It has become something of a hackneyed phrase, but in the case of NASA’s Cassini spacecraft it is rather fitting: an epic mission of exploration of Saturn that has single-handedly changed our view of the ringed planet, its moons, and their potential habitability, yet like all good things it must come to an end. Having nearly completed two full decades in space, Cassini has now entered its final 18 months around Saturn on what has been a tremendously successful and productive mission, full of unexpected and ground-breaking discoveries. Last week the mission’s science team officially began the one-year countdown toward the start of Cassini’s “Grand Finale,” which will culminate with an end-of-mission daring plunge on Saturn’s cloud tops on Sept. 15, 2017.
Launched on October 1997, Cassini undertook a seven-year journey of 3.5 billion km which included two flybys of Venus, one of Earth, and one of Jupiter, before finally arriving and entering orbit around Saturn in July 2004, becoming humanity’s first ever robotic spacecraft to do so. Since that time, Cassini has literally made history with every covered mile of space around the ringed giant planet and its assortage of fascinating moons, while slowly uncovering many of their long-held secrets and returning hundreds of thousands of images of unparalleled beauty. While the mission’s overall science results are too many to mention, highlights include the dispatch of the European-built Huygens probe which made humanity’s historic first soft-landing on the surface of the moon Titan, the ground-breaking discovery of water ice geysers erupting from Enceladus, the discovery of seas and lakes of liquid ethane and methane on Titan, as well as the detailed study of Saturn’s atmosphere and rings, just to name a few.
Having completed its initial four-year primary mission in 2008, Cassini was given the go-ahead by NASA for an extended mission called the Cassini Equinox Mission and a third and final extension in 2010 called the Cassini Solstice Mission. The names for these extended missions weren’t arbitrary. Since one Saturnian year equals 29.4 Earth years, Cassini has been able to study the planet and its moons essentially through half of its orbit around the Sun, allowing planetary scientists to have a first close-up view of how the change of seasons affects the climate and atmospheric circulation on both Saturn and its largest moon Titan.
Yet, now more than a decade after it entered orbit around the ringed giant, Cassini has spent almost all of its onboard fuel which allowed it to maneuver through the Saturn system by way of hundreds of close flybys of its largest moon Titan. Cassini’s mission planners had been preparing for the inevitable end ever since the conclusion of the spacecraft’s primary mission in 2008, while evaluating several scenarios as to the exact way with wich Cassini would make its farewell. Some of the options that were considered for Cassini’s end of mission by ground teams included an escape from Saturn toward Uranus or Neptune, an escape toward a heliocentric orbit, or an aerobraking and eventual placement in a stable orbit around Titan. All of these options were evaluated against certain factors like the time needed for the completion of each scenario, the delta-v required for the orbital changes, the fuel that would be available, as well as the best overall science return for each option. In the end, the only options that satisfied all of the required criteria were either an impact on one of Saturn’s icy satellites, or an impact on the giant planet itself. In the best interests of planetary protection, Cassini’s science team eventually chose the latter, in order to prevent the biological contamination of Saturn’s moons from any terrestrial microorganisms that could have been carried from Earth onboard the spacecraft.
To that end and after shifting through a list of names that were submitted by more than 2,000 members of the public, the mission’s science team chose to appropriately name the final leg of Cassini’s trek around Saturn “The Grand Finale.” The latter is comprised of two parts. The first one, which will begin this year on Nov. 30 following the spacecraft’s penultimate Titan flyby, consists of a set of 20 elongated polar orbits around the planet which will bring Cassini within 10,000 km of Saturn’s F ring (the planet’s outermost discrete ring). This will be followed by The Grand Finale’s second phase beginning on April 22, 2017, when Cassini will use its final Titan flyby in order to change its orbital orientation and execute a daring loop that will bring it just 3,800 km above Saturn’s cloud tops, thus positioning it inside the planet’s entire ring system! From this vantage point, Cassini will complete a total of 22 highly elliptical polar orbits around Saturn before finally plunging onto the gas giant’s atmosphere on Sept. 15, 2017, putting an end to its spectacular 20-year mission.
One of the reasons that this scenario was chosen for Cassini’s end of mission was that it would return the most science compared to all the other options that had been evaluated. In fact, for Cassini’s science team, The Grand Finale represents an entire new mission on its own, which hadn’t even been considered as a possibility when the spacecraft was still on the drawing board back in the 1980s. One of the mysteries that have remained unresolved to this day involve the composition of Saturn’s internal structure as well as the planet’s exact rotation rate, which has not been measured to a great precision. Cassini’s position inside the main ring system during the second phase of The Grand Finale will allow the spacecraft to make very detail measurements of Saturn’s gravity and magnetic fields which will help scientists to better answer the remaining questions regarding the planet’s interior and overall rotation. Furthermore, a different spacecraft, Juno, which is scheduled to arrive on Jupiter this summer, will be making similar measurements regarding Jupiter’s interior at the same time, which will provide planetary scientists with great insights as to the inner workings of the Solar System’s gas giant planet’s during the same point in time. “We’ll have a much better understanding of how the planet works,” says Dr. Jonathan Fortney, a professor of astronomy at the University of California, Santa Cruz, who is part of the science team that has been selected by NASA to coordinate Cassini’s Grand Finale mission. “We really don’t know what their interiors are like. What’s great is that, in the space of a year, we’ll have once-in-a-lifetime data sets for both Jupiter and Saturn.”
One other important set of science results expected to come during Cassini’s Grand Finale is the exact mass determination of Saturn’s ring system. For the entire duration of its mission to date, the spacecraft has operated solely outside of Saturn’s rings which has limited its ability to determine their mass, since it had to take into account the gravitational effects of the giant planet on the ring system. By positioning itself inside the rings, Cassini will be able to differentiate between these gravitational effects and the mass of the rings themselves. In addition, Cassini will have an unprecedented close-up view at the rings’ overall structure and composition, which might help scientists shed more light on their age and determine whether they have formed late in the planet’s history or have the same age as Saturn itself.
NASA has already set officially the countdown clock ticking toward the start of Cassini’s Grand Finale, as announced late last week by Ron Baalke, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, Calif., with a post on his Twitter account. This countdown marks the final 18 months of life for Cassini, which for many has been the epitome and highlight of NASA’s Planetary Science Division for the last two decades.
Both Cassini and Juno will end their missions around the same time, with the latter scheduled to make a final plunge onto Jupiter’s cloud tops in February 2018. After that, and save for the Europa Clipper mission which is still in the conceptual stage, no other missions toward the outer Solar System exist in the NASA pipeline for the foreseeable future. This unfortunate reality, which is a result of cuts in the space agency’s planetary science budgets in recent years, means that we can probably not expect to see another dedicated probe toward the outer planets for the next couple of decades. This doesn’t mean that there hasn’t been a lack of proposals from the planetary science community, including mission concepts for the exploration of the ice giants Uranus and Neptune, both of which have been largely neglected ever since the Voyager 2 fly bys of the 1980s. Yet, as things stand at the moment, with the coming conclusion of the Cassini mission in 2017 and Juno in early 2018, the outer Solar System will likely remain out of reach for at least the next couple of decades.
A computer rendering showing the view from the perspective of the Cassini spacecraft as it dives between the rings and Saturn’s cloud tops during the Grand Finale. Video Credit: NASA/JPL