The Comet Interceptor Mission

Abstract

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA's F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum $\Delta$V capability of $600{\text{ms}}^{-1}$. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes - B1, provided by the Japanese space agency, JAXA, and B2 - that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission's science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

Keywords: Comets; Instruments – spaceborne and space research; Spacecraft.

Fig. 1

Sketch of the Comet Interceptor flyby, not to scale. Spacecraft A will pass furthest from the nucleus, with probes B1 and B2 passing closer. Both probes will relay their data in real time to be stored on spacecraft A for later transmission

Fig. 2

Classifications of comets by Levison (1996). T refers to the Tisserand parameter, whilst parameter $a$ refers to orbital semi-major axis

Fig. 3

Histograms showing the range of observed comet properties (JFCs in blue and Halley-type/LPCs in grey hashing). The figure provides an overview of the range of measured values and the sample size of each parameter. In case a comet has multiple measurements of the same property, the most recently reported sufficiently precise measurement is displayed. The effective radius histogram is limited to thermal-IR measurements from Fernández et al. (2013) and Bauer et al. (2017). The axis ratios plotted are lower limits for all comets except for those visited by spacecraft. Adapted from Knight et al. (2023)

Fig. 4

A subset of the cometary nuclei that have been visited by spacecraft and on the right an image of Arrokoth, a Kuiper Belt Object, which is a member of the Cold Classical population which is not believed to be a significant source of JFCs. Objects are not shown to scale

Fig. 5

Pits on the surface of comet 67P/Churyumov-Gerasimenko (left). Hathor cliff above the ‘neck’ region between the two lobes of 67P (right). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM

Fig. 6

A water ice-filled depression on the surface of 67P’s nucleus. This image is a false-color composite, where the pale blue patches highlight the presence and location of water-ice. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Fig. 7

Surface morphology changes due to fall-back of dust on the surface of 67P/Churyumov-Gerasimenko. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM

Fig. 8

Typical comet spectrum from ground-based visible observations, with key emission features marked. Major components such as water or CO₂ are not observable and require space missions to be characterised. (Image courtesy of C. Opitom)

Fig. 9

Images showing different morphology of gas jets of different species from Rosetta/OSIRIS (from Bodewits et al. 2016)

Fig. 10

Morphology of different types of dust particles, from summary by Güttler et al. (2019)

Fig. 11

Outburst on the surface of 67P/Churyumov-Gerasimenko observed on 3 July 2016 (left) and on 29 July 2015 (right). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM

Fig. 12

Image from NASA’s EPOXI mission shows part of the nucleus of Comet 103P/Hartley 2. The sun is illuminating the nucleus from the right. A distinct cloud of individual particles is visible, gas release from which is responsible for the high apparent activity level given this nucleus’s size. Image Credit: NASA/JPL-Caltech/UMD

Fig. 13

Multi-point measurements will determine the scale and shape of several structures in the comet-solar wind interaction. The time at which each of the three spacecraft/probes cross (or do not cross) the bow shock (green) and diamagnetic cavity (blue) will determine their shapes and scales. The magnetic field (red) will also be probed using magnetometers on all the three platforms

Fig. 14

The highly-structured ion tail of Comet C/2016 R2 (Pan-STARRS). This particular comet was an atypical dust-poor LPC (e.g. Biver et al. 2018) which reached perihelion at 2.6 au from the Sun. Image: ESO, under license Attribution 4.0 Interactional (CC BY 4.0)

Fig. 15

The remote sensing instruments aboard all three platforms will return complementary views of the nucleus from different directions. Inset images show representative complementary views of the single nucleus from two of the three spacecraft platforms

Fig. 16

CAD/CAM of the CoCa instrument. Left: The Camera Support Unit (CSU). Light coming from the Rotating Mirror Assembly (RMA, Fig. 18) enters the instrument through the baffle (purple cylinder) and is reflected by the four mirrors of the telescope (yellow) onto the filter wheel assembly (Fig. 17) Right: The Electronics Unit (ELU). Centre: The Proximity Electronics Unit (PEU)

Fig. 17

The CoCa filter wheel assembly with four filters. The detector is shown in pink below one of the filters. The filter wheel includes a launch lock to prevent motion during launch

Fig. 18

CAD/CAM drawing of the RMA showing the opening (top) and the fold mirror mount (turquoise colour) which rotates and reflects light toward CoCa (left). The mounting feet (yellow colour) attach the RMA to the exterior of the spacecraft while the entire CoCa instrument (Fig. 16) is inside with their optical axes aligned

Fig. 19

MIRMIS TIRI/MIR/NIR mounted on a common optical bench (548.5 × 282.0 × 126.8, in mm)

Fig. 20

MANiaC consisting of a time-of-flight mass spectrometer (SHU, Sensor Head Unit), the Neutral Density Gauge (NDG), and the ELectronic Unit (ELU). For reference the long axis of the SHU corresponds to ∼470 mm. Only the antechamber spheres of both the NDG and the SHU (marked yellow) are exposed to the gas and dust flow of the coma and are covered by dedicated dust shields. The rest is enclosed and protected inside the spacecraft

Fig. 21

DISC Unit. Left: assembled DISC breadboard. Right: Sensing plate with glued PZTs at 3 corners

Fig. 22

Integration of the FGM-A sensor within the COMPLIMENT merged probe

Fig. 23

The SCIENA fully operational Technology Model, excluding some thermal hardware

Fig. 24

LEES instrument CAD model

Fig. 25

Design overview of HI

Fig. 26

(a) PS structure including electronics boxes and (b) a magnetometer breadboard model

Fig. 27

Components of the NAC/WAC system

Fig. 28

EnVisS instrument present mechanical layout (Courtesy of Leonardo SpA-IT)

Fig. 29

OPIC engineering model with internals exposed

Fig. 30

Picture of its sensing elements (left) and CAD rendering (right) of one FGM-B2 sensor

Fig. 31

Empirical CDFs of LPCs with perihelion <2 au

Fig. 32

Sample transfer to SEL2 quasi-Halo orbit and waiting phase in the Sun-Earth rotating frame

Fig. 33

Reachable $R_c$-$\theta$ regions with 750 m/s for sample transfer times of 1 year (top) and 3 years (bottom)

Fig. 34

Example geometry of sample transfers to comet 26P, which was an original backup target for the mission. a) Projection on Sun-Earth rotating frame. Solid blue line: minimum $\Delta$V; dashed orange line: minimum time of flight. b) Zoom of SEL2 departure. c) Distances to Earth and Sun; dots every 15 days; 6 months post-encounter phase

Fig. 35

Influence of $\Delta$V and mission duration on the probability of at least one LPC target. Time between launch and target detection <2 years, mission duration includes 6 months post-encounter phase

Fig. 36

Statistics of time parameters relevant for the mission. a) Shortest wait at SEL2; b) Longest wait at SEL2; c) Transfer time from SEL2 to comet encounter; d) Duration from launch to comet encounter

Fig. 37

Relative encounter speed (left) and flyby solar aspect angle (right) for reachable LPCs (0.7 au < Rc < 1.3 au)

Fig. 38

Allowed regions of relative velocities and flyby solar aspect angles

Fig. 39

Timeline of operations for the comet flyby

Fig. 40

Summary of backup target selected as a function of the launch date. As the launch date at the time of writing is in Q4 of 2029, 26P is no longer reachable

Fig. 41

Comet Interceptor spacecraft A and probe B2 design concepts (OHB)

Fig. 42

Comet Interceptor probe B1 after separation from spacecraft A (JAXA)