From Royal Astronomical Society (UK) : “Tail without a comet-the dusty remains of Comet ATLAS”

From Royal Astronomical Society (UK)

7.20.21

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A serendipitous flythrough of the tail of a disintegrated comet has offered scientists a unique opportunity to study these remarkable structures, in new research presented today at the National Astronomy Meeting 2021.

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Hubble Space Telescope image of comet C/2019 Y4 (ATLAS), taken on April 20 2020, providing the sharpest view to date of the breakup of the solid nucleus of the comet. Hubble’s eagle-eye view identifies as many as 30 separate fragments, and distinguishes pieces that are roughly the size of a house. Before the breakup, the entire nucleus of the comet may have been the length of one or two football fields. The comet was approximately 91 million miles (146 million kilometres) from Earth when the image was taken. Credit: National Aeronautics Space Agency (US) / European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) / Space Telescope Science Institute (US) / D. Jewitt (University of California-Los Angeles (US)). Licence type Attribution (CC BY 4.0)

Comet ATLAS fragmented just before its closest approach to the Sun last year, leaving its former tail trailing through space in the form of wispy clouds of dust and charged particles. The disintegration was observed by the Hubble Space Telescope in April 2020 [see story below], but more recently the ESA spacecraft Solar Orbiter has flown close to the tail remnants in the course of its ongoing mission.
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ESA-United Space for Europe

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)
“Solar Orbiter to pass through the tails of Comet ATLAS”
29/05/2020

ESA’s Solar Orbiter will cross through the tails of Comet ATLAS during the next few days. Although the recently launched spacecraft was not due to be taking science data at this time, mission experts have worked to ensure that the four most relevant instruments will be switched on during the unique encounter.

Solar Orbiter was launched on 10 February 2020. Since then, and with the exception of a brief shutdown due to the coronavirus pandemic, scientists and engineers have been conducting a series of tests and set-up routines known as commissioning.

The completion date for this phase was set at 15 June, so that the spacecraft could be fully functional for its first close pass of the Sun, or perihelion, in mid-June. However, the discovery of the chance encounter with the comet made things more urgent.

Serendipitously flying through a comet’s tail is a rare event for a space mission, something scientists know to have happened only six times before for missions that were not specifically chasing comets. All such encounters have been discovered in the spacecraft data after the event. Solar Orbiter’s upcoming crossing is the first to be predicted in advance.

It was noticed by Geraint Jones of the UCL Mullard Space Science Laboratory (UK), who has a 20-year history of investigating such encounters. He discovered the first accidental tail crossing in 2000, while investigating a strange disturbance in data recorded by the ESA/NASA Ulysses Sun-studying spacecraft in 1996. This study revealed that the spacecraft had passed through the tail of Comet Hyakutake, also known as ‘The Great Comet of 1996’. Soon after the announcement, Ulysses crossed the tail of another comet, and then a third one in 2007.

Earlier this month, realising that Solar Orbiter was going to be 44 million kilometres downstream of Comet C/2019 Y4 (ATLAS) in just a matter of weeks, Geraint immediately alerted the ESA team.

Solar Orbiter is equipped with a suite of 10 in-situ and remote-sensing instruments to investigate the Sun and the flow of charged particles it releases into space – the solar wind. Fortuitously, the four in-situ instruments are also perfect for detecting the comet’s tails because they measure the conditions around the spacecraft, and so they could return data about the dust grains and the electrically charged particles given off by the comet. These emissions create the comet’s two tails: the dust tail that is left behind in the comet’s orbit and the ion tail that points straight away from the Sun.

Solar Orbiter will cross the ion tail of Comet ATLAS on 31 May–1 June, and the dust tail on 6 June. If the ion tail is dense enough, Solar Orbiter’s magnetometer (MAG) might detect the variation of the interplanetary magnetic field because of its interaction with ions in the comet’s tail, while the Solar Wind Analyser (SWA) could directly capture some of the tail particles.

When Solar Orbiter crosses the dust tail, depending on its density – which is extremely difficult to predict – it is possible that one or more tiny dust grains may hit the spacecraft at speeds of tens of kilometres per second. While there is no significant risk to the spacecraft from this, the dust grains themselves will be vaporised on impact, forming tiny clouds of electrically charged gas, or plasma, which could be detected by the Radio and Plasma Waves (RPW) instrument.

“An unexpected encounter like this provides a mission with unique opportunities and challenges, but that’s good! Chances like this are all part of the adventure of science,” says Günther Hasinger, ESA Director of Science.

One of those challenges was that the instruments seemed unlikely to all be ready in time because of the commissioning. Now, thanks to a special effort by the instrument teams and ESA’s mission operations team, all four in-situ instruments will be on and collecting data, even though at certain times the instruments will need to be switched back into commissioning mode to ensure that the 15 June deadline is met.

“With these caveats, we are ready for whatever Comet ATLAS has to tell us,” says Daniel Müller, ESA Project Scientist for Solar Orbiter.

Expect the unexpected

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Hubble captures breakup of Comet ATLAS in April 2020.

Another challenge entails the comet’s behaviour. Comet ATLAS was discovered on 28 December 2019. During the next few months, it brightened so much that astronomers wondered whether it would become visible to the naked eye in May.

Unfortunately, in early April the comet fragmented. As a result, its brightness dropped significantly too, robbing sky watchers of the view. A further fragmentation in mid-May has diminished the comet even more, making it less likely to be detectable by Solar Orbiter.

Although the chances of detection have reduced, the effort is still worth making according to Geraint.

“With each encounter with a comet, we learn more about these intriguing objects. If Solar Orbiter detects Comet ATLAS’s presence, then we’ll learn more about how comets interact with the solar wind, and we can check, for example, whether our expectations of dust tail behaviour agree with our models,” he explains. “All missions that encounter comets provide pieces of the jigsaw puzzle.”

Geraint is the principal investigator of ESA’s future Comet Interceptor mission, which consists of three spacecraft and is scheduled for launch in 2028. It will make a much closer flyby of an as yet unknown comet that will be selected from the newly discovered comets nearer the time of launch (or even after that).

Grazing the Sun

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Solar Orbiter: journey around the Sun.

Solar Orbiter is currently circling our parent star between the orbits of Venus and Mercury, with its first perihelion to take place on 15 June, around 77 million kilometres from the Sun. In coming years, it will get much closer, within the orbit of Mercury, around 42 million kilometres from the solar surface. Meanwhile, Comet ATLAS is already there, approaching its own perihelion, which is expected on 31 May, around 37 million kilometres from the Sun.

“This tail crossing is also exciting because it will happen for the first time at such close distances from the Sun, with the comet nucleus being inside the orbit of Mercury,” says Yannis Zouganelis, ESA Deputy Project Scientist for Solar Orbiter.

Understanding the dust environment in the innermost region of the Solar System is one of Solar Orbiter’s scientific objectives.

“Near-Sun comets like Comet ATLAS are sources of dust in the inner heliosphere and so this study will not only help us understand the comet, but also the dust environment of our star,” adds Yannis.

Looking at an icy object rather than the scorching Sun is certainly an exciting – and unexpected – way for Solar Orbiter to start its scientific mission, but that’s the nature of science.

“Scientific discovery is built on good planning and serendipity. In the three months since launch, the Solar Orbiter team has already proved that it’s ready for both,” says Daniel.

Science paper:
Research Notes of the AAS

See the full article here .
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National Aeronautics and Space Administration(US)/European Space Agency [Agence spatiale européenne] [Europäische Weltraumorganisation] (EU) Hubble Space Telescope

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/National Aeronautics and Space Administration (US) Solar Orbiter

His lucky encounter has presented researchers with a unique opportunity to investigate the structure of an isolated cometary tail. Using combined measurements from all of Solar Orbiter’s in-situ instruments, the scientists have reconstructed the encounter with ATLAS’s tail. 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.

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 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.

Lorenzo Matteini, a solar physicist at Imperial College London and leader of the work, says: “This is quite a unique event, and an exciting opportunity for us to study the makeup and structure of comet tails in unprecedented detail. Hopefully with the Parker Solar Probe and Solar Orbiter now orbiting the Sun closer than ever before, these events may become much more common in future!”

NASA Parker Solar Probe Plus named to honor Pioneering Physicist Eugene Parker.

This is the first comet tail detection occurring so close to the Sun – well inside the orbit of Venus. It is also one of the very few cases where scientists have been able to make direct measurements from a fragmented comet. Data from this encounter is expected to contribute greatly to our understanding of the interaction of comets with the solar wind and the structure and formation of their ion tails.

See the full article here .

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