Like The Avengers of outer space, an all-star cast of satellites and spacecraft have teamed up from across the Solar System for a common goal. After a coronal mass ejection (CME) erupted from the Sun in October 2014, its journey through space was tracked by Curiosity, Rosetta, Cassini, New Horizons and possibly even Voyager 2, allowing astronomers to study the phenomenon in much greater detail than ever before.
During periods of high solar activity, CMEs often burst from the Sun as waves of particles and plasma, triggering auroras and occasional electronic chaos when they wash over Earth. Normally, NASA watches for this wild space weather using solar observatories like SOHO and the STEREO mission, a pair of solar-orbiting satellites pointed at the Sun from two different angles.
But over a 17-month period from October 2014 to March 2016, astronomers watched a CME travel from the Sun to the outer reaches of the Solar System with the help of 10 different NASA and ESA spacecraft. That allowed them to better study how the material moves through space, and how that movement changes over vast distances.
"If you have just one data point, you can simulate that easily, because you only have to validate that one point," says Leila Mays, an author of the study. "Once you get more data, you can put together more pieces of the puzzle."
The CME in question first came into focus while astronomers were observing comet Siding Spring as it swung past Mars close enough to disrupt the Red Planet's magnetosphere. Unfortunately, this once-in-a-million years event was rained out, thanks to a solar storm.
"We found at the time of the comet's passing, there was some solar wind disturbance around Mars," says Olivier Witasse, lead author of the new study. "Which was a bit of a shame because we wanted to see the effects of the comet on the atmosphere."
But there was a silver lining to that disappointment. The team used experimental forecasting from NASA's Community Coordinated Modeling Center to figure out the path of the intruding CME, and realized that this particular ejection would also have washed over the Rosetta spacecraft, which was at the time orbiting comet 67P.
"From there, it was an exciting chase to see where else the CME might have hit," says Mays. "Sometimes the impacted spacecraft's instruments weren't on, but we were able to gather other housekeeping data."
In the end, by searching through existing data the researchers were able to confirm seven direct detections of the same event, one indirect detection and two possible but unconfirmed observations. Armed with all that data, the team pieced together the lifespan of the CME.
It was launched from the surface of the Sun on October 14, 2014, and two days later ESA's Venus Express was the first to pick it up – that's the indirect detection, confirmed through so-called "housekeeping data." That same day, it was also spotted by STEREO-A, which orbits the Sun at roughly the same distance as Earth.
The following day, October 17, the wave washed over Mars, where it was detected by ESA's Mars Express satellite, NASA's MAVEN and Odyssey orbiters, and the Curiosity rover on the surface. After that, on October 22, the CME was detected by the Rosetta spacecraft's magnetometer and other instruments. On November 12 it was Cassini's turn, as the spacecraft zipped around Saturn.
After that, judging by the CME's path, the team believes the wave would have caught up to New Horizons – which was at the time en route to its flyby of Pluto – at some point between January 18 and February 14, 2015. Unfortunately, the craft isn't carrying a magnetometer, but there was an increase in the solar wind speed and temperature, which the team says "very likely" indicates a CME passing by.
And finally, the CME may have swept over Voyager 2 on its way out of the Solar System. Being so far out in space – 110 times the distance between Earth and the Sun – it's hard for astronomers to be sure if what they picked up was the same event, but a sudden change in particles in late March 2016 does seem to correlate with the expected arrival time of the CME.
"Once a CME travels that far from the Sun, it gets squeezed between large, merged interaction regions in the solar wind, so it's not as easy to determine exactly what's going on," says Mays.
We still have a lot to learn about the natural hazards of space exploration, and the researchers say that watching how the solar winds change over those vast distances can help us develop ways to better protect our equipment and astronauts.
The study was published in the Journal of Geophysical Research, and the CME's journey through the Solar System can be seen in the video below.
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