Remember Comet Halley — the famous “hairy star” that has been observed with great regularity once every 76 years? 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 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, currently 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 its orbit. It will reach that turning point — called its aphelion — in December 2023. Then it will be headed on the long return trip back 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.
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 four percent of the sunlight falling on it, which makes it a very “dirty” snowball indeed. We know from the images obtained by the ESA Giotto spacecraft in 1986 that Halley is avocado-shaped and on the average measures about 10 km across. The VLT observation is therefore equivalent to seeing a 5-cm piece of coal at a distance of 20,500 kilometers during 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.
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. 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!
This entry is based on a press release sent out from the European Southern Observatory. If you’re interested in more information about this observation, point your browser to: Comet Halley release.