Simulated Gravity Field Estimation for the Main Belt Comet 133P/Elst-Pizarro Based on a Satellite-to-satellite Tracking Mode

Most asteroids and comets are formed in the early stages of the solar system and therefore contain a wealth of information about their birth.The asteroid exploration mission planned in the coming years by China will likely target the celestial body named 133P/Elst-Pizarro(estimated diameter of about 4 km).The orbit of this asteroid stays within the asteroid belt,but nevertheless,it displays a comet-like dust tail.In this study,we used differential tracking data between two simulated probes and the data from an Earth station to estimate 133P gravity field model.This observation mode is similar to how the gravity field was estimated for large celestial objects in the GRAIL and GRACE missions,but here the object is the very small 133P asteroid.We compared the estimated gravity fields obtained for 133P from the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode,with only the Earth-based two-way range rate mode.The results show that the accuracy of the low-degree(4 degree and order)estimate of the gravity field is improved by one order of magnitude by using the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode with respect to the Earthonly tracking.Furthermore,another order of magnitude improvement in the gravity field solution is gained by decreasing the orbit altitude from 12 to 8 km.