NASA mission lines up to 'touch the Sun'

 NASA's Parker Solar Probe lines up to 'touch the Sun', Bringing New Discoveries

It promises to be a remarkable moment in the history of space exploration. A year from now, on 24 December, NASA's Parker Solar Probe will race past the Sun at the astonishing speed of 195 km/s, or 435,000 mph. No human-made object will have moved so fast nor, indeed, got so close to our star - just 6.1 million km, or 3.8 million miles from the Sun's "surface". "We are basically almost landing on a star," said Parker project scientist Dr Nour Raouafi. "This will be a monumental achievement for all humanity. This is equivalent to the Moon landing of 1969," the Johns Hopkins University Applied Physics Laboratory scientist said. Parker's speed will come from the immense gravitational pull it feels as it falls towards the Sun. It will be akin to flying from New York to London in under 30 seconds. Parker will make the closest ever approach.

The US space agency's Parker Solar Probe is one of the most audacious missions ever conceived. Launched in 2018, it has the goal of making repeated, and ever closer, passes of the Sun. The late 2024 manoeuvre will take Parker to just 4% of the Sun-Earth distance (149 million km/93 million miles). The challenge Parker faces in doing this will be huge. At perihelion, the point in the probe's orbit nearest the the star, the temperature on the front of the spacecraft will probably reach 1,400C. Parker's strategy is to get in quick and get out quick, making measurements of the solar environment with a suite of instruments deployed from behind a thick heat shield.

“Parker Solar Probe “touching the Sun” is a monumental moment for solar science and a truly remarkable feat,” said Thomas Zurbuchen, the associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. “Not only does this milestone provide us with deeper insights into our Sun’s evolution and its impacts on our solar system, but everything we learn about our own star also teaches us more about stars in the rest of the universe.”

The reward, researchers hope, will be breakthrough knowledge on some key solar processes. Chief among these is a clearer explanation of the workings of the corona, the Sun's outer atmosphere. Parker will be sitting in the corona, which is only visible to us on Earth during a total solar eclipse. It experiences what seems to be counter-intuitive superheating. The temperature of the Sun at its photosphere, the surface, is roughly 6,000C but within the corona it can reach a staggering million degrees and more.

You'd think temperature would decrease with distance from the star's nuclear core. It's also within the corona region that the outward flow of charged particles, electrons, protons and heavy ions, suddenly gets accelerated into a supersonic wind moving at 400 km/s, or 1,000,000 mph. Parker has a sideways-looking camera to detect light scattered off particles in the Sun's corona. Scientists still can't fully explain this either. But it's critical to improving forecasts of solar behaviour and the phenomenon of "space weather". The latter refers to the powerful eruptions of particles and magnetic fields from the Sun that can degrade communications on Earth and even knock over power grids. The radiation also poses health risks to astronauts.

As it circles closer to the solar surface, Parker is making new discoveries that other spacecraft were too far away to see, including from within the solar wind – the flow of particles from the Sun that can influence us at Earth. In 2019, Parker discovered that magnetic zig-zag structures in the solar wind, called switchbacks, are plentiful close to the Sun. But how and where they form remained a mystery.  "This takes on a new dimension, especially now that we're thinking of sending women and men back to the Moon and even setting up a permanent presence on the lunar surface," Dr Raouafi said. Parker's knowledge will inform space weather forecasters for future crewed Moon missions. Parker made one of its close approaches to the Sun on Friday. It has three more planned in 2024 before it then swings around Venus on 6 November to help bend its orbit and make 24 December a historic occasion. Parker Solar Probe launched in 2018 to explore the mysteries of the Sun by traveling closer to it than any spacecraft before. 

Unlike Earth, the Sun doesn’t have a solid surface. But it does have a superheated atmosphere, made of solar material bound to the Sun by gravity and magnetic forces. As rising heat and pressure push that material away from the Sun, it reaches a point where gravity and magnetic fields are too weak to contain it. The point is known as the Alfvén critical surface, marks the end of the solar atmosphere and beginning of the solar wind. Solar material with the energy to make it across that boundary becomes the solar wind, which drags the magnetic field of the Sun with it as it races across the solar system, to Earth and beyond. Importantly, beyond the Alfvén critical surface, the solar wind moves so fast that waves within the wind cannot ever travel fast enough to make it back to the Sun – severing their connection. Until now, researchers were unsure exactly where the Alfvén critical surface lay. Based on remote images of the corona, estimates had put it somewhere between 10 to 20 solar radii from the surface of the Sun – 4.3 to 8.6 million miles. Parker’s spiral trajectory brings it slowly closer to the Sun and during the last few passes, the spacecraft was consistently below 20 solar radii (91 percent of Earth’s distance from the Sun), putting it in the position to cross the boundary – if the estimates were correct.

The head of science at NASA is Dr Nicky Fox. She was the lead scientist on Parker before taking up her current role. She said the major plus of the 24 December flyby would be the length of time the probe got to sit in the corona, far longer than on any previous pass. "We don't know what we'll find, but we'll be looking for waves in the solar wind associated with the heating," she said. "I suspect we'll sense lots of different types of waves which would point to a mix of processes that people have been arguing over for years." The coming year will be the apex of Parker's mission; it won't be able to get any closer to the Sun beyond December, not least because its orbit will no longer afford swing bys of Venus to train its trajectory onto an even tighter path. But to go any closer would also risk shortening the shadow cast by Parker's big shield, exposing the rear of the spacecraft to intolerable temperatures.

“Flying so close to the Sun, Parker Solar Probe now senses conditions in the magnetically dominated layer of the solar atmosphere – the corona – that we never could before,” said Nour Raouafi, the Parker project scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “We see evidence of being in the corona in magnetic field data, solar wind data, and visually in images. We can actually see the spacecraft flying through coronal structures that can be observed during a total solar eclipse.”

On April 28, 2021, during its eighth flyby of the Sun, Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii (around 8.1 million miles) above the solar surface that told scientists it had crossed the Alfvén critical surface for the first time and finally entered the solar atmosphere. For the first time, the spacecraft found itself in a region where the magnetic fields were strong enough to dominate the movement of particles there. These conditions were the definitive proof the spacecraft had passed the Alfvén critical surface and entered the solar atmosphere where magnetic fields shape the movement of everything in the region. The first passage through the corona, which lasted only a few hours, is one of many planned for the mission.

By the time it reaches Earth, 93 million miles away, the solar wind is an unrelenting headwind of particles and magnetic fields. But as it escapes the Sun, the solar wind is structured and patchy. In the mid-1990s, the NASA-European Space Agency mission Ulysses flew over the Sun’s poles and discovered a handful of bizarre S-shaped kinks in the solar wind’s magnetic field lines, which detoured charged particles on a zig-zag path as they escaped the Sun. For decades, scientists thought these occasional switchbacks were oddities confined to the Sun’s polar regions.  Understanding where and how the components of the fast solar wind emerge, and if they’re linked to switchbacks, could help scientists answer a longstanding solar mystery: how the corona is heated to millions of degrees, far hotter than the solar surface below.

Parker Solar Probe is part of NASA’s Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living with a Star program is managed by the agency’s Goddard Space Flight Centre in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, manages the Parker Solar Probe mission for NASA and designed, built, and operates the spacecraft.