A unique astronomical anniversary for the ARI: The Return of Comet Longmore

A comet discovered by an astronomer over 40 years ago has been rediscovered by his son at LJMU and photographed at its closest approach to earth in seven years.

Comet Longmore was discovered in 1975 by Andrew Longmore, the father of LJMU astronomer Dr Steven Longmore based at the Astrophysics Research Institute (ARI). It’s a so-called ‘short-period comet’, a ball of dirty ice that follows an elongated path as it orbits around the sun, only coming close enough to be visible for a few months each orbit - about every 7 years in the case of Longmore. Since short-period comets can take anywhere from a few years to a couple of centuries to complete each orbit, Longmore is a relatively quick one, but still a rare visitor.

Although the comet has returned to the inner solar system near us and the sun several times since 1975, this is the first time Dr Steven Longmore has had the opportunity to see the section of the solar system named after his dad Andrew.

Due to the unique occasion, the Liverpool Telescope was utilised, perfect for catching such fleeting events, and on the night of 21st May a set of observations were taken with the help of Professor Andrew Newsam that clearly show the fuzzy comet and its 'tail' of dust and gas boiling off the surface and being blown off into space by the radiation of the sun. Steven was then able to send the images of the 77 P/Comet Longmore to his dad to remind him of his discovery.

The comet was one of Andrew Longmore’s earliest discoveries during his first ever research position in the mid 1970s as an astronomer at the UK Schmidt Telescope at Siding Spring Observatory in Australia. The telescope was largely commissioned to do a Southern Sky Survey to complement  the earlier northern survey done by the Palomar Schmidt Telescope.

Andrew Longmore, who retired from the Astronomy Technology Centre at the Royal Observatory Edinburgh a few years ago, said of his early years as an astronomer: “The primary role of the observing staff was to take the 6.25 degree field-of-view photographs, develop them, do daily quality control inspections and to archive them. A lot of exciting discoveries were made during the quality control stages, which was always done as soon as possible after the plates were taken, usually the next day. It involved manually scanning the plates using a small hand-held magnifier while they were mounted on a light table. I spotted the comet on one of these inspections. It was a fun job, so much so that one of the very last things I did at the Schmidt - following up an X-ray burst confirmation - nearly made me late for my wedding."

As for the comet, Andy added: “The Anglo-Australian Telescope (AAT) had at that time just been completed on the same site, so we were able to take confirmation images very quickly. I have never seen the comet since then. And then recently it came to my attention that it was at a relatively close approach to earth. I mentioned this to my son, and a short while later - bingo, a link to several beautiful clear images of the comet appears in my inbox. After nearly 40 years, I am absolutely delighted with this and impressed that the Liverpool Telescope, operated remotely, got such a good picture so close to mid-summer and taken with the moon quite full.”

Dr Steven Longmore said about the comet: “I was thrilled to have the chance to re-observe and actually see a part of our solar system that my dad had discovered decades ago, especially given the challenging observing conditions so close to mid-summer with the moon relatively full and at only a small angle from the comet.”

Did you know:

The digitised versions of the Schmidt images were, amongst other data, later used to create Google Sky. It was Google Sky that was used to identify the star field around the Liverpool Telescope Comet Longmore image.

Other 'fortuitous' discoveries made thanks to quality control scanning included planetary nebulae, interacting and peculiar galaxies, and minor planets. Occasionally the practice is able to confirm novae or supernovae, including X-ray bursts, either with pre-outburst images or post-discovery checks. 

The P in 77/P Comet Longmore stands for Periodic, which means that the comet's closest approach to the sun happens at a regular interval, in this case every 7 years. The 77 refers to the fact it was the 77th periodic comet that was discovered.

A comet is typically a 'ball' off rocky material held together by frozen gases and water ice. Usually, unless it is significantly closer to the sun than is Jupiter, its appearance in the sky is not much different from an asteroid, so would just appear as a line on a picture taken to track stellar images.  However as soon as it is close enough to the sun for the ices to vapourise, the comet's image becomes fuzzy and can eventually develop substantial tails. Often the tail splits, with a part formed by scattered sunlight from ejected dust mostly following the path of the comet (but also affected by the sun's radiation pressure).  The direction of any ionised gas tail is instead controlled by  the solar wind and local magnetic fields. This loss of material from a periodic comet means that it has a finite lifetime. Their orbits are also unstable because of the influences of planets, so they can be ejected from the solar system or drawn into the sun. Don't worry though, it is thought that there is a reservoir of billions of substantial-sized comets in the outer reaches of the solar system, and occasionally our stock is replaced by one of these whose orbit has been perturbed as well.

Photo credit: 

The image 77/P Comet Longmore reproduced with kind permission by Professor Andrew Newsam and Dr Steven Longmore of the ARI, LJMU using the Liverpool Telescope. The comet itself is in the centre and has visible Œtail¹.