NASA’s Europa Clipper, a spacecraft en route to conduct the first detailed study of Jupiter’s moon Europa, detected something strange on December 19, 2024. It had been about two months since the orbiter launched from the Kennedy Space Center, and the mission team was conducting a routine check of its Plasma Instrument for Magnetic Survey (PIMS). It turned out to be anything but routine.
PIMS, designed to study plasma density, temperature and flow near Europa, detected a fast solar wind with hotter and less dense plasma than usual, prompting the team to take a closer look.
“At this point in the mission, one of our main goals was just to make sure the instrument behaved the way it was supposed to,” PIMS principal investigator Adrienn Luspay-Kuti told Gizmodo. “So when we saw an unexpected plasma signal, we needed to understand it well enough to be sure that the PIMS was actually measuring the space environment and not an artifact of the instrument or spacecraft.”
After confirming that PIMS detected a signal from the space environment, Luspay-Kuti conducted an investigation to determine what exactly Europa Clipper had gone through. The findings, published today in the journal Science Advances, are based on observations from 17 different spacecraft across the inner solar system and reveal a potential blind spot in space weather forecasting that could endanger future crewed missions.
Solve a solar mystery
When Europa Clipper’s PIMS detected this unexpected signal, the probe was heading toward Mars and positioned itself approximately in alignment with Earth. The spacecraft was more than 27 million kilometers from the Sun, and the plasma characteristics measured by PIMS were atypical for the solar wind at this distance. However, they are generally associated with structures produced by coronal mass ejections (CME).
CMEs are enormous expulsions of plasma and magnetic field from the Sun’s corona, the outermost layer of its atmosphere. When aimed at Earth, they can trigger disturbances in the planet’s magnetosphere called geomagnetic storms. Space meteorologists monitor the Sun’s activity to predict these storms, because they can disrupt power grids, jam satellite communications, cause radio outages and pose radiation risks to astronauts in orbit.
“Once we knew the PIMS signal was real, we started looking back to determine where it was coming from,” Luspay-Kuti explained. “We discovered a large filament flare on the Sun a few days earlier that produced a CME, and then we looked at observations from spacecraft at different locations in the inner solar system.”
The researchers used these 17 spacecraft as a giant network of observation points to map the shape and size of the CME, track its movement and measure its physical properties. This wealth of data revealed that this was a more complex process than a traditional CME. Rather than behaving like a bubble extending evenly from the Sun in all directions, it had developed a very irregular shape.
From Earth’s view of the Sun, most of the CME appeared to be moving south and away from our planet, so space weather forecasters expected us to miss it. But NASA’s STEREO-A solar orbiter observed the flare from the side.
This vantage point allowed him to spot another part of the CME heading toward Earth, something that was not visible from our planet. This is what triggered the unusual PIMS detection. Scientists could not have predicted this part of the CME without observations from spacecraft positioned away from the direct line between Earth and the Sun, revealing a concerning limitation in space weather forecasting.
Planetary missions can cover the blind spot
This Earth-directed component of the CME also reached Mars. NASA’s MAVEN orbiter, which studied the Red Planet’s atmosphere from 2014 to late 2025, picked up its signal.
The researchers report no damage to Europa Clipper or MAVEN caused by this part of the CME, but “the important point is that Europa Clipper was traveling through the region between Earth and Mars where future astronauts might one day travel,” Luspay-Kuti said. “This gave us a real-world example of the type of space weather environment a crewed mission can encounter, and the importance of getting good forecasts.”
The greatest risk to astronauts would be a sudden increase in radiation exposure. “Fast CMEs can generate shock waves that accelerate charged particles to very high energies,” Luspay-Kuti explained. “These particles can penetrate spacecraft shielding and pose a serious radiation risk to astronauts once they are outside of Earth’s protective magnetic field.” CMEs can also disrupt spacecraft systems and communications.
Advance warning of the arrival of a CME would give the crew time to shelter in a more protected part of the spacecraft, adjust operations, or take other steps to protect themselves. “If the event is not planned, you lose that preparation time,” Luspay-Kuti said.
Next, she and her colleagues hope to determine whether highly distorted CMEs are rare or simply overlooked because scientists don’t have enough observation points to detect them. This will help them understand how much of a threat they might pose to future missions. They also want to see if current forecast models can reproduce this type of CME behavior.
“This study highlights the role that planetary missions can play in the space weather observation network,” Luspay-Kuti said. A spacecraft positioned away from the Sun-Earth line could fill observation gaps. This will prove particularly valuable as humanity ventures deeper into the solar system.
