The Entire Visible Sky (EnVisS) instrument for the Comet Interceptor ESA mission: an update

Comet Interceptor is the first Fast (F-class) mission in the European Space Agency (ESA) Cosmic Vision program and has been conceived to study a long-period comet. The mission concept includes a spin-stabilized probe venturing close to a yet-to-be selected, and possibly dynamically new, comet. On this probe, the Entire Visible Sky (EnVisS) camera will be hosted. EnVisS will address several fields of cometary science by carrying out observations, close to, and within, a comet’s coma. Both intensity and polarimetric measurements are foreseen. In this work, the up-to-date instrument concept, design and scientific capabilities of EnVisS will be presented.

1) Introduction

The Comet Interceptor spacecraft mission configuration includes a spacecraft (called A) and two probes (called PB1 and PB2). Spacecraft A will carry on remote and in-situ observations of the target from afar; while PB1, by the Japan Aerospace Space Agency (JAXA), and PB2, by ESA, will perform close fly-bys of the target [1].

PB2 spin-stabilized solution allows EnVisS to adopt a rotational push-broom or push-frame imaging technique to scan and image the whole environment around the probe. The filter strip assembly [2] mounted as close as possible to the detector grants the possibility to observe and perform polarimetric imaging in the visible range.

EnVisS will map the intensity and the degree of linear polarization and polarization angle orientation of the light scattered by the dust particles in the comet coma with an extended phase angle coverage. Linear polarization is directly linked with dust size distribution, morphology, porosity and composition [3]. Monitoring linear polarization will provide insights into how these parameters correlate.

2) Instrument concept

The EnVisS instrument works in the visible wavelength range from 550-800 nm. To acquire the full sky, it features an extremely wide Field of View (FoV).

EnVisS adopts a flexible push-broom/push-frame imaging technique: as the probe rotates (see Figure 1 ), slices of the sky are acquired and later stitched together on-ground to reconstruct a full-sky image.

Figure 1: In (a) placement of the EnVisS camera on the B2 probe. In (b) illustration of EnVisS full sky imaging scanning concept. In (c) schematics of the filter strips images on the 2k x 2k detector.

 

The probe spin-axis will point to the comet nucleus for most of the time, except at the closest approach when the comet nucleus will fall inside the camera FoV (see Figure 1 (a) for the EnVisS placement on PB2).

3) EnVisS: Instrument Design

The EnVisS instrument consists of different parts (see Figure 2 ):

• a fish-eye optical head [4];
• a commercial space-qualified detector package from 3D-Plus [5] equipped with an ad hoc filter strip package (FSA) [6];
• ad-hoc electronics (power and data handling units);
• software.

Figure 2: In (a) EnVisS CAD model. In (b) Optical head and camera components are highlighted [4].

 

The FSA contains three broad-band filters, all working in the same wavelength range (i.e., 550-800 nm):

• one broadband intensity filter centred on the detector (see Figure 1 (c) blue central strip I);
• two linear polarizing filters with transmission axis angles oriented at 45° one to the other; positioned on the sides (see Figure 1 (c) the red and yellow strips P1 and P2).

A flexible approach is considered to achieve the required SNR. Depending on the target object activity, the map of the coma will be taken with different spatial resolutions, i.e. smearing and pixel binning.

Along track, the signal from the coma is not expected to vary too much, high spatial resolution is not required and smearing can be tolerated. The spatial resolution is retained in the across-track direction and ensures a sampling of the comet phase function every 0.2°. This strategy will also allow for an adjustment of the exposure time if the radiance of the coma differs from expected.

Further pixel binning on-board, or co-adding, on-ground, of the images over different rotations, could be considered if the signal is extremely low.

 

Acknowledgements

This work has been supported by: the Italian Space Agency (ASI) through contracts to the Istituto Nazionale di Astrofisica (2020-4-HH and 2023-14-HH.0), and the European Space Agency (ESA) through a Contract to the Italian National Research Council (CNR) (Contract n. 4000136673/21/NL/IB/ig); and Instituto de Astrofísica de Andalucía (IAA-CSIC, Granada, Spain) with SENER (Barcelona, Spain) being supported by the Spanish Ministerio de Ciencia e Innovación (MCIN) through ESA PRODEX and the Spanish National Plan Ref PID2021-126365NB-C21 respectively.

 

References

[1] Jones, et al., “The Comet Interceptor Mission”, Space Sci Rev 220, 9 (2024).

[2] Naletto et al., “Characterization of the polarizing filters for the EnVisS camera”. Proc. SPIE paper 13092-225 (2024).

[3] Fulle, A.C. Levasseur-Regourd, N. McBride, E. Hadamcik "In situ dust measurements from within the coma of 1P/Halley, The Astronomical Journal, 119:1968-1977, (2000).

[4] Tofani, et al., “Design of the EnVisS instrument optical head”, SPIE Proc. 12777, International Conference on Space Optics — ICSO 2022; 127772P (2023).

[5] https://www.3d-plus.com/

[6] Nordera, et al., “Ghost analysis of the EnVisS camera for the Comet Interceptor ESA mission”, SPIE Proc. 12180, 1218036 (2022).