According to data harvested by NASA's Cassini spacecraft during its historic first Grand Finale dive on April 26, the gap between Saturn's upper atmosphere and inner rings is a near-empty expanse of space. Very few particles struck the spacecraft during its maiden plunge, and the findings are likely to significantly alter how Cassini behaves in subsequent Grand Finale dives.
Despite their diminutive size, tiny dust and ring particles have the capacity to seriously damage a spacecraft. Although it is likely that most particles inhabiting the gap between Saturn and its rings are very small, many less than 1 micron across, an impact with a larger specimen and Cassini could seriously risk the health of the aging wanderer.
During its first Grand Finale dive, Cassini was traveling at roughly 77,000 mph (124,000 kph) as it passed through Saturn's ring plane. As no spacecraft had traversed the 1,500 mile (2,000 km) wide expanse prior to April 26, the risk of the unknown and a potential particle encounter was considered significant enough to warrant extra precautions.
To better protect Cassini, the spacecraft's handlers orientated the probe's 4-meter-wide (13-ft) antenna to face the direction that any potential particles would strike – essentially using the dish as an ad hock shield. This measure limited when and how Cassini could observe Saturn and its rings, as the spacecraft was essentially locked with its antenna facing forward.
However, despite the impressive size of the dish, two of Cassini's instruments, the Radio and Plasma Wave Science (RPWS) instrument, and the spacecraft's magnetometer, extend beyond the antenna shield. By listening out for particles striking the RPWS sensor, Cassini scientists were able to roughly gauge the levels of dust and ring matter present in the environment.
When compressed into an audio format, particle impacts recorded in RPWS data can be heard as a pop or crack, amongst the background whistling and squeaking of the charged-particle environment of space.
Prior to embarking on its Grand Finale, Cassini undertook a number of ring grazing orbits that saw the spacecraft pass close to the outer edge of Saturn's main rings. During this mission phase, RPWS detected hundreds of particle collisions, yet, when Cassini passed between the planet and the rings, the instrument recorded only a handful of the encounters.
"The region between the rings and Saturn is 'the big empty,' apparently," said Cassini Project Manager Earl Maize of NASA's Jet Propulsion Laboratory in Pasadena, California. "Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected."
Whilst the revelation may have confused ring scientists in Cassini's mission team, it will also be a huge relief. With the threat of particle collision reduced, the probe's handlers can abandon the restrictive shielding approach used in the first dive, allowing for greater freedom of movement, and therefore data harvesting for most of the 21 remaining passes.
Only four of the remaining dives will definitely require the shielding measure, as these passes will take the probe close to the inner edge of Saturn's iconic ring system – an environment likely to harbor a larger population of ring particles.
Cassini will pass through Saturn's ring plane on its next Grand Finale dive at 3:38 p.m. EDT on May 2, and communications with the spacecraft are expected to be restored the following day.
Scroll down to listen to an audio representation of data collected by the RPWS instrument during the April 26 dive, and another recording created from a December ring-grazing flyby.