  The near-Earth asteroid (99942) Apophis is a potentially hazardous asteroid.
We obtained far-infrared observations of this asteroid with the Herschel Space
Observatory's PACS instrument at 70, 100, and 160 micron. These were taken at
two epochs in January and March 2013 during a close Earth encounter. These
first thermal measurements of Apophis were taken at similar phase angles before
and after opposition. We performed a detailed thermophysical model analysis by
using the spin and shape model recently derived from applying a 2-period
Fourier series method to a large sample of well-calibrated photometric
observations. We find that the tumbling asteroid Apophis has an elongated shape
with a mean diameter of 375$^{+14}_{-10}$ m (of an equal volume sphere) and a
geometric V-band albedo of 0.30$^{+0.05}_{-0.06}$. We find a thermal inertia in
the range 250-800 Jm$^{-2}$s$^{-0.5}$K$^{-1}$ (best solution at 600
Jm$^{-2}$s$^{-0.5}$K$^{-1}$), which can be explained by a mixture of low
conductivity fine regolith with larger rocks and boulders of high thermal
inertia on the surface. The thermal inertia, and other similarities with
(25143) Itokawa indicate that Apophis might also have a rubble-pile structure.
If we combine the new size value with the assumption of an Itokawa-like density
and porosity we estimate a mass between 4.4 and 6.2 10$^{10}$ kg which is more
than 2-3 times larger than previous estimates. We expect that the newly derived
properties will influence impact scenario studies and influence the long-term
orbit predictions of Apophis.
