Due to the immensity of interstellar distances, research on the powerful shocks created by exploding stars, or supernovae, was always limited to computer modelling and observations across entire galaxies.
Now, however, scientists might finally get a chance to explore the phenomenon from up close – the Cassini-Huygens space probe, sent on a mission to orbit Saturn for 11 years, had picked up a handful of unusually strong shock waves, called “bow shocks” due to their resembling the wave that forms in front of a moving ship, generated by the supersonic solar wind interacting with the planet’s magnetosphere, a finding which might yield some new insight into the nature of plasma (the most abundant form of ordinary matter in the Universe) and the remnants of dying stars.
Cassini-Huygens is the only spacecraft currently orbiting a planet in a region where Mach values (a measure of shock strength) are significantly higher than those recorded at Earth – the Mach value of Earth’s bow shock is typically less than 10, whereas supernova shocks are in the 100s.
Details of the study were published in the journal Physical Review Letters, where the research team, headed by scientists at the Imperial College of London, discuss their analysis of 54 crossings of Saturn’s bow shock where Mach numbers above 25 were recorded, including events that reached approximately Mach 100.
“Saturn’s bow shock is proving to be a unique laboratory, giving us a rare insight into the physical processes driving ultra-strong shock waves. With Cassini’s ongoing exploration of Saturn’s environment, we have had the opportunity to encounter some of the strongest shocks ever,” said project leader Ali Sulaiman from the Department of Physics at Imperial.
Thanks to this riveting opportunity, researchers had been able to better determine the timescales of ions as they are reflected at the shock front – one of the keys to understanding the processes behind this phenomenon.
The shock waves in question could also help the team gain a fresh perspective on plasma – a notoriously volatile “soup” of free ions and electrons that has proved difficult to probe into back on Earth. “Like studying waves in air or water, studying shock waves in plasma is fundamental to understanding its collective behaviour,” explained Sulaiman.
Plasma shock waves are abundant in the Universe, formed as comets speed through the solar wind or manifesting as solar flares erupting from the Sun. According to Sulaiman, the new data streaming from Cassini could help the researchers finally bridge the gap between interplanetary and intergalactic regimes.