How a huge fleet of 17 spacecraft discovered something surprising about solar eruptions

A misbehaving coronal mass ejection (CME) that blew out a hidden cloud of charged particles aimed at Earth has been tracked by a record 17 spacecraft spread throughout the solar system, revealing the CME to be surprisingly lopsided.

CMEs are "burps" from the sun – huge clouds of magnetized plasma belched out from the sun's hot outer atmosphere, the corona, by the energy resulting from a solar flare. The magnetized CME cloud then expands out into the solar system, and the charged particles that the cloud contains are a significant radiation hazard to astronauts, spacecraft and even passengers on jet liners, but they also have a beautiful side as they can trigger the beautiful lights of the aurora when they intercept Earth.

CMEs occur on a regular basis, but one that erupted from the sun at 00:48 UT (7:48 p.m. ET) on Dec. 15, 2024 proved to be rather special.

"We used observations from 17 spacecraft to track and characterize this CME," Adrienn Luspay-Kuti of Johns Hopkins University Applied Physics Laboratory, who led the research that drew all the observations together, told Space.com. "This was a record number of spacecraft for tracking and characterizing a single CME, and gave us an exceptionally detailed view of how the CME evolved."

The previous record had been 10 spacecraft, but they had mostly been in a rough line from the sun through to the Earth and beyond, meaning that their measurements were somewhat one-dimensional. This time, for the December 2024 CME, the spacecraft were spread far and wide, not just at different distances from the sun, but also substantially wide of the Earth–sun line.

They revealed that the CME had two asymmetric lobes, one moving faster than the other. One of those lobes, which headed for Earth and Mars, would have gone unseen, obscured by the larger but slower lobe that left the sun at a tangent, were it not for the wide spread of the spacecraft.

"Our observations showed a fast lobe propagating through the Earth–Mars sector and a much slower lobe farther west toward STEREO-A," said Luspay-Kuti. STEREO-A is one half of NASA's two-spacecraft space weather monitoring system known as the Solar Terrestrial Relations Observatory.

The onset of the CME was seen by the joint NASA–ESA Solar and Heliospheric Observer (SOHO), which has been constantly monitoring the sun for more than 30 years. Yet it only saw the slower lobe that erupted at an angle to Earth; the faster lobe heading for our planet was missed, obscured by the slower, larger lobe.

The CME was next detected at a distance from the sun of 0.35 astronomical units (AU; 1 AU is the distance of Earth from the sun) on Dec. 16 moving through space near Mercury and the European Space Agency's BepiColombo mission (which finally arrives into orbit around the innermost planet in November 2026).

The next detection, on Dec. 17, was of the "hidden" component arriving at Earth, where a multitude of spacecraft picked it up. The CME was not strong enough to produce significant aurorae. Intriguingly Europe's Solar Orbiter mission, on an elongated orbit around the sun and at the time 0.94 AU from the sun and just 10 degrees off the Earth–sun line, did not detect the CME. This non-detection was actually vital since it helped to constrain the shape of the CME.

Then on Dec. 18, the slower moving lobe reached NASA's STEREO-A spacecraft, which orbits the sun at the same distance as Earth — 1 AU —- but substantially ahead of Earth in its orbit. The lobe that intersected the Earth had an average velocity of 522 miles (840 kilometers) per second, but the other lobe dragged its feet, moving at an average of 332 miles (534 km) per second from the sun and past BepiColombo and STEREO-A. In fact, by the time it reached STEREO-A, it had slowed to about 248.5 miles (400 km) per second. Both lobes decelerated as a result of friction with the regular solar wind that the CME was overtaking, and the range of velocities measured in the CME told researchers that the solar eruption was not traveling as a single, unified front.

Beyond Earth NASA's Europa Clipper mission, on which Luspay-Kuti is the Principal Investigator for the spacecraft's Plasma Instrument for Magnetic Sounding (PIMS) experiment, detected the faster moving lobe at 1.19 AU as the spacecraft was cruising to Mars for a gravity assist to help it on its journey to Jupiter. At the red planet, the now-defunct MAVEN mission also detected the CME on Dec. 19 to Dec. 20.

To have not only so many spacecraft follow the progress of a CME but also enough spacecraft off the Earth–sun line to measure the shape of the CME, at least in two dimensions, is a significant step forward in understanding and forecasting the propagation of CMEs.

"This matters for future human exploration because a missed CME can mean losing valuable warning time," said Luspay-Kuti. "Fast CMEs can drive shocks that accelerate high-energy particles, which can pose a radiation hazard to astronauts outside Earth's protective magnetic field. This is why observations from multiple viewpoints, including spacecraft away from the sun–Earth line and planetary missions operating during their cruise phase, will become increasingly important for space-weather forecasting as human exploration moves further from Earth."

The asymmetrical double-lobed structure of this particular CME was certainly a surprise. The cause of the asymmetry remains unclear at present, though Luspay-Kuti told Space.com that researchers are investigating. The main question is, how frequent are asymmetric CMEs?

"In the context of previous observations, this event is at the most extreme end of observed CME variability," said Luspay-Kuti. "Are highly asymmetric CMEs actually fairly common but we fail to recognize them because we don't have enough observational coverage, or are they genuinely rare?"

The 17 spacecraft that detected or imaged the CME across some or all of its various stages – the bow shock and turbulent sheath ahead of the magnetic cloud, the CME itself and the turbulent wake left in the solar wind after its passing – were the following: SOHO (initial imaging), BepiColombo, NASA's Solar Dynamics Observatory, STEREO-A, the four spacecraft of the Magnetospheric Multiscale (MMS) mission, the two spacecraft of the ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the sun) mission, NASA's Wind, ACE (Advanced Composition Explorer), GOES (Geostationary Operational Environmental Satellite) and DSCOVR (Deep Space Climate Observatory) missions, Europa Clipper, MAVEN and Solar Orbiter, the latter of which made measurements but did not detect the CME.

In the future, these missions will be joined by the European Space Agency's Vigil mission that will watch for space weather when it launches in 2031 for the sun–Earth L5 Lagrange point, 60 degrees behind Earth in its orbit and therefore providing additional off-axis monitoring.

The findings were published on Aug. 19 in the journal Science Advances.