NASA snaps first-ever pic of a long-period comet's core
NASA/JPL-Caltech/University of Arizona

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NASA snaps first-ever pic of a long-period comet's core

Planetary scientists have lifted the veil from the comet Siding Spring, which brushed past Mars on 19 October. NASA has now released an image of the comet’s nucleus (pictured), the part that is usually hidden in a cloud of gas and dust. The image was taken by the Mars Reconnaissance Orbiter at a distance of 138,000 kilometers, or nearly a third of the distance between Earth and the moon. It is the first-ever picture of a nucleus of a long-period comet, one that hails on an orbit of a million years or more from the Oort cloud, a distant region of trillions of comets. At about half a kilometer across, the nucleus is smaller than expected. Siding Spring is thus about half the size of a typical short-period comet, which hails from the Kuiper belt, a region past Neptune. Because these two comet families originated in different parts of the solar system more than 4.5 billion years ago, the size difference might point to different formation mechanisms. A next task is to figure out the albedo, or reflectivity, of the nucleus. Although the image makes Siding Spring seem bright and white, comets are really the darkest objects in the solar system, with albedos of about 4%.