ESO Annual Report 2004

Quadrant 1: 1.600 µm Quadrant 2: 1.575 µm Quadrant 3: 1.625 µm Quadrant 4: 1.625 µm
To obtain unique images of the surface
of Titan, the simultaneous “atmospheric”
images (at waveband 1.625 µm) were
“subtracted” from the “surface” images
(1.575 and 1.600 µm) in order to remove
any residual atmospheric features
present in the latter.
Titan is tidally locked to Saturn, and hence
always presents the same face towards
the planet. To image all sides of Titan (from
the Earth) therefore requires observations
during almost one entire orbital peri-
od, 16 days. Still, the present week-long
observing campaign enabled the mapping
of approximately three-quarters of the
surface of Titan.
Over the range of longitudes which have
been mapped during the present observations,
it is obvious that the southern
hemisphere of Titan is dominated by
The parallax of Asteroid Toutatis
On September 29, 2004, the asteroid Toutatis
approached the Earth to within 1550 000 km,
or just four times the distance of the Moon.
Simultaneous images obtained with telescopes
at ESO’s two observatories at La Silla and
Paranal were taken just over 12 hours before
the closest approach. They clearly demonstrate
the closeness of Toutatis to the Earth. As
can be seen on the unique photo that combines
two of the exposures from the two observatories,
the sighting angle to Toutatis from
the two observatories, 513 km apart, is quite
different.
The measured angular distance in the sky of the
beginnings (or the ends) of the two trails
(about 40 arcsec), together with the known distance
between the two observatories and the
position of Toutatis in the sky at the moment of
the exposures fully define the triangle “Paranal-
Toutatis-La Silla” and thus allow the calculation
of the exact distance to the asteroid.
It is found to be very close to that predicted
from the asteroid's position in its orbit and
that of the Earth at the moment of this unique
observation, 1607900 km.
a single bright region centred at approximately
15° longitude. The equatorial
area displays well-defined dark (low-reflection)
structures.
The imaging and monitoring of Titan will
continue, with the goal of assisting the
Cassini-Huygens team in the interpretation
and understanding of what will certainly
be a rich and complex flow of information
about this enigmatic moon.
ESO Annual Report 2004
Four images of Titan,
obtained simultaneously
with the NACO adaptive
optics instrument in
the SDI observing mode.
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