Editing Phobos (moon) (section)

=== Shklovsky's "Hollow Phobos" hypothesis ===
In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.<ref>{{cite web |date=12 July 2017 |title=A Convenient Truth - One Universe at a Time |url=https://archive.briankoberlein.com/2017/07/12/a-convenient-truth/index.html |access-date=14 July 2020}}</ref> Around 1958, Russian astrophysicist [[Iosif Samuilovich Shklovsky]], studying the [[Secular phenomena|secular]] [[acceleration]] of Phobos's orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.<ref name="Shklovsky">Shklovsky, Iosif Samuilovich; ''The Universe, Life, and Mind'', Academy of Sciences USSR, Moscow, 1962</ref> 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 {{Convert|16|km|sp=us}} across but less than {{convert|6|cm}} thick.<ref name="Shklovsky" /><ref>{{Cite journal |last=Öpik |first=Ernst Julius |author-link=Ernst Julius Öpik |date=September 1964 |title=Is Phobos Artificial? |journal=Irish Astronomical Journal |volume=6 |pages=281–283 |bibcode=1964IrAJ....6..281.}}</ref> In a February 1960 letter to the journal ''Astronautics'',<ref name="Singer">[[Fred Singer|Singer, S. Fred]]; ''Astronautics'', February 1960</ref> [[Fred Singer]], then science advisor to U.S. President [[Dwight D. Eisenhower]], said of Shklovsky's theory:
<blockquote>If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.<ref name="Singer" /></blockquote>

Subsequently, the systematic data errors that Singer predicted were found to exist, the claim was called into doubt,<ref name="Öpik1963">{{Cite journal |last=Öpik |first=Ernst Julius |date=March 1963 |title=News and Comments: Phobos, Nature of Acceleration |journal=Irish Astronomical Journal |volume=6 |page=40 |bibcode=1963IrAJ....6R..40.}}</ref> and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.<ref>{{Citation |last=Singer |first=S. Fred |title=On the Origin of the Martian Satellites Phobos and Deimos |date=1967 |journal=The Moon and the Planets |pages=317 |bibcode=1967mopl.conf..317S}}</ref> Singer's critique was justified when earlier studies were discovered to have used an overestimated value of {{convert|5|cm}} per year for the rate of altitude loss, which was later revised to {{convert|1.8|cm}} per year.<ref>Singer, S. Fred; "More on the Moons of Mars", ''Astronautics'', February 1960. [[American Astronautical Society]], page 16</ref> The secular acceleration is now attributed to tidal effects, which create drag on the moon and therefore cause it to spiral inward.<ref name="JGR-Planets">{{cite news |last1=Efroimsky |first1=Michael |last2=Lainey |first2=Valéry |date=29 December 2007 |title=Physics of bodily tides in terrestrial planets and the appropriate scales of dynamical evolution |work=Journal of Geophysical Research—Planets|volume=112|page=E12003 |doi=10.1029/2007JE002908}}</ref>

The density of Phobos has now been directly measured by spacecraft to be {{convert|1.887| g/cm3||abbr=on}}.<ref name="dlr2008">{{cite web |date=16 October 2008 |title=Mars Express closes in on the origin of Mars' larger moon |url=http://www.dlr.de/mars/en/desktopdefault.aspx/tabid-207/422_read-13776/ |access-date=16 October 2008 |publisher=[[German Aerospace Center|DLR]] |archive-date=4 June 2011 |archive-url=https://web.archive.org/web/20110604233019/http://www.dlr.de/mars/en/desktopdefault.aspx/tabid-207/422_read-13776/ |url-status=dead }}</ref> Current observations are consistent with Phobos being a [[rubble pile]].<ref name="dlr2008" /> Images obtained by the [[Viking probes|''Viking'' probes]] in the 1970s showed a natural object, not an artificial one. Nevertheless, mapping by the ''Mars Express'' probe and subsequent volume calculations do suggest the presence of voids and indicate that it is not a solid chunk of rock but a porous body.<ref>Clark, Stuart; "Cheap Flights to Phobos" in ''New Scientist'' magazine, 30 January 2010</ref> The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty.<ref name="Andert" />