In DepthPlanetary Science

New Horizons inspects a distant time capsule

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Science  11 Jan 2019:
Vol. 363, Issue 6423, pp. 115
DOI: 10.1126/science.363.6423.115

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NASA's New Horizons spacecraft has given humanity its first good look at a pristine relic of the solar system's earliest days, a planetary building block in the Kuiper belt named MU69. On New Year's morning, New Horizons skimmed past its target at a distance of just 3500 kilometers. The first photos it sent back revealed two nearly perfect spheres of ice and rock nestled together—a 33-kilometer-long interplanetary snowman. MU69's shape bolsters a new view of how planetesimals formed at the solar system's birth, first clinging together through static electricity and then clumping as pebbles as they drag behind each other like cyclists in a peloton. Much remains to be learned about MU69, however; the best imagery won't arrive until February, and the spacecraft will take 20 months to beam all its data back.