TETHER ORIENTATION The problem of deriving the 3-D orientation is relatively easy to describe, but will need a bit of effort to code it. The simplest thing to recognise is that if it is vertical, then it will be seen as perpendicular to the horizon no matter where you see it from, at what angle or distance. If you were lucky enough to have it pass high in the sky and the tether diminished to a point, then that defines the orientation. Some simple geometry would then define the in-orbit orientation. In general terms, however, you could define a rectangular coord system centred on the satellite with Z up (extension of the radius vector) and X forward in the orbital plane, and Y normal (perpendicular) to the plane. Two estimates of the tether orientation (angle relative to the observer's zenith) are needed to make a determination and these should be from as wide an arc as possible during one pass, or from well separated observers during the same pass. (It is assumed that tether orientation is stable over a period of several minutes). The observer's spatial coordinates and the observed orientation for each observation have to be transformed into the satellite coordinate system as defined above. These define two planes that contain the tether. It is then a simple procedure of finding the intersection of the two planes. Other than the transformation of coordinates, this is formally equivalent to determining the radiant of a meteor. Of course, more than two observations can be combined, in which case a least squares solution is used for the intersection of all the planes. Regarding Paul Maley's request for the magnitude of the tether, it is not possible to give a number that is comparable with stellar magnitudes. The brightness to the naked eye is greater than in binoculars, as the brightness decreases linearly with magnification until the seeing disk is resolved. It may be possible to define mags/arcmin or some such, but this is not trivial. Naked-eye visibility relative to the sky background may be the easiest way of defining the visibility, but you will require estimates of the naked-eye limiting magnitude and how obvious the tether is. The tether is a strange intermediate between a point source (star) and an extended source (sky background) as it is only resolved in one dimension. Rob McNaught