A close encounter with the Sun

In December, the Parker Solar Probe spacecraft made the closest controlled approach to the sun of any manmade object.

Moving at 192 kilometres a second, it was also the fastest. It got to within 6.5 million kilometres of the solar surface (the photosphere). That is very close.

Our planet orbits the sun at a distance that varies little from 150 million kilometres. At our distance a square metre of non-reflective material located above the atmosphere will collect about 1,400 Watts of solar energy. At its point of closest approach, the Parker Solar Probe had to deal with about 745,000 Watts of energy per square metre.

Designing a shield to protect the spacecraft from this amount of heat was a major engineering challenge. Getting the spacecraft to the sun was another. The Earth is moving around the sun at roughly 30 kilometres per second. If we want to fall toward the sun, we will have to launch the spacecraft at that speed in the opposite direction, cancelling out that 30 km/s. Unfortunately, we have no launcher capable of that, so getting to the sun involved launching the spacecraft towards Venus, which we can do. Multiple swings by that world put the probe onto a path taking it close to the sun.

The effort was worth it for two reasons. Firstly the sun is the only star close enough for easy study, and secondly, our growing dependence on technology for our daily lives and our highest-ever degree of exploitation of our planet is making us more and more vulnerable to the Sun's bad behaviour. We need to address this issue.

The spacecraft was named after Eugene Parker, an American physicist who contributed hugely to our knowledge of how solar plasmas and magnetic fields interact. This might sound esoteric, but it is these interactions that cause solar flares and coronal mass ejections, both of which are extremely disruptive to us.

If you were to take a long iron bar, and put one end in a pile of ice cubes and the other in hot water, you would expect the temperature to vary gradually along its length, from cold at the icy end to hot at the hot water end. You would not expect the middle of the rod to glow red hot. Our understanding of the nature of heat, and simple common sense would rule that out. However, we have exactly that sort of problem with the sun.

The visible surface of the sun, the photosphere, has a temperature of about 6,000 C. Above that is a thin layer called the chromosphere, which can be a little hotter and above that is a layer extending all the way out to Earth and probably beyond, called the corona, which has a temperature of a million degrees or more.

Beyond the corona is the cold vacuum of space. One side of the corona is in contact with a medium with a temperature of 6,000 C and the other is exposed to cold space. Since heat only flows from warmer to colder things, where is the corona getting its heat? The only way the corona could be hotter than the substances with which it is in contact is if there is something generating heat within it.

In 1957, Parker proposed that whatever is heating the corona forces it to expand, driving a "solar wind". Back then, his idea was treated with derision. However, he was proven right.

Solar flares, coronal mass ejections and other explosive solar events are all driven by processes in that hot corona. Understanding what is going on there is not only a matter of scientific curiosity but could have enormous value in minimizing the disruption of our transportation, energy and communications infrastructure the sun causes.

Our growing dependence on instant world-wide communication is making us more vulnerable. It will take a while to digest the data coming back from the Parker Solar Probe, but we know it will teach us a lot about our star, especially what drives its bad behaviour and how to predict it.

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• After sunset, Venus shines low in the southwest with Saturn nearby. Jupiter shines high in the east. At the same time Mars is rising in the northeast.

• The moon will be full on Jan. 13.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.