When did Phobos disappear??? I don't see how this is even possible, and I haven't heard of anything unusual happening to it.
I looked on Wikipedia and they have an animation of Phobos and Deimos orbiting Mars. Both are moving counterclockwise.
I'm not sure why there is anything wrong with Phobos orbiting at the altitude it is. Phobos is (slowly!) getting closer to Mars and eventually is
going to crash into the surface, so it's hardly a stable orbit, but I don't see any other problem with it. I've heard a theory that both Martian
moons are rogue asteroids captured by gravity; not sure if it's true or not, but the moons are the right size and Mars is relatively close to the
asteroid belt.
The Wikipedia entry for Phobos
here mentions a Russian physicist who hypothesized that
Phobos was hollow to fit his secular acceleration calculations, but that others have cast doubt on those calculations.
From Wikipedia - Phobos
"Hollow Phobos" claims
Around 1958, the distinguished Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos' orbital motion,
suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations on Phobos' artificial origin. Shklovsky based his
analysis on estimates of the upper martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular
acceleration, Phobos had to be very light —one calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick.
Competing explanations were based on the land tides Phobos could raise on Mars. The reality of the secular acceleration itself (corresponding to an
altitude loss of about 5×10−12 per revolution, about 5 cm per year) was later subjected to doubt, and the problem vanished on its own by 1969. In a
February 1960 letter to the journal Astronautics, however, Siegfried Frederick Singer, then science advisor to President Eisenhower, came out in
support of Shklovsky's theory, going as far as stating that "[Phobos'] purpose would probably be to sweep up radiation in Mars' atmosphere, so
that Martians could safely operate around their planet". A few years later, in 1963, Raymond H. Wilson Jr., Chief of Applied Mathematics at NASA,
allegedly announced to the Institute of Aerospace Sciences that "Phobos might be a colossal base orbiting Mars", and that NASA itself was
considering the possibility.
Similar "hollow Moon" claims occurred at around the same time.
