NEAR Voyage 5-1: Waltzing w/Mathilde

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Waltzing with Mathilde: June 27, 1997
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All told, 3 words best describe Mathilde: BIG, BLACK, and BATTERED. Mathilde is by far the largest asteroid yet encountered by a spacecraft. It dwarfs tiny Gaspra, and is as wide as the state of Rhode Island. One of its huge craters could easily swallow Washington, DC.

This montage of asteroids imaged by spacecraft highlights Mathilde's enormous size.

Mathilde, the largest asteroid visited by a spacecraft, is as wide as the state of Rhode Island.

Mathilde's largest craters are big enough to swallow Washington, DC.

Mathilde is by far the darkest asteroid yet encountered by a spacecraft, darker than the soot inside a chimney! And it is as plain as it is black. In true color, it appears bland and colorless. Even computer enhancement of color pictures reveals only subtle color differences that can be explained away as illumination effects and tiny color aberrations in NEAR's camera. After Mathilde, the next darkest asteroid-like body visited by a spacecraft is Mars's inner Moon Phobos. Compared with Mathilde, Phobos is nearly twice as bright and its colors are spectacular.

Asteroids Mathilde and Ida, shown at their correct relative brightness.

True color (left) and high enhanced color (right) images of Mathilde, from the sixth image mosaic.

Color variation on Mathilde compared to color variation on the Mars
Moon Phobos, imaged by the Russian spacecraft Phobos-2 in 1989.

And Mathilde is battered. The density of its smaller craters rivals that on the oldest parts of the Moon, Mars, and Mercury - all ancient worlds - but the density of its largest craters exceeds that of any other known planet or asteroid. Mathilde is "saturated" with large craters - no more could fit even if a giant cookie-cutter began to dismember the surface, because new craters would just obliterate the old ones.

The relative densities of craters of different size on Mathilde

How can such a battered body still exist, not having been blown to smithereens? NEAR's radio science experiment provided a clue. The density of the Mathilde is only 1.3 times the density of water, only half the density of even the lightest rock type that could make up its interior. The inside of Mathilde must be full of pore space - a giant pile of loose rubble! Mathilde probably withstood its pummeling because this kind of a loose interior effectively absorbs the seismic energy from impacts. In much the same way, a sandbag absorbs the blow from a sledgehammer that would shatter a solid rock.

First and last images of Mathilde acquired by NEAR on June 27, 1997

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