Distant activity of 67P/Churyumov-Gerasimenko in 2014: Ground-based results during the Rosetta pre-landing phase

Abstract

Context. As the ESA Rosetta mission approached, orbited, and sent a lander to comet 67P/Churyumov-Gerasimenko in 2014, a large campaign of ground-based observations also followed the comet. Aims. We constrain the total activity level of the comet by photometry and spectroscopy to place Rosetta results in context and to understand the large-scale structure of the comet's coma pre-perihelion. Methods. We performed observations using a number of telescopes, but concentrate on results from the 8 m VLT and Gemini South telescopes in Chile. We use R-band imaging to measure the dust coma contribution to the comet's brightness and UV-visible spectroscopy to search for gas emissions, primarily using VLT/FORS. In addition we imaged the comet in near-infrared wavelengths (JHK) in late 2014 with Gemini-S/Flamingos-2. Results. We find that the comet was already active in early 2014 at heliocentric distances beyond 4 au. The evolution of the total activity (measured by dust) followed previous predictions. No gas emissions were detected despite sensitive searches. Conclusions. The comet maintains a similar level of activity from orbit to orbit, and is in that sense predictable, meaning that Rosetta results correspond to typical behaviour for this comet. The gas production (for CN at least) is highly asymmetric with respect to perihelion, as our upper limits are below the measured production rates for similar distances post-perihelion in previous orbits.

Searching for water ice in the coma of interstellar object 2I/Borisov

“…As discussed previously in section 3.2, unlike the spectrum of 1I, the spectrum of 2I is not from a bare nucleus but is dominated by the coma surrounding the nucleus. For the case of comet 67P, Snodgrass et al (2016) noted that the spectral slopes of the coma spectra, obtained with ground-based facilities, are consistent with the values found for the nucleus by Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer. As such, it is not surprising that the two ISOs show comparable spectra and are both lacking the ultra-red matter.…”

The Main Belt Comets and ice in the Solar System

“…The first detection of the gas coma was through sub-mm observations of the 557 GHz water band by the MIRO instrument, in June 2014 at 3.9 au from the Sun, when the spacecraft was around half a million km from the comet . At this time the water production rate was slightly below the strongest limits on MBCs to date, at 1 × 10 25 mol s −1 , and outgassing was not detectable from Earth (via sensitive searches for CN with large telescopes- Snodgrass et al 2016;Opitom et al 2017). This is the lowest activity comet environment ever visited by a spacecraft, with all previous mission performing fast flybys at ∼ 1 au, and there were some surprising results: the Rosetta Plasma Consortium instruments discovered oscillations in the magnetic field at around 40 mHz attributed to interactions with cometary ions and the solar wind (Richter et al 2015), the so-called 'singing comet' based on the public release of an audio version of this interaction.…”

Gemini and Lowell observations of 67P/Churyumov−Gerasimenko during the Rosetta mission

“…Preliminary results from this observing campaign have already been published in synopses of 67P's distant activity as observed with large telescopes (Snodgrass et al 2016) and of the worldwide ground-based observation campaign ). Here we present comprehensive analysis of our observations.…”