Phobos

The hollow-sounding moon. The Soviet probe photographed something, then went silent.

Mars' inner moon is anomalously light (grooved, possibly rubble-pile with large voids — 'hollow' in the loose sense of ESA's MARSIS porosity findings). The Soviet probe Phobos 2 was lost in 1989 just before deploying landers; its final frames included an elongated shadow-like feature, prosaically explainable, endlessly reproduced. Shklovsky's 1959 calculation that Phobos' orbital decay implied a hollow artificial body was based on erroneous drag data and retracted — but it put 'artificial moon' into the literature. Links Mars anomalies and the hollow-moon file.