ESA’s Solar Orbiter Encounters Tail of Fragmented Comet ATLAS

The long-period comet C/2019 Y4 (ATLAS) fragmented during its perihelion in April 2020. About two months later, ESA’s Solar Orbiter observed the dusty tail of the fragmented comet when the spacecraft was at a distance of approximately 0.5 AU from the Sun.

Hubble resolved roughly 25 fragments of comet C/2019 Y4 (ATLAS) on April 23, 2020. Image credit: NASA / ESA / Hubble / D. Jewitt, University of California, Los Angeles / Q. Ye, University of Maryland.

C/2019 Y4 (ATLAS) was a comet with a near-parabolic orbit and an orbital period of about 6,000 years.

Also known as comet ATLAS, the object was discovered on December 28, 2019 by a reflecting telescope atop Mauna Loa in Hawaii as part of the Asteroid Terrestrial-impact Last Alert System (ATLAS).

At the time of its discovery, the comet was about 3 AU from the Sun and shone at magnitude of 19.6 in the constellation of Ursa Major.

In April 2020, it fragmented just before its closest approach to the Sun, leaving its former tail trailing through space in the form of wispy clouds of dust and charged particles. The disintegration was observed by the NASA/ESA Hubble Space Telescope.

In June 2002, ESA’s Solar Orbiter approached close to the tail remnants in the course of its ongoing mission.

Using combined measurements from all of Solar Orbiter’s in-situ instruments, Imperial College London solar physicist Lorenzo Matteini and colleagues reconstructed this encounter.

The resulting model indicates that the ambient interplanetary magnetic field carried by the solar wind ‘drapes’ around the comet, and surrounds a central tail region with a weaker magnetic field.

Schematic reconstruction of a Solar Orbiter encounter with the ion gas tail of the fragmented comet C/2019 Y4 (ATLAS): lines identify interplanetary magnetic field lines in the solar wind; these are draped around the comet and form the typical magneto-tail with opposing orientation at the two sides; Solar Orbiter data from the Imperial College MAG magnetometer are shown along the spacecraft trajectory through the structure; arrows indicate the direction of the magnetic field, and length its intensity; the identified sharp boundary between the tail and the ambient solar wind on the draped side is indicated by the magenta dashed line. Image credit: L. Matteini / Imperial College London.

“Comets are typically characterized by two separate tails; one is the well-known bright and curved dust tail, the other — typically fainter — is the ion tail,” the researchers said.

“The ion tail originates from the interaction between the cometary gas and the surrounding solar wind, the hot gas of charged particles that constantly blows from the Sun and permeates the whole Solar System.”

“When the solar wind interacts with a solid obstacle, like a comet, its magnetic field is thought to bend and ‘drape’ around it.”

The simultaneous presence of magnetic field draping and cometary ions released by the melting of the icy nucleus then produces the characteristic second ion tail, which can extend for large distances downstream from the comet’s nucleus.

“This is quite a unique event, and an exciting opportunity for us to study the makeup and structure of comet tails in unprecedented detail,” Dr. Matteini said.

“Hopefully with NASA’s Parker Solar Probe and ESA’s Solar Orbiter now orbiting the Sun closer than ever before, these events may become much more common in future!”

The scientists presented the results this week at the Royal Astronomical Society’s National Astronomy Meeting 2021 (NAM 2021).

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L. Matteini et al. Waves and structures from Solar Orbiter’s encounter with the tail of comet C/2019 Y4 (ATLAS); signatures from magnetic field draping and cometary pick-up ion instabilities. NAM 2021

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