by NASA’s sun-hitting Parker Solar Probe spacecraft will celebrate St. Patrick’s Day (March 17) by making another close approach to our star. As people around the globe enjoy a cold beer, the spacecraft will brave sweltering temperatures of up to 1,400 degrees Celsius as it makes its 15th approach to the sun, or perihelion.
According to NASA’s Parker Solar Probe website, (opens in new tab) the exact time of the close approach is 4:30 p.m. EDT (2030 GMT) when the spacecraft comes within about 8.5 million km of the sun’s surface, the photosphere.
That’s closer than the innermost planet to the sun, Mercury, which orbits the planet more than 6 times farther, about 34 million miles (54 million kilometers) from the sun. This close approach means that Parker will get close to the sun’s outer atmosphere, known as the corona.
Related: Parker Solar Probe: First spacecraft to ‘touch’ the sun
One of the main missions of the Parker Solar Probe, which launched on August 12, 2018, is to investigate why the corona is hundreds of times hotter than the photosphere below it.
The corona reaches temperatures of more than 1.8 million degrees Fahrenheit (1 million degrees Celsius) compared to the photosphere’s average temperature of about 10,000 degrees Fahrenheit (5,800 degrees Celsius). Because most of the sun’s heat comes from the nuclear fusion processes at its heart, stellar models say the inner regions should be hotter. Moreover, the photosphere’s plasma is a whopping 10 million times denser than the corona’s plasma, which means the pressure inside the star should also be greater, so it’s a mystery why the corona is so blisteringly hot.
The corona is difficult to study from Earth because the light it produces is “washed out” by light from the aptly named photosphere, which isn’t really visible until the moon covers the sun’s surface during an eclipse. At these times, the corona appears as a burning ring of white light.
To solve this lingering solar puzzle, the Parker Solar Probe must hit the corona and race through this wispy and fuzzy plasma at speeds of up to 365,000 miles per hour (587,000 kilometers per hour). That makes the spacecraft the fastest object built by mankind, capable of flying some 250 times faster than the top speed of a Lockheed Martin F-16 jet.
Braving the corona’s heat doesn’t depend on the luck of the Irish, however, with the Parker Solar Probe relying instead on pioneering engineering to beat the heat. Specifically, the spacecraft is equipped with a 4.5-inch-thick (11.4-centimeter) carbon-composite shield that keeps its scientific payload at room temperature even at perihelion.
The last time the spacecraft reached perihelion was during its 14th close pass on December 11, 2022, when it came within about the same distance from the solar surface as it did on St. Patrick’s Day 2023. This is not the closest the Parker Solar However, Probe has come to the sun. On November 21, 2021, the spacecraft passed just a fraction closer to the sun.
Later this year, Parker will swing past Venus and adjust its orbit to get it even closer to the sun. The probe’s planned mission will include 24 close approaches to the sun before ending in 2025, and eventually it will get as close as 3.8 million miles (6.1 million km) to the photosphere. This is closer than any other spacecraft that has approached our star and is only one-tenth the distance between our star and its inner planet Mercury.
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