An artist’s conception of the Parker solar probe in a stream of solar particles. Image: NASA
2021 was a year of many firsts. One of them was that NASA Parker solar probe entered the Sun’s outer atmosphere, or the solar corona.
NASA announced this mission in 2009 to study solar storms and the behaviour of the star’s magnetic field. It launched in 2018. On April 28, 2021, it entered the solar corona for the first time, coming within 13.08 million km of the Sun’s surface. It got even closer in December, up to 7.86 million km from the surface. By 2025, the probe plans to make its closest approach – of up to 6.16 million km.
The Sun’s surface has a temperature of around 5,500º C. The solar corona is actually much hotter – around a million degrees – for unknown reasons. This is known as the coronal heating problem, and is one of the Parker probe’s study goals. So how come the probe hasn’t melted yet?
As the definition goes, temperature is a unit that defines how fast particles are moving. Put another way, a body’s heat is the average kinetic energy of the particles that make it up. Heat, on the other hand, is an expression of the amount of energy that fast-moving particles are transferring.
This is why it’s possible for a body to have a high temperature but low heat. That is, the body’s particles could have a lot of kinetic energy, but there may be too few of them to transfer much energy. A good example is space, which has a constant background temperature, but because it is so vast, its density is very low and therefore transfers almost no heat to the objects in it.
Another way to understand this is by sticking a spoon in a pot of boiling water and separately in an oven, and then measuring the temperature of the spoon. It will have a higher temperature after being placed in boiling water simply because it interacts with more particles there than in the oven.
The solar corona is the Sun’s outermost layer. It has low density but a high temperature. As a result, the Parker probe interacts with a smaller number of particles.
This said, the solar corona is still at a million degrees celsius, which is tremendously high even for a star. To overcome this issue, the Parker probe has a heat shield in front of it, called the thermal protection system (TPS)[footnote]Visualised in the image above[/footnote]. It is 115 mm thick and 2.4 m wide, and is made of a composite foam sandwiched between two carbon plates. Its Sun-facing plate is polished with white ceramic paint to reflect as much heat as it can. White-coloured surfaces are in general better reflectors of electromagnetic radiation than surfaces of darker colours. The TPS can withstand up to 1,650º C.
As the Parker probe gets closer to the Sun’s surface in the coming years, its instruments and circuits will need to be protected as well. For this, the engineers who built it have installed a cooling system. It consists of a heated tank filled with 3.7 litres of deionised water as a coolant, two radiators, aluminium fins and a pump. The coolant circulates and removes heat from the probe’s components and loses it through the radiators and fins.
The entire probe is designed to work autonomously and to keep itself safe. It will autonomously change its position according to instructions from its central computer, which has also been programmed to ensure the probe can perform its scientific studies without any human intervention.
Nishant Tiwari is a master’s student of physics and astronomy.