Coma for Halley’s Comet

Distant Halley
The faint, star-like image of Comet Halley (center, inside pink circle), 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 arcsec2; North is up and East is left. Credit: ESO

Remember Halley’s Comet – 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). 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 incoming 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 astronomical units 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. 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 Very Large Telescope, for the first time in the long 2000-year observational history of this comet, the astronomers now possess the means to watch it at any point in its 76-year orbit!

A census of faint Transneptunian Objects

halley_comet
Halley’s Comet becomes visible to the unaided eye about every 76 years as it nears the sun. Credit: Lick Observatory

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. When these frames are added directly, the composite shows very faint stars and galaxies. Also this addition 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.

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. 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.

What’s Next

Extreme Explorers' Hall of Fame
This artist’s impression shows the Rosetta Lander anchored to a comet’s surface with instruments, legs and solar panels. Comet-chasing mission Rosetta has refocused its sights on Comet Churyumov-Gerasimenko. The spacecraft will be launched in February 2004 from Kourou, French Guiana, using an Ariane-5 G+ launcher. The rendezvous with the new target comet is expected in November 2014. Credit: ESA 2001. Illustration by Medialab

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.

In the next 5 or so years, there will be multiple encounters of spacecraft with comets and asteroids. In May 2003, a Japanese probe [called Muses-C] lifted off on the world’s first mission to collect samples from the surface of an asteroid, part of a four-year journey covering nearly 400 million miles.

On Jan. 2, 2004, the spacecraft called Stardust will fly within 75 miles of a cometary main body (called Wild-2), close enough to trap small particles from the coma, the gas-and-dust envelope surrounding the comet’s nucleus. Stardust will be traveling at about 13,400 miles per hour and will capture comet particles traveling at the speed of a bullet fired from a rifle. Launched in February 1999, Stardust was designed to capture particles from Wild 2 and return them to Earth for analysis. The spacecraft already has collected grains of interstellar dust. It is the first U.S. sample-return mission since the last moon landing in 1972.

All the following missions are slated:

2001 Sept. 22 Comet Borrelly Deep Space One (simple flyby)  
2004 Jan. 1 Comet Wild 2 Stardust (coma sample return)
2005 July 3 Comet Tempel 1 Deep Impact (big mass impact)
2005 Sept. Asteroid 1998 SF36 Muses-C (sample return)

The first 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). 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.