[Overview]
The mission targets a long-period comet (or an interstellar object) to observe the cometary nucleus and coma through simultaneous, multi-point observations. The spacecraft will initially be placed on stand-by at the Lagrangian (L2) point in the Sun-Earth system for a period of up to 3 to 4 years. During this time, ground observations will be conducted to search for previously unknown comets that would be reachable by the spacecraft. When a target is selected, multi-point observations will be performed by flying past the chosen comet and utilising the spacecraft’s multi-probe configuration that will consist of a main spacecraft and two ultra-small probes. JAXA will provide one of the two probes, and observe the comet using the equipped visible-light camera, hydrogen coma imager, and plasma measurement suite (ion mass spectrometer and magnetometer).

[Expected outcome]
Outcome and impact from the project
This is humankind’s first exploration of a long-period comet (or an interstellar object). The comet’s nucleus surface and coma will be imaged from different angles and spectroscopically analysed to clarify properties such as the shape and structure, and the composition of the coma. Simultaneous multi-point observations in the vicinity of the comet will also explore cometary plasma-solar wind interactions. These observations will address two scientific themes in the Solar System science community:
　(1) Understanding the formation of habitable environments in the Solar System.
　(2) Elucidation of the universal laws that govern space gases.
The mission will provide important knowledge that cannot be obtained through alternative Solar System body exploration missions.

Outcome and impact from participation from Japan
The Comet Interceptor mission builds on Japan’s small body exploration program that focusses on understanding the formation of the Solar System through exploration of the Solar System’s primordial celestial bodies. Hayabusa, Hayabusa2 and the DESTINY+ mission are focussed on the inside of the snow line (where water cannot freeze into ice), while Comet Interceptor will extend this to a comprehensive understanding beyond the snow line with the exploration of comets.
This international cooperation additionally utilises the small probe and CubeSat technology that Japan has cultivated, and allows Japan to further enhance its international strength in this area.

[Scientific objectives of the mission]
(1) Science of comet nuclei: investigation of the truly primordial features of comet nuclei
Comets are samples of the planetesimals that formed in the early Solar System. To understand the origin and evolution of planetesimals and the environment of the primitive Solar System, close-up imaging of comets has been carried out by spacecraft. However, past exploration has targeted short-period comets (period < 200 years), where it is difficult to determine which of the characteristics of the nucleus surface were primitive, and which have been acquired during alteration from the comet’s repeated close passes around the Sun.
The Comet Interceptor mission will explore a long-period comet, which has never or rarely approached the Sun before to extract the truly primordial features of cometary nuclei.

(2) Science of the comet coma: investigation of the interaction between the comet coma and the solar wind
A comet forms a coma consisting of dust and gas that is partially converted into a weakly ionised plasma (electrically charged electrons and ions) and interacts with the solar wind that streams from the Sun. In-situ observations have previously been conducted during fly-by surveys to understand the dynamic physical and chemical processes of the coma. However, these past observations have been carried out with a single spacecraft, whose single point of view made it difficult to separate the spatial structure of the plasma from temporal evolution of the phenomenon. The Comet Interceptor will be able to explore the spatial structure and temporal evolution of the comet coma-solar wind interactions through the use of simultaneous multi-point observations made by the three probes.
(Image Credit: Geraint Jones, UCL Mullard Space Science Laboratory)