Helios spacecraft data revisited: Detection of cometary meteoroid trails by in-situ dust impacts

Cometary meteoroid trails exist in the vicinity of comets, forming fine structure of the interplanetary dust cloud. The trails consist predominantly of the largest cometary particles (with sizes of approximately 0.1 mm to 1 cm) which are ejected at low speeds and remain very close to the comet orbit for several revolutions around the Sun. In the 1970s two Helios spacecraft were launched towards the inner solar system. The spacecraft were equipped with in-situ dust sensors which measured the distribution of interplanetary dust in the inner solar system for the first time. 

When re-analysing the Helios data, Altobelli et al. (Astron. Astrophys., 448, 243-252, 2006) recognized a clustering of seven impacts, detected by Helios in a very narrow region of space at a true anomaly angle of 135 +/- 1 degrees, which the authors considered as potential cometary meteoroid trail particles. At the time, however, this hypothesis could not be studied further.

We re-analyse these candidate cometary trail particles in the Helios dust data to investigate the possibility that some or all of them indeed  originate from cometary trails and we constrain their source comets.

The Interplanetary Meteoroid Environment for eXploration (IMEX) dust streams in space model is a new universal model for cometary meteoroid streams in the inner solar system, developed by Soja et al. (Astron. Astrophys., 583, A18, 2015). We use IMEX to study cometary trail traverses by  Helios. 

During ten revolutions around the Sun, the Helios spacecraft intersected 13 cometary meteoroid trails. For the majority of these traverses the predicted dust fluxes are very low. In the narrow region of space where Helios detected the candidate dust particles, however, the spacecraft repeatedly traversed the trails of comets 45P/Honda-Mrkos-Pajdusakova and 72P/Denning-Fujikawa with relatively high predicted dust fluxes. 

The analysis of the detection times and particle impact directions shows that four detected particles are compatible with an origin from these two comets. By combining measurements and simulations we find a dust spatial density in these trails of approximately 10^-8 to 10^-7 m^-3.

The identification of potential cometary meteoroid trail particles in the Helios data greatly benefitted from the clustering of trail traverses in a rather narrow region of space. The in-situ detection and analysis of meteoroid trail particles which can be traced back to their source bodies by spacecraft-based dust analysers opens a new window to remote compositional analysis of comets and asteroids without the necessity to fly a spacecraft to or even land on those celestial bodies. This provides new science opportunities for future space missions like Destiny+, Europa Clipper and IMAP.