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Q: What sorts of visible things are shed by a space vehicle?
A: The vehicle may have dropped a booster stage or structural support elements, such as the objects seen by moon-bound Apollo crews, or the Skylab crews (the station’s S-II booster). Insulation fragments had a tendency to ‘shed’ on Gemini and Apollo and Skylab [which regularly released small reddish fragments seen through the on-board solar telescope, out the wardroom window, and on space walks), and spacewalkers on occasion manually jettisoned excess equipment during hatch openings. During payload deploys, retaining straps and pyrobolt shells could be seen and imaged. On shuttles, right after reaching orbit a lot of ice associated with the cryogenic main engines [including a particularly weird-shaped ice sculpture that often formed at the interface of the shuttle and its external fuel tank feed line] came off and was clearly seen. Later on shuttle flights, small hardware items would float out of the payload bay, or become detached from mechanical structures outside. Tile fragments and strips of polyurethane ‘gap filler’ material were also noticed on a number of flights. Several deployed payloads, including inflatable structures and spherical free-flying camera pods, have been inaccurately described on ‘youtube’ as ‘unknowns’. During spacewalks, packing materials might be jettisoned, or tools come loose accidentally [and once, several golf balls swatted off into space]. But by far the largest population of sources of videotaped ‘dots’ has been effluent from inside the vehicles, such as water and propellant [hydrazine and nitrogen tetroxide] ice, from more than a hundred external valves – some deliberate, such as water dumps and flash evaporator operation and hydraulic pressure pump testing, but most accidental from seeping thruster valves.
Q: What directions could such stuff be seen to fly in?
A: Because of the large size of the space shuttle and its sources of effluent or other shedding, and the distribution of external cameras, ‘dandruff’ can drift across a camera’s field of view in practically any direction. But it doesn’t change the big picture that stuff visible to a shuttle camera is orbiting very close to – and hence probably originated from – the shuttle itself.
Q: What are typical motions of nearby objects?
A: Three major features dictate the expected motion of small, light objects near a spacecraft such as the shuttle or the space station. They are “orbital mechanics” effects [sometimes called ‘astrodynamics’], differential air drag [at the altitudes typical of shuttle and station flights], and vehicle plume/outgassing events. As a result, objects can drift backwards, reverse course, zip below the vehicle and vanish ahead within minutes or an hour or two. They can abruptly change angular rate and course. They can behave in genuinely bizarre-looking motion that is truly ‘unearthly’ – which is only to be expected because they are NOT on Earth [or in Kansas].
Q: Explain “differential air drag”
A: Moving at 25,000 feet per second, orbiting objects do hit some air molecules that reach up that high, and this slows the orbiting object. But objects that are less dense – more frontal area but less mass – are slowed down more significantly that ‘thicker’ objects – high mass concentrated into smaller volume – are slowed. As a result, ‘lighter’ stuff such as insulation or ice flakes slow down more quickly, drop into lower orbits, and then pull ahead [because of speed gained in the descent] of the objects that are denser, such as the main spacecraft. In extreme cases this effect can be detected over periods of 10 to 20 minutes.
Q: Explain “orbital mechanics”
A: Objects at different altitudes need different velocities to remain in circular orbits, and lower objects thus pull ahead of higher ones. A ‘rule of thumb’ in Mission Control said that something 100 feet lower would pull ahead by ten times 100 feet every full circuit of Earth [every ‘orbit’, or ‘revolution’]. That “10:1” rule applies to a wide range of altitude differences.
An even more bizarre behavior is when nearby objects are slowly moving in relatively different directions – although both are flying NEARLY parallel at 25,000 ft/sec. Something moving off to either SIDE of a central object will depart for 15 to 20 minutes, then come to a stop and begin moving BACK towards its point of origin, arriving there with the original departure speed, in about half an orbit, or 45 minutes. Something pushed away UPWARDS will rise, then slowly fall behind and pass downwards behind the spacecraft, but then swoop lower and catch up with the spacecraft and return to it a full orbit later. Something pushed FORWARD will pull ahead and gradually climb in height, slowing down and passing overhead headed backwards, and vanish aft from sight – but if influenced by differential drag, it can then slip back into a lower, faster orbit and overtake the spacecraft from behind in the hours ahead. Only something pushed BACKWARDS will safely depart the neighborhood of the spacecraft, as it falls behind, slips into a lower orbit, and then can be seen passing below the spacecraft moving out ahead of it – but safely below it.
Q: Explain “vehicle pluming”
A: Attitude control thrusters fire under manual or autopilot command, and create a 10,000 ft/sec effluent plume that packs plenty of punch. Unlike such plumes in an atmosphere, plumes in a vacuum spread to an amazing degree. Half of a shuttle’s thruster plume flow spread out at angles greater than 30 degrees off centerline, and some is still present at 90 degrees off centerline and even higher. Also, plume flow bounces off vehicle structure that it hits. This is not 'reflective' [angle out = angle in] but random. This effect is most noticeable for the shuttle’s aft down-firing jets, which seriously impinge on structure such as elevons and the ‘body flap’ [losing about 30 % of their effective thrust in this impingement].
Another important affect that must be appreciated is ‘plume shadowing’, where objects closest to a camera may be over or inside the shuttle’s payload bay. Since it is impossible to judge range from merely observing a dot on a screen, it can be puzzling to observe that some of the dots may be affected by a plume field and others not. But this can be because some are far enough away from the camera to be out of the structural plume shadow, and others are not.
It’s also important to realize [to be continued]
It’s also important to realize that plumes are not continuously visible even when an engine is firing. This is most evident in watching plumes from the shuttle’s three main engines and its larger ‘Orbital Maneuvering System’ [OMS] engines, which ‘flash’ when starting up and stopping but largely burn invisibly. Smaller ‘Reaction Control System’ [RCS] thrusters [both ‘Primary’ and the smaller ‘Vernier’ thrusters] can display bright centerline plumes but the visibility of the plume drops off rapidly with greater angle off centerline, even though plume flow is present. The visibility is usually due to propellant mixture deviations that occur when both valves open or close nearly but not precisely simultaneously, or otherwise during a burn when there is a slight ‘burp’ in one of the flow lines. For long [5-10 second] RCS burns, they also can be mostly invisible during the stable portion of the burn.
Q: Explain “vehicle outgassing”
A: Cabin air can be vented through specific valves, to adjust interior pressure or dump waste gas such as carbon dioxide or methane. Water [or waste water] can also be released, and depending on flow rate can generate an invisible gentle plume all the way to a blizzard of flash-frozen ice crystals. On shuttles, hydraulic power units expel vapor when ‘burning’ hydrazine to provide the push needed to move aerosurfaces. Also on shuttles, a ‘flash evaporator’ near the forward base of the tail sometimes expels water to dump heat from electrical usage.
There are special phenomena during spacewalks. Spacesuits can release gas, and they also release water vapor from cooling devices. Although it’s invisible, this flow is powerful and when sometimes aimed in the ‘wrong’ direction can gradually shift the entire space vehicle off its desired orientation. Also, residual air in an airlock will rush out the hatch as soon as it is opened, often entraining floating debris from inside the airlock. And although rarely used, EVA mobility units can fire pulses of nitrogen or other materials to provide propulsion to free-flying astronauts.
The most serious angle to outgassing is when it is accidental, usually involving either the failure of a valve or of a pressurized line – or even, conceivably, a hole in a window seal or cabin pressure hull or [on the shuttle] a tire.
Q: Are these the only factors that can influence relative motion in space?
A: Probably not. Small particles of ice, for example, seem to be influenced by the sublimation of water [or fuel] molecules off their sunlit sides, that over a period of several minutes can slightly bend their drifting paths. The paths of larger objects, such as a dropped tool kit or discarded spacesuit, can be affected by escape of small amounts of gas or liquids trapped inside them. Ice particles in a swarm such as from a water dump can collide, sending them in different directions, and spinning ice particles can break apart, sending fragments off in widely different directions. It is truly weird out there, even without recourse to alien visitations.