Parker Solar Probe's Sun‑skimming triumph redefines space exploration

We are concerned about climate change on Earth, economic upheavals, and military and social conflicts. However, these are matters over which we have some influence and that occur relatively close to us. Meanwhile, at an average distance of 150 million kilometres from Earth, sits a celestial body whose existence not only enabled our civilization but may also shape its fate on a global scale and over the long term. The Sun is the most familiar star in the cosmos, yet so little known that we continue to send space missions toward it.

Humans are gradually starting to prepare to leave their habitat, Earth, which has been home for millions of years. As we venture into space, protection against solar radiation and wind, plasma ejected by our star, will become a key challenge in astronautics.

But even on Earth, we must remain vigilant. Already, thousands of satellites are 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 a gentle "strike," we need to have the best possible understanding of how it operates. That’s why the Parker Solar Probe was developed, achieving a record approach on December 24, 2024.

To appreciate the astronomers' achievement, consider that sunlight reaches Earth in eight minutes. Solar wind propagates much slower, but the ejected matter also reaches us quite 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, this comparison highlights an extraordinary opportunity now given to heliophysicists to study the Sun and its activities.

Currently, we know the probe has survived its closest approach and continues its mission. Telemetry data is expected by New Year’s Day (January 1, 2025), but this is not the end. Scientists will be analyzing the observations for many weeks, or even longer, before publishing their conclusions. Earlier discoveries suggest that they will once again be groundbreaking for our knowledge about the Sun.

Initially, astronomers hoped the Parker Solar Probe would come even closer to the Sun. One of the first probe designs from the 1990s planned to use gravitational assistance from Jupiter to position the vehicle at about 1.9 million kilometres from the Sun. However, costs and vehicle complexity stood in the way. During its extended mission starting in the second half of 2025, the Parker Solar Probe may continue close flybys as long as we are able to control it, but it will maintain its current orbit.

A mission almost touching the surface of the Sun was envisioned from the start of the space flight era in 1957. In December 1974, the Helios 1 probe was sent toward the Sun, and a little over a year later, in January 1976, Helios 2 followed. They approached to 46.5 and 42.6 million kilometres respectively. It was still a significant distance, just slightly less than one-third of the Earth-Sun distance, but also about 9.7 million kilometres less than the size of Mercury's orbit. A similar distance was achieved by the European Solar Orbiter, which currently studies the Sun over its poles.

The record set by Parker Solar Probe was enabled by technologies developed in the late 1990s, including carbon composite foam resistant to high temperatures. It took years before we could use it to create an 11-centimetre shield protecting the valuable electronics of the probe from intense radiation. Such strong protection is necessary because, at a distance of 6.1 million kilometres from the surface of the Sun, every 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.

The surface of the Sun has a temperature of approximately 5,500 degrees Celsius, while its surrounding corona is heated to over a million degrees. According to forecasts, the Parker Solar Probe's thermal shield reached 980 degrees Celsius. At this temperature, silver melts, and gold and copper at only about 100 degrees higher. These aren't the highest temperatures experienced by Earth vehicles—during re-entry, the Space Shuttle's ceramic tiles reached 1,650 degrees Celsius, but the shield of the Orion vehicle in the Artemis 1 mission experienced even 2,760 degrees Celsius, due to its more than 40% higher re-entry speed than the shuttle.

An Earth landing vehicle doesn't have to perform many tasks, while the Parker Solar Probe, when exposed to the greatest inferno, must fulfil an observation plan. The shield must 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 don’t fare well under excessive radiation even on Earth. The photovoltaic panels were designed so they could dissipate 13 watts of unnecessary heat for every watt of power obtained during approach to the Sun, 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 touched the cells at a slight angle when the probe is close to the Sun. Meanwhile, the probe can generate energy more efficiently when it reaches the farthest point of its orbit. Heat is dissipated from the bottom of the panels through channels with ultra-pure water to cooling systems.

This is not all. Near the Sun, we encounter emerging and accelerating solar wind, as well as dust left from comets that break apart under the Sun's immense gravity, moving at speeds of hundreds of kilometres per second. The probe may also encounter a coronal mass ejection (CME), where plasma can reach speeds of up to 3,000 kilometres per second. This occurred in September 2022, but the CME, in which matter moved at 1,350 kilometres per second, did not damage the probe.

If a similar stream of matter hit Earth, we would experience a geomagnetic storm akin to the largest documented one from 1859 (the so-called Carrington Event). Nour Raouafi, a scientist on the Parker Solar Probe project, believes that the potential destruction from such a large and fast CME could be enormous. That’s why the Parker Solar Probe continues its mission: to help humanity learn to decode the signals the Sun emits, which we can record from orbit and Earth, and integrate these into computer models and space weather forecasts for greater safety of our civilization.