Sky Rivers

At 4 a.m. on Zero Day, Ana Bonaca woke up in her Manhattan hotel in a state of excitement. She made her way through the hushed streets of New York City with several other early bird astronomers. “It was still dark, and raining,” says Bonaca, a Harvard University postdoc. At the Flatiron Institute, a new and lavishly funded center for computational science, they settled into a conference room on the empty third floor. The hunt for galactic ghosts—witnesses to the Milky Way’s violent history—was about to begin.

Zero Day was the name for an impending data release from Gaia, a European Space Agency satellite mapping the Milky Way. At 6 a.m. on 25 April, the mission was to release positions and motions for more than a billion stars, all at once, as a model of open science and a way to prod publication-hungry astronomers into action. “It was a radical way to release the data,” says Adrian Price-Whelan, a Princeton University postdoc who also arrived early at the Flatiron Institute. “Everyone felt pressure.” By day’s end, nearly 70 astronomers would converge there, some from as far away as Australia, to sift through the data together.

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