The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission

Abstract

We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of the mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively 'well-behaved' comet, typical of Jupiter family comets and with activity patterns that repeat from orbit to orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends-in this paper, we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies.This article is part of the themed issue 'Cometary science after Rosetta'.

Keywords: Rosetta; comet 67P/Churyumov–Gerasimenko; observations.

Figure 1.

Observability of the comet, as seen from Earth, during the Rosetta mission. The observability of the comet from Earth is shown by hatched, cross-hatched and solid grey areas marking when the solar elongation is less than 50°, 30° and 15°, respectively. Perihelion (in August 2015) is marked by a vertical dashed line. At that time the comet was 43° from the Sun. Dash-dot vertical lines show the boundaries between the years 2014 and 2016. Upper panel: Solar elongation ϵ (solid line) and phase angle α (dashed line); middle panel: declination (Dec); lower panel: heliocentric r (solid line) and geocentric Δ (dashed line) distances.

Figure 2.

Map of locations of contributing observatories. (Online version in colour.)

Figure 3.

R-band images of the comet, 1 arcminute on each side. Arrows indicate the direction of the orbital velocity (v) and the Sun (⊙) directions, i.e. opposite the expected direction of the dust trail and ion tail, respectively. Image dates, telescopes and exposure times: 2014/02/27, VLT/FORS, 10×50 s; 2014/07/01, VLT/FORS, 31×50s; 2014/10/22, VLT/FORS, 39×50s; 2015/05/21, VLT/FORS, 2×30s; 2015/07/18, LT/IO:O, 10×20s; 2015/10/07, LT/IO:O, 9×15s; 2016/01/10, LT/IO:O, 3×120s; 2016/03/10, LT/IO:O, 14×180s; 2016/06/03, LT/IO:O, 3×180s. May 2015 image shows reflection from a bright star out of the field of view (above comet).

Figure 4.

Wide field image taken with the 2.5 m INT in March 2016, showing the long trail (approx. 2°).

Figure 5.

Wide field image taken with the 8 m Subaru telescope and the Hyper Suprime-Cam, taken on 2016/03/08 when the comet was at 2.5 AU from the Sun. (a) Full field of view of the Hyper Suprime-Cam (single frame), showing the region containing the comet. (b) Extracted comet region (62.3×14.2 arcmin), total of 10 × 6 min exposures stacked. (c) Same images median combined after shifting to account for comet motion.

Figure 6.

(a) ‘Jets’ in the coma (labelled J1, J2), as seen from the 6 m BTA telescope of the SAO (Russia), on 2015/11/08. Image is approximately 100 000 km across. (b) Enhanced Gemini NIRI J-band images of the comet monthly from August 2015 through to January 2016 (date give as YYMMDD). Images are centred on the comet and an azimuthal median profile has been subtracted to reveal the fainter underlying structure. At times (August, January) two distinct structures can be discerned that match those labelled J1, J2 in the SAO image while at other times they overlap to appear as a single larger structure towards the southeast. All images have the same colour scheme with red/orange bright and blue/purple/black faint, but different colour scales. Each image is 50 000 km on a side and has north up and east to the left. The Sun and the direction of the comet’s orbital velocity are towards the southeast in all panels, and do notchange significantly over this period (figure 3). The red blob within a few pixels of the centre in all panels is an artefact of the enhancement; trailed stars can be seen as streaks in August, October and January. (Online version in colour.)

Figure 7.

Spectrum taken with the blue arm of ISIS on the WHT, on 2015/08/19, with the comet just past perihelion. The narrow red lineshows a scaled solar analogue (i.e. the continuum/dust signal) for comparison. The strongest emission bands are identified. (Online version in colour.)

Figure 8.

Spitzer/IRAC images of the comet at 3.6 and 4.5 μm (a,b). The CO₂ coma (c) is apparent after the 3.6 μm image (dust) is subtracted from the 4.5 μm image (dust and gas). Celestial North (N), the projected orbital velocity (v) and the projected direction of the Sun (⊙) are marked with arrows. Each image is approximately 200 000 km on a side. (Online version in colour.)

Figure 9.

Distribution of linear polarization (P, %) in comet 67P ((a) coma and tail; (b) zoom in on coma). Observation obtained with the SAO 6 m telescope on 2015/11/08. (Online version in colour.)

Figure 10.

Total R-band magnitude of the comet, compared with prediction from previous orbits (solid line). Solar elongation is indicated with hatching as in figure 1. (Online version in colour.)

Figure 11.

CN production rate (in molecules s⁻¹) as a function of heliocentric distance (negative values indicate pre-perihelion data, positive post-perihelion). We include data from previous orbits [3,7,12,73], narrowband photometry from TRAPPIST and the Lowell 1.1 m around perihelion, and spectroscopy from the VLT, SAO 6 m and LT (see key in plot for symbols). Error bars are not included for clarity. The dashed line shows a scaled version of the dust scaling law plotted in figure 10. In general, pre-perihelion data are only upper limits (marked with arrows) until relatively low r, when the rate climbs quickly pre-perihelion, with a similar slope to the dust fit. CN emission can be detected to larger distance post-perihelion, with a shallow decrease in Q(CN). Datasets from different telescopes mostly agree, although the LT production rates post-perihelion are generally lower than those measured with larger telescopes, but with significant scatter. This behaviour appears to be seen in data from previous orbits too, with little change in total CN production (there appears to be a reasonable match between the ‘previous’ points and those taken in this campaign). (Online version in colour.)