By Sheer Luck, NASA’s Planet-Hunting Satellite Captured A Comet As It Exploded

For your viewing pleasure: comet ISON (not the one that exploded) snapped by the Marshall Space Flight Center 20' telescope on November 19, 2013. NASA/MSFC/MEO/Cameron McCarty/Flickr CC BY-2.0

While out planet-hunting for NASA, the Transiting Exoplanet Survey Satelite, or TESS, managed to observe a rarely seen space curiosity: a comet exploding.

While being trained on a specific spot in the sky for a month, TESS captured the entire event from beginning to end as comet 46P/Wirtanen exploded, flinging out dust, ice, and gas into the cosmos, during a close approach in 2018.

Comets are big chunks of icy rock left over from the formation of stars and planets billions of years ago. As they float around the Oort cloud, the most distant region of the Solar System, the gravity from passing celestial bodies can snag them, setting them on a path towards the Sun. As they get closer, they get more active, the Sun’s heat vaporizing the icy surface and trailing behind a gaseous tail.

Occasionally, however, the comets spontaneously combust, and as we are still not entirely sure what causes this, it’s hard to predict when it may happen.

“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,” said lead author Tony Farnham, from the University of Maryland, in a statement. “We did so and were very surprised!”

TESS managed to catch the outburst in unprecedented detail. The comet exploded on September 26, 2018, and dissipated over the following 20 days. TESS takes a picture every 30 minutes, so the team was able to view the entire sequence. The initial outburst happened in two phases; the first an hour-long flash, and then for the next 8 hours a gradual brightening. To show the combustion in all its explosive glory, the team has created a composite animation from images snapped every three hours over the first three days of the outburst.

Farnham et al./NASA

There are a number of theories about what mechanism triggers a comet outburst, but we know they are related to conditions on the comet’s surface. It could be a heatwave from the Sun penetrating a pocket of highly volatile gases, causing the ice to vaporize quickly and creating a burst of activity, or it could be something as simple as part of the rocky surface collapsing and revealing previously unexposed ice to direct sunlight. Capturing what happens in those early brightening stages will help us understand more about the physical properties of comets.

By their own admission, the timing of the event couldn't have been more fortuitous. "The outburst happened mere days after the observations started,” Farnham said. "We can’t predict when comet outbursts will happen. But even if we somehow had the opportunity to schedule these observations, we couldn’t have done any better in terms of timing."

In a paper published in The Astrophysical Journal Letters, the team calculated how much material may have been ejected in the outburst, estimating it would have been about 1 million kilograms (2.2 million pounds), which would have created a crater on Wirtanen’s surface about 20 meters (about 65 feet) across.

They also managed to capture the comet’s dust trail for the first time. Unlike a comet’s normal tail of fine particles, which changes direction as it is blown about by solar winds, a dust trail is a field of larger debris – “more like sand and pebbles,” said co-author Micheal Kelly – that remains more or less constant in direction.

It’s this trail that gives us meteor showers as Earth’s orbit crosses that of a comet’s and some of the debris enters Earth’s atmosphere. As the space rocks encounter drag in the air, they burn up hot and leave behind a streak of hot air that we call “shooting stars”.

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