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.

PR Photo 27a/03: VLT image (cleaned) of Comet Halley
PR Photo 27b/03: Sky field in which Comet Halley was observed
PR Photo 27c/03: Combined VLT image with star trails and Comet
Halley

The Halley image

ESO PR Photo 27a/03

Caption: PR Photo 27a/03 shows the faint, star-like image of
Comet Halley (centre), observed with the ESO Very Large
Telescope (VLT) at the Paranal Observatory on March 6-8, 2003.
81 individual exposures from three of the four 8.2-m VLT
telescopes with a total exposure time of about 9 hours were
combined to show the magnitude 28.2 object. At this time,
Comet Halley was about 4200 million km from the Sun (28.06 AU)
and 4080 million km (27.26 AU) from the Earth. All images of
stars and galaxies in the field were removed during the
extensive image processing needed to produce this unique
image. Due to the remaining, unavoidable "background noise",
it is best to view the comet image from some distance. The
field measures 60 x 40 arcsec^2 ; North is up and East is
left.

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

ESO PR Photo 27b/03 ESO PR Photo 27c/03

Caption: PR Photo 27b/03 shows the sky field in which Comet
Halley was observed with the ESO Very Large Telescope (VLT)
at the Paranal Observatory on March 6-8, 2003. 81 individual
exposures with a total exposure time of 32284 sec (almost 9
hours) from three of the four 8.2-m telescopes were cleaned
and combined to produce this composite photo, displaying
numerous faint stars and galaxies in the field. The predicted
motion of Comet Halley during the three nights is indicated
by short red lines. The long straight lines at the top and
to the right were caused by artificial satellites in orbit
around the Earth that passed through the field during the
exposure. The field measures 300 x 180 arcsec^2.

PR Photo 27c/03 was produced by adding the same frames, however,
while shifting their positions according to the motion of the
comet. The faint, star-like image of Comet Halley is now visible
(in circle, at centre); all other objects (stars, galaxies) in
the field are "trailed". A satellite trail is visible at the
very top. The field measures 60 x 40 arcsec^2 ; North is up and
East is left in both photos.

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 Kinoshita
(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.