This animation shows an explosive outburst of dust, ice and gases from comet 46P/Wirtanen that occurred on September 26, 2018 and dissipated over the next 20 days. The images, from NASA’s TESS spacecraft, were taken every three hours during the first three days of the outburst. Credits: Farnham et al./NASA. View enlarged image
Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS),
astronomers at the University of Maryland (UMD), in College Park,
Maryland, have captured a clear start-to-finish image sequence of an
explosive emission of dust, ice and gases during the close approach of
comet 46P/Wirtanen in late 2018. This is the most complete and detailed
observation to date of the formation and dissipation of a
naturally-occurring comet outburst. The team members reported their
results in the November 22 issue of The Astrophysical Journal Letters.
“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,” said Tony Farnham, a research scientist in the UMD Department of Astronomy
and the lead author of the research paper. “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!”
Normal comet activity is driven by sunlight vaporizing the ices near
the surface of the nucleus, and the outflowing gases drag dust off the
nucleus to form the coma. However, many comets are known to experience
occasional spontaneous outbursts that can significantly, but temporarily
increase the comet's activity. It is not currently known what causes
outbursts, but they are related to the conditions on the comet's
surface. A number of potential trigger mechanisms have been proposed,
including a thermal event, in which a heat wave penetrates into a pocket
of highly volatile ices, causing the ice to rapidly vaporize and
produce an explosion of activity, and a mechanical event, where a cliff
collapses, exposing fresh ice to direct sunlight. Thus, studies of the
outburst behavior, especially in the early brightening stages that are
difficult to capture, can help us understand the physical and thermal
properties of the comet.
Although Wirtanen came closest to Earth on December 16, 2018, the
outburst occurred earlier in its approach, beginning on September 26,
2018. 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. Because TESS takes detailed, composite images
every 30 minutes, the team was able to view each phase in exquisite
detail.
“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.”
The team has generated a rough estimate of how much material may have
been ejected in the outburst, about one million kilograms (2.2 million
pounds), which could have left a crater on the comet of around 20 meters
(about 65 feet) across. Further analysis of the estimated particle
sizes in the dust tail may help improve this estimate. Observing more
comets will also help to determine whether multi-stage brightening is
rare or commonplace in comet outbursts.
TESS has also detected for the first time Wirtanen’s dust trail.
Unlike a comet’s tail—the spray of gas and fine dust that follows behind
a comet, growing as it approaches the sun—a comet’s trail is a field of
larger debris that traces the comet’s orbital path as it travels around
the sun. Unlike a tail, which changes direction as it is blown by the
solar wind, the orientation of the trail stays more or less constant
over time.
“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 the current study describes initial results, Farnham, Kelley
and their colleagues look forward to further analyses of Wirtanen, as
well as other comets in TESS’ field of view. “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.”
TESS is a NASA Astrophysics Explorer mission led and operated by MIT
in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight
Center. Additional partners include Northrop Grumman, based in Falls
Church, Virginia; NASA’s Ames Research Center in California’s Silicon
Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science
Institute in Baltimore. More than a dozen universities, research
institutes and observatories worldwide are participants in the mission.
Claire Andreoli
NASA’s Goddard Space Flight Center
301-286 -1940
claire.andreoli@nasa.gov
Matthew Wright
University of Maryland, College Park
301-405-9267
mewright@umd.edu
Source: NASA/TESS
