There have been a handful of occasions throughout recent history when someone has discovered a comet while looking for, or examining, another one. Such an incident happened in late 1969 in the former Soviet Union, when on September 11 Svetlana Gerasimenko with the Alma Ata Observatory (in what is present-day Kazakhstan) took photographs of Comet 32P/Comas Sola. Klim Churyumov, then with the Kiev Observatory, performed the astrometric measurements of what he believed was that comet, but a month later realized that the positions were far off. Upon re-examining the photographs he found that he had in fact measured a previously-unknown comet that was some two magnitudes brighter than Comet Comas Sola (that was also on the photographs).

Comet 67P achieved fame in the mid-2000s when it was selected as the destination for ESA's ambitious Rosetta mission. Rosetta had originally been scheduled for launch in January 2003 with its destination being Comet 46P/Wirtanen, however a failure of the Ariane 5 launch rocket a month earlier grounded all flights of that vehicle until the necessary failure analysis could be conducted. Comet 67P was accordingly chosen as the new destination.

The approach photographs showed that the comet’s nucleus is made up of two discrete hemispheres, one distinctly larger than the other, making it a “contact binary” similar to several of the near-Earth asteroids that have been successfully radar-imaged (and also the Kuiper Belt object (486958) Arrokoth that was visited by the New Horizons mission at the beginning of 2019). The evidence that has been collected suggests that the two hemispheres were at one time two separate objects that collided and stuck together a long time ago, and based upon the various other objects that exhibit such a structure it would appear that this is a relatively common occurrence amongst the “small bodies” of our solar system.

  LEFT: The inactive nucleus of Comet 67P, showing the two hemispheres of its "contact binary" structure, as imaged by Rosetta on August 3, 2014. Courtesy ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA. RIGHT: Colorized and slightly processed image of Comet 67P's surface, taken by Rosetta on September 22, 2014. Within a year part of the cliff on the lower left had collapsed due to the comet's activity. Courtesy ESA/ Rosetta/MPS for OSIRIS Team/MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/Jacint Roger.

At the time of Rosetta’s arrival Comet 67P was a full year away from perihelion passage, and the overall rationale for the mission was to examine the comet as it approached perihelion and became active, and then as it began to shut down as it receded from perihelion. With an onboard complement of eleven scientific instruments Rosetta was able to conduct numerous scientific studies throughout that time, and among its findings were numerous organic compounds, including four for the first time (one of these being acetone), a deuterium-to-hydrogen ratio in its water that is three times higher than that in Earth’s seawater – the significance of these findings being part of the subject of a future “Special Topics” presentation – large amounts of free oxygen (which was quite unexpected), a lack of a magnetic field, and the appearances of numerous “sinkholes” and the crumbling of cliffs as the comet’s activity proceeded.

Rosetta also carried a separate probe, Philae, that was designed to soft-land upon the comet's nucleus. Rosetta released Philae on November 12, 2014, for a seven-hour descent to the comet's surface, but unfortunately the planned anchoring system failed and Philae bounced twice before coming to rest, apparently on its side in a hole next to some high cliffs. It was nevertheless able to carry out various scientific observations during the 60 hours before its battery power ran out, and while the cliffs were

Rosetta’s mission was formally scheduled to end at the end of 2015, however before that time the mission was extended until the end of September 2016, i.e., a full year after the comet’s perihelion passage and two full years after the spacecraft’s arrival. As that time approached Rosetta was placed into progressively lower orbits, and on September 2 it successfully photographed Philae, on its side and wedged against a large overhanging cliff. On September 30 Rosetta touched down upon the comet’s surface, and contact was terminated.

The large collection of data collected by Rosetta during the two years it spent orbiting Comet Churyumov-Gerasimenko is still being analyzed, and will continue to be for some time yet to come. Just last year, for example, a Spanish amateur astronomer, Jacint Roger, was examining Rosetta images when he found a four-meter-wide moon in images that had been taken on October 21, 2015. This “Churyumoon,” as it was dubbed, was not a permanent feature, but did orbit the nucleus at a distance of 2 to 3 km for the next two days before disappearing.

  LEFT: The active nucleus of Comet 67P on October 21, 2015. The “Churyumoon” is circled. Courtesy ESA/Rosetta/MPS/OSIRIS/UPD/LAM/IAA/SSO/ INTA/UMP/DASP/IDA/Jacint Roger. RIGHT: Comet 67P as imaged on June 24, 2015, from Siding Spring Observatory in New South Wales. Courtesy Jose Chambo.

The viewing geometry during Comet 67P’s 2015 return to perihelion was not especially favorable, with the comet for the most part remaining at a moderately low elongation in the morning sky and reaching a peak brightness of about 12th magnitude. Meanwhile, although it won’t have the fanfare that accompanied this recent return, the comet’s next return, in 2021, is a very favorable one, similar to that of 1982. It passes through perihelion on November 2 and is closest to Earth (0.42 AU) less than two weeks later, and once again should reach a peak brightness around 9th magnitude.

Next Week

Previous Week

"Comet of the Week" archives

Ice and Stone 2020 home page

Earthrise Institute home page