Nasa's Parker solar probe is attempting its closest ever flyby of the sun, passing 3.8m miles from its surface on Christmas Eve.
The spacecraft was scheduled to make the record-breaking approach, known as a perihelion, at 6.53am US eastern time (11.53 GMT).
The mission team lost contact with their ship and aren't due to receive a "beacon tone" until Friday 27 December. On 20 December, they received a transmission indicating everything was operating normally, via Nasa's Deep Space Network complex in Canberra, Australia.
The Parker probe was launched in August 2018 on a seven-year mission to deepen scientific understanding of the sun, as well as helping to forecast space weather events that can impact life on Earth. It is named after Eugene Parker, who pioneered scientific understanding of the sun and died in 2022 at the age of 94.
"No human-made object has ever passed this close to a star, so Parker will truly be returning data from uncharted territory," Nick Pinkine, the probe's mission operations manager at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said in a statement. "We're excited to hear back from the spacecraft when it swings back around the sun."
While the 3.8m mile distance may sound far away, the probe will be in the sun's outer atmosphere, the corona. If the 93m mile distance between Earth and the sun's surface was 100 metres, the spacecraft was 4 metres away at its closest point.
The ship's 4.5-in thick (11.43cm) carbon-composite shield is designed to protect it from blistering temperatures of about 1,600 to 1,700F (870 to 930C), ensuring its internal instruments stay close to room temperature.
As it ventured into the unknown, the probe was flying at a staggering speed of about 430,000mph (690,000km/h), more than 550 times the speed of sound and fast enough to fly from Washington DC to Tokyo in less than a minute.
"As we're looking for planets in other solar systems that could actually harbour life, we need to understand how our star works so that we can know what kind of stars we're looking for in other galaxies, as we search for more and more exoplanets," Dr Nicola Fox, Nasa's head of science, told the BBC.
Parker was already helping to shed light on some of sun's biggest mysteries, from the origins of solar wind to the formation of coronal mass ejections - massive clouds of plasma flung through space - and why the corona is hotter than the sun's surface beneath.
Related: Parker Solar Probe: set the controls for the edge of the sun...
The spaceship has been using flybys of Venus to move it closer into the sun's orbit, the most recent on 6 November. These have enabled it to send back new data about the planet, such as capturing visible and near-infrared light, giving scientists a new way to see through its thick clouds to the surface. Previously, that had only been done with radar and infrared imagery.
This Christmas Eve journey to the edge of the sun is the first of three record-setting flybys due to be made by the probe. The next two - on 22 March 2025 and 19 June 2025 - are expected to bring it back to a similarly close distance.
"This is one example of Nasa's bold missions, doing something that no one else has ever done before to answer longstanding questions about our universe," said Arik Posner, the probe's programme scientist at Nasa headquarters in Washington.