Mathematics – Logic
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994eso..pres....4.&link_type=abstract
ESO Press Release, 02/1994
Mathematics
Logic
Scientific paper
Eight years after the passage of Comet Halley in
early 1986, astronomers at the European Southern
Observatory have succeeded in obtaining an image [1]
of this famous object at a distance of no less than
2,820 million km from the Sun. The comet is now about
as far away as giant planet Uranus. It recently
passed the halfway mark towards the most distant
point of its very elongated 76-year orbit.
The image shows the 6 x 15 km avocado-shaped nucleus
as an extremely faint point of light without any
surrounding dust cloud. It appears that the surface
is now completely frozen and the comet has ceased to
emit dust and gas.
This observation was made with the ESO 3.58 metre New
Technology Telescope (NTT). It is by far the faintest
and most distant image ever recorded of this comet.
A DIFFICULT OBSERVATION
The new Halley image was obtained in the course of an
observational programme by a small group of
astronomers [2], aimed at the investigation of
distant solar system objects. The observation was
difficult to perform and is close to the limit of
what is possible, even with the NTT, one of the
technologically most advanced astronomical
telescopes.
In fact, this observation may be compared to viewing
a black golfball, used during a late evening game,
from a distance of 12,000 km.
At Halley's present, very large distance from the
Sun, the intensity of the solar light is over 350
times fainter than here on Earth. The surface of the
cometary nucleus is very dark; it reflects only 4 %
of the infalling sunlight. The amount of light
received from Halley is therefore extremely small:
the recorded star-like image of the nucleus is about
160 million times fainter than the faintest star that
can be seen with the unaided eye. A long exposure was
needed to catch enough light to show the object; even
with the very sensitive SuSI CCD camera at the NTT,
the shutter had to be kept open for a total of 3
hours 45 minutes. During this time, of the order of
9000 photons from Comet Halley were registered. The
extreme faintness of its image is illustrated by the
fact that almost 1 million, or 100 times as many
photons were simultaneously received in this
direction from the luminous atmosphere of the
Earth. They must be carefully "subtracted", before
the comet can be seen.
There is another complication. Due to the motions of
the comet and the Earth, the direction to the comet
(as seen against the stars in the background)
continuously changes during the observation. The
movement of the telescope must therefore be
accurately offset to "follow" the motion of the comet
in order to keep the sparse photons falling on the
same spot of the detector during the long exposure.
IS HALLEY NOW FROZEN?
The measured brightness of the Halley image (visual
magnitude 26.5 +- 0.2) closely corresponds to what
would be expected, if it results from sunlight being
reflected from the nucleus alone. This indicates that
there is little, if any, dust left around the nucleus
and it must be assumed that its surface layers are
now completely frozen.
The observation therefore shows that nothing is left
of the great mass of dusty material, estimated at 1
million tonnes, that was thrown out during the
completely unexpected outburst observed at ESO in
February 1991. Nevertheless, the astronomers intend
to continue to monitor the behaviour of Halley during
the next years - it cannot be excluded that this
comet may be good for another surprise!
FUTURE OBSERVATIONS WITH THE VLT
Comet Halley will continue to move outwards through
the solar system at decreasing speed. Thirty years
from now it reaches the turning point (the
"aphelion") of its elongated orbit, almost 5,300
million kilometres from the Sun. Although the light
reflected from its nucleus will then be 15 times
fainter than at the present time, it should still be
possible to register its image with one of the 8.2
metre unit telescopes of the ESO Very Large Telescope
(VLT) during exposures of only a few hours' duration.
Comet Halley's next return to our neighbourhood will
take place in the year 2061.
1 A B/W photo accompanies this Press Release.
2 The members are Olivier Hainaut and Richard West
(ESO), Brian Marsden (Harvard-Smithsonian Center
for Astrophysics, Cambridge, Massachusetts, U.S.A.)
and Karen Meech (Institute for Astronomy, Honolulu,
Hawaii, U.S.A.). The Halley observation is also
described on a Circular of the International Astronomical
Union, published today.
3 See ESO Press Release 03/91 of 22 February 1991.
FIGURE CAPTION
ESO PR PHOTO 04/94-1: COMET HALLEY AT 2,820 MILLION KM
This negative photo shows the faint image of periodic
comet Halley (in the circle) at the record
heliocentric distance 18.82 AU (= 2,820 million km,
about the distance of Uranus). It was obtained with
the SuSI CCD camera at the ESO 3.58 m New Technology
Telescope (NTT) during the night of January 10--11,
1994.
Nine individual exposures, each lasting 25 minutes,
were used to produce this picture. They were cleaned
to remove various sky and instrumental noise, shifted
according to the predicted motion of the comet and
then co-added. This ensures that all recorded light
from the comet is concentrated in one place. At the
same time, the images of the other objects that do
not share the motion of the comet, are not superposed
and will therefore be seen as long trails. The
non-uniformities of these trails arise because of
varying sky conditions and also due to the time
intervals between the individual exposures.
In addition to the comet, the picture contains the
images of three very different types of objects:
stars with relatively sharp trails (e.g. the
comparatively bright one, just below the comet
image), several extended (diffuse) galaxies, and an
artificial Earth satellite which happened to cross
the field during one of the exposures (its trail
extends from the middle of the left edge to the lower
edge).
The measured magnitude of P/Halley is V = 26.5
+-0.2. The position in the sky is less than 1 arcsec
from that predicted on the basis of the comet's very
well-determined orbit.
Technical information: The CCD frames were cleaned of
cosmics and flat-fielded, but they were neither
filtered, nor smoothed. Total exposure time: 13,500
seconds. The seeing varied from 0.6 - 0.9 arcsec. One
pixel = 0.13 arcsec. Field size: 310 x 430 pixels or
40 x 56 arcsec. North is up and East is to the left.
This photo (ESO PR PHOTO 04/94-1) accompanies ESO
Press Release 04/94 and may be reproduced, if credit
is given to the European Southern Observatory.
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