Parker Solar Probe breaks barriers in Sun's corona quest
On Tuesday, 24 December 2024, NASA's Parker Solar Probe achieved something unprecedented. It became the first Earth-originating vehicle to approach within about 6.5 million kilometres of the Sun, while simultaneously racing through its surrounding corona at a speed of 192 kilometres per second. This success might not be fully appreciated today, but future generations will surely understand its value.
We are concerned about climate change on Earth, economic upheavals, and military and social conflicts. However, these are matters over which we can exert some influence and which occur relatively close to us. Meanwhile, at an average distance of 150 million kilometres from Earth, there is a celestial body whose existence not only enabled the rise of our civilization but may also shape its future on a global scale and in the long term. The Sun is the best-known star in the cosmos, yet still so mysterious that we continue to send space missions towards it.
Such an important success that not everyone realises it
Humans are gradually preparing to leave their habitat, which has been Earth for millions of years. As we venture into space, protecting against the effects of solar radiation and wind, ejecta from our star, will become a key challenge in astronautics.
Even on Earth, vigilance is necessary—there are already thousands of satellites in orbit, and Earth's energy or telecommunications infrastructure is susceptible to solar activity. To predict when the Sun will "strike," even if it's just a gentle "strike," we need the best possible understanding of how it operates. That’s why the Parker Solar Probe was developed, which on 24 December 2024, got very close to the Sun.
To grasp the significance of the astronomers' achievement, consider that sunlight reaches Earth in eight minutes. Solar wind propagates much slower, but the ejected matter also reaches us relatively quickly, within several hours to a few days. The record approach distance of 6.1 million kilometres is covered by photons in just under 21 seconds. While the physics of matter and energy propagation from the Sun is not as straightforward as a straight line, the above comparison highlights the extraordinary opportunity now provided for heliophysicists to study the Sun and its activities.
Currently, we know the probe has survived the closest approach and continues its mission. Telemetry data are expected to be transmitted by New Year's Day (1 January 2025), but this is just the beginning. Scientists will analyse the observations over many weeks or even longer before publishing their first conclusions. Earlier discoveries suggest they will once again be groundbreaking for our understanding of the Sun.
Why only now?
Initially, astronomers hoped the Parker Solar Probe would approach the Sun even closer. One of the early designs from the 1990s planned to use gravitational assistance from Jupiter to place the vehicle into orbit at a distance of about 1.9 million kilometres from the Sun. However, costs and the complexity of the vehicle stood in the way. During the extended mission, starting from the latter half of 2025, the Parker Solar Probe may continue close flybys, as long as we can control it, but it will maintain its current orbit.
A mission almost touching the surface of the Sun was envisioned from the early space flight era in 1957. In December 1974, the Helios 1 probe was sent towards the Sun, and a little over a year later, in January 1976, Helios 2 followed. They approached to 46.5 and 42.7 million kilometres respectively. It is still a significant distance, just slightly less than one-third of the Earth-Sun distance, but also about 10 million kilometres less than the size of Mercury's orbit. A similar distance was achieved by the Helios 2, and the European Solar Orbiter, which currently studies the Sun from a vantage point over the poles of our daytime star.
The record set by the Parker Solar Probe was made possible by technologies developed at the end of the 1990s, including a carbon composite foam resistant to high temperatures. However, it took further years to learn to use it to make an 11-centimetre shield protecting the valuable electronics of the probe from intense radiation. Such robust protection is necessary because at a distance of 6.1 million kilometres from the surface of the Sun, each square metre perpendicular to the Sun-Earth direction is illuminated with a power of 650 kW. On Earth, in ideal conditions, when the Sun is directly overhead, it would be 500 times less.
Hell near the Sun is not just radiation
The surface of the Sun has a temperature of about 5,500 degrees Celsius, and the surrounding corona is heated to over a million degrees Celsius. According to forecasts, the thermal shield of the Parker Solar Probe reached 980 degrees Celsius. At this temperature, silver melts, and gold and copper at only about 100 degrees higher. These are not the highest temperatures experienced by Earth vehicles—during reentry into Earth's atmosphere, the ceramic tiles of the shuttle reached 1,650 degrees Celsius (theoretical limit for the Parker Solar Probe shield), and the shield of the Orion vehicle in the Artemis 1 mission even reached 2,760 degrees Celsius (due to, among other factors, more than 40% higher reentry speed than the shuttle).
A landing Earth vehicle does not have many tasks to perform, whereas the Parker Solar Probe, when exposed to the greatest inferno, must fulfil an observation plan. The shield has to survive repeated, not just one-time, close approaches to the Sun. Yet the entire probe at launch weighed 685 kilograms, several times less than just the thermal shield of the Orion capsule.
The observation plan requires power, which is supplied by solar panels that do not fare well with excessive radiation even on Earth. The photovoltaic panels were designed so that during approach to the Sun, they could dissipate 13 watts of unnecessary heat for every 1 watt of power obtained, with minimal degradation during the mission. We will only find out how unfavourable the record flyby was for them.
Foldable and tiltable panels were used, so that radiation only struck the cells at a slight angle when the probe is close to the Sun. At the same time, the probe can generate energy more efficiently when it reaches the farthest point of the orbit. Heat is dissipated from the bottom of the panels through channels with ultra-pure water to cooling systems.
This is not all—in the vicinity of the Sun, we deal with emerging and accelerating solar winds, dust left from comets that disintegrated under the immense gravity of the star, moving at speeds of hundreds of kilometres per second. The probe may also encounter a coronal mass ejection (CME), in which plasma can reach speeds of up to 3,000 kilometres per second. This happened in September 2022, but the CME, where the matter had a speed of 1,350 kilometres per second, did not damage the probe.
However, if a similar stream of matter encountered Earth, we would experience a geomagnetic storm similar to the largest documented one from 1859 (the so-called Carrington Event). Nour Raouafi, a scientist on the Parker Solar Probe project, believes the potential destruction from such a large and very fast CME would be colossal. That’s why Parker Solar Probe continues its mission, to help humanity learn to interpret the signals the Sun emits, which we can record in orbit, from Earth. We can then incorporate them into computer models and space weather forecasts for greater safety of our civilization.
