When did the mission end? After almost 20 years in space, the Cassini mission ended on Sept. 15, 2017.
- Why was the end of mission called the Grand Finale? With input from more than 2,000 members of the public, Cassini team members chose the name for the final phase of the mission: the Cassini Grand Finale. Starting in late 2016, the Cassini spacecraft began a daring set of orbits that was, in some ways, like a whole new mission. The spacecraft repeatedly climbed high above Saturn's north pole, flying just outside its narrow F ring. Cassini probed the water-rich plume of the active geysers on the planet's intriguing moon Enceladus, and then hopped the rings and dove between the planet and innermost ring 22 times. Because the spacecraft was in close proximity to Saturn, the team was calling this phase "the proximal orbits," but they felt the public could help decide on a more exciting moniker. The Cassini mission invited the public to vote on a list of alternative names provided by team members or to suggest ideas of their own. "We chose a name for this mission phase that would reflect the exciting journey ahead while acknowledging that it's a big finish for what has been a truly great show," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.
- Why dispose of the spacecraft in Saturn’s atmosphere instead of other possible ends for the mission (for example, send Cassini somewhere else in the solar system, attempt a landing on one of the moons, crash into the rings and watch the impact with telescopes)? Concepts were evaluated for parking Cassini in an orbit around Saturn that would have been stable for a long time, along with a variety of other mission scenarios. However, the Grand Finale of close dives past the outer and inner edges of the rings, and ultra-close brushes with the planet and its small, inner moons, offered such enormous scientific value that this scenario was chosen for the mission’s conclusion.
Did Cassini continue to transmit data as it entered Saturn’s atmosphere? The Cassini spacecraft continued to collect and transmit scientific data as it entered Saturn’s atmosphere.
- Could microbes really have survived onboard Cassini for this long in space? Is this truly a concern that influenced the decision to deorbit into Saturn? Based on exposure experiments on the International Space Station, it is known that some microbes and microbial spores from Earth are able to survive many years in the space environment – even with no air or water, and minimal protection from radiation. Therefore, NASA decided to dispose of the spacecraft in Saturn’s atmosphere to avoid the possibility that microbes from Cassini could potentially contaminate Saturn’s moons at some time in the future.
What challenges did the Grand Finale present?
While the Grand Finale offered unprecedented opportunities for new discoveries during Cassini’s final orbits and a thrilling end to the Cassini mission, it was not without risk. The Cassini spacecraft and instruments were designed to observe Saturn, its rings and satellites from a relatively benign vantage point away from Saturn’s rings and atmosphere. A primary concern was the effect of Saturn’s atmosphere. If it was denser than modeled, it could cause the spacecraft to lose its ability to maintain a fixed attitude. Though recoverable, this could have curtailed science observations while the spacecraft team reestablished attitude control. Another, more significant, concern was the potential impact with ring material. The Cassini spacecraft successfully made numerous passes through the G ring. While the engineers and scientists believe that the flight path during the Grand Finale will pose no greater threat, the spacecraft will be traveling at over 76,000 mph (34 kilometers per second) in uncharted regions; an impact with even a grain-of-sand-sized piece of ring material could seriously damage the spacecraft or an instrument.- When did the Grand Finale actually start? The first orbit in Cassini’s Grand Finale phase began when the spacecraft is at the farthest point from Saturn in its orbit (called apoapsis) on April 22, 2017. But the real excitement of the Grand Finale began on April 26, when Cassini made its first dive between Saturn’s rings and the planet itself.
- Was it always planned that Cassini would end its mission by plunging into Saturn? The preferred end-of-mission plan for Cassini was always been to safely dispose of the spacecraft in the upper atmosphere of Saturn. The exact “when” and “how” of the mission’s conclusion has evolved over the years as the scientifically productive mission has been granted three extensions by NASA. The current “Grand Finale” scenario – to send the spacecraft on a series of orbits between the planet and its rings – has been part of the mission plan since 2010 and was developed in detail over the past four years.
Why was it safe to dispose of a spacecraft by burning it up in Saturn’s atmosphere? Disposing of Cassini in Saturn’s atmosphere was safe. The spacecraft entered Saturn’s atmosphere at high speed and burned up like a meteor. Any spacecraft material that survived atmospheric entry, potentially including its radioisotope fuel, sank deep into the planet where melted and became completely diluted as it mixed with the hot, high-pressure atmosphere of the giant planet. Saturn’s atmosphere does not have conditions that would be favorable to life as we know it, according to evaluations by the Committee on Space Research of the International Council for Science.
