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We report the results of an analysis of all Spitzer/MIPS 24 micronobservations of HD 209458b, one of the touchstone objects in the study ofirradiated giant planet atmospheres. Altogether we analyze 2.5 transits, 3eclipses, and a 58-hour near-continuous observation designed to detect theplanet's thermal phase curve. The results of our analysis are: (1) A meantransit depth of 1.484% +/- 0.035%, consistent with previous measurements andshowing no evidence of variability in transit depth at the 3% level. (2) A meaneclipse depth of 0.332% +/- 0.026%, somewhat higher than that previouslyreported for this system; this new value brings observations into betteragreement with models. The dayside flux shows no evidence of variability at the12% level. (3) Eclipses in the system occur 32 s +/- 129 s earlier than wouldbe expected from a circular orbit, which constrains the orbital quantity (e cosomega) to be 0.00004 +/- 0.00033. This result is fully consistent with acircular orbit and sets an upper limit of 140 m/s (3 sigma) on anyeccentricity-induced velocity offset during transit. The phase curveobservations (including one of the transits) exhibit an anomalous trend similarto the detector ramp seen in previous Spitzer/IRAC observations; by modelingthis ramp we recover the system parameters. The photometry which follows theramp and transit exhibits a gradual, ~0.2% decrease in flux, similar to thatseen in pre-launch calibration data. The large uncertainties associated withthis poorly-understood, likely instrumental effect prevent us from usefullyconstraining the planet's thermal phase curve. Our observations highlight theneed for a thorough understanding of detector-related instrumental effects onlong time scales when making the high-precision mid-infrared measurementsplanned for future missions such as EChO, SPICA, and JWST. [abridged]

Spitzer/MIPS 24 micron Observations of HD 209458b: 3 eclipses, 2.5 transits, and a Phase Curve Corrupted by Instrumental Sensitivity Variations