Design and execution of the asteroid mathilde flyby

Abstract

On June 27, 1997, the Near Earth Asteroid Rendezvous (NEAR) spacecraft flew within 1200 km of mainbelt asteroid 253 Mathilde. This first-ever flyby of a C-type asteroid produced high resolution images of Mathilde's surface and an accurate determination of the mean density of its interior. We describe the essential steps that were needed to accomplish these objectives with a spacecraft designed not for fast flybys, but rather for orbital operations at NEAR's ultimate goal, asteroid 433 Eros. Principal challenges included carrying out the flyby in an orientation in which the solar panels received adequate sunlight to power the spacecraft and the camera, while at the same time pointing the whole spacecraft to ensure that the asteroid was captured in the camera's field of view. Making use of approach imaging to refine the estimate of Mathilde's position relative to the spacecraft near closest approach still left an area of the sky some 20 times the camera's field of view to be scanned to capture the highest resolution images. Unlike the Galileo spacecraft which previously carried out flybys of S-asteroids 954 Gaspra and 243 Ida, NEAR does not have a scan platform, and the entire spacecraft must be slewed to change the camera's field of view.