ESA's Solar Orbiter sneaks in bonus science by choking on the dust of a comet tail (again)

Plucky probe due to make closest pass to the Sun in March

The European Space Agency's Solar Orbiter followed up its whizz past Earth as 2021 drew to a close by passing through the tail of a comet. Again.

While eyes were turned to French Guiana and the impending launch of the James Webb Space Telescope, for a few days around 17 December the spacecraft flew through the tail of Comet C/2021 A1 Leonard.

It's not the first time; the spacecraft also passed through the tail of the fragmenting comet C/2019 Y4 ATLAS in May and June 2020, a few short months after its launch.

While the primary mission of the Solar Orbiter is related to the Sun (the probe is on course to get up close and personal in March 2022), ESA is not averse to a bit of bonus science from such encounters, even if the nucleus of the comet was some 44.5 million kilometres away at the time. As well as snapping images, the probe also detected atoms, ions, and molecules attributable to the comet rather than the solar wind.

Other spacecraft were also keeping an eye on the crossing. The NASA/ESA SOHO spacecraft, NASA's STEREO-A, and the Parker Solar Probe were all observing from afar, meaning astronomers got not only data from within the tail, but also contextual images from outside.

The crossings of the Solar Orbiter have thus far been predicted in advance thanks to work by Geraint Jones of the University College London Mullard Space Science Laboratory. This was built upon by Samuel Grant, a postgraduate student at the laboratory, who came up with the Comet Leonard prediction.

Grant, who is now looking at archive data for other crossings that might have gone unnoticed, said: "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."

Daniel Müller, ESA Project Scientist for Solar Orbiter, said: "This kind of additional science is always an exciting part of a space mission." While the previous encounter was impressive, the team were still working through the calibration of the instruments and the comet fragmented just before the probe reached it. "But with Comet Leonard we were totally ready," said Müller, "and the comet didn't fall apart."

In March, the Solar Orbiter will make its closest pass to the Sun yet (at around 50 million kilometres), one of hopefully many encounters during its mission. ®

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