In 2018, Nour Raouafi and his colleagues launched a space probe towards the Sun. At 6:53 a.m., on the morning of December 24, 2024, this probe was to come closer to the star than ever before. “By nature, this is a high-risk mission. We travel in an extreme environment: the atmosphere of a star”, reminds the Duty Mr. Raouafi, the scientific manager of the Parker probe, developed by NASA. Thursday noon, his team did not yet know if the craft had burned its wings during its approach, like Icarus. “The atmosphere is a little festive, but we know there are risks,” says the astrophysicist attached to Johns Hopkins University, awaiting the first sign of life from his toy, expected around midnight .
For six years, the trajectory of the small probe (685 kilograms, 3 meters) has been twisting around the Sun and Venus. Each ride brings her a little closer to the ball of fire. This week's approach — called “perihelion” — is the most ambitious in the program. On this Christmas Eve, it plunged just 6.1 million kilometers from the surface of the day star: the equivalent of 9 times the radius of the star, or 4% of the distance between Earth and the Sun. Traveling at nearly 700,000 km/h, the Parker probe was to become the fastest human-made object of all time.
The probe, equipped with a range of sensors (magnetic field, electric field, plasma density, images), is hidden behind carbon and ceramic armor, capable of withstanding the 1000°C that awaited it. The atmosphere of our star, called the solar corona, is very hot: at some of its points, the mercury climbs up to 1.5 million °C. By comparison, the surface of the Sun is almost cold, at 5500°C. This is perhaps the main enigma of heliophysics: why is the periphery hotter than the core? By penetrating deeper into this terra incognitathe Parker probe could provide answers.
Parker's first 21 low flights have already delivered a wealth of goods. Starting with the “laces” (our translation of switchbacks), which are probably the most important discovery of the mission so far. Powerful magnetic fields propagate around the Sun. Some field lines go out to sea, others attach to its surface. However, with the new probe, astrophysicists have observed violent magnetic whiplashes which occur when two field lines recombine. The result is brief waves of solar wind at 500,000 km/h. The switchbacks are now thought to play a key role in transmitting energy between the Sun and its corona.
Geomagnetic storms
Beyond the switchbacks, the probe named in honor of astrophysicist Eugene Parker (1927-2022) observed that an almost perfect vacuum forms in the wake of coronal mass ejections (CMEs), these plasma bubbles violently projected into space by solar flares. The scientific machine also confirmed that a dust-free zone exists around our star. The project contributes to the advancement of fundamental science, but also allows us to better prepare for geomagnetic storms. Indeed, when an EMC shoots towards the earth, it can induce rogue currents in electrical cables and disrupt the ionosphere, where the radio waves essential for piloting modern aircraft bounce.
Such a storm hit the Earth in March 1989. Overloads in the Hydro-Québec network caused a general power outage. These events occur more frequently when solar activity — which follows an 11-year cycle — is at its peak. The Parker mission was precisely designed so that the shortest perihelion coincides with the maximum of solar cycle #25, which reaches its peak between 2024 and 2026. “For us, the greatest gift would be to see the Sun produce a strong explosion” while the probe is in the crown, Mr. Raouafi dreams. According to telescopes, the solar flares of recent days tend to fall more towards normality. Nevertheless, indicates the specialist, “we learn a lot from these events”.
Christmas data is expected to be reported at the end of January. Hundreds of scientists around the world are working with the Parker probe's harvest. It will take years, or even decades, to comb through everything. “Every time the Parker probe sends us fresh data, every three months, we are like spoiled little kids,” explains the astrophysicist of Tunisian origin. Happy unboxing!
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