A coronal mass ejection observed in December 2024 was tracked by 17 spacecraft across the solar system, revealing an unexpected double-lobed structure with significant speed differences between its components
In December 2024, a coronal mass ejection (CME) from the Sun was observed and tracked by an unprecedented network of 17 spacecraft distributed throughout the inner solar system. This coordinated observation campaign provided the most detailed multi-point measurements yet of a single CME, revealing that the event was far from symmetrical. Instead, the ejection consisted of two distinct lobes, each moving at a different speed and following separate trajectories-an arrangement not previously resolved in such detail.
Multi-Spacecraft Tracking
The CME was first detected on December 15, 2024, at 00:48 UT, when it erupted from the Sun's corona. The Solar and Heliospheric Observatory (SOHO), a joint NASA-ESA mission that has monitored the Sun for over three decades, recorded the initial outburst. However, SOHO's vantage point only captured the slower, more prominent lobe, missing a faster-moving component that was obscured along the Sun-Earth line. The wide spatial distribution of spacecraft-including BepiColombo near Mercury, STEREO-A ahead of Earth's orbit, and Europa Clipper en route to Jupiter-enabled researchers to reconstruct the CME's full three-dimensional structure.
By December 16, the CME had reached 0.35 astronomical units (AU) from the Sun, passing near BepiColombo. On December 17, the faster, previously hidden lobe arrived at Earth, where multiple spacecraft-including Wind, ACE, GOES, and the Magnetospheric Multiscale mission-recorded its passage. The slower lobe, meanwhile, continued outward and was detected by STEREO-A on December 18. The ability to observe the CME from multiple off-axis positions was critical for distinguishing the two lobes and measuring their differing velocities.
Velocity and Structure Measurements
Analysis of the data revealed that the lobe intersecting Earth and Mars traveled at an average speed of 840 kilometers per second, while the slower lobe, moving toward STEREO-A, averaged 534 kilometers per second and decelerated further as it propagated. By the time it reached STEREO-A, its speed had dropped to approximately 400 kilometers per second. Both lobes experienced deceleration due to interaction with the ambient solar wind, but the marked difference in their initial velocities and spatial separation indicated that the CME was not a single, coherent front.
Europa Clipper, at 1.19 AU and on its way to a Mars gravity assist, detected the faster lobe as it moved through the solar system. At Mars, the now-retired MAVEN mission also recorded the CME's arrival between December 19 and 20. Notably, the Solar Orbiter, positioned just 10 degrees off the Sun-Earth line at 0.94 AU, did not detect the CME, providing a crucial non-detection that helped constrain the CME's geometry. The event's double-lobed, asymmetric structure stands out as one of the most extreme examples of CME variability yet documented.
Implications for Space Weather Forecasting
The ability to track a CME with such a large and widely distributed fleet of spacecraft marks a significant advance in space weather monitoring. Previous multi-spacecraft studies were limited by the alignment of available probes, often providing only one-dimensional information along the Sun-Earth line. In this case, the off-axis observations were essential for identifying the hidden, faster lobe that would otherwise have gone undetected. This has direct implications for space weather forecasting, as missing a fast-moving CME component could reduce warning times for astronauts and spacecraft operating beyond Earth's magnetic protection.
The findings also highlight the importance of planetary missions and spacecraft in cruise phase for heliophysics research. As human exploration extends further from Earth, the need for multi-point, multi-directional monitoring of solar activity will become increasingly critical for mission safety and operational planning. The study's results were published in Science Advances on August 19, 2026, and add to a growing body of work on the complex dynamics of solar eruptions. For context on how multi-instrument observations have advanced our understanding of extreme astrophysical phenomena, see this report on the fastest known star orbiting the Milky Way's black hole.
Unresolved Questions and Future Missions
The cause of the CME's pronounced asymmetry remains uncertain. Researchers are now investigating whether such double-lobed structures are rare or simply underreported due to limited observational coverage in past events. The December 2024 CME represents the most extreme end of observed variability, but it is not yet clear how common these asymmetric ejections may be. The upcoming European Space Agency Vigil mission, scheduled to launch in 2031 to the Sun-Earth L5 Lagrange point, is expected to provide additional off-axis monitoring that could help resolve these questions.
The 17 spacecraft involved in this study included SOHO, BepiColombo, Solar Dynamics Observatory, STEREO-A, the Magnetospheric Multiscale mission, ARTEMIS, Wind, ACE, GOES, DSCOVR, Europa Clipper, MAVEN, and Solar Orbiter. Their combined measurements covered the CME's bow shock, turbulent sheath, magnetic cloud, and wake, offering an unprecedented view of the event's evolution across the inner solar system.
Understanding the true frequency and structure of asymmetric CMEs will be essential for improving predictive models and safeguarding future crewed and robotic missions beyond Earth orbit.
Coronal mass ejections are immense clouds of magnetized plasma expelled from the Sun's outer atmosphere, often following solar flares. When a CME is launched, it travels through the solar system, interacting with the solar wind and planetary magnetic fields. Instruments on spacecraft detect CMEs by measuring changes in plasma density, magnetic field orientation, and energetic particle flux. The ability to observe a CME from multiple vantage points allows scientists to reconstruct its three-dimensional structure and propagation, revealing complexities-such as asymmetric lobes-that are invisible from a single line of sight. This multi-point approach is increasingly vital for understanding and forecasting the impact of solar activity on space-based infrastructure and exploration.
