Navigation and Flight Mechanics Section at the Jet Propulsion Laboratory

The terminal navigation of the NEAR spacecraft during its close flyby of asteroid 253 Mathilde involved coordinated efforts first to determine the heliocentric orbits of the spacecraft and Mathilde and then to determine the relative trajectory of the spacecraft with respect to Mathilde. From ground-based optical observations alone, the error of Mathilde’s ephemeris at the time of encounter was less than the size of the asteroid itself. Optical navigation images of Mathilde made from the NEAR spacecraft were utilized to reduce the impact plane targeting uncertainty to less than 9 kilometers at closest approach and to less than 4 seconds in the time of closest approach. The gravitational perturbation of Mathilde upon the passing spacecraft was apparent in the spacecraft Doppler tracking data. Asa result of the accurate targeting achieved, this tracking data could be used to detern]ine Mathilde’s mass as (1.024 +/0.058) x 1020 g. Coupled with a preliminary volume estimate provided by the NEAR imaging team, this mass suggests a surprising low bulk density for Mathilde of 1.3 g/cm3 and a relatively high porosity of over 50%.