253 Mathilde is seriously battered. Not as in breadcrumbs and oil vats, but that’s not completely off, either.
And finally- we have an up-close visit, by a recognizably-modern spacecraft. The Galileo mission had a digital camera, but of a late ’70s vintage. NEAR Shoemaker (Near-Earth Asteroid Rendezvous, later honoring scientist Eugene Shoemaker) launched in 1996, and after a fast development program. Its camera was 12-bit (not 8-bit) plus antiblooming features. This came in handy at Mathilde, known to be a dark object. NEAR Shoemaker also had an X-band transmitter; the Doppler shift of its signal told us of Mathilde’s gravitational pull, and therefore its mass. Galileo had such a radio too, but… the antenna stuck. Galileo only used a backup, S-band transmitter, not as useful. Still, Galileo got first-mover advantage: seeing a new type of body, up close, for the first time, upended many of our assumptions even via obsolete instruments. NEAR Shoemaker’s only first-mover benefit was the first C-type asteroid. (Galileo flew by two S-asteroids.) Even then, the real flood of data only came at NEAR Shoemaker’s primary target, 433 Eros. NEAR’s trajectory only aligned with Mathilde in the Main Belt, where the Sun appears farther and dimmer. With less light on its solar arrays, mission managers operated just the camera, not the other instruments.
As this was the wild west of small-body space flight, even just a camera reaped a bonanza. We have carbonaceous chondrite meteorites, thought to be Mathilde-like, and full of organic chemicals. NEAR Shoemaker passed within 1212 km (757 mi) of Mathilde on June 27, 1997, approaching from the night side and crossing into day. NEAR covered 60% of Mathilde’s surface, with 160 meter/pixel resolution at peak (the target was 500m/pixel). Only 60% was covered, because by chance Mathilde was a slow rotator. A day on Mathilde is about 417 Earth hours, and in the 25.2 minute encounter period the night side never came out of shadow.
Mathilde turned out to be slightly smaller and brighter, at least for a C-type. Its albedo (reflectivity) was found to be .047 instead of an assumed .035, and its diameter roughly 53 km instead of 60 km. This is darker than coal, darker than Mars’ moons Phobos and Deimos, and darker than carbonaceous chondrite meteorites (widely thought to come from C-type asteroids) but not much off other chondrite asteroids (C-type and most in the S-type).
The big surprise at Mathilde was big impacts. Not only is the top crater about equal to the radius of the asteroid, but… there are three more such impacts.