Goodbye, Cassini. And thank you. After a 20-year mission, 13 in orbit of Saturn, Cassini dived one last time for Saturn’s atmosphere. Over the course of its years at Saturn, Cassini has watched the seasons change on the majestic ringed planet and Titan, made unparalleled discoveries, and opened our eyes about the possibilities of life in our solar system beyond Earth. Now, Cassini’s final act will be to preserve that potential life by destroying itself in Saturn’s atmosphere rather than risk contamination of Titan and Enceladus.
Two decades of development:
What would ultimately become the Cassini-Huygens mission dates to the early 1980s when the National Academy of Sciences in the United States and the European Science Foundation came together to investigate the potential of cooperative missions.
The venture quickly suggested a joint Saturnian orbiter and Titan lander mission.
The following year, 1983, the Solar System Exploration Committee of NASA recommended a similar joint Saturnian orbiter and Titan lander mission.
Throughout 1984-5, NASA and the European Space Agency (ESA) jointly investigated the feasibility of such a mission; however, after 1985, NASA backed out of the venture, which continued forward, in a study capacity, via the ESA.
By 1988, then-NASA Associate Administrator for Space Science and Applications agreed that NASA would commit to a Saturn mission with ESA as long as ESA chose the Cassini mission from the three missions under consideration.
And as much as the Cassini-Huygens mission has been about science, it is also very importantly a solidifying mission of cooperation and equality for NASA and ESA.
In the years preceding 1988, ESA and NASA had developed a strained relationship because of NASA’s inequitable treatment of ESA during previous partnership missions.
Cassini-Huygens would be the mission to correct that relationship.
With joint NASA and ESA funded secured, spacecraft design and construction began following the selection of the primary mission objectives.
In all, Cassini-Huygens’ scientists developed seven primary objectives that could be reasonably met within the planned four Earth-year duration of the primary science mission.
Those objectives were to determine the three-dimensional structure and dynamic behavior of Saturn’s rings; determine the composition of the satellite surfaces and the geological history of each object; and determine the nature and origin of the dark material on Iapetus’s (third-largest moon of Saturn) leading atmosphere.
Further mission objectives included the measurement of the three-dimensional structure and dynamic behavior of Saturn’s magnetosphere; the study of the dynamic behaviors of Saturn’s atmosphere at cloud level; the study of the time variability of Titan’s clouds and hazes; and the characterization of Titan’s surface on a regional scale.
In collaboration with ESA, NASA took responsibility for assembly of Cassini at the agency’s Jet Propulsion Laboratory – the same center that would manage the mission.
ESA conversely took responsibility for building a large number of Cassini’s constituent parts, as well as nearly all of the Huygens Titan lander (save the batteries and two scientific instruments which came from NASA).
Once assembled, Cassini-Huygens was 2,500 kg (with Huygens accounting for just 350 kg), 6.8 meters in height, 4 meters in width, and included 14 kilometers of wiring, 22,000 wire connections, and 1,630 interconnected electronics components.
To power all of Cassini’s systems, as Saturn is too far from the sun to effectively use solar power, NASA designed a 32.7 kg plutonium-238 power source – with electrical power derived from heat from the material’s radioactive decay.
Huygens was designed to use this same power source while in transit to Saturn, transitioning to chemical batteries for deployment, entry, and landing on Titan.
Launch and cruise to Saturn:
NASA’s decision to power Cassini with plutonium-238 led to a higher-than-usual interest in the probe’s launch – specifically from protesters who turned out to draw attention to, what they called, reckless behavior and public endangerment by NASA.
At the core of their protests was the idea that millions of people in Central Florida would be in danger of radioactive poisoning in the event of a launch failure.
The claims were quickly discounted by NASA and the U.S. Department of Energy. NASA designed Cassini’s Radioisotope Thermoelectric Generators (RTGs) to withstand intense heat, heat that might be encountered during a launch failure.
Likewise, the US Department of Energy stated that “It cannot be exploded like a bomb. It is an alpha emitter. Alpha radiation can be stopped by a piece of paper.”
Prior to Cassini’s launch, several dozen protesters were arrested for violating government-controlled facilities, throwing pieces of carpet at guards, and attempting to climb over security fences.
NASA proceeded uninterrupted toward launch.
Cassini-Huygens lifted off without issue atop a Titan IV (401) B rocket from SLC-40 at the Cape Canaveral Air Force Station, FL, on 15 October 1997 at 04:43:00 local time (08:43:00 UTC).
The Titan IV-B rocket delivered Cassini into a multi-planet encounter, heliocentric orbit, beginning the near 7-year cruise to Saturn.
Cassini-Huygens performed two gravity assist flybys of Venus on 26 April 1998 and 24 June 1999, and one gravity assist flyby of Earth on 18 August 1999.
On 23 January 2000, Cassini-Huygens performed an unexpected flyby of asteroid 2685 Masursky – an encounter mission controllers used to help further calibrate Cassini-Huygens’s cameras (following initial calibrations during the Earth-Moon flyby).
The Jovian encounter occurred on 30 December 2000, providing the final gravity assist to place Cassini-Huygens on course for rendezvous with Saturn.
During the Jovian flyby, Cassini performed scientific observations of the planet, showing that Jupiter’s cloud belts were areas of “net-rising atmospheric motion.”
This observation contradicted previous hypotheses about Jupiter’s dark and light belts and served to highlight differences in planetary weather systems.
During the flyby, Cassini was also able to study Jupiter’s thin ring system, revealing that Jupiter’s rings were composed of irregularly shaped particles that likely originated as ejecta from micrometeorite impacts with the moons Metis and Adrastea.
Following its encounter with Jupiter, Cassini-Huygens enjoyed a smooth cruise to Saturn, during which mission scientists revealed the results of Cassini’s test of Einstein’s Theory of General Relativity.
During periods when radio waves in transit from Cassini to Earth passed near the Sun, scientists measured the shift in these radio waves and were able to verify the predicted result of General Relativity.
General Relativity states that an object as massive as the Sun causes space-time to curve.
Thus, a radio wave passing close to the Sun will have to travel a greater distance as it travels around that curvature.
The experiment using Cassini verified the predicted General Relativity model to within one part in 51,000 – the most accurate measurement to date.
Arrival at Saturn:
In the final weeks leading up to Cassini-Huygens’s arrival, instruments and cameras aboard Cassini began continuous observation of the Saturnian system – observations that yielded the detection of three unknown moons.
On 11 June 2004, Cassini performed its first Saturnian moon flyby when it encountered Phoebe (for the first and only time due to orbital mechanics of the mission).
The encounter provided fresh and highly detailed images of Phoebe, leading scientists to realize that the moon probably has large amounts of water ice under its proximal surface.
As Cassini got closer to Saturn, it was able to identify something peculiar about the planet’s rotation rate: It was different from Voyager 2’s observations in 1980.
In fact, the rotation rate was 6 minutes longer than previously observed.
Since there are no permanent, solid features on Saturn, scientists calculated the planet’s rotation rate on observable radio waves emanating from the planet.
The different rotation rate observed by Cassini was determined to be due to the latitude at which those waves emanated.
Finally, 30 June 2004 arrived: Saturn Orbit Insertion day.
The insertion of Cassini into orbit of Saturn would prove historic not only because it made Cassini the first spacecraft to orbit the planet, but also because of the highly dangerous route Cassini had to take through Saturn’s rings.
To prepare, Cassini oriented itself with its high-gain antenna facing away from Earth and along the vehicle’s flight path to shield its instruments from the particles in Saturn’s rings.
Crossing through the gap between the F and G rings, Cassini transited the rings of Saturn and came through the crossing without damage.
The probe then reoriented itself, with its high-gain antenna toward Earth and its engine along the flight path.
Cassini then fired its engine – a maneuver which slowed the spacecraft by 622 meters per second to allow capture by Saturn’s gravity.
With the maneuver complete, Cassini was officially captured by Saturn’s massive gravity field at 20:44 PDT on 30 June 2004 – 03:44 UTC on 1 July 2004.
Just one day after arriving at Saturn, Cassini-Huygens performed their first flyby of Titan.
Approaching to a distance of 339,000km, Cassini’s cameras and instruments peered through the methane clouds to reveal a south polar surface of different brightness – indicating areas of rivers and drainage channels on the surface of the moon.
A subsequent flyby (just 1,200km above the moon’s surface), produced the first radar images of Titan’s surface.
The radar images showed a relatively smooth surface – paving the way for the Huygens mission.
On 25 December 2004, Huygens detached from Cassini, and on 14 January 2005, the probe entered Titan’s atmosphere for descent to the moon’s surface.
During the descent and landing, Huygens transmitted 700 pictures to Cassini (350 of which were transmitted to Earth).
As the craft descended by parachute through the atmosphere, it gathered information on winds, cloud composition, and general atmospheric characteristics.
After a 2.5hr descent, Huygens successfully touched down on the surface of Titan – making the farthest to date landing ever accomplished from Earth.
Surface characteristics were analyzed and, while indications that chucks of water ice were present at the landing site were returned by Huygens, the surface features were determined to be clay-like (because Huygens displaced a rock during its landing) and sandy with a thin layer of methane haze.
Information from Huygens suggested that while liquid definitely played a role in the shaping of the local terrain, actual collections of liquid methane and hydrocarbon lakes probably were not currently present on Titan’s surface.
That determination from Huygens data was categorically disproved by Cassini’s the 127 flybys of Titan.
In 2007, Cassini made the unprecedented discovery of huge, permanent lakes and seas of liquid methane, ethane, and propane in the polar regions of the moon.
The discovery confirmed a long-standing hypothesis and marked Titan as the only other place in the solar system (aside from Earth) with (known) permanent, ambient temperature liquid lakes and seas on its surface.
Throughout its 13 years of Titan observations, Cassini has further confirmed the presence of methane rain and seasonal floods on the moon.
Overall, Cassini completed its 127 flybys of Titan – with the final encounter coming just days ago on 12 September 2017.
Enceladus – Reexamining the best-possible place for life on other worlds:
Despite the amazing finds and confirmations at Titan, the standout surprise of the Cassini mission has hands down been the moon Enceladus.
Since arriving in 2004, Cassini has confirmed that Enceladus is the likely cause of Saturn’s E-ring – which is formed via eruptions of water vapor from cryo-geysers at the southern polar region of the moon.
The discovery of potential water vapor eruptions from Enceladus quickly catapulted the moon up the list of places to examine in more detail.
Cassini’s orbital characteristics were tweaked to send the probe through the vapor ejecta from Enceladus on 12 March 2008.
The flyby confirmed the presence of water, carbon dioxide and various hydrocarbons in the ejecta – furthering the growing hypotheses that Enceladus harbored a subterranean ocean.
Subsequent flybys between 2009 and 2012 confirmed that Enceladus harbors a large, salt-water ocean beneath its surface – a confirmation announced by NASA in April 2014.
This significant discovery, along with the confirmation (through analysis of the ejecta material from the cryo-geysers) that the ocean harbors organic molecules and nutrients and has a permanent heat source threw Enceladus to the near top of the list of “best places in the solar system to host alien microbial life.”
As the final year of the primary science mission (2008) arrived, NASA requested additional funding for the continuation of the Cassini mission.
On 15 April 2008, Congress approved a 27-month extension and additional funds for Cassini through September 2010.
This mission, called the Cassini Equinox Mission, allowed the probe to make 60 additional orbits of Saturn and observe the planet’s (and Titan’s) changing weather features as they transitioned through the Saturnian equinox (August 2009) and as the seasons changed across the hemispheres.
Additionally, 21 more flybys of Titan were added to the mission, as well as seven more flybys of Enceladus.
As the end of this mission extension drew near, another mission extension proposal, through 2017, was submitted to Congress and approved with appropriations.
Cassini’s mission, in 2010, was once again renamed to the Cassini Solstice Mission – as an extension of the mission to 2017 would allow mission scientists to observe Saturn through its northern hemisphere summer/southern hemisphere winter solstice in May 2017.
As Cassini’s arrival in 2004 was just a few months after the planet’s northern hemisphere winter solstice/southern hemisphere summer solstice, Cassini operations through 2017 have now allowed scientists to observe the planet and its moons through almost half of Saturn’s orbit.
As it mission now comes to an end, Cassini has been in orbit of Saturn for about 13 years 2 months 15 days, just shy of the 14.7 years it takes Saturn to complete half of its orbit.
Moreover, the extension through 2017 has allowed for 155 more orbits (from year 2010) of Saturn, 54 more flybys of Titan, and 11 more flybys of Enceladus.
The kiss good-bye – farewell, Cassini:
“The mission has exceeded all expectations,” said Curt Niebur, Cassini program scientist at NASA Headquarters during a media teleconference Tuesday.
“We’ve been shocked by things we never expected. We’ve seen one of the truly weirdest features of the solar system – a hexagonal structure the size of Earth that’s been at Saturn’s north pole for decades. We’ve learned that Enceladus has all of the ingredients needed to support life right now.
“Cassini has opened our eyes as to what the possibilities of life outside Earth might actually be, and the Grand Finale is using the last dregs of fuel to answer new questions and new science.
“I find great comfort that Cassini will keep teaching us new things right up to its final moment on September 15th.”
Now, 13 years 2 months and 15 days after it first entered orbit of Saturn, the final hours of Cassini are at hand.
Currently, Cassini is undertaking its 293rd and final orbit of Saturn – an orbit that will bring the craft to its final encounter with the ringed planet on Friday morning, 15 September.
Cassini’s final trajectory was irrevocably shaped on 11 September when it passed within 73,974 miles (119,049 km) of Titan.
The distance was enough to just slightly nudge Cassini into a new orbit – one that will take it into Saturn’s atmosphere.
The maneuver, dubbed “the kiss goodbye” set up Cassini’s final days.
Downlink of all final Titan data began at 16:56 PDT (23:56 UTC) on 12 September.
The final image to be taken by Cassini is set to occur at 12:58 PDT (19:58 GMT) on Thursday, after which Cassini will reorient itself to aim its antenna at Earth.
Final playback of all remaining, stored data in Cassini’s computers began transmitting back to Earth at 13:22 PDT (20:22 UTC) 14 September – being received back on Earth at 14:45 PDT (21:45 UTC).
Communications from Cassini through the Deep Space Network will – at this moment – was continuous from this point until End Of Mission (EOM).
At 20:15 PDT, 14 September (03:15 UTC, 15 September), Canberra, Australia’s Deep Space Network location took over all tracking of Cassini for the remainder of the mission.
At 00:14 PDT (07:14 UTC) on 15 September, Cassini executed a 5 minute roll to aim its INMS (Ion and Neutral Mass Spectrometer) at Saturn’s atmosphere to collect as much data as possible as the probe enters Saturn atmosphere.
At this time, Cassini also reconfigured its systems for real-time data transmission at 27 kilobits per second (3.4 kilobytes per second).
Here, the final, real-time relay of data commenced, and Cassini was in constant downlink communication with Earth – communication that continued for as long as possible.
At 03:31 PDT (10:31 UTC), Cassini reached the upper traces of Saturn’s atmosphere, and Entry Interface began. The signal from Cassini that this had commenced will be received through Canberra at 04:54 PDT (11:54 UTC).
To maintain its communications lock with Canberra, Cassini’s thrusters will fire at 10% of their capacity to maintain the craft’s orientation in the initial seconds of entry.
From this moment on, Cassini will teach us all the final things it can about Saturn as temperatures and stresses on the outside of the craft rise rapidly over the next minute.
While the exact Loss Of Signal (LOS) moment cannot be precisely determined, NASA expects Cassini’s thrusters to be firing at 100% capacity just 1 minute after Entry Interface at 03:32 PDT (10:32 UTC) in a final grasp to keep Cassini’s antenna aimed perfectly back at Earth.
At Saturn, Cassini is expected to bid farewell at 03:32 PDT (10:32 UTC).
After Cassini signals its last, the valiant probe’s good-bye transmission will reached Earth 83 minutes later at 04:55 PDT (11:55 UTC) – marking the end of an era for a craft that has long outlasted its original mission and that has taught us more than most will ever know about Saturn, Titan, Enceladus, and our place in the larger structure of the universe.
To you, your engineers, designers, builders, scientists, operators and all those inspired by the discoveries you have enabled…
Farewell, Cassini. And thank you.
(Images: NASA, JPL, ESA and L2 Artist Nathan Koga)