The enormously successful Cassini-Huygens Mission is ending, but it has made many important discoveries during its 20 years of service.
After a two-month break over the summer, I think it’s most fitting to start off with an article celebrating one of the most wondrous journeys of discovery of recent years. I remember looking forward with great anticipation to the 2004 Huygens landing on Titan, the first and, so far, the only time any spacecraft has landed on a body in the outer solar system. Roll on 2017, and the Cassini-Huygens mission has blessed us with many great discoveries, some of the most exciting of which I’ll be looking at in this week’s listicle. As I write, Cassini is in the Grand Finale phrase of its mission, where it will dive into Saturn’s atmosphere to meet its destruction on September 15.
#1. Putting General Relativity to the Test
Mysid, Wikimedia Commons
First published by Albert Einstein in 1915, the theory of general relativity has become one of the most important foundations of modern science. In 2003, while Cassini was en route to Saturn, a team of Italian scientists put the theory to the test, successfully calculating the curvature of spacetime with a precision some 50 times closer than previous measurements. By measuring the shifts in radio wave frequencies between the spacecraft and Earth, the scientists witnessed first-hand how the gravitational force of a massive object like the Sun literally bends time and space to a measurable degree. Previously, the best calculations came from the Voyager space probe.
#2. Exploring Jupiter’s Atmosphere
Just over three years into its journey, Cassini-Huygens reached Jupiter, using the gas giant for a gravity assist to provide it with enough momentum to reach Saturn, which lies almost two times further away. However, over the course of its six-month flyby, the ambitious space probe wasn’t about to waste any opportunities. It made numerous scientific measurements and took an unprecedented 26,000 photos, including the most detailed global true-colour portrait of Jupiter ever. This mosaic, composed of images taken from 6.2 million miles (10 million km) away, displays surface details as small as 37 miles (60 km) across.
#3. Discovering New Saturnian Moons
Like all the gas giants, Saturn comes with a plethora of moons in addition to its famous rings. Aside from countless tiny moonlets, the gas giant now has 61 moons with formal designations, and seven of them were discovered upon Cassini’s arrival in 2004 and during the following years. The moons named so far are Methone, Pallene, Polydeuces, Daphnis, Anthe and Aegaeon. They’re all rather small though, with the largest being less than four miles (6 km) in diameter. Another moon, still known only by its scientific label S/2009 S 1, was revealed in a press release published in 2009.
#4. Landing on the Dunes of Titan
As the only extraterrestrial object in the solar system with permanent surface liquids and weather systems like Earth, Saturn’s largest moon was easily one of the most compelling targets for the Cassini-Huygens mission. After conducting a flyby of the mysterious orange world, Cassini released the Huygens lander, which entered the Titanian atmosphere on January 14, 2005. After nearly three hours of descending through the dense, soup-like atmosphere, Huygens became the first and, so far, the only spacecraft to land on an object in the outer solar system. There it found a soft orange surface scattered with rocks topped with a methane haze.
#5. Revealing the Icy Plumes of Enceladus
Although only a little over 300 miles in diameter (500 km), Saturn’s sixth largest moon is anything but a boring and frozen chunk of rock. Aside from having the most reflective surface of any object in the solar system, Enceladus also appears to have a subsurface ocean, making it one of the most likely places to find extraterrestrial life. Kept liquid by the heat caused by the tidal friction of Saturn’s immense gravity field, the little moon regularly spews out icy geysers through breaches in its surface. Passing within 30 miles (50 km) of the surface in 2008, Cassini flew right through one of these geysers, detecting both water and carbon dioxide.
#6. Analysing Saturn’s Dynamic Rings
Saturn is easily most famous for its elaborate ring system, which you can view yourself through any reasonable telescope. In total, the gas giant has four main rings and three fainter and narrower ones. Upon its arrival in the Saturnian system, Cassini discovered that Enceladus is the source of one of the smaller rings, with its geysers constantly fuelling it with new material. Using radio occultations, Cassini also measured the size and distribution of the particles that make up the rings, while also exploring the atmosphere of the planet itself. Thanks to this mission, we now have an incredibly detailed overview of the inner workings of Saturn’s rings
#7. Unveiling the Hydrocarbon Lakes of Titan
Although the Huygens landing itself, in addition to previous studies, pointed to the existence of surface liquids on Titan, radar images taken in 2006 finally confirmed that there were multiple lakes in the moon’s northern hemisphere. A year later, JPL announced conclusive evidence of entire seas composed of methane and ethane, which remain liquid in Titan’s frigid −179.5 °C surface temperatures. In addition to this incredible revelation is the fact that the moon also sports rain, snow and flowing rivers but, instead of having a hydrological cycle like Earth, its system relies on an atmospheric chemistry that produces hydrocarbons such as methane and ethane.
#8. Witnessing Saturn’s Ferocious Storms
Saturn isn’t known for its calm weather. In fact, with wind speeds reaching 1,100 miles per hour (1,800 kph), it’s the second windiest place in the solar system after Neptune. During its exploration of the planet’s atmosphere, Cassini discovered more about its ferocious storms, including the detection of a 5,000-mile-wide (8,000 km) cyclone in the southern hemisphere. This peculiar hurricane featured an eyewall, a characteristic previously only seen in terrestrial storms although, at the same time, the storm was stationary at the pole. Cassini also took the clearest ever photos of Saturn’s bizarre hexagonal storm at the north pole.
#9. Showcasing the Saturnian System
On July 19, 2013, NASA released the most splendid image of Saturn to date. Now one of the most famous space photos of all time, this spectacular panorama features all the rings of Saturn, numerous moons and even Earth, Mars and Venus in the background. Known as ‘The Day the Earth Smiled’, the photo was taken during a solar eclipse while the probe was in orbit of Saturn, some 900 million miles (1.45 billion km) from home. The photo is also notable for being one of only three images ever taken of the Earth as seen from the outer solar system, reminding us just how small our pale blue dot really is.
#10. Diving into Saturn’s Clouds
Having already gone above and beyond its original mission plan, Cassini-Huygens is now being retired. The probe will be disposed of during a controlled descent into Saturn’s atmosphere. During its Grand Finale mission, the probe has been diving between the planet’s rings, taking photos and scientific measurements until its last minute. At the time of writing, Cassini was 614,000 miles (988,000 km) from Saturn, hurtling towards its spectacular end, in which it will be crushed out of existence by Saturn’s immense atmospheric pressure. This might sound brutal, but it’s also to prevent contamination of any potentially habitable environments, such as Enceladus.