Space exploration often consists of projects that can sometimes verge on the extreme, such as controlling a robot on Mars from Earth or attaching a miniature rover onto the surface of a comet. In this same vein, NASA is preparing to launch a spacecraft none other than the Parker probe straight towards the Sun. It will go into orbit around our star, close enough to plunge into its atmosphere.
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The launch of the probe is scheduled for August 11th from the Kennedy Space Center in Florida. Then a long adventure lies ahead for the Parker probe. After passing past Venus to get onto the right trajectory, the probe should make its first visit to the Sun in November 2018. In total, 24 flybys of this type are planned between 2018 and 2024, in the part of the atmosphere called the solar corona.
The closest one of these flights to the sun will be 0.04 astronomical units, or almost 4 million miles from the surface of the Sun. That’s 10 times closer than Mercury’s orbit and 7 times closer than the current record holders, the Helios probes that were launched in the mid 70's. ‘The probe will travel through the atmosphere of the Sun, closer to its surface than any other spacecraft before it,’ says NASA in a statement.
This manoeuvre will expose the probe ‘to extreme heat and strong radiation’ but it will also allow ‘humanity to learn from the closest observations of a star ever made,’ the US space agency continued. To withstand these extreme conditions and relay the long-awaited information, the probe is equipped with a high-performance heat shield.
A shield to withstand temperatures exceeding 1000 ° C
One of NASA's engineers, Betsy Congdon, has been working on this mission for 10 years, building an object that will fly dangerously close to the Sun. For the sake of the Parker mission, Betsy and her team made a thermal shield measuring 8 feet in diameter and about 4 inches thick, made of a carbon alloy.
During its flights near our star, the role of the heat shield is to protect the fragile electronics on board the probe. The temperatures at the surface of the shield will climb up to 1370 ° C. This heat comes from the radiation of the nearby Sun, whose surface reaches 5500 ° C. Betsy Congdon is confident; ‘in our tests, we showed what [the probe] was made of.’
As heavy as an adult human, the shield consists of several layers. The interior is made up of what is called a ‘carbon foam,’ made of carbon molecules that are spaced apart enough to create an aeronautic material. This foam is placed between two sheets of carbon fibres made up of specific properties, which become more resistant when the temperature reaches one thousand degrees. This same element is found on the nose of space shuttles, to protect them when they enter or leave the Earth's atmosphere.
It is therefore behind this protection that the instruments are hidden. But the probe also has solar panels that, when exposed to radiation, will power the satellite when its orbit takes it to less hot areas a little further from the Sun. Despite the conditions, the objective remains to make observations. The Parker mission is very important for astronomers, as it could provide answers to questions that have been asked for decades.
Parker's goal: to find an explanation for solar winds
How do solar winds, these currents of particles from the solar corona, form and escape into space? This question remains unanswered since they were first described in 1958 by Eugene Parker, whom the probe is named after.
The surface of the Sun is perpetually deformed by ejections of matter in the form of plasma. Plasma is a certain state of matter similar to that of gas, but composed of charged and extremely hot particles. Sometimes, some of these particles escape from our star’s atmosphere, and spread throughout the solar system. The problem is that the solar winds seem to challenge the basic laws of physics.
Once expelled from the solar corona, the particles should cool down and slow down as they move away from the Sun. But this is not the case, an unexplained mechanism always pushes them further. The Parker probe will be able to identify how these solar winds are formed, and perhaps explain the mechanism that allows the particles to escape.
Better understanding this phenomenon could help improve the predictions of when solar storms will occur, which cause problems on our planet. When they reach the Earth, most particles are deflected by the earth's magnetic field. But the strongest solar storms can cause problems for satellites in orbit and their power grids. However the solar corona from which the solar winds are derived also defies the principles of physics.
How can the solar corona exceed one million degrees Celsius?
What heats the plasma to a temperature 200 times higher than that of the Sun's surface? The latter reaches ‘only' 5500 ° C while the plasma located in the solar corona can exceed one million degrees. The corona, however, is further away from the heat source, and should thus not be as hot.
Scientists have proposed several scenarios that could explain how the magnetic field could lead to this extreme warming of the gas. Unlike everyday materials, plasmas are guided by the magnetic field, and move along its lines. In one way or another, the energy that heats the solar corona passes through the lines of the magnetic field.
To explain this strange phenomenon, several theories have been put forward. Either the magnetic field allows for large energy conductions between the surface of the Sun and the corona, and when large energy discharges occur, they heat the plasma, or the heat conduction is produced by oscillations of this same magnetic field which will heat the plasma particles in the corona.
Two other instruments will participate in this survey
The Parker probe will not be the only instrument which will be carefully studying the Sun's atmosphere in order to shed light on these questions over the next 6 years. The new Daniel K. Inouye Solar Telescope (DKIST) installed in Hawaii, capable of taking very accurate images, will be usable from 2020 on. Solar Orbiter, another probe launched by the European Space Agency, will observe the radiation that propagates in the solar corona from a little further away.
‘This is an absolutely seminal moment for the physics of the Sun,’ says Valentin Martinez Pillet, director of the US National Solar Observatory, which is responsible for building the DKIST telescope. ‘This is a once in a lifetime opportunity to combine these instruments along with scientific knowledge.’ The prospects are far reaching, but the Parker probe must still be able to resist the infernal heat of our star.
