The NEAR flyby of Mainbelt Asteroid 253 Mathilde: Science Objectives and Encounter Strategy

A. Harch, J. Veverka, J.F. Bell, C. Chapman, M. Malin, L.A. McFadden, S. Murchie, M. Robinson, P.C. Thomas, D.K. Yeomans, B.G. Williams, S. Squyres, R.W. Farquhar, A. Cheng, D.W. Dunham

On June 27, 1997 the NEAR spacecraft passed within about 1200 km of main belt asteroid 253 Mathilde. Complementing the Galileo flyby's of S-asteroids Gaspra and Ida, this was the first ever close observation of a C-asteroid. Mathilde has attracted recent attention due to its extremely slow rotation period of 17.5 days. Primary science objectives during this 10 km/sec flyby include high-resolution imaging, as well as albedo and spectral mapping of the illuminated surface of the large (50x50x70 km) asteroid. The best monochrome images will achieve resolutions of 200 meters/pixel. Global imaging in seven colors between 0.4 and 1.1 micron were carried out at resolutions of 400-500 m/pixel. On departure a satellite search was made in which bodies as small as 100 meters across could be detected. A determinationof the mass of Mathilde to about +/- 10% will be carried out by the Radio Science experiment.

Due to the encounter geometry (approach phase angle 139 degrees, departure at 39 degrees) the best imaging of Mathilde will occur around and just after closest approach. Locating Mathilde with sufficient accuracy to insure the highest resolution observations are obtained near closest approach required optical navigation updates of Mathilde's position as late as 12 hours before encounter. This was the first ever fast flyby of an asteroid with a spacecraft which (unlike Galileo) does not have a scan platform.

Modified from Eos, Trans. AGU, 77, F449 with permission from the publisher.

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