Solar orbiter spacecraft picks up a second comet from the tail

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For a spacecraft designed to conduct unique studies of the Sun, the Solar Orbiter is making a name for itself as a comet exploration. For several days centered on 1200-1300 UT on December 17, 2021, the spacecraft found itself flying through the tail of Comet C/2021 A1 Leonard.

Composite images of Comet Leonard from December 15-16, 2021 in visible light were captured by the Metis instrument on board the ESA/NASA Solar Orbiter spacecraft. The comet crossed through the field of view with its ionic tail and dust pointing toward the instrument. Credit: ESA/Solar Orbiter/Metis Team

The encounter captured information about the particles and magnetic field in the comet’s tail. This will allow astronomers to study the way a comet interacts with the solar wind, a shifting wind of particles, and a magnetic field emanating from the sun and extending across the solar system.

This crossing was predicted by Samuel Grant, a graduate student in the Mullard Space Science Laboratory at University College London. He adapted an existing computer program that compares spacecraft orbits with those of a comet to include the effects of the solar wind and its ability to form a comet’s tail.

This plot series represents data collected by the Solar Wind Analyzer Heavy Ion Sensor as the ESA/NASA Solar Orbiter spacecraft passed through the tail of Comet Leonard in December 2021. The data covers December 11-20, with the first and last plot before and after the tail transit, on Respectively, it is characterized by the absence of single ionized ions. While crossing the tail, the instrument detected particles that were attributed to the comet rather than the solar wind, for example, ions of oxygen, carbon, molecular nitrogen, and molecules of carbon monoxide, carbon dioxide and water. (Ions are atoms or molecules that have been stripped of one or more electrons and now carry a net positive electric charge.) Credit: ESA / Solar Orbiter / SWA team

“I ran it with Comet Leonard and Solar Orbiter with some guesses of solar wind speed. That’s when I saw that even for a very large range of solar wind speeds, it looked like there would be a crossover,” he says.

At the time of the transit, the solar orbiter was relatively close to Earth after it passed on November 27, 2021, for the gravitational-assisted maneuver that marked the beginning of the mission’s science phase, and set the spacecraft on course for its close in March 2022 to approach the Sun. The comet’s nucleus was 44.5 million kilometers away, close to Venus, but its giant tail extended through space to Earth’s orbit and beyond.

This data plot uses solar wind speed and direction data from the Solar Wind Analyzer’s Alpha Sensor (SWA-PAS) data to estimate how close the ESA/NASA Solar Orbiter spacecraft will be to the center of comet C/2021 A1 Leonard’s ion tail during December 2021. The chart records how far Near each band is a solar wind detected by SWA-PAS that is believed to have reached the comet’s nucleus during its journey from the Sun to the spacecraft. The left axis gives the scale in astronomical units (au), where 1 au is the distance from the Sun to the Earth, and the same distance in kilometers is shown on the right axis. for differences in the plotted distance. There are short data acquisition gaps on December 15 and 17. Credit: ESA/Solar Orbiter/SWA team & S. Grant (UCL)

So far, the best detection of a comet’s tail from the Solar Orbiter has come from the Solar Wind Analyzer (SWA) instrument cluster. The Heavy Ion Sensor (HIS) has measured atoms, ions, and even molecules that are attributed to the comet rather than the solar wind.

Ions are atoms or molecules that have been stripped of one or more electrons and now carry a net positive electrical charge. SWA-HIS detected ions of oxygen, carbon, molecular nitrogen, carbon monoxide, carbon dioxide and possibly water molecules. “Because of their small charge, these ions are clearly of comet origin,” says Stefano Levi, senior researcher at SWA-HIS from Southwest Research Institute, Texas.

Composite images of Comet Leonard from December 15-16, 2021, captured in ultraviolet light by the Metis instrument on board the ESA/NASA Solar Orbiter spacecraft. The comet crossed through the field of view with its ionic tail and dust pointing toward the instrument. Credit: ESA/Solar Orbiter/Metis Team

As a comet moves through space, it tends to wrap the sun’s magnetic field around it. This magnetic field is carried by the solar wind, and the winding causes discontinuities as the polarity of the magnetic field changes sharply from north to south and vice versa.

Magnetometer instrument (MAG) data already indicates coiled magnetic field structures, but more analysis needs to be done to be absolutely certain. “We are in the process of investigating some of the smaller magnetic perturbations that appear in our data and combine them with measurements from the Solar Orbiter’s particle sensors to understand their possible cometary origin,” says Lorenzo Matini, MAG Research Associate from Imperial College London. .

In addition to the particle data, the Solar Orbiter also took pictures.

Composite images of Comet Leonard from December 15-16, 2021 in visible light were captured by the Metis instrument on board the ESA/NASA Solar Orbiter spacecraft. The comet crossed through the field of view with its ionic tail and dust pointing toward the instrument. Credit: ESA/Solar Orbiter/Metis Team NASA’s Parker Solar Probe was almost on the far side of the sun from the ESA/NASA Solar Orbiter spacecraft when the WIPSR instrument recorded images of Comet Leonard on December 7, 2021. From the viewpoint of the Parker Solar Probe it appears that the comet It passes directly between Venus and Earth. Credit: NASA/USNRL/G.Stenborg/K.Battams NASA’s Parker Solar Probe was almost on the far side of the sun from the ESA/NASA Solar Orbiter spacecraft when its WIPSR instrument recorded images of Comet Leonard on Dec. 7, 2021. From its point of view Looking at the Parker Solar Probe, the comet appears to pass directly between Venus and Earth. Credit: NASA/USNRL/G.Stenborg/K.Battams

Metis is a coronal vertebra of multiple wavelengths in the solar orbit. He can make ultraviolet observations that see Lyman’s alpha emission from hydrogen, and he can measure the polarization of visible light. During December 15 and 16, it captured the head of a distant comet simultaneously in both visible and ultraviolet light. These images are now being analyzed by the device team. “Visible light images can indicate the rate at which a comet is spewing dust, while ultraviolet images can give the rate at which water is produced,” says Alain Corso, Metis co-investigator at CNR-Istituto di Fotonica e Nanotecnologie, Padua, Italy.

The data was also captured by the Solar Orbital Heliosphere Imager (SoloHI). These images show large portions of the comet’s ionic tail taken while the spacecraft itself was inside the tail. As the image sequence progresses, changes in the tail can be seen in response to changes in the speed and direction of the solar wind.

And it wasn’t just the Solar Orbiter that monitored the crossing. The European Space Agency/NASA’s SOHO mission and NASA’s STEREO-A spacecraft and Parker Solar Probe were watching from afar. This means that astronomers now not only have data from inside the tail, but also contextual images from these other spacecraft (see images above).

The SOHO spacecraft discovered Comet Leonard in the Lyman Alpha Solar Wind Variation (SWAN) instrument. SWAN doesn’t look directly at the sun, but rather paints the entire sky in ultraviolet light, and sees interstellar hydrogen interacting with the solar wind. The maps can be used to detect bright spots in ultraviolet light that indicate a comet, for example, as is the case here. Credit: SOHO (European Space Agency and NASA); W Bonblood

Comet tail crossings are relatively rare events. Of those discovered, most were not noticed until after the event. ESA/NASA’s Ulysses mission encountered three ionic tails of the comet, including C/1996 B2 Hyakutake in May 1996, and C/2006 P1 McNaught in early 2007. The same solar orbiter also crossed the segmented comet tail C/2019 Y4. ATLAS in May and June 2020, shortly after launch.

While the early crossings were a surprise, the two Solar Orbiter encounters were previously predicted thanks to computer code developed by Geraint Jones, University of London Mullard’s space science laboratory, and extended by Samuel.

“The big advantage is that basically with no effort on the part of the spacecraft, you can sample a comet at a huge distance. That’s very exciting,” says Samuel, who is now looking at archiving data from other spacecraft looking for comet tail crossings that haven’t yet. Nobody notices it yet.

The work also helps build expertise for ESA’s Comet Interceptor mission, of which Geraint is the science team leader. The mission will visit a yet-to-be-discovered comet, making a target flyby of three spacecraft to create a 3D profile of a “dynamically new” object containing unprocessed material surviving from the dawn of the solar system.

The ESA/NASA Solar Orbiter spacecraft flew through the tail of Comet C/2021 A1 Leonard in December 2021, collecting images, wind and solar particle data at the site. At the same time, SOHO (ESA/NASA), Parker Solar Probe (NASA), and STEREO-A (NASA) have also been observing the comet’s evolution from other angles. The graphic shows the approximate relative positions of the planets, comets, and spacecraft on December 17, 2021, not a scale. Very approximate fields of view are indicated for the selected devices: SoloHI on the Solar Orbiter and SECCHI on the STEREO-A. Credit: G. Jones & S. Grant (UCL)

In the meantime, instrument teams on the Solar Orbiter are busy analyzing Comet Leonard’s data to not only see what it can tell them about the comet but about the solar wind as well.

“This kind of extra science is always an exciting part of a space mission,” says Daniel Muller, ESA’s solar module project scientist. “When Comet Atlas’ transit was predicted, we were still calibrating the spacecraft and its instruments. Also, the comet fragmented before we got there. But with Comet Leonard we were pretty much ready — and the comet didn’t collapse.”

Solar orbit. Credit: ESA/ATG medialab

In March, the Solar Orbiter makes its closest path to the Sun but at a distance of 0.32 AU (about a third of the distance between the Earth and the Sun, or about 50 million km). It’s one of about 20 passes close to the Sun that will occur over the next decade. This will result in unprecedented images and data, not only from close range, but also from the Sun’s never-before-seen polar regions.

“There is so much to look forward to with the Solar Orbiter, we are just beginning,” Daniel says.

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