NASA's Parker Solar Probe was set to break new ground by entering the sun's outer atmosphere, the corona, as part of a mission to advance scientific knowledge about Earth's closest star.

"No human-made object has ever passed this close to a star, so Parker will truly be returning data from uncharted territory," Nick Pinkine, mission operations manager at Johns Hopkins Applied Physics Laboratory, said in a NASA blog.

Parker was on course to fly 3.8 million miles (6.1 million km) from the sun's surface at 6:53 a.m. EST (1153 GMT). With the spacecraft out of contact, it will be Friday before mission operators confirm its health following the close flyby.

Moving at up to 430,000 mph (692,000 kph), the spacecraft will endure temperatures of up to 1,800 degrees Fahrenheit (982 degrees Celsius), NASA said on its website.

When the probe first passed into the solar atmosphere in 2021 it found new details about the boundaries of the sun's atmosphere and collected close-up images of coronal streamers, cusp-like structures seen during solar eclipses.

Since the spacecraft launched in 2018, the probe has been gradually circling closer toward the sun, using flybys of Venus to gravitationally pull it into a tighter orbit with the sun.

One instrument aboard the spacecraft captured visible light from Venus, giving scientists a new way to see through the planet's thick clouds to the surface below, NASA said.