A 2018 artist’s concept shows the Parker Solar Probe flying into the sun’s outer atmosphere, called the corona, on a mission to help scientists learn more about the star.
| Photo Credit: NASA
Among the various places humans have aspired to visit in the solar system, the sun remains the most foreboding. On December 24, 2024, NASA’s Parker Solar Probe arrived within 6.1 million km from the star’s surface. This is a short distance to be from the sun: no spacecraft has ever made such a close approach. Even the Parker Solar Probe took seven years to get here.
The probe made another approach to the sun on March 22 and will do so again on June 19 this year.
Watching the sun
Sunlight is the main source of energy for earthlife. The sun’s core produces this energy using nuclear fusion. The star also has strong, dynamic magnetic fields crisscrossing its surface, and sudden changes in the way they’re arranged give rise to intense explosions called solar flares. Numerous electrons, protons, and heavy nuclei are spit out of the solar corona — the uppermost layer of the sun’s atmosphere — at about 900 km/s.
These particles carry an enormous amount of energy and sometimes rush towards the earth at tremendous speed in an event called a coronal mass ejection. Their effects on the earth constitute a solar storm, including electric grid failures, loss of telecommunication channels, and damage to the ozone layer. They can also damage instruments onboard satellites.
To understand the dynamics of the corona over time and their effects on the solar system at large, scientists need to observe the sun closely. This is also why the Indian Space Research Organisation launched and is currently operating the Aditya-L1 probe, stationed at about 150 million km from the star.
An able heat shield
Around six decades ago, a scientist named Eugene Parker predicted the existence of the solar wind: a stream of charged particles flowing out from the sun in all directions. NASA named the Parker Solar Probe in his honour.
The probe was launched on board a Delta IV rocket from Cape Canaveral in Florida in August 2018. Once in space, the probe’s maximum speed was an astounding 692,000 km/hr.
To protect against the sun’s intense heat, the probe has an 8-foot-wide, 4.5-inch thick carbon-carbon composite material shield that can withstand up to 1,370º C while weighing only 73 kg. This shield was built by researchers at the Johns Hopkins Applied Physics Laboratory. It consists of a carbon composite foam sandwiched between two carbon plates. Its sun-facing side is coated with white ceramic paint to reflect as much sunlight as possible instead of absorbing it.
Just a few metres behind the shield, in its shadow, the ambient temperature drops to 29º C, allowing the probe’s scientific instruments to operate without special provisions to maintain the temperature. The probe also has two sets of solar power arrays: one in the shield’s shadow that supplies power to the instruments and the other on the sun-facing side, which uses a special fluid pump to cool itself while powering the probe during its close approaches.
Touching the sun
Curiously, the first obstacle to the mission’s success wasn’t the sun’s heat but its gravity. Since the probe flew through space at a very high speed, it had to decelerate significantly as it got close to the sun. If it didn’t, the sun’s gravity would have encouraged it to dive right into the star. This is why Parker’s first mission profile had the probe fly towards Jupiter and swing around the gas giant to achieve an optimal speed to fly towards the sun. This idea fell out of favour because of the large travel distance.
The probe’s final mission profile was more direct: to use the combined gravitational forces of the earth and Venus to slowly spiral closer to the sun’s surface, in the process spending more than 2,000 hours flying through the corona and 24 times along the solar equator.
The probe has four scientific instruments: FIELDS, Integrated Science Investigation of the Sun (ISoIS), Wide-Field Imager (WISPR), and Solar Wind Electrons Alphas and Protons (SWEAP). FIELDS measures the electric and magnetic fields of the sun’s atmosphere; ISoIS observes the energetic particles that cause solar storms while SWEAP records their properties; and WISPR takes pictures as it passes through the corona.
A ‘sun-touching’ event occurred in April 2021 when Parker moved closer to the sun than its Alfvén surface — the height beyond which the solar wind can’t affect the star’s surface — as ascertained by the FIELDS and SWEAP instruments.
A fifth instrument, called a Faraday cup, lies outside the shadow of the heat shield and measures the density of ions and electrons in the solar wind. It is made of a molybdenum alloy with a melting point of 2,349º C.
Blowin’ in the (solar) wind
Parker data has already revealed many new details about the sun. The solar system is ceaselessly coated in dust particles created when space rocks weather and scientists believed the dust ought to be everywhere. Yet Parker revealed dust-free pockets near the sun. The probe also detected magnetic switchbacks: parts of the solar wind where the magnetic field (created by the collective of charged particles) abruptly bent back on itself.
An important open question in solar astrophysics is why the sun’s surface is only 6,000º C or so whereas the solar corona is 200-times hotter. Based on Parker’s data of magnetic switchbacks and other related phenomena, scientists believe the answer to the mystery lies in Alfvén waves: an oscillation of ions in the plasma released by the sun, set in motion by forces in the surrounding magnetic field.
During its close-approach on March 22, the Parker Solar Probe once again attempted to get within around 6 million km of the sun’s surface. What did it find this time?
Shamim Haque Mondal is a researcher in the Physics Division, State Forensic Science Laboratory, Kolkata.
Published – March 25, 2025 09:00 am IST
