In a significant moment for South African astronomical heritage, the University of the Free State (UFS) received historic photographic plates of comet 55P/Tempel-Tuttle during the 2024 International Astronomical Union (IAU) General Assembly in Cape Town, held from 6 to 15 August. These plates, which were essential in confirming the comet’s rediscovery, trace back to the 1960s when German astronomer Joachim Schubart and international collaborators identified the elusive comet using archival data from Boyden Observatory in Bloemfontein.
The delivery of these plates, facilitated by Dr Markus Pössel, Senior Outreach Scientist at the Max Planck Institute for Astronomy and Director of the IAU's Office of Astronomy for Education, marks the culmination of a cometary saga that spans over 650 years. It also underscores the university’s pivotal role in this extraordinary astronomical journey.
Dawid van Jaarsveldt, Curator of the Boyden Observatory, highlighted that the images were captured using the Metcalf 10-inch telescope, which remains at the Observatory. He says the timing is perfect, as he plans an exhibition featuring the 13 comets discovered at Boyden Observatory, six of which were identified by observer Michiel Bester, who may also have been the observer for the Tempel-Tuttle plates.
Closing a historical chapter in the comet’s journey
Dr Pössel wrote in an article for the Umnyele wezulu, the IAU General Assembly 2024's daily newspaper, that if you have ever watched the shooting stars of the annual Leonid meteor shower, visible every year in November, you have witnessed microscopic remnants of comet 55P/Tempel-Tuttle.
This comet, like others, leaves a trail of dust in its orbit, which causes the Leonids when Earth passes through these debris fields. “The comet itself proved considerably more elusive than the dust it had left behind. After its discovery in 1865/1866 by the Franco-German astronomer Wilhelm Tempel and the American astronomer Horace P. Tuttle, it became lost – the orbital elements were not known with sufficient precision to locate the comet again. In 1867, Giovanni Schiaparelli linked the comet to the Leonids,” Dr Pössel elaborates.
In 1872, British astronomer John Russell Hind suggested that 55P/Tempel-Tuttle might be a periodic comet, similar to those observed in 868 by Europeans and in 1366 by the Chinese. A 1933 analysis by Shigeru Kanda supported the 1366 connection but not the 868 one, says Dr Pössel.
“At that time, it wasn’t possible to make more definite predictions and orbital reconstructions,” Dr Pössel notes. With the advent of electronic computers, German astronomer Joachim Schubart from the Astronomisches Recheninstitut (ARI) began reconstructing the comet’s orbit in the 1960s. Together with Peter Stumpff, he matched the orbit with the 1366 observations and linked it to a 1699 observation by Gottfried Kirch. They predicted the comet’s close approach to the Sun in 1965, but the prediction came too late for optimal Northern Hemisphere observations.
“While today, astronomers routinely share data via the internet, back in the 1960s, standard observations were recorded on photo plates,” says Dr Pössel.
Schubart sought assistance from astronomers in New Zealand, Argentina, and South Africa. Despite initial difficulties, Schubart eventually found traces of the comet on photographic plates from Boyden Observatory and later on plates from the Mount Palomar Schmidt camera in California. These observations confirmed the rediscovery of the 55P/Tempel-Tuttle, writes Dr Pössel.
When Schubart retired, he rediscovered the plates and sought assistance to return them to South Africa, thus closing another chapter in a cometary discovery voyage that involved astronomers from various countries, continents, and generations.