Twin tails

January 31, 2016
Amateur and professional astronomers alike have been monitoring changes in Comet 67P/Churyumov-Gerasimenko’s tail, which, since December, has been exhibiting two prominent structures.

“Current indications from the data we’re collecting of Comet 67P/C-G is that both features are dust structures,” says professional astronomer Alan Fitzsimmons, who has recently spent time observing the comet with the Isaac Newton Telescope in La Palma.

Multiple tail structures are not uncommon in comets, and indeed have been observed during previous apparitions of Comet 67P/C-G.

The two portions of the tail are attributed to different populations of dust grains swept away from the comet’s nucleus by the radiation pressure of the Sun over the course of its 6.5 year orbit around the Sun.

In the image above, taken with the 2.5m Isaac Newton Telescope on La Palma, the upper ‘streamer’ is precisely aligned along the projected orbit of the comet, implying it is made of large and/or old dust grains moving slowly along the orbit of comet. This part is called a comet dust trail because it is formed from the particles trailing along the path of a comet. This dust trail of Comet 67P/C-G has been seen several times before by both ground-based telescopes and space-based infrared observatories.

The lower portion of the dust tail exhibits a thin central core and resembles a feature called a neckline structure. This is formed from dust grains released on the “opposite side” of the orbit from the time of the observation, and all lining up as seen from Earth. This has also been seen before in previous returns of Comet 67P/C-G.

“We need to do more calculations and modelling, but if our interpretation is correct, then the dust grains forming the neckline in December 2015 were ejected from the comet nucleus around March last year, prior to perihelion,” says Alan.

The dust coma can also be seen all around the comet nucleus, with a peak extending just ahead of the comet, and with a broader tail of smaller dust grains swept out below the neckline.

The amateur community is also acquiring regular images of the comet.

“Because the large telescopes have nowhere near the time-coverage that the amateur/pro-am community have, it’s fantastic to see the amount of valuable data people are collecting,” adds Alan.

For example, the images shown left were taken by Tony Angel and Caisey Harlingten through a 4” telescope of the Searchlight Observatory Network at the Observatorio Sierra Contraviesa, Spain, on 22 December 2015. The image was made by stacking six images taken with an exposure of 300 seconds each in order to bring out deep details of the faint, extended tail structures.

Astronomer Damian Peach has also been keeping a regular eye on the comet as it moves across the sky. The colour composite below captures the comet between September and November last year, showing the comet’s tail and its fuzzy coma. (Click here for an image taken more recently by Damian, on 18 January.)


Comet 67P/C-G between September and November 2015 seen in six LRGB images acquired with a 24" CDK telescope with FLI camera by Damian Peach. Each section is composed of 8 x 120 second exposure images. During this time, the tail is estimated to measure around 20–30 arc mins. Credit: D. Peach.


“Collecting data from Earth while Rosetta is flying alongside the comet is providing a unique and complementary dataset that will help both Rosetta mission scientists and ground-based astronomers understand processes relating to the comet’s activity at a range of scales,” says Rosetta project scientist Matt Taylor. “We’re hoping that ground based images will continue to be obtained until later this summer when the comet gets fainter and too close to the Sun in the sky to observe.”

Astronomers wishing to contribute to the imaging campaign of Comet 67P/C-G can find out more here.

Read more about the amateur observing campaign in our blog post here, and keep up to date with latest images from the PACA (Pro-Am Collaborative Astronomy) project community here.
Read more about the professional observing campaign in recent blog posts here and here.