NASA’s TESS Watched an Outburst from Comet 46P/Wirtanen

TESS, the Transiting Exoplanet Survey Satellite, has imaged an outburst from the comet 46P/Wirtanen. It caught the outburst in what NASA is calling the clearest images yet of a comet outburst from start to finish. A comet outburst is a significant but temporary increase in the comet’s activity, outside of the normal sunlight-driven vaporization of ices that creates a comet’s coma and tail.

Astronomers aren’t certain what causes them, but a new study based on this observation is shedding some light on them.

TESS is not meant to study comets. Its job is to spot dips in starlight when an exoplanet passes, or transits, in front of a star. But because it has such a wide field of view, and because it watches one section of sky for over 27 days, it also captures other transient phenomena like this comet outburst.

The study outlining this comet outburst was published in The Astrophysical Journal Letters on November 22nd. The study is titled “First Results from TESS Observations of Comet 46P/Wirtanen.” Tony Farnham, a research scientist at the University of Maryland’s Department of Astronomy is the lead author.

“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,” Farnham said in a press release.

“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!,” said Farnham.

Comets are small bodies in the Solar System with significant ice content. They normally follow long elliptical orbits that take them close to the Sun. As they approach the Sun, the comet warms and some of the ice out-gasses, dragging dust along with it. This creates a coma and a tail that stretches away from the Sun.

A comet outburst is a distinct phenomenon separate from the normal out-gassing that creates the tail and coma. While out-gassing is an ongoing gradual process, an outburst is a more immediate effect. They’re related to the conditions on the comet’s surface.

Astronomers know of a number of potential triggers for comet outbursts. Comets can contain pockets of volatile ices. If a heat wave penetrates into one of those pockets, the rapid heating could vaporize those ices, causing an outburst of gas and dust. There are also mechanical events where a cliff or other feature on the comet’s surface collapses, subjecting freshly-exposed ice to the heat from the Sun, causing an outburst. Both could work together, and there could be other causes.

To understand these outbursts, it’s especially important to capture images of a comet in the days leading up to the outburst itself. Those observations can help astronomers understand the thermal and physical characteristics of the comet.

Thanks to TESS, that’s what happened in the case of 46P/Wirtanen.

The outburst began on September 26th 2018, almost three months before the comet’s closest approach to the Sun. The initial brightening occurred in two phases: first there was a bright flash that lasted about an hour, and then there was a second 8-hour long phase of gradual brightening. The team behind this study thinks that the second phase was dust and gas from the initial phase spreading out and reflecting more sunlight. After the brightness peaked, the outburst faded over a period of about two weeks.

Thanks to TESS’s observational capabilities, astronomers had access to composite images of the outburst at 30 minute intervals.

“With 20 days’ worth of very frequent images, we were able to assess changes in brightness very easily. That’s what TESS was designed for, to perform its primary job as an exoplanet surveyor,” 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. The outburst happened mere days after the observations started.”

Those detailed observations allowed the team to estimate the amount of material ejected during the outburst. They think that Wirtanen lost about 1 million kg (2.2 million lbs) of material, and that the outburst created a crater about 20 meters (65 ft.) across. That estimate will likely become more accurate when they analyze the size of the dust particles in the tail.

This is not the first time that 46P/Wirtanen has been observed during an outburst. It’s happened at least three other times since its discovery in 1948: in October 1991, September 2002, and May 2008. Since none of those outbursts were correlated with the comet’s orbital position, scientists think that the outbursts are not related to a persistently volatile region of the comet.

This makes Wirtanen different than outbursts seen on some other comets. Comet 29P/Schwassmann–Wachmann 1 (SW1) seems to experience outbursts frequently. Half of those outbursts are periodic, which suggests that it has volatile ice hot-spots that outburst according to diurnal heating.

The team of astronomers also detected the comet’s trail for the first time. The trail is separate from the tail. While the tail is shaped by the solar wind and always points away from the Sun, the trail is always behind the comet, tracing its orbital path. The trail is also made of larger debris than the tail. The trail is where a comet loses most of its mass, and is responsible for the meteor showers seen from Earth.

“The trail more closely follows the orbit of the comet, while the tail is offset from it, as it gets pushed around by the sun’s radiation pressure. What’s significant about the trail is that it contains the largest material,” said Michael Kelley, an associate research scientist in the UMD Department of Astronomy and a co-author of the research paper. “Tail dust is very fine, a lot like smoke. But trail dust is much larger—more like sand and pebbles. We think comets lose most of their mass through their dust trails. When the Earth runs into a comet’s dust trail, we get meteor showers.”

While this study is fascinating, it’s really just a beginning. Further analysis of this comet, and of other ones in TESS’s field of view, should shed more light on comets, and on their outbursts. Observing more comets will also help to determine whether multi-stage brightening is rare or commonplace in comet outbursts.

“We also don’t know what causes natural outbursts and that’s ultimately what we want to find,” Farnham said. “There are at least four other comets in the same area of the sky where TESS made these observations, with a total of about 50 comets expected in the first two years’ worth of TESS data. There’s a lot that can come of these data.”

More:

• Press Release: NASA’s Exoplanet-Hunting Mission Catches a Natural Comet Outburst in Unprecedented Detail
• Research Paper: First Results from TESS Observations of Comet 46P/Wirtanen
• A 1955 paper on comet outbursts: Comet Outbursts