September 17th, 2017
PASADENA, Calif. — Just one month shy of twenty years in space, NASA’s Cassini spacecraft dramatically ended its mission in the early morning hours at approximately 4:55 a.m. PDT (7:55 a.m. EDT / 11:55 GMT) Earth-Received Time (ERT) on Friday, September 15, 2017.
Cassini was a remarkable mission, returning some of the most surprising science and breathtaking images from any voyage undertaken thus far into the distant reaches of the Solar System. The discoveries and understandings that have resulted from the Cassini spacecraft will continue to emerge from the mountains of data for many years to come.
As the inevitable end of this workhorse of a mission approached, millions of people across the globe tuned in for the 12,593-pound (5,712 kg) vehicle’s plunge into Saturn’s upper atmosphere.
Ever science’s soldier, the spacecraft conducted science right up until the end.
“There are two sensors. Fortunately, the instrument was built to have a large dynamic range. And so it actually has two independent detectors, and one of them is about, let’s say is about 3,000 times less sensitive than the other,” Hunter Waite, the team lead for Cassini’s Ion and Neutral Mass Spectrometer from the Southwest Research Institute in San Antonio, Texas, told SpaceFlight Insider. “So if one saturates, and it has happened on these close flybys, then the other one takes over. And so we’ve been able to – we did this – we repeatedly used this technique at Titan, and we’re using it again here, so we’re pretty sure that up to the time probably that we lose the signal, which is the first thing that is going to happen, we will be able to disentangle the data. ”
It has been argued that one of the greatest discoveries that Cassini has helped to make has been the surprising revelation that the tiny, icy moon of Enceladus possesses a global subsurface ocean – and that its ocean was spewing out jets of water reaching more than 100 miles (161 kilometers) from its surface.
The tiny Saturnian moon, which measures a mere 310 miles across (500 kilometers), was found to not only have more than 100 geysers but also that they have salty water containing particles of silica indicating the likelihood of subsurface hydrothermal vents – the first of their kind believed to exist outside of Earth.
Practically everything we believe to be necessary for life to form on a world, we now know exists within the tiny, frozen world of Enceladus.
Also, Enceladus isn’t the only Ocean World in the Saturnian system. Titan, Saturn’s hazy moon was found to not only have a thick hydrocarbon-rich atmosphere but also to have an ethane-/methane-based cycle; it is thought that ethane/methane plays the same role on Titan as water does on Earth.
“Life is either a daring adventure or nothing.” – Helen Keller
On December 25, 2004, Cassini released the European Space Agency’s (ESA) Huygens probe, which was built by Aérospatiale and sent to enter into Titan’s atmosphere and land on the moon’s surface. The probe completed this part of its mission a few weeks later on January 14, 2005 – becoming the first probe to land on a world in the outer Solar System.
What the Huygens probe observed was remarkable. It was a landscape not unfamiliar to so many places on Earth. It looked like a dry lake or streambed with rounded, fluid eroded “rocks” and “pebbles” composed of water ice that had naturally been sorted by their mass. It also saw clouds in the atmosphere. This helped to confirm that, yes, Titan did have an active liquid cycle – not one based on water, like on Earth, but on liquid ethane/methane, due to the frigid temperatures (94 K / −179.2 °C).
However, that wasn’t all.
Between the Huygens probe and the instruments on Cassini, it was finally possible to generate a detailed map of Titan’s surface. A map that included globally frozen hydrocarbon dunes as well as some of the largest bodies of liquid found in the Solar System, some of which rival the size of the Great Lakes here on Earth.
These instruments also confirmed that Titan is itself one of the ocean worlds with what is believed to be its own subsurface global ocean deep beneath the surface of Titan’s frigid, hydrocarbon dunes. Titan also possesses so-called “magical islands” that appear and disappear, and they could be a chain reaction of a unique chemistry occurring within the ethane/methane seas and creating “islands” of bubbles.
Because of the Cassini-Huygens mission, we now know that shy Titan, which has for eons hidden behind its shroud of haze, has turned out to be one of the most curious and mysterious worlds in the Solar System. While it possesses passing similarities to Earth, it does so at the whim of its extreme environment.
The Road Goes Ever On
With Cassini’s epic voyage now in the history books, the U.S. space agency is eyeing other missions to follow up on the discoveries and lessons learned during the 13-year-long mission to the “Lord of the Rings”. While Cassini provided clues and tantalizing hints of what other mysteries Saturn might contain, NASA is looking to up the ante in terms of its exploration efforts.
Onward to Europa
The Europa Clipper mission, which is currently slated to launch sometime in the 2020s, is currently under development. Many scientists and engineers involved with Cassini are currently hip-deep working on the Europa Clipper mission.
From the design of new and improved instruments such as the Ion and Neutral Mass Spectrometer (INMS) and the Cosmic Dust Analyzer instruments on Cassini, members of the Cassini team are now developing extremely sensitive, next-generation versions of these same instruments for Europa Clipper in the hopes that they can confirm plumes that are thought to be on Jupiter’s ice-encrusted moon.
Possibly one of the most important aspects of the Europa Clipper mission that can trace its origin directly back to the Cassini mission could be the manner in which the new spacecraft will maneuver. Mission planners learned how to harness the finer points of orbital mechanics and gravitational assists from Cassini’s Solstice mission.
Instead of a simple orbit around the tidally locked Jovian satellite, which would have ended in the spacecraft being bathed in some of the worst radiation in the Solar System, Europa Clipper’s planners went the route of Cassini and opted to use the gravitational influence of two of Jupiter’s large moons – Callisto and Ganymede – to influence Europa Clipper’s movement.
Return to Saturn
Few can argue that the Cassini mission has been, in every sense of the word, remarkable. Much like it was influenced by the Voyager and Pioneer missions, Cassini is influencing the missions of tomorrow.
The next NASA New Frontiers mission is being considered now. The themes which were chosen, based on the Planetary Sciences Decadal Survey, range from comet and lunar sample return missions to trojan asteroid rendezvous missions, Venus exploration, probes to Saturn, and to the Saturnian ocean worlds of Titan and Enceladus.
Two of the proposed missions, the Enceladus Life Finder (ELF) and the Enceladus Life Signatures and Habitability (ELSAH) are both missions inspired by Cassini’s discovery of the water geysers venting from Enceladus’ southern polar “tiger stripes” region. The Oceanus (orbiter) and Dragonfly (roto quad-copter) missions both focus on Saturn’s giant hazy moon Titan. Finally, the Saturn PRobe Interior and aTmosphere Explorer (SPRITE) would be a probe to enter into Saturn’s atmosphere and take direct samples.
Each of these potential future missions, as well as so many others that have are being considered, have been directly inspired by the results of Cassini’s nearly two-decade-long journey in space and thirteen years above Saturn’s cloud tops.
The Science Continues
Even with the fiery end of Cassini, the sheer volume of scientific data from the mission will likely take years if not decades to be fully understood. Some of the answers that scientists hoped to uncover not only haven’t been answered but also have left them scratching their heads and trying to consider entirely new ways of thinking for geophysics and beyond.
“Over the next decade or so [of] new discoveries – you know, we sort of have this fire hose of data that we skim the cream off the top of, and when you really dig down, there’s probably more Ph.D. theses in there. There’s going to be a lot more coming out,” Linda Spilker, Cassini’s Project Scientist told SpaceFlight Insider.
Models that were once considered standard now seem to be impossible, and right up until the very end before the big, beautiful Cassini merged with Saturn’s atmosphere, new ideas were being considered. This wellspring of new notions and considerations would not have been possible without the bus-sized (Cassini measured some 22 feet, or 7 meters, in length) spacecraft or the men and women who cared for her for almost two decades.
Video courtesy of NASA / Jet propulsion Laboratory
A native of the Greater Los Angeles area, Ocean McIntyre’s writing is focused primarily on science (STEM and STEAM) education and public outreach. McIntyre is a NASA/JPL Solar System Ambassador as well as holding memberships with The Planetary Society, Los Angeles Astronomical Society, and is a founding member of SafePlaceForSpace.org. McIntyre is currently studying astrophysics and planetary science with additional interests in astrobiology, cosmology and directed energy propulsion technology. With SpaceFlight Insider seeking to expand the amount of science articles it produces, McIntyre was a welcomed addition to our growing team.