Cassini finds ‘big empty’ near Saturn

Artist’s concepts of Cassini spacecraft diving through the gap between Saturn’s outer atmosphere and inner rings. Cassini will do this 22 times in 2017, before ending its mission at Saturn.

There’s much less dust between Saturn and its inner rings than expected, said NASA engineers, after last week’s historic dive through this gap by the Cassini spacecraft. Astronomers have been contemplating this maneuver by a spacecraft for decades, since the two Voyager spacecraft passed Saturn in the early 1980s. The fear was that a spacecraft might encounter debris that would suddenly end its mission! But Cassini – which is running out of fuel after orbiting Saturn since 2004 – not only passed through the gap successfully but also found it surprising debris-free. Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California said:

The region between the rings and Saturn is ‘the big empty,’ apparently. Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.

Cassini will make its second dive through the gap today (May 2, 2017) at 12:38 p.m. PDT (3:38 p.m. EDT, 19:38 UTC; translate UTC to your time zone)

With information from the first dive in hand, the Cassini team will now move forward with its preferred plan of science observations. NASA said:

A dustier environment in the gap might have meant the spacecraft’s saucer-shaped main antenna would be needed as a shield during most future dives through the ring plane. This would have forced changes to how and when Cassini’s instruments would be able to make observations. Fortunately, it appears that the “plan B” option is no longer needed. (There are 21 dives remaining. Four of them pass through the innermost fringes of Saturn’s rings, necessitating that the antenna be used as a shield on those orbits.)

Based on images from Cassini, models of the ring particle environment in the approximately 1,200-mile-wide (2,000-kilometer-wide) region between Saturn and its rings suggested the area would not have large particles that would pose a danger to the spacecraft.

But because no spacecraft had ever passed through the region before, Cassini engineers oriented the spacecraft so that its 13-foot-wide (4-meter-wide) antenna pointed in the direction of oncoming ring particles, shielding its delicate instruments as a protective measure during its April 26 dive.

The video below represents data collected by Cassini’s Radio and Plasma Wave Science instrument, as it crossed through the gap between Saturn and its rings on April 26. The instrument is able to record ring particles striking the spacecraft in its data. In the data from this dive, there is virtually no detectable peak in pops and cracks that represent ring particles striking the spacecraft. The lack of discernible pops and cracks indicates the region is largely free of small particles. William Kurth, RPWS team lead at the University of Iowa, Iowa City said:

It was a bit disorienting — we weren’t hearing what we expected to hear. I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.

The team’s analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (about 1 micron across).

Today’s ring crossing will occur in a region very close to where Cassini passed on last week’s dive. Prior to today’s crossing, Cassini’s cameras have been looking closely at the rings; in addition, the spacecraft was rotated (or “rolled”) faster than engineers have ever allowed it to before, in order to calibrate the magnetometer.

As with the first finale dive, Cassini will be out of contact during closest approach to Saturn, and is scheduled to transmit data from this dive on May 3.

Bottom line: During its April 26, 2017 dive between Saturn and its rings – its first of 22 dives in its Grand Finale this year – the Cassini spacecraft found a relatively dust-free region. Scientists are calling it The Big Empty.


May 2, 2017

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