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On the current Reference Mission design, the spacecraft are bristling with small video cameras. The external video cameras are relatively inexpensive and lightweight, especially in comparison to the equipment available just a few years ago.
Data compression allows us to reduce the downlink bandwidth consumed by even dozens of these cameras to a small portion of the total bandwith available to the spacecraft. Most of the bandwidth will be reserved or real-time productions-quality video channels that photograph internal video of the crew -- both scripted scenes and stock footage. The external scene, on the other hand, changes so slowly that real-time, full-motion video would be wasted.
Their primary task would be to get some really neat footage of the spacecraft, space, nearby celestial objects, and footage of EVAs and assembly. This video would be indispensable, and would be valued at many times the cost of the video setup.
These systems have several other applications, such as science. Imaging Earth and Moon would be useful, and could have scientific value. Depending on the design of the cameras, it would be possible to do imaging of the Moon and Earth, and possibly the solar corona using careful application of the camera's movable sunshield, with a variety of wavelengths and filters. Resolution is an unknown and important factor; however, a scientific application is a possibility.
Also, using the various cameras for star tracking is another possibility. Either as a redundant backup system, or to reduce some or most of the need for special guidence star-trackers, the cameras would have a use.
With careful design, the electronics and computing power involved in the cameras, for their recording and transmission to mission control, could find other applications in non-peak use times. Having extra processing power for compression would allow more channels of video to be transmitted, increasing the quantity of footage produced by the mission.
However, these cameras have another important spin-off: monitoring the spacecraft. When not collecting any new or useful footage, the cameras can go into low-bandwidth mode (higher compression, low frame rate, less resolution) and image the external spacecraft. This includes assembly, docking, and engine burns, as well as possible contingencies.
These cameras would be extremely useful in contingency situation. As an example, imagine how valuable external video would have been on Apollo 13. Having these cameras would provide much more information for the ground support team and crew alike, and would lead to a faster, better analysis fo the situation and course of action.
External video equipment would be very valuable even in its primary function of capturing footage of the flight; however, they are made indispensable by the secondary applications of the video systems.
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