Over the past three months, NASA’s New Horizons spacecraft has been taking images to measure the brightness of its next flyby target — a Kuiper belt object nicknamed Ultima Thule (also known as 2014 MU69) — and how the brightness varies as the object rotates. Even though New Horizons team members determined in 2017 that Ultima Thule isn’t shaped like a sphere — that it is probably elongated or maybe even two objects — they haven’t seen the repeated pulsations in brightness that they’d expect from a rotating object of that shape. The periodic variation in brightness during every rotation produces what astronomers refer to as a light curve.

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