© NASA/JPL-Caltech
This artist's impression depicts Comet Siding Spring narrowly miss Mars in 2014.
When Comet C/2013 A1 (Siding Spring)
swung past the Red Planet in October 2014, it was an unprecedented opportunity
for an armada of Mars robots to have a ringside seat of the interplanetary spectacle.
But as dazzling as the flyby was, the real drama wasn't seen by the cameras of Mars orbiters or rovers; it was detected by a magnetometer. And that magnetometer, located 100 miles above the Martian surface, detected chaos.
"Comet Siding Spring plunged the magnetic field around Mars into chaos," said Jared Espley, of NASA's Goddard Space Flight Center in Greenbelt, Md. and science team member of NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft,
in a NASA press release. "We think the encounter blew away part of Mars' upper atmosphere, much like a strong solar storm would."
Although Mars' magnetic field is weak and patchy (unlike Earth's strong, global magnetosphere), MAVEN's sensitive magnetometer detected a huge upheaval in orbit as Siding Spring's own magnetism rattled the planet's magnetic field.
The comet's nucleus may only be a third of a mile wide, but the atmosphere surrounding the nucleus (known as the coma) was as wide as 600,000 miles when it encountered Mars. (The coma is formed through solar heating — the ices contained within a comet's nucleus sublimate into space, pumping the coma with gas.) Through interactions with the solar wind, comets also generate their own magnetic fields that loop their way through the coma.
So when Siding Spring buzzed Mars, coming as close as 87,000 miles, the cometary magnetism punched Mars' weak magnetic field, sending it into violent turmoil for several hours.
Comment: See also: Who are the real machines? People follow robots in emergency situations, even when proven untrustworthy