NASA’s Cassini spacecraft is about to begin its “Grand Finale” that will bring an end to its 20 years in space, 13 of which have been spent exploring Saturn, its rings and its moons. It is running out of fuel and to avoid any possible contamination of those moons — some of which may have environments that could support life — the spacecraft is being commanded to enter Saturn’s atmosphere where it will be destroyed. The last drops of fuel will be used to make 22 dives through the unexplored gap between the planet and its rings to extract some last morsels of scientific data. The first is on April 26; the last on September 15.

Launched on October 15, 1997, Cassini arrived at Saturn in 2004. Seventeen nations and three space agencies participated in the mission. Perhaps the best known international contribution is the European Space Agency’s (ESA’s) Huygens probe that separated from Cassini and landed on the moon Titan in 2005, the first spacecraft to land on a surface in the outer solar system. Huygens sent back amazing data about Titan and its methane lakes. Cassini itself revealed that another moon, Enceladus, has a salty ocean under its icy crust. As Cassini project scientist Linda Spilker explained at a press conference today, cracks in the surface allow geysers of water vapor and organics to spew out. Cassini flew through one of the plumes in 2015 obtaining tantalizing scientific data. (Jupiter’s moon Europa is similar. NASA’s Europa Clipper mission is under

development to investigate it in the 2020s.)

Jim Green, NASA’s planetary science division director, added that the agency is soliciting ideas for future missions to “ocean worlds” like Titan and Enceldaus. Such a mission would be one of NASA’s “New Frontiers” series of planetary science missions. The Announcement of Opportunity (AO) opened on December 12, 2016. Green enthused that Titan, Earth-like in some ways with liquid on the surface and a “water cycle” but with methane instead, and extremely cold temperatures, could harbor an entirely different form of life, so-called “weird life,” based on something other than DNA. If so, it could hint at other types of life on planets outside our solar system (exoplanets).

A 2007 report from the National Academies of Sciences, Engineering, and Medicine, The Limits of Organic Life in Planetary Systems, explores the concepts of “weird life.” One of the report’s authors, Steve Benner, summarized it in lay terms at an Academies workshop in 2010 on sharing the adventure of space science with the public and the “grand questions” yet to be answered.

Saturn and some of its rings are visible from Earth, but it was NASA’s Voyager 1 and 2 flyby missions in the early 1980s that provided the first close-up pictures of the planet and the extent of the ring system. The rings are made of dust and ice particles that could damage Cassini and its instruments. The 22 dives between the top of Saturn’s atmosphere and the lowest ring pose a risk to the spacecraft, but mission managers decided that since the spacecraft would have to be destroyed in any case, they would use it to obtain every last piece of scientific data possible.

Cassini project manager Earl Maize said today that models suggest the spacecraft has a 98.8 percent chance of surviving the first 21 dives. It is intended to enter the atmosphere on the 22nd dive on September 15 and be destroyed by atmospheric forces. Saturn is a gaseous planet so it will not “crash.” Maize offered an ethereal description — Cassini will “become part of the planet itself.”

Cassini will make one last flyby of Titan to get a gravity assist to put it into the correct position to pass through the gap between the planet and its rings. Joan Stupak, a Cassini guidance and navigation engineer, demonstrated NASA’s interactive “Eyes” website where anyone can “fly along” with Cassini and other NASA spacecraft. She showed the trajectories she and her team have

devised for the dives and Titan’s influence on them. The first dive is on April 26.



Illustration of Cassini’s “Grand Finale” orbits of Saturn. Credit: NASA JPL-CalTech

Maize pointed out that everyone involved in the project has mixed emotions as the end is in sight — excitement about new discoveries that will be made from the 22 dives, pride in a successful mission that for some has consumed their entire careers, and a sense of loss. “Humankind has been at Saturn for 13 years. … We’re connected. … That’s going to go away and there’s no substitute for some time to come.” Spilker has worked on the project since it began three decades ago, starting when her daughter was in kindergarten until now when her daughter has a daughter herself. It will be “hard to say goodbye to this plucky, capable” spacecraft and the family of scientists, engineers, technicians and others involved in the mission. Stupak noted that Cassini received its first funding in 1989, the year she was born, so they are the same age and it has been an “incredible privilege” to work on the mission for the past four years.

Cassini cost $3.27 billion including launch, of which $2.6 billion was paid by the United States and $660 million by European partners. These “flagship” missions are NASA’s most expensive and are sometimes criticized for their high cost. Supporters argue that they are robustly designed to obtain cutting edge science. Maize said today Cassini validates the flagship model, demonstrating time and again that it was “ready for anything” and able to obtain unexpected scientific results. Flagships are “expensive, but the payoff is well worth it.”

Cassini’s cost was controversial in the early 1990s, however. Originally, it was paired with the Comet Rendezvous Asteroid Flyby (CRAF) mission. NASA believed that by using the same spacecraft design (the Mariner Mark II “bus”), two spacecraft, CRAF and Cassini, could be built for $1.6 billion. Congress was skeptical and set $1.6 billion as a cost cap, insisting that if the total grew beyond that point, one would have to be cancelled. That is, indeed, what happened. Cassini was the survivor.