Cassini’s dead dive into the planet Saturn

The most spectacular, ambitious space mission Cassini waved final goodbye, plunging into Saturn’s atmosphere on September 15^th, 2017. The orbiter commissioned to study Saturn system began its journey on October 15^th, 1997 with Titan IVB/Centaur carrying the orbiter and European Space Agency’s (ESA) Huygens Probe lifting off from the Cape Canaveral. The mission a cooperative effort of NASA, ESA and Italian Space Agency performed 127 close flybys of Saturn’s moon-Titan in the past 20 years has provided valuable information about the sixth planet of the Solar System.

The spacecraft had two elements- Cassini Orbiter and Huygens probe that reached Saturn in July 2004. The probe-orbiter duo was designed to carry out 27 investigations. Orbiter has 12 instruments which included spectrometers, imaging systems, dust analyzer, radio, radar, both optical and microwave sensing instruments for receiving, analyzing data, making precise distance measurements, measuring quantity and composition of dust particles, strength of plasma and radio waves. Three Radioisotope Thermoelectric Generators (RTG)s power the Orbiter and instruments on board.

Huygens Probe designed by ESA was a hard shellfish like structure with two modules: Entry Assembly Module and Descent Module. Probe descended on the surface of Titan on January 15^th, 2005 in two hours twenty-seven minutes. While Entry Assembly Module contained all equipment needed to control Huygens after its separation from Cassini. Descent Module had all the instruments and three parachutes. Huygens had six probes to measure thermal properties of atmosphere, wind speed, flow of radiation, chemical composition of Titan, collect aerosols and an acoustic sounder.

During the long momentous journey, Cassini orbited Saturn 293 times with its trajectories corrected hundreds of times. A major portion of the propellant was expended in lifting off the spacecraft from the Earth. By the time, Cassini reached Saturn, it had less than 1/30^th of the propellant needed for trajectory changes. With a fraction of fuel left to carry out the assignments of the mission, navigation team used Saturn’s moon Titan for lifting and swinging the spacecraft up and down. A single flyby of Titan at an altitude of 1000 kilometers gave it a velocity of 800 meters per second, equal to velocity provided by one third of the Cassini’s total propellant making “Titan the engine of the tour”. Thus, by the end of the mission, Cassini achieved a change in velocity of 90,000 meters per second from Titan flybys, equal to 37 times the velocity provided by the propellant. The gravitational force of Titan helped in making sharp turn whenever Cassini flew closer to it and acted as pivot point for Cassini. Propellant was judiciously used to make small corrections or for bringing back spacecraft to tread along the intended trajectory. Each Titan flyby provided the necessary speed and directionality to orbit Saturn that lasted for several months. While Titan played a crucial role in changing the trajectories of Cassini, its exact position is still a mystery. Besides, space navigation team also encountered difficulties in tracking location of Cassini. By calculating the time taken by the signal sent from Earth to Cassini and received from the spacecraft, navigation team would calculate the relative distance of spacecraft from Earth.

Prior to Cassini mission, earliest observations of Titan and its imaging were done by Voyager-1 and 2 in 1980 and 1981. Cassini is the first spacecraft that orbited Saturn and has been explored the planet for the past 13 years. One Saturn year is equal to Earth’s 29 years. Cassini meandered through 60 moons of Saturn and provided vital details of its two moons-Titan and Enceladus which caught the attention of scientists. The fierce jets emanating from the ridges in the Southern Hemisphere of Enceladus, moon of Saturn fascinated astronomers. The plumes referred to as “Tiger Stripes”, upon analysis were found to contain molecular Hydrogen, traces of methane, ammonia, carbon monoxide, carbon dioxide, salts and simple organic molecules, indicating probable existence of simple life forms. Cassini’s images revealed existence of a frozen ocean on Enceladus. While images of the seas, lakes, rivers and clouds filled with liquid methane and ethane on Titan’s surface provided by Huygens Probe changed the perspectives of scientists towards the gas giant Saturn’s planet system. Titan and Enceladus were in fact found to contain ingredients for supporting life. Titan is the only place on Solar System other than the Earth where liquid streams of methane and ethane are found, not water. Scientists therefore decided to plunge Cassini into Saturn’s atmosphere as they believed that a wandering orbiter upon collision with either of these potential habitable planets might contaminate their nascent ecosystem. Further spacecraft can collect crucial data as it moved much closer before eventually getting lost in the Saturn’s atmosphere. This might help scientists to unravel mysteries pertaining to the iconic rings of Saturn. This death dive is expected to yield cryptic clues about the evolution and formation of this giant gas planet as all instruments on board are still working.

Cassini orbiter has provided wonderful images of storms on Saturn with clouds reaching speeds of 150 meters per second and probed the earth-sized jet streams at both poles of Saturn that exist in all seasons but change colors. It studied Saturn’s moon Daphnis, 8 kilometers across, but gravitational pull strong enough to create ripples in Saturn’s rings. Cassini captured beautiful images of Saturn rings, which are “flattest structures known to man”, family portrait of Saturn’s moons-Janus, Pandora, Enceladus, Mimas and Rhea lined up together. Cassini spotted Southern lights on Saturn. The mission found that the haze-shrouded moon of Saturn, Titan is a repertoire of organic chemicals, the precursors of prebiotic chemistry. It investigated cycling of liquid methane through the complex web of clouds enveloping skies of Titan and the great seas on its surface. By unraveling depths of Titan seas, it opened new realms in the era of extra-terrestrial oceanography and reinforced possibility of occurrence of complex chemical process different temperatures and atmospheric conditions. This mission provided invaluable clues to scientists laying foundation for future missions which can collect signs of biology across other giant planets and their moons. Scientists are keen on using the trick of using moon’s gravitation to nudge spacecraft in the Clipper Mission, designed to explore Jupiter’s Moon, Europa. Clipper Mission is scheduled to be launched in 2020.

In its two-decadal momentous journey, Cassini travelled 7.9 billion kilometers, collected 635 GB data that can keep scientists busy for decades. Amount spent on construction, development and maintenance of this mission for 20 years was $3.9 billion. On September 11^th, Cassini using the gravitational push of Titan propelled itself into a collision course with Saturn’s atmosphere. In the early morning hours of September 15^th Cassini will dive into the planet. Like the illustrious Rosetta Spacecraft of ESA launched in 2004 which plunged into 67P/ Churyumov-Gerasimenko Comet, ending the mission on September 30^th, 2016, Cassini will meet its end. 

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