Universe Update
13 Years at the Ringed Planet
In 2004, a schoolbus-sized spacecraft entered orbit around Saturn, the aesthetically pleasing but (at the time) relatively unexplored giant planet.
In the 13 years since, the Cassini spacecraft has explored the Saturn system—the planet, its rings, its moons—relentlessly sending back a treasure trove of data that will keep planetary scientists hard at work for a good long while. In the days leading up to Cassini’s final plunge into the ringed planet next Friday, we’re celebrating Cassini’s many discoveries with a series of articles focussing on its many successes.
Near the beginning of its long stay, Cassini parted ways with the Huygens probe, which touched down on Saturn’s giant moon Titan on January 14, 2005. Huygens became the first spacecraft to land on another planet’s moon, providing a ground level view of its surface, otherwise obscured by a thick, hazy atmosphere. Light at infrared and radio wavelengths can peer through that haze, too, however, and years of subsequent observations from Cassini revealed Titan as world with rain, rivers, lakes, and seas. We’ll have more on Titan tomorrow.
Perhaps the biggest surprise, moon-wise, came when Cassini’s images of Enceladus revealed geysers erupting from its south pole, leading the first detection of hydrothermal activity beyond Earth! Repeated visits to the tiny moon eventually provided evidence for a global subsurface ocean, which might even be an interesting place to look for microscopic life. We’ll share more on Enceladus early next week.
Saturn’s dramatic rings were another significant target for the mission. The rings give Saturn a profile so iconic and recognizable that it formed the central image of the Sci Fi Channel before the cable network rebranded—it even showed up in a previous version of the Academy’s logo affectionately referred to by some as “Barney on a Treadmill.” But there’s much more to the rings than their familiar overall shape. For a long time, we’ve known that the rings are made of smaller objects—tiny “moonlets,” if you will, ranging in size from dust particles to mountains. But Cassini discovered spectacular features—towering peaks and propellers, for example. With these new data, we could see how active and dynamic Saturn’s rings are. Indeed, in many ways, we now see them as a laboratory for planetary formation. More on Saturn’s rings next week!
And finally, Cassini has taught us a lot about Saturn itself (surprise, surprise). From a hexagonal storm persisting at the planet’s north pole to major insights about its magnetic field and rotation, we have discovered many things we didn’t expect, which is why we send spacecraft to far-flung destinations, after all. And again, stay tuned next week for more on Cassini’s revelations about Saturn itself.
The Morrison Planetarium engineering team created a short video recapping the entire mission, showing the trajectory of the spacecraft in its many orbits around the planet. The video starts in August 2004, about a month after Cassini had entered orbit on July 1. The orbits of Saturn’s moons appear in blue, and Cassini’s trajectory shows up as an orange line beginning in the lower left of the screen.
Cassini started its mission orbiting Saturn in pretty much the same plane as the planet’s rings, a configuration that allowed the spacecraft to conduct a rapid reconnaissance of the Saturn system. From February to September 2005, it passed into Saturn’s shadow several times, so it could observe how sunlight interacted with the planet’s backlit atmosphere. From September 2005 through July 2006, it executed a petal rotation (terminology you can easily understand when you watch the video), tracing out a flower shape around Saturn and taking measurements of different parts of the planet’s magnetic field. Starting in August 2006, NASA orchestrated several close flybys of Titan that used repeated slight tugs from the moon’s gravity to shift the inclination of Cassini’s orbit—basically, giving the spacecraft an increasingly top-down view on Saturn and its rings (this, too, you can see very clearly in the video above).
The video also shows a shift in color (from amber to violet) in June 2008, when Cassini finished its “Prime mission” and began its extended mission—codenamed “Equinox,” which sounds both astronomical and mysterious but simply refers to the fact that Saturn’s equinox (when both its northern and southern hemispheres are equally lit by the Sun) occurred on August 11, 2009. This part of the mission lasted only until October 2010, but in that short time, the spacecraft orbited Saturn 60 times, making close flybys of five moons—26 flybys of Titan, seven of Enceladus, and one each of Dione, Rhea, and Helene. The timing of the Equinox mission also allowed Cassini to observe Saturn’s rings as the sun lit them edge on, revealing towering peaks and other previously unseen features.
The next phase of the mission—named “Solstice” because it lasted through the longest day of the year for Saturn’s northern hemisphere—began in November 2010 and will end next week. We color-coded the Solstice mission bright green in the video, up until the dramatic change in Cassini’s orbit starting in November 2016, when the trajectory shifts in color to a dark pink. This final phase, nicknamed the “Grand Finale,” tweaked Cassini’s orbit until the spacecraft was passing between the inner edge of the ring system and the planet’s cloud tops. This wasn’t done without taking some risks (which, by the way, NASA engineers are somewhat averse to), but it has given us a startling new perspective on the planet—even 13 years after Cassini first arrived!
Cassini’s final sacrificial dive into Saturn’s atmosphere will protect Enceladus and Titan from contamination—if life exists on one of these moons, we don’t want to risk infection by any microscopic life carried by Cassini on its long journey, and if the spacecraft remained in orbit around Saturn, there’s a chance it could crash into one of the moons.
When Cassini makes its fateful final plunge into Saturn next Friday, September 15, it will end nearly two decades in space (if you include the nearly seven years it spent in transit from Earth to Saturn). But we will spend many more decades mining its discoveries. Definitely something to celebrate!