How the Parker Solar Probe Flew Into the Sun's Corona and Survived the Extreme Heat

The Sun's surface sits at roughly 5,500 degrees Celsius. The corona, the wispy outer atmosphere that extends millions of kilometres into space, reaches temperatures of 1 to 2 million degrees Celsius. That is not a typo, and it is not intuitive. Move away from a fire and you get colder. Move away from the Sun's surface and, for a stretch, you get dramatically hotter. This reversal has baffled solar physicists for decades and is called the coronal heating problem.

Eugene Parker, the astrophysicist who first predicted the existence of the solar wind in 1958, had a theory about what might be driving this. Magnetic waves and small explosive events called nanoflares, he argued, were dumping energy into the corona from below. NASA named a spacecraft after him in 2017, the first time a living scientist received that honour, and launched it on August 12, 2018, specifically to go find out.

The Parker Solar Probe carries a Thermal Protection System, or TPS, that is 11.4 centimetres thick and made from a carbon composite foam sandwiched between two carbon fibre face sheets. The whole shield weighs about 72 kilograms. On the Sun-facing side it reaches around 1,370 degrees Celsius during the closest passes. On the other side, where the instruments sit, the temperature stays close to room temperature.

The geometry matters as much as the material. The spacecraft is designed so that the shield always faces the Sun. If it tilted even slightly off-angle, the instruments would be exposed and destroyed within seconds. Onboard sensors detect any drift and fire thrusters to correct it automatically, without waiting for a signal from Earth, at closest approach, a radio signal takes about eight minutes each way, far too slow for real-time correction.

Temperature and heat are not the same thing. Temperature measures how fast individual particles are moving. Heat measures how much energy is actually transferred to an object. The corona is extraordinarily hot in the particle-speed sense, but it is also extraordinarily thin. There are so few particles per cubic centimetre in the corona that even though each one moves at extreme speed, very few of them actually strike the spacecraft at any given moment. The total energy delivered to the shield is manageable, intense, but manageable.

Compare this to an oven and a pot of boiling water. Both can be at 100 degrees Celsius, but plunging your hand into the water causes a burn far faster than waving it briefly through oven air at the same temperature. Density determines how much energy actually transfers. The corona's density is so low that the probe's shield handles the thermal load without catastrophic failure.

In April 2022, NASA confirmed that the Parker Solar Probe had, in December 2021, crossed the Alfvén surface, the boundary where the solar wind transitions from being magnetically dominated to flying free into space. This was the formal definition of entering the corona. No spacecraft had done it before.

Inside that boundary, Parker detected structures called pseudostreamers: regions where the magnetic field folds back on itself. It also recorded sudden reversals in the magnetic field direction, called switchbacks, which had been spotted earlier in the mission from farther out but are far more frequent and intense close in. These switchbacks may be one of the mechanisms heating the corona, Parker's data is still being analysed to test that hypothesis.

The spacecraft also measured the solar wind at its source, before it has had millions of kilometres to mix and slow down. That data is changing models of how the wind accelerates and what it carries. India's Aditya-L1, launched by ISRO on September 2, 2023, is studying the Sun from the L1 Lagrange point about 1.5 million kilometres from Earth. It observes the solar wind and corona from a fixed vantage point. Parker goes inside. The two missions are asking the same questions from very different positions, and the answers from each sharpen the other.

Parker's closest planned perihelion brings it to within about 6.1 million kilometres of the Sun's surface, at speeds approaching 690,000 kilometres per hour, the fastest any human-made object has ever travelled. At that speed, it would cover the distance between Mumbai and Delhi in under half a second.

The coronal heating problem is not solved. But Parker has eliminated several explanations and strengthened others. The answer, when it comes, will have been measured from inside the thing being explained, which is a different kind of knowing than any telescope at a safe distance could provide.