Sun Probe Passes Through Comet Leonard's Tail as Space Rock Leaves Solar System

A spacecraft intended to study the boiling atmosphere of the sun has completed additional tasks by passing through the tail of the comet C/2021 A1 Leonard.

Comet Leonard, as it is known, made headlines late last year as it passed relatively close by Earth and provided avid skywatchers with a unique viewing opportunity.

The comet was particularly exciting as predictions showed that after making its close pass by the Earth on December 12, the comet would swing around the sun and head out of the solar system, never to be seen again.

Leonard is now speeding away from us, but the Solar Orbiter probe managed to say a final goodbye last month after it found itself passing through the comet's tail—referring to the trail of dust and ice left in the comet's wake.

Comet tail crossings are quite rare, and most of the time they're only detected after the event has actually happened. Indeed, the Solar Orbiter itself passed through the tail of another fragmenting comet shortly after launching in 2020.

This time, astronomers had actually predicted that the encounter would happen by calculating the effect of solar winds on the comet's tail as well as the expected path of the Solar Orbiter spacecraft.

In a way the meeting was also good luck. The Solar Orbiter wasn't specifically designed to study comets; it's designed to take the closest images of the sun ever recorded, and with a suite of 10 instruments, answer burning questions about its atmosphere and the generation of solar winds.

Using these same instruments, the Solar Orbiter was able to taste the tail of Leonard. Its Heavy Ion Sensor (HIS) measured atoms, ions and molecules that were attributable to the comet rather than the solar winds.

Via an on-board light sensor the orbiter was also able to take photos of the comet in the distance, giving scientists a hint about the rate at which Leonard is ejecting dust and how much water is was giving off.

Comet Leonard
Comet Leonard, as seen from the Solar Orbiter. ESA/Solar Orbiter/Metis Team

"The big advantage is that for basically no effort on the spacecraft's part, you get to sample a comet at a massive distance. That's pretty exciting," said Samuel Grant, a postgraduate student at University College London's Mullard Space Science Laboratory, who predicted the comet crossing in a European Space Agency (ESA) press release.

Daniel Müller, the ESA project scientist for the Solar Orbiter, said the measurements were an "additional science" on top of the orbiter's main objective of studying the sun and its winds.

In March, the Solar Orbiter is due to make its closest pass to the sun yet, traveling one-third of the distance between the star and Earth. It will continue to make nearly 20 more close passes in the coming years.

Comet Leonard
Comet Leonard, photographed passing overhead on December 4, 2021. The comet, which is not expected to return, caught attention due to its visibility. Franco Tognarini/Getty