PRESS RELEASE FROM THE AMERICAN ASTRONOMICAL SOCIETY
************************************************** ****



New Image of Comet Halley in the Cold

VLT Observes Famous Traveller at Record Distance

Summary

Seventeen years after the last passage of Comet Halley, the ESO Very
Large Telescope at Paranal (Chile) has captured a unique image of this
famous object as it cruises through the outer solar system. It is
completely inactive in this cold environment.

No other comet has ever been observed this far - 4200 million km from
the Sun - or that faint - nearly 1000 million times fainter than what
can be perceived with the unaided eye.

This observation is a byproduct of a dedicated search [1] for small
Trans-Neptunian Objects, a population of icy bodies of which more than
600 have been found during the past decade.

The full text of this Press Release, with three photos (ESO PR Photo
27a-c/03) and all related links, is available at:

http://www.eso.org/outreach/press-re...hot-27-03.html

The Halley image

Remember Comet Halley - the famous "haired star" that has been
observed with great regularity - about once every 76 years - during
more than two millennia? Which was visited by an international
spacecraft armada when it last passed through the inner solar system
in 1986? And which put on a fine display in the sky at that time?

Now, 17 years after that passage, this cosmic traveller has again been
observed at the European Southern Observatory. Moving outward along
its elongated orbit into the deep-freeze outer regions of the solar
system, it is now almost as far away as Neptune, the most distant
giant planet in our system. At 4,200 million km from the Sun, Comet
Halley has now completed four-fifths of its travel towards the most
distant point of this orbit. As the motion is getting ever slower, it
will reach that turning point in December 2023, after which it begins
its long return towards the next passage through the inner solar
system in 2062.

The new image of Halley was taken with the Very Large Telescope (VLT)
at Paranal (Chile); a "cleaned" version is shown in PR Photo
27a/03. It was obtained as a byproduct of an observing program aimed
at studying the population of icy bodies at the rim of the solar
system. The image shows the raven-black, 10-km cometary nucleus of ice
and dust as an unresolved faint point of light, without any signs of
activity.

A cold and inactive "dirty snowball"

The brightness of the comet was measured as visual magnitude V = 28.2,
or nearly 1000 million times fainter than the faintest objects that
can be perceived in a dark sky with the unaided eye.

The pitch black nucleus of Halley reflects about 4% of the sunlight;
it is a very "dirty" snowball indeed. We know from the images obtained
by the ESA Giotto spacecraft in 1986 that it is avocado-shaped and on
the average measures about 10 km diameter across. The VLT observation
is therefore equivalent to seeing a 5-cm piece of coal at a distance
of 20,500 km (about the distance between the Earth's poles) and to do
so in the evening twilight. This is because at the large distance of
Comet Halley, the infalling sunlight is 800 times fainter than here on
Earth.

The measured brightness of the cometary image perfectly matches that
expected for the nucleus alone, taking into account the distance, the
solar illumination and the reflectivity of the surface. This shows
that all cometary activity has now ceased. The nucleus is now an inert
ball of ice and dust, and is likely to remain so until it again
returns to the solar neighbourhood, more than half a century from now.

A record observation

At 28.06 AU heliocentric distance (1 AU = 149,600,000 km - the mean
distance between the Earth and the Sun), this is by far the most
distant observation ever made of a comet [2]. It is also the faintest
comet ever detected (by a factor of about 5); the previous record,
magnitude 26.5, was co-held by comet Halley at 18.8 AU (with the ESO
New Technology Telescope in 1994) and Comet Sanguin at 8.5 AU (with
the Keck II telescope in 1997).

Interestingly, when Comet Halley reaches its largest distance from the
Sun in December 2023, about 35 AU, it will only be 2.5 times fainter
than it is now. The comet would still have been detected within the
present exposure time. This means that with the VLT, for the first
time in the long history of this comet, the astronomers now possess
the means to observe it at any point in its 76-year orbit!

A census of faint Transneptunian Objects

The image of Halley was obtained by combining a series of exposures
obtained simultaneously with three of the 8.2-m telescopes (ANTU,
MELIPAL and YEPUN) during 3 consecutive nights with the main goal to
count the number of small icy bodies orbiting the Sun beyond Neptune,
known as Transneptunian Objects (TNOs). Since the discovery of the
first TNO in 1992, more than 600 have been found, most of these
measuring several hundred km across. The VLT observations aim at a
census of smaller TNOs - the incorporation of the sky field with Comet
Halley allows verification of the associated, extensive data
processing. Similar TNO-surveys have been performed before, but this
is the first time that several very large telescopes are used
simultaneously in order to observe extremely faint, hitherto
inaccessible objects.

The VLT observations will provide very useful information about the
frequency of (smaller) TNOs of different sizes and thereby,
indirectly, about the rate of collisions they have suffered since
their formation. This study will also cast more light on the mystery
of the apparent "emptiness" of the very distant solar system. Why are
so few objects found beyond 45 AU? It is not known whether this is
because there are no objects out there or if they are simply too small
or too dark, or both, to have been detected so far.

How to extract a very faint comet image

The combination of the images from three 8.2-m telescopes obtained
during three consecutive nights is not straightforward. The individual
characteristics of the imaging instruments (FORS1 on ANTU, VIMOS on
MELIPAL and FORS2 on YEPUN) must be taken into account and
corrected. Moreover, the motion of the very faint moving objects has
to be compensated for, even though they are too faint to be seen on
individual exposures; they only reveal themselves when several (many!)
frames are combined during the final steps of the process. It is for
this reason that the presence of a known, faint object like Comet
Halley in the field-of-view provides a powerful control of the data
processing. If Halley is visible at the end, it has been done
properly. The extensive data processing is now under way and the
intensive search for new Transneptunian objects has started.

The field with Comet Halley was observed with the giant telescopes
during each of three consecutive nights, yielding 81 individual
exposures with a total exposure time of almost 9 hours.

The faint comet is completely invisible on the individual images. On
PR Photo 27b/03, these frames have been added directly, showing very
faint stars and galaxies. Also this photo does not show the moving
comet, but by shifting the frames before they are added in such a way
that the comet remains fixed, a faint image does emerge among the
stellar trails, cf. PR Photo 27c/03.

A better, but much more cumbersome method is to "subtract" the images
of all stars and galaxies from the individual exposures, before they
are added. PR Photo 27a/03 has been produced in this way and shows the
image of Comet Halley more clearly. In total, about 20,000 photons
were detected from the comet, i.e. about one photon per 8.2-m
telescope every 1.6 second. However, during the same time, the
telescopes collected about one thousand times more photons from
molecular emission in the Earth's atmosphere within the sky area
covered by the comet's image. The presence of this considerable
"noise" calls for very careful image processing in order to detect the
faint comet signal.

The identity of the comet is beyond doubt: the image is faintly
visible on composite photos obtained during a single night,
demonstrating that the direction and rate of motion of the detected
object perfectly matches that predicted for Comet Halley from its
well-known orbit. Moreover, the image is located within 1 arcsec from
the predicted position in the sky.

Outlook

After its passage in 1910, Comet Halley was again seen in 1982, when
David Jewitt first observed its faint image with the 5-m Palomar
telescope at a time when it was 11 AU from the Sun, a little further
than planet Saturn. It was observed from La Silla two months later. As
the comet approached, the ice in the nucleus began to evaporate
(sublimate), and the comet soon became surrounded by a cloud of dust
and gas (the "coma"). It developed the tail that is typical of comets
and was extensively observed, also from several spacecraft passing
close to its nucleus in early 1986.

Observations have since been made of Comet Halley as it moves away
from the Sun, documenting a steady decrease of activity. When it
reached the distance of Saturn, the tail and coma had disappeared
completely, leaving only the 5 x 5 x 15 km avocado-shaped "dirty
snowball" nucleus. However, Halley was still good for a major
surprise: in 1991, a gigantic explosion happened, providing it with an
expanding, extensive cloud of dust for several months. It is not
known whether this event was caused by a collision with an unknown
piece of rock or by internal processes (a last "sigh" on the way out).

Until now, the most recent observation of Comet Halley was done in
1994 with the New Technology Telescope (NTT) at La Silla, at that time
the most powerful ESO telescope. It showed the comet to be completely
inactive. Nine years later, so does the present VLT observation. It is
unlikely that any activity will be seen until this famous object again
approaches the Sun, more than 50 years from now.

Notes

[1]: The Halley image was obtained during a study of faint
Transneptunian Objects, conducted by a team of astronomers lead by
Olivier Hainaut (ESO-Chile) and including Audrey C. Delsanti
(ESO-Chile and Paris Observatory, Meudon), Daisuke Kino****a
(ESO-Chile and National Astronomical Observatory, Japan) Karen
J. Meech (Institute for Astronomy, University of Hawaii, Honolulu,
USA), Emanuela Pompei (ESO-Chile) and Richard West (ESO-Garching).

[2]: The previous, most distant cometary observation was that of comet
Shoemaker 1987H1 (at 20 AU from the Sun) with the Keck-2 telescope on
Mauna Kea (Hawaii, USA), obtained by Karen Meech and collaborators in
1997. It revealed that this comet - following its first passage ever
near the Sun - still displayed some cometary activity this far away.