In early heritage, Saturn was the god of time, an agricultural deity whose harvesting scythe finally took on a different connotation in the sense that all good things must eventually come to an end. This week, this proverb is proving particularly apt for Cassini, the intrepid spacecraft which has buzzed around Saturn’s planetary namesake for the last 13 years and, in the process, utterly transformed scientists’ understanding of the solar system’s most visually spectacularnbsp;world.
First envisioned by the U.S. space agency, NASA, over three years ago, Cassini was launched in the autumn of 1997 and arrived at Saturn in 2004, becoming the first spacecraft ever to orbit the enigmatic ringed world and spy on its many unusual moons. Since that time, it has fulfilled its promise to a remarkable degree, notching discovery upon discovery and rewriting the textbooks on the way. For many, it rates as the most successful planetary mission innbsp;background.
On Friday morning, Cassini’s mission will eventually come to a conclusion. With fuel for manoeuvring and control exercising, flight controllers were confronted with a difficult choice. Even after 13 years in space, there is a very good chance that Cassini, named after a 17th-century astronomer who studied Saturn, could be hauling bacteria from Earth. To prevent the risk that it might one day contaminate one of Saturn’s moons (especially Enceladus, which, as a result of Cassini, scientists have understood could have some native life of its own), the assignment is ending with a suicide dive into the atmosphere of Saturn. There, it is going to burn as Saturn’s strong gravity pulls it in and friction builds. Torn apart atom by atom, Cassini will become one with the world it’s illuminated sonbsp;definitively.
“There is no backing out now,” said Joan Stupik, a guidance and control engineer with NASA’s Jet Propulsion Laboratory in Pasadena, Calif.. On Monday, Ms. Stupik was a part of the mission team that led Cassini to make a close pass of Saturn’s giant moon, Titan. The moon’s gravity shifted the probe’s trajectory only enough to place it on a collision course with thenbsp;world.
“That is what sealed our destiny,” she told The Globe andnbsp;Mail.
Between now and then, scientists are scrambling to catch Cassini’s final bursts of information, including an estimated measurement of Saturn’s atmospheric composition as it disappears into the world’s impenetrable clouds, it is antenna pointed toward Earth before the end. For people who have been with the assignment for most of their professions, receiving Cassini’s final readings will indicate a bittersweetnbsp;second.
“It has been an irreplaceably valuable experience for me,” said Jonathan Lunine, a planetary scientist and professor at Cornell University who had been chosen to be a member of Cassini’s science group 27 yearsnbsp;past.
“After the week is over, it is going to be fine. It is just going through it that is going to be a little bit tough,” henbsp;additional.
However, what is most significant is that Cassini has left an unparalleled legacy of data and images from a world which has shown timeless in its capability to elicitnbsp;wonder.
A world for allnbsp;seasons
A vast, gaseous orb surrounded by a stunning collection of concentric rings, Saturn has always been a showstopper for astronomers. By taking a look at Earth from every possible angle over several years, Cassini revealed that Saturn’s visual richness consists of great diversity and change. When the spacecraft came in July, 2004, Saturn was illuminated from below, marking the winter solstice for the northern hemisphere. Since that time, Cassini has seen close to a half of Saturn’s 30-year cycle of seasons. During this time, summer has returned to the north, triggering dramatic ammonia storms which revealed the turbulence lurking inside the world’s pastel cloud layers. Cassini also found a strangely hexagonal zone of clouds, which appears as a somewhat dark region on top of thenbsp;world.
Beginning with its first trajectory, which required passing through Saturn’s ring plane before settling into orbit, Cassini was able to show a world of complexity in the patterns and movement of the trillions of icy particles which individually compose the planet’s rings. Alternating bands of dark and light found inside the thousands of person “ringlets” which compose the system would be the mathematical effect of a gravitational tug-of-war between Saturn and its various moons. Cassini also seen tiny moons orbiting inside the ring plane, such as Daphnis, just eight kilometres across, which the spacecraft recorded as it skirted along one edge of a ringlet, creating ripples like the wake from a small boat. By any measure, Cassini’s detailed observations “have altered our knowledge of rings,” says Doug Hamilton, a researcher who specialized in ring dynamics in the University of Maryland. “Every component of the area has beennbsp;redefined.”
“Titan is an explorer’s paradise,” says Alexander Hayes, a Cornell University astronomer that has been working with Cassini information for over 10 years to attempt and comprehend the intricate processes that have formed Titan’s surface. The only moon in the solar system with a considerable atmosphere, Titan is too cold for water except in solid shape. However, its surface was carved by rivers of liquid methane that falls as rain and finally gathers in seas and lakes. Covered with a thick orange smog that results from sun reacting with the methane, Titan was eventually revealed by Cassini’s infrared camera and radar pictures. These days, the smooth dark seas specify alien coastlines that scientists are eager to explore later on with amphibious robotic landers. John Moores, a planetary scientist at York University in Toronto, still remembers his time as a graduate student working on the Huygens lander, which Cassini carried with it from Earth and which dropped by parachute onto Titan’s surface. After so many years of wondering what was down there, assistant Prof. Moores compares seeing the first glimpse of ice boulders on Titan’s surface with viewing the runway materialize when shooting his native St. John’s on a foggynbsp;afternoon.
Along with the massive and complicated Titan (which is bigger than the planet Mercury), Saturn is circled by dozens of small, icy moons, each with its own unexpected capabilities. One of the strangest is Iapetus, with one side dark and one side glowing. Cassini revealed the comparison in extraordinary detail and quantified the composition of the darker, leading edge of the moon. Differences in temperature and sublimation of ice are regarded as the cause for Iapetus’s two-tone look, but the specifics of the process that generated it are still a matter of debate. Equally odd is the irregularly shaped moon Hyperion, whose low density and porous inside has given it a honeycomb look after countless years of battering bynbsp;meteorites.
An ocean ofnbsp;puzzle
For many scientists that worked with Cassini through over three decades such as planning, development, launch and implementation, the mission’s main discovery was also its most surprising. While coming up on Saturn’s small icy moon, Enceladus, Cassini observed jets of material shooting out from a region close to the moon’s south pole. After many following close passes by the spacecraft, scientists could determine that the jets are brought on by water vapour escaping from a salty, underground sea and shooting up through cracks in the moon’s intensely lineated surface. The occurrence of the sea — a spectacular find — has contributed to a few of the most pressing questions that Cassini has left for another probe to see Saturn, mission scientist Jonathan Lunine says: “Is there life in thenbsp;sea?”
While orbiting Saturn, Cassini sometimes was in a position to find the planet blocking sunlight, with its rings brightly backlit by the sun’s light in what amounts to a Saturnian variant of a whole eclipse. Sometimes, Cassini has been able to spot Earth as a little dot among countless background stars. It is not clear when any spacecraft will return to Saturn. Due to its space, assignments there cost billions and take decades to meet. But on July 19, 2013, Cassini captured this view of its remote home, where its graphics have inspired surprise, amazement and countless questions. Asked what he found most memorable about working on the assignment, Dr. Moores pauses and says: “I always enjoy how character differs from that which we cannbsp;envision.”