Sunday, November 13, 2011

The Lacrosse 5 "disappearance trick" captured on video

Note: the video below was featured (with my permission) on Unfortunately, it was initially suggested there (and this definitely did not come from me) that the discussed "disappearance trick" is a deliberate "stealth" feature of this satellite.
It almost certainly is not: it is something brough about by accident from something specific in the satellite design.

I also want to make clear, as it kept popping up in the YouTube comments (which I have now disabled), that this is not the moment the satellite disappears in earth shadow! The drop in magnitude happened at 17:35:20 UTC: shadow entry was much later, 17:38:55 UTC.

The video above was shot by me Friday evening (11 Nov). It shows Lacrosse 5 (2005-016A), the latest of the Lacrosse SAR satellites.  It was launched in 2005. In the movie, it is doing it's infamous "disappearance trick" (also note the old Russian rocket stage visible in the second part of the footage).

The brightness behaviour of this satellite is different from that of the previous Lacrosse satellites. Apart from that it is brighter overall and a bit yellowish in colour (the others are distinctly orange-reddish), it shows a variable brightness behaviour that the other Lacrosse satellites do not show (or at least not to this extreme extend).

Lacrosse 5 can sometimes drop several magnitudes in brightness, typically from +1 (easy naked eye) to +5 or +6 (naked eye invisibility or near-invisibility), in a matter of seconds.

After observing this a couple of times, I coined it the "disappearance trick", a term that has stuck in the amateur satellite observer's community.

While many satellites can flare briefly (and the Lacrosses do), this opposite effect of one suddenly dropping in brightness other than due to normal phase angle changes or entry into earth shadow (which is not the case here!!!), is not quite common. And Lacrosse 5 does it that frequently, that it stands out.

Normally, when a satellite or spent rocket stage shows sudden changes in brightness, it is due to either:

a) the satellite entering earth shadow;
b) the satellite is tumbling.

Both are not the case here. These "disappearance tricks" of Lacrosse 5 happen well before the point of shadow entry. In addition, the behaviour is not the typical "flashing" behaviour of a tumbling or spinning satellite. There is no periodicity, and the drop in brightness happens after a long period of stable brightness.

The behaviour is interesting, because the sister ships of Lacrosse 5 (the other Lacrosses) do not typically show this behaviour. The implication is, that Lacrosse 5 is different in design than Lacrosses 1 to 4.

I have photographically documented the phenomena several times, including brightness curves (see here, and a comparison of several curves showing the phenomena here, featuring the comparative diagram shown below).

click diagram to enlarge

Philip Masding has been documenting the phenomena as well, his results can be seen here. His curves also show, and I have seen this happen as well, that Lacrosse 5 can sometimes "re-appear" (and, as I have seen occasionally, next "disappear" again...).

One point is, that we so far cannot find a clear pattern in this all. The satellite does not seem to do this at specific phase angles for example.

We are still at a loss to explain this behaviour. Please note: we don't think it is an intentional "stealth" characteristic. Yet it must have something to do with the satellite design or operation.

Is it a matter of strongly differing reflectance properties of the satellite body with illumination angle? Is it some brightly reflecting appendage on the satellite disappearing from view? Is it a dark appendage on the satellite starting to block view of the illuminated satellite body, or casting a shadow on it? Is it due to some moving part of the satellite, e.g. a moving dish antenna?

We simply do not know. And it is giving us a nice puzzle.

The photograph below, taken in addition to the video footage above, shows Lacrosse 5 in the bright phase of Friday's trajectory.

click image to enlarge

Apart from Lacrosse 5, I observed a couple of other satellites last Friday, including the NOSS 3-5 duo (11-014A & B) and USA 32 (88-078A)

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