Astronomers Use Spacecraft, Distant Star to Study Neptune’s Largest Moon

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A stupendously rare astronomical event occurred last year when Triton, Neptune’s largest moon, passed in front of a distant star. The extreme distances involved made this event almost impossible to observe, but the European Space Agency (ESA) helped astronomers pin down exactly where they needed to be to observe it. Thanks to the Gaia spacecraft, researchers managed to capture a “central flash” from Triton, carrying important hints about conditions on the moon.

Triton is large for a moon at more than 20 percent the diameter of Earth. It has a thin atmosphere composed mostly of nitrogen and methane, but we’ve never been able to study it in detail. In October of 2017, the path of Triton passed directly in front of a distant star called UCAC4 410-143659. This event, known as an occultation, presented astronomers with a unique chance to learn about Triton’s atmosphere. However, they needed to be in just the right place.

As the occultation approached, astronomers calculated the path of the moon’s shadow on Earth. Since Triton is so far away, the shadow would cross Europe and head off toward North America in just three minutes. To gather data about Triton’s atmosphere as the light from the star passed through it, astronomers knew they’d need to be directly in the shadow’s path. That’s the only way you can see the central flash. The strip of land where it would be visible was just 62 miles (100 kilometers) wide.

Bruno Sicardy from Pierre and Marie Curie University in France worked to refine calculations of the occultation’s path, and it’s a good thing, too. The original estimates turned out to be off by a wide margin. Sicardy employed data from the ESA’s Gaia space observatory, which launched in 2013 in order to track the positions of stars with unprecedented accuracy. One of the stars it tracks is UCAC4 410-143659, and the data was included in a Gaia release from 2016. Sicardy also asked the ESA for early access to data slated for release later this year.

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With the aid of Gaia, astronomers found the optimal path to observe the occultation. As you can see in the video above, the team successfully captured the central flash. At that moment, the light from UCAC4 410-143659 focused through Triton’s atmosphere, carrying important data about the moon to scientists on Earth. Researchers are currently analyzing the data from Triton’s occultation, which might never have been captured without the Gaia spacecraft.