On October 28, 2021, the universe put on an unexpected show. A massive solar eruption, known as a coronal mass ejection (CME), burst from the Sun and spread across such an immense area that it reached Earth, the Moon, and Mars. Though all three celestial bodies orbit the Sun, they were on opposite sides of it during this occurrence. The distance between Mars and Earth during this event was roughly 250 million kilometers.
Notably, this marks the first recorded instance of a solar event simultaneously measured on the surfaces of Earth, Moon, and Mars, as detailed in a recently published Geographical Research Letters paper. The data was collected by an international array of spacecraft, including ESA's ExoMars Trace Gas Orbiter (TGO), NASA's Curiosity Mars rover, the CNSA Chang'e-4 Moon lander, NASA's Lunar Reconnaissance Orbiter (LRO), and DLR's Eu:CROPIS Earth orbiter. These measurements provide crucial insights into the impact of solar eruptions and how a planet's magnetic field and atmosphere can offer protection.
A rare 'ground level enhancement' event
The incident on October 28, 2021, is categorized as a rare 'ground level enhancement' event. During these events, particles from the Sun have enough energy to penetrate Earth's protective magnetic bubble. While Earth has experienced 73 such instances since the 1940s, this event was unprecedented.
Unlike Earth, the Moon and Mars do not generate their own magnetic fields. This allows solar particles to reach their surfaces directly, interacting with the soil and producing secondary radiation. Mars, however, does have a thin atmosphere that can block the lower-energy solar particles and decelerate the highly energetic ones.
Understanding solar events and their potential impacts on human health is of paramount importance, especially as the Moon and Mars are targets for future human exploration. Astronauts risk radiation sickness, with radiation doses above 700 milligray potentially causing symptoms such as infection and internal bleeding via destruction of bone marrow.
On the extreme end, a dose exceeding 10 gray can be lethal within two weeks. For instance, a solar outburst in August 1972 could have given an astronaut on the lunar surface such a high radiation dose, but it occurred between the Apollo 16 and 17 missions, averting potential disaster.
However, during the October 28, 2021 event, the dose in lunar orbit, as measured by NASA's LRO, was a relatively benign 31 milligray. Jingnan Guo, the scientist researching the October event, cautions that the safe dose level on the Moon may be exceeded by an average of one event every 5.5 years if no radiation protection is in place.
The measurements taken by the ExoMars TGO and the Curiosity rover underscore the significance of Mars's atmosphere as a shield, with the rover recording a dose 30 times smaller than the one measured by TGO.
This recent event's impact was also felt by other ESA missions, including Solar Orbiter, SOHO, and BepiColombo, giving us even more perspectives to study this solar event.
The reality is that we are living in an era of profound exploration of our solar system, as commented by Marco Pinto, an ESA research fellow. Missions to Mercury and Jupiter provide a crucial view to study solar energetic particles' acceleration and propagation.
The importance of understanding and predicting such intense radiation events cannot be overstated, as this knowledge is fundamental to protecting astronauts as they venture deeper into space. Current protective measures on the International Space Station involve retreating to the sleeping quarters or the kitchen, where walls shield against radiation.
As we look toward a future of increased space exploration, incidents such as the solar event on October 28, 2021, emphasize the need for greater understanding of our solar system and the implementation of effective protective measures for our explorers in the final frontier.
