite, with a large orbital height, passing through the observer's zenith can have a smaller range than a satellite in a low orbit which appears only a few degrees above the observer's horizon. The higher satellite can thus be brighter than the low satellite, especially since objects close to the horizon suffer from additional dimming. This is due to the fact that their light has to travel a longer path through the atmosphere, which scatters the light, to reach the observer. This is called atmospheric extinction. According to the spacecraft type and its intended mission, the altitude may vary between (roughly) 150 km and 40,000 km. For diffusely reflecting objects (many satellites), the satellite's magnitude depends logarithmically on the range: m ~ -5 * log(Range) in which m is the magnitude and the symbol ~ is shorthand for "is directly proportional to". ---- 6.1.2 Size And RCS Size is an obvious factor. The larger a satellite is, the brighter it will be, in general, although we have to bear in mind the satellite's attitude or orientation in space. More light will reach the observer if the cross-sectional area available for reflection is larger. Radar Cross Section (RCS) is often used to estimate the average cross-section a satellite will offer for reflection. The RCS is routinely determined by the radars of USSPACECOM and is expressed in square meters. With all other factors staying the same, the magnitude depends logarithmically on the radar cross section: m ~ 2.5 * log(RCS) in which m is the magnitude and the symbol ~ is shorthand for "is directly proportional to". ---- 6.1.3 Surface Characteristics A highly reflective and polished satellite will naturally act more as a mirror than a satellite that is painted in dull or matte colors. It will thus appear brighter, since mirror-like (also called specular) reflection does not scatter the light as much as diffuse reflection, and so more light can reach the observer's eyes. Over time, a polished finish may become tarnished while a dull surface may ablate and appear brighter. Functional parts of the spacecraft contribute, too, such as large solar arrays or cylindrical antennae, which make nice mirrors. ---- 6.1.4 Phase Angle And Attitude Just as the Moon's brightness depends on how "full" it is, the satellite's magnitude depends on the so-called Phase Angle. This angle is defined as the angle between the two lines connecting, respectively, the satellite with the sun and the satellite with the observer. If this angle is 0 (or more generally, smaller than 90 degrees) the satellite will be "fully lit", and just like the full Moon, will show the largest illuminated area to the observer. A satellite will be brighter for small phase angles than when the angle is closer to 180 degrees. At large phase angles, as with a "new Moon", much less light is reflected, thus making the satellite fainter. During one pass, the phase angle (and thus the magnitude) typically changes gradually and smoothly. This gradual change of brightness should not be confused with flashing satellites. The brightness changes for flashing satellites are usually more extreme and frequent. Phase angle also explains why, during the evenings, the satellites will usually be at their brightest in the east (since the sun is in the west), and why, during the mornings, satellites will usually be at their brightest in the west (since the sun is in the east). If all satellites were spheres, predicting their magnitude would be easy, as easy as predicting our natural Moon's magnitude. (Un?)fortunately, most satellites are shaped like cylinders or boxes. This means that the attitude of a satellite is also a very important factor in brightness. Take the case of an irregular spacecraft such as the Space Shuttle with its large wing plane area. If the observer sees it edge on, it will be much fainter than if the wings are seen fully. The attitude of many satellites is highly variable over time, which explains why so many satellites show signs of flashing behavior. ---- 6.2 "Flashing" Satellites Aside from the influence of the phase angle, many satellites do not show a constant brightness. They give off flashes. Many of these flashing satellites exhibit regular flashing (such as a flash every 5 seconds). An equally large group of satellites give off occasional flashes or exhibit wildly irregular flashing. A flash typically occurs when the satellite or a piece of the satellite acts as some kind of mirror for the sun's light and reflects the light directly to the observer for a brief period of time. ---- 6.2.1 Glints And Irregular Flashing Whether or not flashing will occur at regular intervals critically depends on whether or not the satellite shows simple rotational behavior (e.g. with most of its rotation being around one principal rotation axis). A satellite with large rotational components along several principal axes (i.e. tumbling about several axes at the same time) will generally exhibit a flash pattern, which can appear quite chaotic. The rotational state of the satellite can, of course, change with time, so that some satellites that exhibit irregular flashing at one time can sometimes show regular flashes at other times. Typically, the rotational state of payloads (after their useful life!) causes more irregular flashing. This is because a payload usually has quite an irregular shape, so that the torques acting on the satellite (which change its rotational state) cannot bring the payload into a stable rotational regime with most of the rotation about one of the principal axes. This situation is different for rockets, which are usually cylindrically shaped, and for which a stable rotational regime usually implies end-over-end tumbling. In such a mode of rotation, the rockets give off regular flashes. The presence or absence of irregular flashing also depends on which piece of the satellite causes the reflections. If the reflecting piece of the satellite is flat (such as a solar panel) or itself irregularly shaped (such as a piece of debris), flashes will be visible to the observer only under specific geometrical conditions, i.e. typically not during the whole pass. Taken to the extreme, solar panels, or other flat surfaces of active payloads, can cause as few as one flash per pass. This phenomena is called a "glint", which can briefly cause the satellite to appear several magnitudes brighter than it usually is. Satellites such as Mir or the Hubble Space Telescope, both of which have large flat reflecting surfaces, are known for their bright glints of negative magnitude! The attitude of active payloads is usually stabilized in some way. A payload stabilized along all three principal axes will usually not show much flashing. Only occasional glints will be seen. A spin-stabilized payload, i.e., a payload which is stabilized by spinning it about one of its principal axes, can give off regular flashes by virtue of antennae or solar panels attached to the spinning body. The period between two flashes will then reflect the spin period. Some satellites are active and spin stabilized and still manage to give off irregular flashes. These satellites are usually geodetic satellites, i.e. to be used as reflectors of laser-light from lasers on Earth, so that the continental drift can be measured. The laser reflectors on board such satellites can cause quite irregular or chaotic flashing. One of the more well-behaving geodetic satellites is EGP (86- 61 A), which is simply marvelous to watch! It is obvious from the above that flashing behavior is governed by a combination of factors, some of which are hard to predict or analyze. This means that even regular flashers can sometimes be irregular, and vice versa. But this is part of what makes tracking flashing satellites so much fun. :-) The best example of how complex the situation can become is illustrated by the story of the geostationary satellite Superbird A, which was flashing only during certain portions of the night during September 1996, due to the fact that the flashes were caused by flat pieces of the satellite. This story can be found in the SeeSat-L archives. The earliest reports are at: Initial Report: http://www.satobs.org/seesat/3632.html Satellite Identification Report: http://www.satobs.org/seesat/3661.html ---- 6.2.2 "Regular" Flashing Third stages usually stay in orbit after having delivered a payload to Earth orbit. During separation from the payload, they can receive a little kick, which can lead to tumbling, since there is very little friction in space. Some third stages are commanded to vent the left-over fuel into space, which can lead to spinning up and, ultimately, tumbling. These long and hollow cylinders are ubiquitous and usually give off regular flashes, due to the rotational motion that is imparted to them after payload separation. Many of the spin-stabilized payloads also give off regular flashes, by virtue of their long, thin, cylindrical antennae attached perpendicularly to the spinning body. These regular "flashers" very much resemble tiny lighthouses, as they travel across the observer's sky and give off a flash (or rather, show a maximum in their brightness) every few seconds. For some satellites the maxima in the light curve are distinct, narrow spikes. Usually these satellites reflect light more like mirrors (also called specularly). On the other hand, many satellites are painted in a matte finish and reflect light in a more diffuse way. The light curve of such satellites will be more smooth, and the maxima will be less distinct. Measuring the period between two flashes, or maxima (or minima) in the light curve, can yield a good approximation for the satellite's rotation period (or half a period). The flash period is defined as the time interval between two flashes. Measuring the total time interval during which the rocket has flashed some tens of times can, after dividing this total time by the number of flash periods, give a good approximation for the true rotation period. (Note that the number of flash periods is equal to the number of flashes counted minus 1.) By counting at least one hundred flashes, an accuracy of up to 0.001 seconds can be obtained. Regular observations of the flash period can shed light on the rotational behavior of satellites. Very little research has been done on the rotational behavior of satellites, and the accelerations, especially, prove to be very puzzling (see below). The amateur observer can contribute to the observation of satellites through flash period measurements. About forty enthusiastic observers worldwide make regular measurements of flash periods. The Belgian Working Group for Satellites (BWGS) of the VVS (Vereniging voor Sterrenkunde, the Belgian Astronomical Society) has specialized in coordinating such observations, collecting, and analyzing flash periods. More information on flash period measurements and the activities and publications of the BWGS can be found in section 7.2 of this FAQ. ---- 6.2.2.1 Is The Rotation Period Constant? Immediately after the launch, most flash periods of rocket stages are on the order of a few seconds. However, the rotation period changes with time. Several torques act upon the satellite and (generally) slow down the rotation, until the flash period has grown so large, that after months or years, the satellite no longer shows brightness variations. It has become "steady". ---- 6.2.2.2 What Causes The Rotation Period To Increase? Rocket stages, usually hollow metal cylinders, tumble through the Earth's magnetic field. This induces eddy currents in the skin of the structure. The Earth's magnetic field acts upon these currents, which creates a torque. This torque acts on the tumbling rocket. Due to this torque, the rate of rotation will slowly decrease (equivalent to rotation period increase). This torque will also change the mode of rotation from spinning (rotation about the longitudinal axis) to end-over-end tumbling (rotation about a transverse principal axis). A rough analogy is a toy top which starts out upright, spinning rapidly about its long axis, but then, due to friction, it begins to spin more slowly and to wobble, tracing out an increasingly large circular path closer to the floor. Another factor contributing to rotation period increase is the air resistance, which the satellite experiences in its orbit. Although the atmosphere is very thin at those heights (a few molecules per cubic meter at some hundred kilometers height), it does brake the rotation of the satellite around its own axis. Above a height of approximately 250 km, however, the torque due to eddy currents is the dominant factor. ---- 6.2.2.3 What Causes The Rotation Period To Decline? Often there is some leftover fuel in the rocket's tanks, which can slowly escape through the nozzle or through a puncture that is caused by a micrometeorite or space debris. The rotation period of the rocket can suddenly change strongly due to such an event. Depending on the geometry, either an acceleration or a deceleration in the rate of rotation can take place. "Accelerating rockets" are poorly understood. Some rockets exhibit recurring bouts of "accelerations", many years after launch. There also seems to be a link between accelerations and changes of the orbital altitude. A tentative link between accelerations and the season of the year has yet to be proven but, if proven, would be quite mysterious. Understanding these rocket accelerations could also shed light on the influence of collisions of space debris with large satellites. Many more observations are needed to improve our understanding of these accelerations. The BWGS has a special program that is devoted to accelerations. See section 7.2 of this FAQ for more details. ---- 6.2.2.4 Synodic Effect As has been indicated before, the flash period of a satellite is not necessarily equal to its rotation period. This is due to changing illumination conditions, as the satellite moves across the sky. To understand this so-called synodic effect we have to understand why satellites flash. ---- 6.2.2.4.1 Why And How Satellites Flash Most rockets and payloads give off specular flashes. Their metallic surfaces act as a mirror. Imagine a line that is perpendicular to one of the metallic surfaces. In specular reflections, the rays of incoming sunlight and the rays of the reflected light are symmetric about this perpendicular. That is to say, the angle between the perpendicular and the inbound sunlight rays is equal to the angle between the perpendicular and outbound reflected light rays. In order to see the reflected sunlight, the observer must be in the path of the outbound light rays. Another way to say this is that the observer can see the reflected sunlight only if the reflecting surface is perpendicular to the bisectrix of the angle between the line from satellite to sun and the line from satellite to observer. Let's assume the satellite-sun-observer geometry is a given and constant in time (which it never is in reality). The condition for an observer to witness a specular reflection would hardly ever be satisfied, if it wasn't for the fact that most satellites rotate about some axis. Due to this rotation, the reflecting surfaces continuously change their orientation during one rotation period, which creates more possibilities to fulfill the reflection condition. If the reflecting surfaces are flat (as for box shaped satellites), there is no guarantee that the reflection condition will be fulfilled during one rotation. Cylindrical objects, however, have reflecting surfaces in many more directions at any time than satellites with flat sides. Most rockets and payloads are basically cylindrical. It can be proven that rotating cylindrical objects will give off zero, one, or two flashes per rotation period (the ends of the cylinder are assumed to be non-reflective). If the rotation axis is perpendicular to the longitudinal axis of the cylinder (i.e., if the satellite is tumbling end-over-end), there will always be two flashes per rotation period. Moreover, the two flashes will always be symmetrical. This means that the time period between the two flashes will be exactly one-half of the rotation period. If the sun-satellite-observer bisectrix is parallel to the rotation axis of the tumbling cylinder, the rocket will give off one long and continuous "flash", i.e. it will be steady. (Note, most third stage rockets are tumbling.) Spinning cylinders (i.e. the rotation axis is coincident with the longitudinal body axis) don't give off regular flashes. Then again, spinning satellites usually have booms or solar panels or other structures, which cause asymmetries in the cylindrical body and, hence, regular flashes can occur. Cylinders that have a rotational state between spinning and end-over-end tumbling (i.e., the angle between the rotation axis and the longitudinal body axis is something other than 0 or 90 degrees, respectively) do not necessarily give off two flashes per period, nor are the flashes necessarily symmetrical. It is because of this typically asymmetric flashing behavior that, when measuring flash periods, an even number of periods should be counted. A more detailed explanation for this is given in section 7.2 of this FAQ. If secondary structures are visible in the light curve, it is wise to count an even number of flashes, as well, since it is unknown what causes the secondary flashes. It is quite common to see secondary flashes "appear" or "disappear" during a pass. This is due to the changing illumination of the object. ---- 6.2.2.4.2 Origin And Size Of The Synodic Effect So far we have assumed the satellite-sun-observer geometry to be constant in time. In reality the satellite moves with respect to the observer and the geometry changes continuously during a pass. In the above, we saw that a tumbling rocket would give off a new flash after exactly one half of the rotation period, *if* the geometry was constant! However, if the satellite-observer line changes direction, which it does because of the satellite's motion, the flash will not necessarily occur. It will probably occur a little later or earlier than expected. This means the measured flash period is no longer the same as the rotation period (or one-half the rotation period). This is what we call the synodic effect. The size of the synodic effect depends on the attitude of the satellite with respect to the sun, the observer and the rotation axis. In general, the size of the synodic effect is hard to predict since we don't know the direction of the rotation axis. However, as a general rule, one can say that the synodic effect will usually be larger for objects at low orbital heights, since they move faster across the sky and the geometry can thus change more drastically during one rotation period. Objects with long rotation periods are also more prone to a larger synodic effect, for essentially the same reason (more change of geometry per period since the period is long). Satellites with rotation periods less than 10 seconds are usually not very much influenced by the synodic effect, i.e. the synodic effect is smaller than the accuracy with which the flash period is measured. Objects with longer periods can show flash periods which are different from the rotation period by several seconds. For certain extreme geometries, the synodic effect can become as large as half a rotation period. The flashes will then (for a short time) appear at seemingly irregular intervals. This is called a "synodic anomaly". Studying such occurrences allows for a precise determination of the direction of the rotation axis vector. This is discussed more fully in section 7.3 of this FAQ. ---- 6.3 Molniya And Geosynchronous Satellites Surprisingly, given dark enough skies, it is possible, armed with binoculars or a telescope, to spot some of the satellites nestling in the far-distant geostationary ring (known as a Clarke orbit, after Arthur C. Clarke who first suggested the usefulness of such an orbit). Most of these birds are communications satellites of one description or another. A common design is that of a spin stabilized cylinder. The Hughes HS376 series are typical, the main body being some 3 meters long and 2 meters in diameter. This grows to around 6 meters in length, once on orbit, with the extension of the communications antennae and an extra skirt of solar panels. These supplement the cells which already cover the main body, making very nice specular reflectors. This skirt and the main body rotate about the long axis, typically at around 55 revolutions per minute (rpm) while the antenna and equipment shelf are de-spun, so as to maintain contact with their ground targets. Unlike objects in low Earth orbit, geostationary satellites are visible throughout every night of the year, only entering the Earth's shadow for up to 70 minutes per day, for a couple of weeks on either side of the fall and spring equinoxes. During the same period, the satellite tends to brighten over several days, twice a year, when the satellite's orientation favors "beaming" of the Sun in the direction of the observer. Typically the satellite will be in the magnitude +11 to +14 range (or dimmer), but brighten by several magnitudes when the geometry is favorable (around magnitude +5 to +6 is not atypical). One satellite is reported to have briefly been visible to the naked eye at magnitude +3 ! Two line element sets can be obtained for nearly all these satellites, bar the classified military ones, such as the MAGNUM/VORTEX signals intelligence and the DSP early warning satellites. These TLEs can be used to generate a series of positions for the satellite in right ascension and declination (RA and Dec) for the time of observation. This can then be plotted on star map to form a finder chart; the guide stars will help identify the satellite's location. Turning off the motor of a driven telescope will maintain the satellite in the field of view while the stars drift out, courtesy of the Earth's rotation. By either doing this or tracking the stars instead during a wide angle photographic exposure, one can obtain a very nice illustration of the geostationary ring, as either the satellites are fixed and the stars trail, or vice versa. As pointed out above, it is more rewarding to carry this out around the equinoxes, when the satellite will be more apparent. Observations over successive nights before and after this time will allow you to view the brightening of the object, plus its entry and exit from the Earth's shadow. This will also avoid the disappointment of searching for it in vain while it is in eclipse! ---- 6.4 Shadow Entry and Emergence It is common to see a satellite gradually fade from view over several seconds during a pass, its light extinguished as it enters the Earth's shadow. (It is equally common to see a satellite suddenly appear in view, when it emerges from the Earth's shadow.) The eclipse of a satellite is similar to what happens to the Moon during a lunar eclipse. First the satellite enters into the by-shadow (or penumbra) of the Earth's shadow cone. During this phase, it is still visible but gradually dimming. Finally it enters the core- shadow (or umbra) and becomes invisible. Contrary to the moon (which is still visible during a total eclipse, due to earthshine, the light reflected from the Earth), artificial satellites are for most practical means invisible when in eclipse. Only with telescopes, and computerized tracking facilities is it possible to see satellites when in eclipse. SeeSat-L member, Ron Dantowitz, has reported seeing Mir under such circumstances. ---- 6.5 Satellites During Re-entry: What do they look like? ---- 6.6 What Does the Space Shuttle Look Like? Note: Measurements used in the following sections are metric. Use the following conversions to obtain the English equivalent: Meter to feet: m x 3.3 = feet Centimeter to inches: cm x 0.4 = inches Kilometer to miles: km x 0.6 = statute miles Kilogram to pounds: kg x 2.2 = pounds Full details of the Shuttle and it's systems can be found in NASA's "1988 News Reference Manual" at the URL: http:www.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html Additional information can be found at the following URLs: * http://shuttle.nasa.gov/ * ftp/telnet/http://spacelink.msfc.nasa.gov/ ---- 6.6.1 Background Information on the Shuttles The Space Shuttle orbiters (there are four: Columbia, Discovery, Atlantis and Endeavour) are some 37 meters long with a wingspan of about 24 meters. Each is powered by three liquid hydrogen/ oxygen Space Shuttle Main Engines (SSMEs) and two solid rocket boosters (SRBs), which separate to be recovered after just over two minutes of flight. The Shuttle then continues under the power of its three main engines until about 8.5 minutes into the flight, whereupon they are shut down and the external tank (ET), which provided the fuel to the SMEs throughout the ascent, is jettisoned. Two Orbital Maneuvering System engines (OMS) are then used to place the Shuttle into its final desired orbit. ---- 6.6.2 Visibility to the Observer The Shuttle's visibility to an observer depends on the orbital inclination targeted for the mission. Inclinations used include 28.5, 39, 51.6, and 57 degrees. The Shuttle normally orbits at an altitude at or below 300 km, but some missions can take it over 567 km in altitude. A launch to a 28.5 degree inclination orbit sees the Shuttle climb to the east of the Kennedy Space Center, directly away from the USA coast. This orbit allows heavy payloads (up to around 29,000 kg) to be lifted, taking advantage of the Earth's eastward rotation. For example, the Hubble Space Telescope and Compton Gamma Ray Observatory were deployed into such orbits. In a 28.5 degree inclination orbit, the Shuttle can be easily seen only at latitudes between 38 degrees North (N) and 38 degrees South (S). A 39 degree inclination orbit is typically used for missions involving life sciences and microgravity experiments. This orbit provides the maximum de-orbit opportunities and optimum repetitive scheduling ability. In such an orbit, the Shuttle can be easily seen only at latitudes between 50 degrees North (N) and 50 degrees South (S). For Mir docking missions, in preparation for the International Space Station missions, the Shuttle is flown to orbits of 51.6 degrees inclination. In such an orbit, the Shuttle can be easily seen only at latitudes between 61 degrees North (N) and 61 degrees South (S). The highest inclination orbit flown is one of 57 degrees, for missions involving Earth observation. In such an orbit, the Shuttle can be easily seen only at latitudes between 67 degrees North (N) and 67 degrees South (S). Naturally, the time of launch and time of year will further dictate whether any good passes can be seen from a particular location. During a mission the orbiter can be typically seen as a very bright object often climbing to magnitude -1 or -2 (thanks to the large flat wing area) with slow 'flashes' up to magnitude -4 or -5, from either the payload bay door radiators or (also) various equipment in the payload bay, as the sun-shuttle- observer angle changes. During the mission various satellites can be seen when deployed from the orbiter such as payloads like the SPARTAN or WAKEFIELD satellite which are deployed during some missions for a few days to be retrieved later. These subsatellites can be seen leading or following the Shuttle by a few seconds as a fainter object around magnitude +3 or fainter. During STS-27, the Lacrosse 1 satellite could even be seen even while in the payload bay due to its reddish coloration. Possibly even spacewalking astronauts would be visible under suitable circumstances with binocular or telescopic magnification. A Mir rendezvous can be watched. Both objects docked together make an interesting and often bright sight, but sometimes not as bright as expected, due to shadowing caused by the docked orbiter on Mir or vice versa. In 11x50 binoculars (during a low elevation pass) one observer has noted that a dipole is apparent, with a bronze-yellow color (Mir) and a magnesium white 'pole' (the Shuttle) distinguishable. Other events to watch for are the various maneuvering burns carried out either for phasing, as part of a rendezvous sequence, or to achieve mission objectives, such as moving to a specific orbit for satellite deployment or to arrive over a target area for Earth observation purposes. Water dumps can also be seen. As waste water is vented to the vacuum of space, it can create the impression of a comet-like cloud trailing from the orbiter. Finally as the mission ends, the OMS engines are fired over the far Western Pacific ocean and the vehicle reenters the atmosphere. A landing at the Kennedy Space Center in Florida results in the orbiter blazing a bright trail of hot ionized gas over portions of the USA or Mexico, easily visible on a clear night. A landing at Edwards Air Force Base in southern California can result in a reentry trail near the Hawaiian island chain. ---- 6.6.2.1 What Can Be Seen on Launch? The ascent itself, if to a 51.6 or 57 degree orbit, can be seen from the USA east coast, especially night launches, as the vehicle's trajectory carries it parallel to the coast a few hundred kilometers offshore. In fine weather the stack (Shuttle, ET, and SRBs) can be followed courtesy of the fiery output of the SRBs. Between 2 and 8 minutes into the launch the main engines may be picked out, pulsing, in binoculars. Following main engine cut off (MECO) and ET separation, the reaction control system (RCS) burns can be seen, building up the separation distance of the orbiter from the ET and correcting the flight attitude and direction. After having separated, the Shuttle and ET continue to coast above the upper atmosphere in close proximity to each other in an "orbit" whose perigee is dangerously low. The Shuttle achieves its target orbit with an Orbital Maneuvering System (OMS) burn over the Pacific Ocean about 45 minutes after launch, whereas the ET crashes to a fiery end in the ocean after it reenters south of Hawaii. In the intervening time the pair can be seen halfway around the world. The two high inclination (51.6 and 57 degree) scenarios allow the Shuttle and ET to be seen over European skies some 16-25 minutes after the launch. The orbiter appears as a bright white object around magnitude 0. The smaller ET (47 meters by 8 meters) is a degree or two from the orbiter and exhibits a definite red/orange tinge but is fainter, around magnitude +2. The pair describe arcs across the sky from the west to the east, before fading into the Earth's shadow. Sometimes a cloud of fuzzy particles has been observed to accompany the pair when viewed with binoculars. Possibly these are ice particles. ---- 6.6.2.2 What Can Be Seen While in Orbit? One of the spectacular effects that can sometimes be witnessed during a Shuttle mission is that of a water dump. Throughout the flight, electrical power on board the orbiter is provided by fuel cells. These combine hydrogen and oxygen in a reaction that yields electricity. One useful by-product is water. In fact, the reaction is not unlike the electrolysis of water, but in reverse. Of course, the water can then be consumed by the astronauts in the course of their daily activities. This spent water and any excess from the original reaction is disposed of by simply venting the liquid to the vacuum of space. In doing so, the water rapidly cools, condensing into a cloud of ice crystals. Under suitable lighting angles and conditions, this has been seen as an almost cometary tail extending from the orbiter. During the STS-73 mission of October 1995, daily water dumps coincided with passes over the southern United States, and a number of observations were made, including this one by Cal Deal at 0626 EDT, 26 October 1995: I watched the Shuttle fly over Fort Lauderdale [Florida] this morning. I noticed what I can only describe as a "headlight" effect, a dim gray glow that extended in front of the Shuttle. I would say that it was not quite as long as the sun is wide [0.5 degree], if that helps. It widened as it got farther away from the Shuttle. It had sort of a rounded, elongated triangular shape that emanated from the Shuttle. The fact that STS-73 was a microgravity mission may have contributed to the spectacle, that is, the persistence of the cloud and its proximity to the orbiter arising from the lack of maneuvers. Cal also described the pass that occurred the following morning: Another unusual observation of STS-73: This morning at 6:34 EDT, it passed over South Florida. Through binoculars I was easily able to discern a dim vaporous stream, or "tail", that seemed to arc forward of its direction of travel, then sweep backward, as though caught by a breeze. This was very different from the triangular cloud I observed with my naked eye yesterday. Today's vapor trail seemed much longer than what I saw yesterday, undoubtedly because of the binoculars. The length and shape of the trail were quite surprising; I've never seen anything like that before. ---- 6.6.2.2.1 External Tank (ET) Reentry The external tank provides the fuel and oxidizer for the Shuttle's main engines during the first 8.5 minutes or so of the ascent. After main engine cut-off (MECO) the tank is separated from the Shuttle. At this point in time both Shuttle and ET are in an elliptical orbit whose apogee is close to the operating orbital altitude of the Shuttle, but with a perilously low perigee. The Shuttle performs an OMS (orbital maneuvering system) burn some 45 minutes into the mission to circularize the orbit and prevent a premature return to Earth. The ET is destined to reenter over the Pacific or Indian Ocean (ascent trajectory dependent) some 80 minutes or so after the launch. The earlier launches (involving two orbital maneuvering system, or OMS, burns) resulted in the ET reentering around 185,000 feet above the Indian Ocean. On one occasion, a South African pilot reported seeing the reentry. The current direct ascent trajectories result in ET reentry over the Pacific Ocean, south of Hawaii. After separation the ET cruises half-way round the world to its reentry point, visible to observers from Newfoundland, across Europe to Africa, as a magnitude +2 object of reddish hue, accompanying the white orbiter of greater brightness. During earlier missions, a tumble was induced (by venting excess gas) to negate aerodynamic lift and help guide the ET towards a predetermined area. A large reentry footprint is planned for, so that no pieces of the tank will come within 370 km of any land mass nor enter foreign air-space, and to avoid areas of human activity and the polar regions. The reentry of the STS-41C's External Tank (on 6 April 1984 from a direct ascent) was observed by Paul Maley of the Johnson Space Center Astronomical Society, from Mauna Kea, Hawaii, in the pre-dawn sky. The following account, written by Paul Maley, is taken from an article in Spaceflight (Vol. 26, December 1984) with the author's permission: At 0519 (local time) a bright glow pierced the horizon, varying slowly in brilliance. Three complete brightness variations were visible before the glow was unexpectedly obscured, as the tank was hidden by a small cinder cone, but the wake remained luminous as three bright lines in the sky broken by two dark areas. This was later found to be due to the tank tumbling; the broad cylindrical side and the small end alternately faced the observer as it tumbled at 45 degrees per second. As the glow emerged from its short eclipse, it had brightened to at least magnitude -6, continually pulsating as it tumbled. Disintegration was beginning. The brilliant mass slowly began to separate into a sparkling cluster of individual pieces, some climbing upward, gaining lift for a short span of new life. The swarm gradually formed into an elliptical group of perhaps as many as 30 to 50 visible pieces, as bright as magnitude 0 and as dim as magnitude +4. The steady stream of glittering fragments covered a span of about 10 degrees long and 5 degrees high as it sped through the southern constellations of Ara, Pavo, Indus and Grus. The orange-yellow "glob" had given birth to a host of brilliant blue-white fragments, some unvarying in brilliance, with others cascading end over end. 137 seconds after it had begun it was all over, but a glance upward revealed the steady light of the magnitude +2, 250 km high, orbiter climbing into its parking orbit. At 0549 there was still an odd bluish cloud in the same spot where the tank had disintegrated. This contrail-like cloud was photographed until 0553. ---- 6.6.2.2.2 Shuttle Reentry Paul Maley also witnessed a reentry of the Shuttle itself during STS-69 on 18 September 1995, as the vehicle streaked across the southern United States on its way to the Kennedy Space Center. The following account, written by Paul Maley, was taken from an article in Spaceflight (Vol. 26, December 1984) with the author's permission: I am happy to report we observed the spectacular reentry of STS-69 at 1122GMT this morning. In its typical fashion (I've seen 6 or 7 reentries of the orbiter before), we observed a red incandescent trail in the wake of Endeavor which itself was -3 magnitude and yellow (oval shape). Skies were mostly cloudy (low clouds) but we had quite excellent views anyway. I was able to get the parking lights in front of the Mission Control Center turned off so that we could better observe the low elevation pass (maximum elevation 17 degrees) as it flew directly over Waco, Texas to our north. The trail, which repeats at certain missions, also took it over McDonald Observatory. This sighting was different from STS-70, which occurred with the sun 3 degrees above the horizon. This time, sunrise was 40 minutes after the predawn pass. Orbiter altitude was 37 nautical miles (62 km), velocity Mach-16. ---- 6.7 What Does the Space Station Mir Look Like? Note: Measurements used in the following sections are metric. Use the following conversions to obtain the English equivalent: Meter to feet: m x 3.3 = feet Centimeter to inches: cm x 0.4 = inches Kilometer to miles: km x 0.6 = statute miles Kilogram to pounds: kg x 2.2 = pounds ---- 6.7.1 Background Information on the Mir Complex The Mir space station (catalog number 16609 or International Designation 86017A) was launched on 19 February 1986, replacing the successful Salyut series of space stations that were run by the Soviets throughout the 1970's and 1980's. Operating in an orbit some 390 km high, inclined at 51.6 degrees, the Mir space station is modular in design and has been slowly constructed in such a manner. The orbital inclination of Mir arises from the optimum launch direction from the Baikonur (Tyuratam) Cosmodrome, which is north of east to avoid flying through Chinese air-space. As of mid-1997 the Mir complex consisted of the original core Mir module (dimensions 15 m x 4.15 m, mass about 20,000 kg) at whose rear axial port (on the +x axis) is docked the Kvant 1 (17845/87030A) astrophysics module (dimensions 5.8 m x 4.15 m, mass 11,000 kg) which was launched on 31 March 1987. This second module is technically known as Kvant, but is identified in most resources as Kvant 1, so as not to confuse it with the later Kvant 2 module. The Kvant 1 module houses the Roentgen astrophysics observatory with the Pulsar x-ray telescopes, a gamma ray detector and the Glazar UV telescope. Also mounted on this module is the Sofora mast structure, 14 meters long, with an attitude maintaining thruster package at one end. A docking port to the rear of Kvant (on the +x axis) is used to accommodate the arrival of either a Progress M or Soyuz TM vehicle. The Progress M re-supply vehicle (dimensions 7.5 m x 2 m, mass 7000 kg) is normally launched every couple of months, carrying water, tools, equipment, mail and fuel. It is capable of free flight for extended periods and normally is docked to Mir for a period of 90 days. The Progress M uses solar arrays to charge its power supply system's batteries. While docked to the Mir, its engine can be used to maintain or alter the space station's orbit. An extended duration flight of a Progress M vehicle occurred during 1993 and 1994. Progress M-17 docked with Mir on 2 April 1993 and finally undocked, after extensive maneuvering of Mir using Progress M-17, on 11 August 1993. There was not enough fuel on board Progress M-17 following the Mir maneuvers to ensure a successful controlled reentry, so the Russians decided to "test" the duration abilities of the Progress M-17, since the Progress M design has many similarities to the Soyuz transport vehicle, and the Soyuz transport vehicle design is planned to be used as the Assured Crew Rescue Vehicle (ACRV) for the International Space Station. Progress M-17 undocked from Mir on 11 August 1993 and flew autonomously for over 7 months in a lower orbit, until it was lowered to an even lower orbit and destructively reentered on 2 March 1994 south of the Philippines. The duration test had been planned for 1.5 years. The reason for the early return was not announced, but continual charging and discharging of the battery power supply as a result of flying in the Earth's shadow or a diminishing fuel supply *may* have been a factor in its early return. In the early 1990's the Progress M vehicle was equipped with a small return capsule called the Reentry Ballistic Capsule (VBK) (mounted to the forward docking hatch) to return material back to Earth. By 1995 the use of this capsule seems to have been discontinued with the periodic docking of the Shuttle to Mir. The expendable Progress M vehicle is loaded with any materials for disposal. The Progress M separates and normally makes a destructive reentry over the South Pacific. The Soyuz TM vehicle (dimensions 7.5 m x 2 m, mass 7000 kg) can deliver 2-3 cosmonauts to Mir, taking some 2-3 days to reach the space station, and is capable of remaining docked for up to 180 days prior to returning to Earth. At the opposite end of the Mir core module is the front axial port that delineates the center of the x axis. It is on this rear (-x) axis that either the Progress or Soyuz vessel is docked. The remaining four side ports (on the z and y axis) are stations for the remaining modules. On one of the side ports (the +y axis) is docked the Kvant 2 (20335/89093A) module (dimensions 12 m x 4.4 m, mass 20,000 kg) which was launched on 26 November 1989. This carries technical support facilities and amenities. It also houses the airlock, which the cosmonauts use to access the outside of the Mir complex in extravehicular activities (EVA). On the opposing side port (-y axis) is the Spektr (23579/95024A) remote sensing module for geophysical sciences (dimensions 12 m x 4.35 m, mass 19,640 kg). It was launched on 20 May 1995 and arrived at Mir on 1 June 1995. The Spektr module suffered solar array and hull damage on 25 June 1997 when Progress M-34 was being manually manuvered from Mir to re-dock to the Kvant module. This module is nearly depressurized (one account had it at 20 mm of Hg) and isolated from the remainder of the Mir complex. Three of the four solar arrays provide power to the Mir complex via temporary power cables that penetrate the modified internal Spektr hatch. On another side port (the -z axis) is the Kristall (20635/ 90048A) industrial processing module (dimensions 12 m x 4.4 m, mass 20,000 kg). Launched 31 May 1990, this module is used for semiconductor and biological experiments. It also houses Earth observation instruments. Attached to the end of Kristall is the Mir-Shuttle docking module. Thus, the Shuttle docks to Mir on the -z axis. Finally, the Priroda remote sensing module (dimensions 12 m x 4.35 m, mass 19,700 kg), which was launched on 23 April 1996, resides at the final vacant side port on the +z axis. The Priroda module houses passive and active radiometers (IKAR), a synthetic aperture radar (Travers), an infrared radiometer (Istok 1), a spectrometer for measuring ozone and aerosol concentrations (Ozon-M), visible and infrared spectrometers (MOZ-Obzor), optical scanners (MSU-SK, MSU-E) and a device for remote interrogation of geophysical stations (Centaur). Thus, the present and final configuration of the Mir complex (32 m long x 30 m wide x 27 m high) is: On the +x axis: Mir module Kvant 1 module Either a Progress or Soyuz vehicle On the -x axis: Either a Progress or Soyuz vehicle On the +y axis: Kvant 2 module with air lock On the -y axis: Spektr module On the +z axis: Priroda module On the -z axis: Kristal module with Mir-Shuttle docking module Further details, photos, and graphics of the Mir complex can be found at the URLs: * http://www.osf.hq.nasa.gov/mir/ * http://shuttle-mir.nasa.gov/ * http://solar.rtd.utk.edu/~jgreen/fpspace.html * http://infothuis.nl/muurkrant/mirmain.html * http://www.mcs.net/~rusaerog/ ---- 6.7.2 Visibility to the Observer The Mir space station and its attendant modules can be seen at intervals throughout the year. Mir can typically be seen for a couple of weeks in evening skies, then is lost for roughly the same time in daylight, reappears for a couple of weeks in morning skies, then is eclipsed by the Earth for a short time, depending upon the observer's location and time of year, before returning to the evening sky. When visible, Mir can be quite bright, reaching magnitude -1 or better, thanks to the large combined surface area of its modules. It is not unusual to catch a glint of magnitude -3 or better off one of its solar arrays. In binoculars, a hint of a bronze-yellow color can often be seen. At various times during the year it is also possible to see (depending upon the observer's location) a closing or departing Soyuz-TM or Progress-M vehicle. Viewing of Progress or Soyuz vehicles prior to docking with Mir is not normally provided to North American observers due to the orbital profile used to catch up to Mir in a two or three day period. Somewhat less predictable are events such as the dumping of rubbish. In the past, a lot of spent materials were dumped overboard to be allowed to reenter, under the influence of various forces, at a later date. Observing such dumpings would have required a dark sky, good binoculars, and luck. These activities have since been curtailed, because the International Space Station is due to be constructed in a similar orbit, and a high speed collision with debris is to be avoided. Rubbish is now routinely placed in the expendable portion of the Progress-M vehicle to be disposed of upon reentry. During extravehicular activities (EVA) additional material has been observed using binoculars in the vicinity of the Mir complex. Most material quickly enters a lower orbit due to atmospheric drag and begins to lead the orbit of Mir. A lot of this material is cataloged and tracked by NASA Goddard's Orbital Information Group (OIG). Occasionally, various craft are deployed from Mir, which can be observed. On 19 April 1995, the Russian-German GFZ-1 satellite (86017JE/23558) was orbited from Mir's airlock. This small, 21 cm diameter, 20 kg ball is being used for orbital geodetic studies and is studded with laser retro-reflectors for range- finding purposes. Detection of it would require a dark sky and binoculars. A different study was conducted on 4 February 1993, when a departing Progress vehicle deployed the Znamya (86017GZ) solar reflector. This 20 meter diameter reflector was reported as an experiment into solar sailing and an investigation of the degree of solar energy that could be reflected to the ground. (The beam appeared brighter than the Moon over Russia, and was spotted on the ground by the then-resident Mir crew. It was still seen as bright as magnitude -5 whilst tumbling out of control just prior to reentry). Solar radiation pressure caused Znamya to decay within a couple of days. ---- 6.8 What are the ten brightest satellites? ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in February 1998. ========================================================================