Mercury’s transit will help Solar Orbiter scientist to calibrate the spacecraft’s instruments.
In January 2023, the Solar Orbiter spacecraft took advantage of a rare astronomical event – the transit of Mercury – to further enhance its observation of the sun. As the closest planet to the sun, Mercury crossed the face of the star, providing an opportunity for Solar Orbiter to sharpen its view. The European Space Agency (ESA) spacecraft made use of several instruments, including the Polarimetric and Helioseismic Imager (PHI), the Extreme Ultraviolet Imager (EUI), and the Spectral Imaging of the Coronal Environment (SPICE), to capture the transit from different perspectives.
PHI’s image revealed Mercury as a small black disk on the lower right-hand side of the sun, distinct from the dark sunspots visible on the star’s surface. EUI, on the other hand, filmed a movie of Mercury as it moved across the face of the sun, showing the tiny planet in greater detail after it had left the disk and was framed by the sun’s atmosphere. SPICE provided a different view of the sun by splitting its light into different colors, which allowed scientists to identify the atoms in the various layers of the star at different temperatures.
“It’s not just looking at Mercury passing in front of the sun, but passing in front of the different layers of the [sun’s] atmosphere,” Miho Janvier, a space physicist at the Institut d’Astrophysique Spatiale in France and SPICE deputy project scientist, said in a statement(opens in new tab).
For Solar Orbiter, the transit of Mercury offered a valuable chance to calibrate the instruments.
“It is a certified black object traveling through your field of view,” Solar Orbiter Project Scientist at ESA, Daniel Müller, said in the statement. “That means that any brightness recorded by the instrument within Mercury’s disc as it transits must be caused by the way that instrument transmits light, a quantity called the point spread function.”
By studying the transit of Mercury, astronomers can improve the accuracy of the point spread function in Solar Orbiter’s data. The more accurately this function is known, the better astronomers can account for it and remove it from data, ultimately leading to higher quality observations.
Planetary transits have been a useful tool for astronomers for centuries. Early astronomers used the passage of planets in front of the sun to estimate the size of the solar system by monitoring the time of the transit from different regions and comparing the timings to calculate the distance between the sun and Earth. More recently, planetary transits have been used to study exoplanets, planets outside our solar system. The transit method of exoplanet detection involves looking for dips in the light from stars, which indicate the passage of a planet. By studying these dips, scientists can estimate the orbital period and size of the planet. This technique has been particularly useful in detecting “hot Jupiters,” massive planets that orbit very close to their stars.
Although Solar Orbiter will have another opportunity to observe Mercury during a joint ESA/NASA mission in June, its next close pass of the sun in April 2023 will be the spacecraft’s next major observation event. By continuing to study the sun and its behavior, Solar Orbiter will help scientists gain a better understanding of the star’s inner workings and how it affects our solar system.
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