Solar Orbiter Catches a 2nd Comet by the Tail

For the second time in its mission so far, the ESA/NASA Solar Orbiter spacecraft has flown through the tail of a comet. Predicted in advance by astronomers at University College London, UK, the spacecraft collected a wealth of science data that now awaits full analysis.

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

The encounter captured information about the particles and magnetic field present in the tail of the comet. This will allow astronomers to study the way the comet interacts with the solar wind, a variable wind of particles and magnetic field that emanate from the Sun and sweep through the solar system.

The crossing had been predicted by Samuel Grant, a post graduate student at University College London’s Mullard Space Science Laboratory. He adapted an existing computer program that compared spacecraft orbits with comet orbits to include the effects of the solar wind and its ability to shape a comet’s tail.

“I ran it with Comet Leonard and Solar Orbiter with a few guesses for the speed of the solar wind. And that's when I saw that even for quite a wide range of solar wind speeds it seemed like there would be a crossing,” he says.

At the time of the crossing, Solar Orbiter was relatively close to the Earth having passed by on 27 November 2021 for a gravity assist manoeuvre that marked the beginning of the mission’s science phase, and placed the spacecraft on course for its March 2022 close approach to the Sun. The comet’s nucleus was 44.5 million kilometres away, near to the planet Venus, but its giant tail stretched across space to Earth’s orbit and beyond.

So far, the best detection of the comet’s tail from Solar Orbiter has come from the Solar Wind Analyser (SWA) instrument suite. Its Heavy Ion Sensor (HIS) clearly measured atoms, ions and even molecules that are attributable to the comet rather than the solar wind.

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

As a comet moves through space, it tends to drape the Sun’s magnetic field around it. This magnetic field is being carried by the solar wind, and the draping creates discontinuities where the polarity of the magnetic field changes sharply from north to south and vice versa.

The magnetometer instrument (MAG) data does indeed suggest the presence of such draped magnetic field structures but there is more analysis to be done to be absolutely sure. “We are in the process of investigating some smaller scale magnetic perturbations seen in our data and combining them with measurements from Solar Orbiter’s particle sensors to understand their possible cometary origin,” says Lorenzo Matteini, a co-investigator on MAG from Imperial College, London.

In addition to the particle data, Solar Orbiter also acquired images.

Metis is Solar Orbiter’s multi-wavelength coronagraph. It can perform ultraviolet observations that see the Lyman alpha emission given out by hydrogen, and it can measure the polarisation of visible light. During 15 and 16 December it captured the distant head of the comet simultaneously in both visible and ultraviolet light. These images are now being analysed by the instrument team. “The visible light images can hint at the rate at which the comet is ejecting dust, while the ultraviolet images can give the water production rate,” says Alain Corso, a Metis co-investigator at the CNR-Istituto di Fotonica e Nanotecnologie, Padova, Italy.