Representation of the Cassini Satellite orbiting Saturn.
Image Credit: NASA/JPL-Caltech.

Named after a famed 17th Century Italian-French astronomer the Cassini spacecraft took 10 years to reach a distant location in the Solar System, that of Saturn and its surrounding neighbourhood. Since its arrival in 2004 it has made a number of remarkable discoveries during its exploratory mission.


Polydeuces: Is it an egg? Is it a snowball? No, it’s a Trojan moon!
Image Credit: NASA/JPL/Space Science Institute.


The Cassini spacecraft has discovered something astounding and totally unique for a planet, that a moon of Saturn has its own companion moon, also known as a ‘Trojan’ moon! The first planet in the Solar System to be able to claim this feature, they can exist near stable points of gravity, in front of or in the wake of a larger moon. Polydeuces is just 2 miles wide and has been found to be accompanying Dione, a moon approximately 700 miles across.  The category is named after the Trojan asteroids, which orbit the planet Jupiter in a similar configuration. 


One of Saturn’s two polar hexagons.
Image Credit;NASA/JPL-Caltech/Space Science Institute


Cassini has also discovered a hexagonal-shaped storm the diameter of 4 Earths at both of Saturn’s poles! It obtained the first complete view of the north pole of the ringed planet, which shows the striking and unforgettable form of a hexagon. This distinctive shape is thought to be a jet stream. Cassini also revealed both of Saturn’s poles as having hurricane-like features, however the dynamo for these storms is still unknown.


Left: forward orbital face of Lapetus. Right: rear of the Saturnian moon.
Image Credit: NASA/JPL/Space Science Institute.


A 300-year mystery on Lapetus was explained. The severe two-tone contrast on this Saturnian moon was a long-standing mystery, however the Cassini Orbiter’s data revealed that the dark reddish dust seen around the equator and stopping short of the moon’s poles originates in the Solar System itself. It appears to get whipped up before settling on the forward face of the planetary satellite as it progresses on its orbital path.


Plumes of icy particles, organic compounds and water vapour erupting from ‘tiger stripe’ fissures near Enceladus’ South pole.
Image Credit: NASA/JPL/Space Science Institute.


A dark back-lit view of Enceladus enabled the robotic traveller to make yet another exciting discovery, icy plumes erupting from its surface. They became visible as they were caught in the sunlight beyond the moon’s dark disk. The spacecraft’s mission was altered to facilitate a better look at these fascinating features. Since life forms within known biology rely on water, scientists were particularly excited to discover water-based ice in the plumes which may imply a sea of liquid water beneath the moon’s icy surface.


Vertical structures rising from Saturn’s 10 metre thick B ring.
Image Credit:NASA/JPL/Space Science Institute.


Approximately every 15 years the Sun happens to shine on the edge of Saturn’s rings and the Cassini spacecraft was in the right place and at the right time to witness this rare event. The angle of the sunlight exposed some frosty-looking vertical structures and shadows towering up to 2.5km along the ring plane.

Two new terrestrial locations visited in the Solar System in the last 50 years: Titan, as seen from the car-sized Huygens lander (left), and the surface of Earth’s own moon (right) photographed during one of the manned American Apollo missions.
Image Credit: ESA/NASA/JPL/University of Arizona.


Named after the Dutch astronomer who discovered Titan in 1655 – the Huygens probe dispatched by the Cassini Orbiter became the first robot to land on a moon in the outer Solar System in January 2005. It revealed Titan to be one of the most Earth-like places in the Solar System to date, featuring an atmosphere, rain, rivers, lakes and seas composed of ethane and methane (effectively oil).


Birth process of new Saturnian moon?
Image Credit: NASA/JPL-Caltech/Space Science Institute.


Although unfortunately for Star Wars fans Earth’s emissary didn’t sense any disturbances in ‘The Force’ while visiting Saturn, it did, just as excitingly, observe a disturbance in Saturn’s rings. The object is thought to potentially be a new moon forming, readying itself to depart the outskirts of the rings.

Commencing on the 26th April 2017 for the Cassini spacecraft’s ‘grand finale’, the final stage of its eventful and lengthy mission as named by the public, it dived through the 2400 km void between the planet and its inner rings, the first probe ever to attempt this manoeuver. It positioned its 4m antennae dish down like a shield to protect its more sensitive instruments from the anticipated collisions with dust particles during descent. The spacecraft’s team was anticipating the RPWS (Radio & Plasma Wave Science instrument’s) usual whistles and squeaks of waves in the charged particle environment to be drowned out by pops and cracks, the audio formatted sound of dust particles colliding with the antennae recorded during its dive. However this big empty zone proved to be far emptier than any of the scientists could have predicted with almost no dust particles detected whatsoever! This was a complete contrast to the zone just beyond Saturn’s outer rings, where hundreds of particles had registered on the antennae. Moreover the few particles the Cassini Orbiter detected between the gas giant planet and its rings also appear to be extremely small, no more than one micron in diameter, the approximate size of fine smoke particles.


The spectacularly huge gas giant planet with the tiny ‘Death Star’ moon, Mimas visible beneath its rings.
Image Credit: NASA/JPL-Caltech/Space Science Institute.


Twenty-one other dives have been scheduled for the epic explorer during which it will gather readings from Saturn’s gravity and magnetic fields, photograph the gas giant and its rings, and estimate the total quantity of material contained in the ring plane. Right up until the end therefore this magnificent orbiter will continue to serve mankind, learning and revealing more about Saturn’s neighbourhood for its distant masters. Extremely proud of its achievements (and to avoid contamination of any life forms on Titan or Enceladus by crashing there), NASA officials have planned a dramatic end for the Cassini Orbiter’s $3.2 billion NASA-ESA-Italian Space Agency mission.  It is due to surrender itself to the gaseous windy embrace of the golden god of the Solar System in September 2017.


 Article written by: Nick Parke, Education Support Officer

Nick Parke, Education Support Officer


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