The Rosetta mission ended more than three years ago. After orbiting comet 67P/Churyumov-Gerasimenko for more than two years Rosetta softly crash-landed on the nucleus on Sept. 30, 2016. However, the data from this mission still hide many gems for cometary science.

A plume of dust from Comet 67P/Churyumov–Gerasimenko, seen by the OSIRIS Wide Angle Camera on ESA's Rosetta spacecraft on 3 July 2016. The shadow of the plume is cast across the basin, which is in the Imhotep region. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

In the present paper we show results which are due to a very lucky event, which could have gone terribly wrong. The ROSINA instrument, two mass spectrometers and a pressure sensor measure densities and composition at the location of the spacecraft. As such it is clear that the closer we were to the comet, the higher our signal to noise ratio. On the other hand, Rosetta could not cope with too much dust in its vicinity as it determined its attitude with the help of star trackers which can get confused by dust grains.

On this star tracker image only the 5 red marked dots are real stars, all the other points are cometary dust.

This meant that the spacecraft had to retract from the comet during perihelion, from 10 km early in the mission to >300 km during perihelion. And it meant that the density at the spacecraft location remained more or less constant for all of this time. Two months before the planned end of mission the operation team at ESOC was willing to take some more risks, the spacecraft was put in 3-day elliptical orbits around the comet with the pericenter slowly being lowered while maintaining the overall size and period of the ellipses. On Sept 5, 2016 it reached its lowest ever distance from the comet nucleus, just 1.9 km above surface. The day started badly with a non-recoverable science data loss due a synchronization error for the antennas on Rosetta and on Earth. Shortly after 16h the communication link could be reestablished. Only an hour later the signature in the pressure sensor showed a very dusty environment and shortly after 18h the current of our filament used for ionizing particles showed a sharp increase, way above normal, which was possible evidence for a failure.. And exactly at that time, the signal was lost again as the antenna link was terminated due to the Earth rotation. Here we were: a very dusty environment, never seen before, 1.9 km from the nucleus, with an instrument where we didn’t know if it would survive. A call to ESOC did not make it better: they were aware of the situation as the star trackers also suffered from the dusty environment and they also no longer received any signal from Rosetta. We didn’t know if we still had an instrument, they didn’t know if they still had a spacecraft. We had to wait untill morning when the spacecraft was supposed to reestablish the communication link. I can assure you: none of us slept well during this night, neither the ROSINA team nor the Rosetta operation team. 

Diversity of comet dust particles seen on a small area on one single target of the COSIMA instrument onboard Rosetta. This image measures 2.5 mm across, with light coming from the right. Examples of compact particle (a), shattered cluster (b), a glued cluster (c) and a large rubble pile (d) are seen in this small area.

• ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S Langevin et al (2016

  Copyright: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S/ Langevin et al (2016)

Rosetta and ROSINA did survive! They behaved perfectly, Rosetta keeping a steady attitude and ROSINA measuring flawlessly and providing the most abundant and most diverse mass spectra, revealing many species not seen any time before. Thanks to a direct hit into the ion source by dust, which almost but not quite killed the filament, we saw the signatures of many of the not very volatile species as the ion source is warm compared to the cometary environment.

The most striking effect was the increase of NH₃ by a factor 100 and this increase remained for several hours. Discussion within the community, especially with laboratory astrochemists, led to the idea of ammonium salts as the cause for the ammonia increase. After considerable laboratory work by Nora Hänni (Ammonium Salts as a Source of Small Molecules Observed with High-Resolution Electron-Impact Ionization Mass Spectrometry by Hänni N. et al) on ammonium salt sublimation, we had a solid basis for our conclusions.

NH₄Cl salt used in the lab

Nora Hänni, our chemist, with the ROSINA flight spare instrument in the Background.

It means we now have a new animal in our existing zoo of molecules found in the coma of 67P. The ROSINA team decided that a salt water crocodile represents these salts best. And the nice result is that this detection probably solves the question of the depleted nitrogen in comets, found more than 30 years ago when the European Giotto mission encountered comet Halley.