How to Catch a Comet

By Dr. Mark Showalter
Senior Research Scientist at the Carl Sagan Center for the Study of Life in the Universe, SETI Institute

Four and a half billion years ago, a fluffy "snowball" coalesced out of the cloud of ice, dust and debris still surrounding our Sun. Most of the snowballs like it later merged to become the planets we know. This one, however, had a chance flyby with a young planet, probably Jupiter. Jupiter's gravity propelled it out into the far reaches of the Solar System, where it remained in deep freeze, among many others like it, as a member of the so-called Oort cloud.

Eventually, the tug of gravity from a passing star slowed it down ever so slightly, and that was enough to send it plummeting back toward the Sun, into the region where it had formed billions of years earlier. By a great quirk of irony Jupiter was again in its path. This time, the planet's gravity captured it into a long, elliptical orbit. On July 7, 1992, it executed a cosmic hole-in-one, passing through Jupiter's slender ring and breaking apart under the planet's ripping tides.

On the night of March 24, 1993, astronomers Eugene and Carolyn Shoemaker and David Levy were searching the skies when they noted an oddly shaped blotch near Jupiter. It was a comet to be sure, but quite unusual in shape. The next morning it became known as Shoemaker-Levy 9, or SL9 for short. With additional detections, its prior history as a body disrupted by Jupiter became clear. As new data accumulated, however, SL9 became even more remarkable, because astronomers realized that it had evaded Jupiter for the last time. In July 1994, the world watched as the broken fragments of SL9 plunged into Jupiter one by one. Few comets are ever featured on the front page of the New York Times, but the July 19 headline read, "Earth-Sized Storm and Fireballs Shake Jupiter as a Comet Dies." SL9 went out with a bang. The impacts left behind blotches in Jupiter's clouds but, after a few months, they faded away. End of story.

Or so we thought. As we have just published in the journal Science Express, this story has an unexpected epilogue.

To tell the rest of the story, however, requires a brief detour to another planet, another spacecraft, and another decade. Cassini arrives at Saturn in 2005, and begins sending back detailed images of that planet's ring system. My colleague and co-author Matt Hedman scours the images of Saturn's innermost ring and finds an obscure detail that, notably, was not present in the Voyager images from 1980 and 1981. He sees "ripples"--vertical corrugations in the ring plane, which repeat every 30 km or so. We do not what to make of this pattern. After a few years, however, we notice a trend: the ripples seem to be getting shorter. Whatever they are, they are winding up like a watchspring. When Matt plays the process backwards, we learn that the pattern began when some "event" tilted Saturn's rings off their axis in late 1983. But what could have caused such an event?

We had seen similar ripples exactly once before. The rings of Jupiter showed vertical ripples when they were first imaged by the Galileo spacecraft in 1996. At the time, we did not know what to make of this pattern. Galileo never detected it again. Nor did New Horizons, a spacecaft that flew by Jupiter in 2007 en route to its 2016 encounter with Pluto.

But what if the pattern in Jupiter's ring was another winding watchspring? I revisit the Galileo and New Horizons data with a more open mind, searching for any pattern of any wavelength. Sure enough, the watchspring pattern was in there after all, winding up tighter in each detection. We had overlooked it previously because we were looking for the wrong pattern. It is evolving in exactly the same way as the pattern at Saturn. This means that, just like at Saturn, we can play the process backwards. When we do, we determine that the rings of Jupiter were tilted off their axis in mid-1994.

Why does this date sound so familiar?

Active comets are typically surrounded by clouds of fine dust, and the Hubble images confirm that SL9 was very active. After many calculations and numerical simulations, we have finally managed to show that, at the same time that the large fragments were hitting Jupiter, SL9's dust cloud missed the planet and carried enough momentum to tilt the entire ring off its axis. SL9 seems to be our "smoking gun." Reasoning by analogy, the event that tilted the rings of Saturn in 1983 was probably also the impact of a comet.

The Jupiter data has shown us something else quite interesting. The pattern produced by SL9 was only one of four different ripple patterns detectable in the data. SL9 was not a freak event; comets hit the rings of Jupiter maybe once or twice per decade, and the rings of Saturn perhaps once or twice per century. As a bigger planet, Jupiter collects more comets. SL9 was not unique, or even unusual.

I posed a question in the title; How do we catch comets? The short answer is, with a really big net. Luckily for us, planetary rings serve as the perfect nets. Our recent studies span decades, planets, and spacecraft, but they tell a simple and unified story: comets hit rings; rings get tilted; tilts become spirals. With the right observations, we can now replay that history years and even decades later, just as if it were embedded in the grooves on an old vinyl record.