In Our Skies: Cassini’s Crash into the history books

Shining in our southwestern sky during the hours after dusk on these gradually cooling mid-September evenings is one of the brighter worlds in our solar system. Because of the time, it takes to complete one trip through the constellations – roughly 29 years – the ancient Greeks gave it the name Cronus, the father of their pantheon of gods and, in some contexts, also the god of time. The Romans named it after their equivalent god, Saturn, the name by which we know it today.

Saturn was one of the celestial objects observed by the Italian astronomer Galileo Galilei in the early 17th Century shortly after the invention of the telescope; he was mystified by what appeared to be accompanying smaller bodies, one on either side, both of which disappeared over time. Half a century later, the Dutch astronomer Christiaan Huygens, utilizing a larger and better telescope, saw that Saturn was in fact accompanied by a dramatic system of rings, which change in orientation over time. Huygens also discovered a moon accompanying Saturn, which has since been named Titan.

Over the intervening decades that have elapsed since then Saturn has held a special fascination for sky-watching humans, no doubt due to its system of rings. We’ve learned that they are in fact made up of thousands of ringlets, and are composed of particles – pebble-size on up to boulder-size and larger – of ice, and are quite possibly due to a moon that wandered too close to Saturn itself and was torn apart by Saturn’s gravity. Meanwhile, many other moons have been discovered, all the way from the edges of the rings themselves out to very distant ones’ tens of millions of miles away; most of these are very small worlds only a few miles across, although a few of the earlier-discovered moons are large. Titan – the second-largest moon in the solar system – is larger than Mercury, and moreover possesses an atmosphere thicker than Earth’s.

The first spacecraft to visit Saturn was Pioneer 11, which passed by in September 1979 following its flyby of Jupiter almost five years earlier. This was followed by the flybys of the Voyagers 1 and 2 spacecraft which took place in November 1980 and August 1981, respectively, and which revealed Saturn itself as well as several of its moons in never-before-seen detail.

Enter the Cassini mission, which was launched almost twenty years ago, on Oct. 15, 1997, from Cape Canaveral, Florida. After two gravity-assist flybys of Venus and one of Earth over the next two years, Cassini headed towards the outer solar system, making a distant passage by the small asteroid Masursky in early 2000 and a gravity-assist flyby of Jupiter at the end of that year.

Cassini arrived at Saturn on July 1, 2004, and has been in orbit around the ringed planet ever since. During this time, it has conducted many observations of the planet itself, including of numerous storms and other temporary “disturbances” in its atmosphere as well as a large hexagonal-shaped cloud structure surrounding its north pole, and of the rings, including their overall structure and composition. Cassini has also made numerous flybys of various moons in the Saturn system; among the most notable of its findings was the detection of erupting water geysers – over a hundred – in the southern hemisphere of the moon Enceladus. The plumes from these geysers contain substantial amounts of water and indicate the existence of a global ocean several tens of miles’ underneath Enceladus’ surface, which in turn suggests the possibility of indigenous life.

Piggy-backed aboard Cassini was the European Space Agency’s Huygens probe, which detached from Cassini in late 2004 and then descended to the surface of Titan on Jan. 14, 2005. Huygens provided us our first detailed view of this cloud-enshrouded world, and revealed an almost Earthlike topography with seas, lakes and mountains. Numerous flybys of Titan by Cassini since then have revealed it to be a geologically and meteorologically active world with many similarities to Earth; indeed, with the exception of its surface temperature – roughly -290 degrees F., which means that the lakes and seas are liquid methane and liquid ethane instead of water – Titan could be considered the most Earthlike world in the solar system, outside Earth itself.

It is a truism that “all good things must come to an end,” and the same is true for Cassini, which has had its mission extended twice since the completion of its primary mission in mid-2008. Beginning in late April of this year Cassini began a series of weekly “Grand Finale” passages between the inner edge of the rings and the top of Saturn’s atmosphere, and among other things has discovered that this region of space is surprisingly free of ring debris. But even this phase of the mission is now ending: in order to avoid any possibility that Cassini might impact Titan or Enceladus, or any other moon, in the distant future and possibly contaminate any indigenous life, it will plunge into Saturn’s atmosphere on Sept. 15 at about 5:54 a.m. MDT and will crushed into nonexistence by the intense atmospheric pressure.

Scientists will continue to plow through Cassini’s rich trove of data for many years to come, and we can expect to see new findings being announced from time to time. But while the past thirteen years that this intrepid explorer has been orbiting and collecting data about Saturn and its retinue of moons has forever changed our views of the ringed planet and those accompanying worlds, Saturn, as always, will continue to shine in our skies at night just as it has done since the dawn of history.

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://www.earthriseinstitute.org.