Cassini’s Deep Dive

Cassini’s Deep Dive[1]

The recording starts with the patter of a summer squall. Later, a drifting tone like that of a not-quite-tuned-in radio station rises and for a while drowns out the patter.

These are the sounds encountered by NASA’s Cassini spacecraft as it dove through the gap between Saturn and its innermost ring on April 26, the first of 22 such encounters before it will plunge into Saturn’s atmosphere in September [2017].

What Cassini did not detect were many of the collisions of dust particles hitting the spacecraft as it passed through the plane of the rings.

“You can hear a couple of clicks,” said William S. Kurth, a research scientist at the University of Iowa who is the principal investigator for Cassini’s radio and plasma science instrument.

The few dust hits that were recorded sounded like the small pops caused by dust on a LP record, he said. What he had expected was something more like the din of “driving through Iowa in a hailstorm,” Dr. Kurth said.

Since Cassini had not passed through this region before, scientists and engineers did not know for certain what it would encounter. Cassini would be traveling at more than 70,000 miles per hour as it passed within 2,000 miles of the cloud tops, and a chance hit with a sand grain could be trouble.

The analysis indicated that the chances of such a collision were slim, but still risky enough that mission managers did not send Cassini here until the mission’s final months. As a better-safe-than-sorry precaution, the spacecraft was pointed with its big radio dish facing forward, like a shield.

Not only was there nothing catastrophic, there was hardly anything at all. The few clicking sounds were generated by dust the size of cigarette smoke particles about a micron, or one-25,000th of an inch, in diameter.

To be clear: Cassini did not actually hear any sounds. It is, after all, flying through space where there is no air and thus no vibrating air molecules to convey sound waves. But space is full of radio waves, recorded by Dr. Kurth’s instrument, and those waves, just like the ones bouncing through the Earth’s atmosphere to broadcast the songs of Bruno Mars, Beyoncé and Taylor Swift, can be converted into audible sounds.

Dr. Kurth said the background patter was likely oscillations of charged particles in the upper part of Saturn’s ionosphere where atoms are broken apart by solar and cosmic radiation. The louder tones were almost certainly “whistler mode emissions” when the charged particles oscillate in unison.

The dust particles create their own distinctive noises.

Upon hitting Cassini, the dust and a bit of the spacecraft vaporize in small clouds of ultrahot gas where electrons are ripped away from atoms and generate radio waves.

The actual physics is still somewhat contentious. “People aren’t really sure what they’re seeing,” said Sigrid Close, an associate professor of aeronautics and astronautics at Stanford.

Her idea, supported by laboratory experiments and computer simulations, is that the lighter electrons initially speed away faster, setting up an electric field that pulls the electrons back toward the ions, and then they oscillate back and forth.

Above is a picture taken of Earth through Saturn’s rings. The arrow points to Earth.

Similar radio frequency pulses are generated by lightning, she said.

For spacecraft closer to home, like commercial and military satellites in orbit around the Earth, this could be an important process to understand, because pulses generated by large dust strikes could cripple their electronic systems.

When Cassini passed through a faint Saturn ring in December, the dust impacts numbered in the hundreds per second.

With the knowledge that the gap between Saturn and its innermost ring is safe, Cassini does not need to use its antenna as a shield, allowing it to make additional scientific measurements.

During its second dive through the gap on Tuesday [May 2, 2017], another instrument, the cosmic dust analyzer, made the first direct analysis of the ring particles. The dust analyzer is able to record particles smaller in size than could have been detected via radio waves during the first pass. In addition, Cassini performed magnetic measurements that will help determine the length of a Saturn day. That is still a mystery, because Saturn’s clouds obscure how quickly the underlying planet is rotating.

Cassini got back in touch with Earth on Wednesday morning [May 3, 2017] and is sending the results of the second dive.

[1] See “The ‘[Sounds’ of Space as NASA’s Cassini Dive by Saturn,” New York Times (May 3, 2017)

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