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New Mercury images offer boost for Solar Orbiter mission

Operated by NASA and the European Space Agency (ESA), the Solar Orbiter mission launched in February 2020.

The goal of the mission is to capture the closest images of the sun to date while also monitoring the solar wind and the sun’s polar regions as part of efforts to better understand the solar cycle. The work could unravel some of the mysteries of our sun and also help make astronauts safer during long-duration missions to deep space.

This week, ESA reported several special image captures by the Solar Orbiter that have had the added bonus of allowing the mission team to better calibrate its instruments, thereby improving the quality of the data delivered by the orbiter.

Shared on Twitter on Monday, one of the clips shows the planet Mercury silhouetted in front of the sun’s atmosphere. It was recorded by the orbiter’s Extreme Ultraviolet Imager (EUI).

The second one was captured by the orbiter’s Polarimetric and Helioseismic Imager (PHI) and shows Mercury as a black circle in the lower-right quadrant of the image. “It is distinctly different from the sunspots that can be seen higher up the sun’s disc,” ESA notes.

In our Polarimetric and Helioseismic Imager (PHI) data Mercury is seen as a black circle in the lower right of the image, distinctly different from the sunspots that can be seen higher up the Sun’s disc.

— ESA's Solar Orbiter (@ESASolarOrbiter) February 20, 2023

“On January 3, 2023, inner planet Mercury crossed the spacecraft’s field of view, resulting in a transit where Mercury appeared as a perfectly black circle moving across the face of the sun,” ESA said in a post on its website.

The fact that the observation shows a certified black object traveling through the field of view means that “any brightness recorded by the instrument within Mercury’s disc must be caused by the way the instrument transmits its light, called the point spread function,” ESA explained. “The better this is known, the better it can be removed. So by studying this event, the quality of the Solar Orbiter data can be ever further improved.”

The space agency explains how planetary transits like this one have proved remarkably useful to astronomers over the centuries, for example, to help calculate the size of our solar system.

“Observers in widely separated locations would time the transit and then compare results,” ESA said. “Because they were observing from different places, the precise time of the event would be slightly different. Knowing the distance between the observers would allow them to use trigonometry to calculate the distance to the sun.”

More recently, transits have proved to be an excellent way to locate planets around stars other than our own sun.

“As the planet moves across the face of the star, the bright surface is marginally covered by the planet’s silhouette and so drops a tiny bit in brightness,” ESA says. “The regular repeating way this happens allows the planet’s size and orbit to be calculated.”

Other achievements during the Solar Orbiter’s mission so far include the capture of a significant solar prominence and incredible footage of the sun’s corona.

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Trevor Mogg
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