Comet Outburst

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Astronomers catch a comet outburst
Deborah Byrd and Eleanor Imster in SPACE | December 6, 2019

Remember comet 46P/Wirtanen? It was a bright comet about this time last year. Around the time it swept near the Earth and sun, the comet entered the field of view NASA’s TESS planet-hunter. And boom! It underwent an outburst, caught by TESS.

This animation shows an explosive outburst of dust, ice and gases from comet 46P/Wirtanen that began on September 26, 2018, and dissipated over the next 20 days. NASA’s TESS spacecraft acquired an image of the comet every 3 hours during the first 3 days of the outburst. Image via Farnham et al./NASA/University of Maryland.

What makes a comet flare up suddenly? No one knows exactly, but astronomers do know that outbursts are sometimes seen when comets are near their parent stars. Now University of Maryland astronomers say they’ve made the most complete and detailed observations to date of the formation and dissipation of a comet outburst. The comet is none other than 46P/Wirtanen, which last swept closest to the Earth and sun in its 5.4-year orbit in late 2018. You might remember this comet. We published many beautiful images of it, captured in November and December 2018 by members of the EarthSky community.

Shortly before earthly astrophotographers began to ogle it, the comet entered the field of view of NASA’s Transiting Exoplanet Survey Satellite (TESS), whose job is to seek exoplanets. As it happened, while TESS was aimed its way, 46P/Wirtanen underwent an outburst. Researchers were able to gain a clear start-to-finish image sequence of the explosive emission of dust, ice and gases. Team members reported their results in the November 22, 2019 issue of The Astrophysical Journal Letters.

University of Maryland astronomer Tony Farnham is the lead author of the study. He said in a statement:

TESS spends nearly a month at a time imaging one portion of the sky. With no day or night breaks and no atmospheric interference, we have a very uniform, long-duration set of observations. As comets orbit the sun, they can pass through TESS’ field of view. Wirtanen was a high priority for us because of its close approach in late 2018, so we decided to use its appearance in the TESS images as a test case to see what we could get out of it.

We did so and were very surprised!

Farnham said that, although Wirtanen came closest to Earth on December 16, 2018, the outburst began in September. The astronomers’ statement explained:

The initial brightening of the outburst occurred in two distinct phases, with an hour-long flash followed by a more gradual second stage that continued to grow brighter for another 8 hours. This second stage was likely caused by the gradual spreading of comet dust from the outburst, which causes the dust cloud to reflect more sunlight overall.

After reaching peak brightness, the comet faded gradually over a period of more than two weeks.