Nasa’s Parker Solar Probe, which has flown closer to the sun than any spacecraft, has beamed back its first observations from the edge of the sun’s scorching atmosphere.
The first tranche of data offers clues to long-standing mysteries, including why the sun’s atmosphere, known as the corona, is hundreds of times hotter than its surface, as well as the precise origins of the solar wind.
“The first three encounters of the solar probe that we have had so far have been spectacular,” said Prof Stuart Bale, a physicist at the University of California, in Berkeley, who led the analysis from one of the craft’s instruments. “We can see the magnetic structure of the corona, which tells us that the solar wind is emerging from small coronal holes; we see impulsive activity, large jets or switchbacks, which we think are related to the origin of the solar wind. And we are also surprised by the ferocity of the dust environment.”
Over the next six years, the car-sized spacecraft will follow an ever-closer elliptical orbit, eventually swooping so near that it will technically “touch” the sun. A drawback of being at such close quarters is that Parker will not be sending pictures home. If it swivelled towards the sun its camera would melt, so the spacecraft’s instruments gaze sideways, measuring the stream of supersonic charged particles that make up the solar wind.
Previously, scientists observed that the sun’s wind appears to have two main components: a “fast” one that travels around 700km per second (and comes from giant coronal holes in the sun’s polar region; and a “slow” wind, travelling below 500km per second, whose origin is unknown.
The Parker probe traced the slow wind back to small coronal holes dappled around the sun’s equator – solar structures that had not previously been observed. Coronal holes are cooler, less dense regions, through which magnetic fields stream out into space, acting as channels for the charged particles to flow along.
The observations also point to an explanation for why the corona is so blisteringly hot.
“The corona is a million degrees, but the sun’s surface is only thousands,” said Prof Tim Horbury, a co-investigator on the Parker Solar Probe Fields instrument at Imperial College London. “It’s as if the Earth’s surface temperature were the same, but its atmosphere was many thousands of degrees. How can that work? You’d expect to get colder as you moved away.”
Parker’s sidelong observations revealed that the particles in the solar wind appeared to be released in explosive jets, rather than being radiated out in a steady stream. “It’s bang, bang, bang,” said Horbury.
This rapid release of energy from the sun’s interior into its atmosphere could help explain why the atmosphere is so staggeringly hot compared to the solar surface, he said.
Another surprise was the dustiness of the area close to the sun. During the nearest approach of its orbit, the probe was peppered with a fine dust, chipping tiny pieces off its heat shield that showed up as white streaks in images captured by the high-resolution camera. It is thought to be the remains of asteroids and comets that came close to the sun, causing them to evaporate, leaving behind just a dusty haze.
The new observations were made when Parker was about 15m miles (24m km) from the sun, but it will eventually fly to about 6m km of its surface — more than seven times closer than the previous closest mission, the Helios 2 spacecraft in 1976.
The extreme conditions faced by Parker has required the use of unconventional materials and spacecraft design. The craft’s white ceramic heat shields will reach a temperature of nearly 1,400C (2,552F) during the mission’s closest approach. As it passes close to the sun, its solar panels are retracted into the shadow of the heat shield, with just a tiny area remaining exposed to generate power. The craft has also broken the record for the fastest moving spacecraft, relative to the sun. It will reach speeds of nearly 435,000 mph (700,000 km/h) in 2024.
“It’s a very bold mission, it’s really extreme and it’s an enormously impressive engineering effort,” said Horbury.
The findings are reported in three papers in the journal Nature.
