A possible selection criterion of the target of Comet Interceptor mission

The ESA mission Comet Interceptor will target an Oort Cloud or interstellar comet during its first approach to the heliocentric distance rh≈1 au, and has been designed to handle the dust loss rate of 1P/Halley as an upper limit of activity level. In the framework of the adopted model of pebble-made nuclei, any extrapolation at rh≈1 au of the dust loss rate observed beyond Jupiter is impossible, because the dust loss rate at rh>3.8 au depends on the nucleus surface area, Ap, composed of water-poor pebbles, whereas at rh<3.8 au the dust loss rate depends on the nucleus surface area, Ar, composed of water-rich pebbles. Pebble-made nuclei are characterized by all possible values Ap/Ar⩾12. The water-driven activity model of pebble-made nuclei has been applied to comets 1P/Halley and C/1995O1 Hale-Bopp. We find that all comets with a nucleus radius smaller than 3.2-0.9+2.3 km will be characterized by a dust loss rate lower than 1P/Halley at rh≈1 au, i.e. will be safe for Comet Interceptor, whichever their ratio Ap/Ar⩾12 will be. We also find that the lower limit of the average refractory-to-water–ice mass ratio of all comets is 3.4.