Space Sunday: Cassini – a journey’s end

An artist’s impression of the Cassini spacecraft entering the upper reaches of Saturn’s atmosphere, high above the cloud tops, and breaking / burning up against the backdrop of the planet’s rings. Credit: NASA

On Friday, September 15th, 2017, just one month short of the 20th anniversary of its launch, the NASA/ Italian Space Agency (ASI) space probe Cassini will plunge into the upper reaches of Saturn’s atmosphere, bringing to a close the momentous NASA / ASI / European Space Agency Cassini-Huygens mission.

It will be a bitter-sweet moment for many the world over – most of all the vast international team who devoted up to fourteen years of their time on the mission – even before it launched. The Cassini vehicle has not only revealed so much about Saturn, its myriad moons, the rich complexities of the gas giant’s ring system- it has also helped inform us on the potential for life to exist elsewhere in the solar system and has even helped test Einstein’s work. It has also over the years returned some of the most stunning and evocative images of other worlds we have yet witnessed. Many of these images have been gathered together by National Geographic and have been put together in a superb interactive web presentation on the mission by Nadia Drake and Brian Jacobs.

A computer model of the hundreds of orbits Cassini has made around Saturn over the years (excluding the more recent orbits of the Grand Finale). Credit: Drake / Jacobs / National Geographic

In all seventeen countries have been directly involved in the conception, design, construction and operation of the Cassini-Huygens mission, both in terms of the Cassini orbiter and the Huygens Titan lander and the science instruments they carry. NASA carries primary responsibility for the orbiter’s design and construction, with the Italian Space Agency providing the all-important, dual-purpose high-gain radio antenna and its associated communications equipment, together with the low-gain communications suite which would provide continuous communications with Earth through the mission. ASI also incorporated a compact radar system in to the high-gain antenna systems, allowing it to function as a synthetic-aperture radar, a radar altimeter, a radiometer, and provide the visible channel portion of the VMS spectrometer package carried by the probe.

ESA was responsible for the Huygens lander, with France designing the vehicle itself, with the descent parachute system provided by Martin-Baker of America, while the science and communications packages were supplied by several European countries and the United States.

Cassini-Huygens stored within the payload fairing of an Atlas 4B rocket on the pad of Launch Complex 40, Canaveral Air Force Station, October 12th, 1997, 3 days ahead of its launch. Credit: NASA

The mission was named for the Italian-French astronomer Giovanni Domenico Cassini, who first observed the divisions within Saturn’s rings system (and after whom one of the divisions is also named) as well as for of the planet’s moons, and  Christiaan Huygens, the Dutch mathematician, physicist and astronomer, who first observed Titan, Saturn’s largest moon.

Work actually commenced on the mission in the 1980s, the goal being to develop a mission which could determine the three-dimensional structure and dynamic behaviour of Saturn’s ring system, investigate Saturn’s atmosphere and magnetosphere, determine the composition and likely structure of Saturn’s moons, including the nature and origin of the dark material on Iapetus‘s leading hemisphere, and, in conjunction with the Huygens lander, characterise Titan’s atmosphere, including the variability of the cloud haze, and characterise the moon’s surface at a regional level.

Initially, the mission was funded for a 10-year period from late 1997 through mid-2008, which included a journey of seven years to reach Saturn. The voyage took so long because at the time of launch, there was no launch vehicle combination capable of sending Cassini directly to Saturn. Instead, it completed a mini-tour of the inner solar system; six months after launch, Cassini flew by Venus, using the planet’s gravity to accelerate it into a wide elliptical orbit. A second encounter in June 2000 again accelerated the spacecraft, slinging it on to a further gravity-assist flyby of Earth in August 2000, which in turn accelerated it and bent it onto an trans-Jovian flight path.

In late 2000, Cassini reached the vicinity of Jupiter, making its closest approach to the planet on December 30th of that year. As well as using Jupiter’s gravity to sling it onwards to its final destination, Cassini used the encounter to study Jupiter and its faint system of rings. In all some 26,000 images of Jupiter, its moons and its rings were taken during the 6-month period of the flyby (October 2000 – March 2001). Cassini’s science suite was powered-up for the flyby, and resulted in some significant discoveries concerning Jupiter’s turbulent atmosphere, including breaking a long-held view. Jupiter’s banded atmosphere comprises a series of alternate bands of darker and lighter zones, in part caused by Jupiter’s rapid rotation. It had also been thought that the lighter bands were the result of the atmosphere rising upwards, giving rise to lighter cloud formations, before circulating downwards once more.

The cratered moon Tethys slips behind Saturn’s largest moon, Titan, as seen by Cassini on November 26th, 2009. Credit: NASA/JPL / Space Science Institute

However, Cassini revealed the dark bands were peppered with individual storm cells of upwelling bright-white clouds too small to see from Earth, suggesting the vertical circulation of Jupiter’s atmosphere to be far more uniform than thought. The probe’s findings also showed that Jupiter’s thin and dusty rings to be made up from small, irregularly shaped particles, most likely created by ejacta from micrometeorites impacting the Jovian moons.

Cassini reached Saturn in 2004, officially entering orbit around the planet on July 1st of the year. Prior to doing so, the vehicle was part of a test of Einstein’s theory of general relativity. This states that any massive object like the Sun causes space-time to curve, causing a beam of light or any other form of electromagnetic radiation that passes close to it to travel farther (the Shapiro time delay). In 2003, with the Sun coming between Earth and Cassini, scientists on Earth measured the frequency shift in radio signals being received from the spacecraft. Similar experiments had been carried out with the Voyager and Viking missions, but Cassini provided for much more refined measurements to be taken, and firmly validated Einstein’s theory.

A geyser sprays water ice and vapour from the south polar region of Saturn’s moon Enceladus. Cassini’s first hint of this plume came during the spacecraft’s first close flyby of the icy moon on February 17, 2005. Credit: NASA/JPL / Space Science Institute

Cassini‘s primary mission at Saturn commenced as it approached the planet for orbital insertion. Although the orbiter was capable of functioning – all things being equal – through until around 2017, this primary mission was scheduled to last just 3 years and 261 days, ending in mid-2008. This was sufficient time for the primary goals of the mission to be achieved, but Cassini was always designed to achieve so much more. With this in mind, the programme was granted two funding extensions. The first, called the Equinox Mission, funded the project through until the end of 2010, and gave a particular focus on Titan (15 flybys) Saturn’s ice-covered moon Enceladus, thought two of the locations in the Saturnian system where life might have taken hold.

The second extension, granted funding in 2010 to the tune of around US $60 million a year, is referred to as the Solstice mission (as it would end a few months past Saturn’s summer solstice). It guaranteed that, avoiding any spacecraft failures, the mission would continue through to the point where Cassini’s manoeuvring propellants would be practically depleted. This phase of the mission allowed for a more extended study of Saturn, its rings and moons. It meant Cassini could witness never before seen seasonal changes in the planet’s atmosphere and study. It also meant Cassini could study Saturn’s atmosphere and magnetosphere at exactly the same time as NASA Juno mission studied Jupiter’s atmosphere and magnetosphere, allowing a direct comparison of the two. Finally, the extension would carry the mission through its 5-month “grand finale”, probing the region between Saturn and its complex ring system.

The surface of Titan as seen by Huygens. On the left, under local lighting conditions. On the right, under close to Earth normal daylight conditions. Credit: ESA

In December 2004, during the primary mission, Cassini released the Huygens Titan probe. The 318 kg (701 lb) lander, protected by a 2.7 metre (8.8 ft) diameter heat shield entered the atmosphere of Titan on January 14th, 2005. Following entry, the heat shield was ejected, and the 1.3 metre (4.2 ft) diameter probe commenced a 2.5 hour parachute decent to Titan’s surface. During the descent, Huygens measured the radiation balance inside Titan’s atmosphere, sought to determine the structure of the atmosphere, measure wind speed (using a Doppler system to compensate for the lander’s own decent through the atmosphere) and identify and quantify the chemical composition of the atmosphere, including organic compounds.

At the end of the descent, which included imaging the final phases, the probe successfully touched-down on Titan, where it operated for around an hour. This phase of the mission used the UK-designed Surface Science Package (SSP) to investigate the physical properties of Titan’s surface at the point of impact, including the surface was solid or liquid. The latter proved to be solid, although images returned by the lander suggested the area it was sitting within was an ancient lake bed, complete with pebbles of water ice scattered across it. However, rather than once having been the home to a liquid water lake, it was suggested the region may have once been the location of a body of liquid hydrocarbons – something findings from the Cassini orbiter had indicated during flybys of Titan.

The lakes and seas of Titan’s northern hemisphere – click for full size. Credit: ESA

In 2012, seven years after Huygens had landed, Cassini imaged long-standing tropical hydrocarbon lakes, including one roughly half the size of Utah’s Great Salt Lake and estimated to be around 1 metre (3 ft) deep in the region dubbed “Shangri-la”, not far from where the probe landed. The hydrocarbon-coated pebbles of water ice imaged by Huygens appeared to be rounded and smooth, suggesting liquid action, while linear striations imaged during the probe’s descent suggested the area may have been subject to periodic flooding or drainage.

The temperature at the landing site was 93.8 Kelvin (−179.3 °C; −290.8 °F) with pressure of 1467.6 millibars (1.448 atm).  Atmospheric moisture content measurements suggested the moon’s weather could feature torrential downpours causing flash floods, interspersed by decades or centuries of drought. Huygens found the brightness of the surface of Titan (at time of landing) to be about one thousand times dimmer than full solar illumination on Earth (e.g. around 10 minutes after sunset). The Sun would be visible as a small, bright spot, one tenth the size of the solar disk seen from Earth, and comparable in size and brightness to a car headlight seen from about 150 metres (487.5 ft).

In all, Huygens took some 700 images of its decent through Titan’s atmosphere and of Titan’s surface. However, a software glitch aboard Cassini meant only 350 of the images were successfully received.

Titan itself played a pivotal role in Cassini’s mission around Saturn. Roughly the size of Mercury, Titan is possessed of its own gravity well which Cassini has utilised to alter its trajectory and orbit around Saturn, allowing to do far more and see so much more than would have been possible if the spacecraft been wholly dependent upon its own fuel supplies.

It is hard to quantify all that a mission like Cassini-Huygens has achieved. We’ve learned so much about Saturn, its moons, the potential for life to arise elsewhere in the solar system, that a list of the mission’s achievements could go on for pages. However, some of the things worth highlighting might be:

  • Titan: the first landing on a probe on a moon beyond Earth’s; the first measurements of Titan’s atmosphere made from within the atmosphere, the first close-up images of the surface of Titan and the first images of liquid hydrocarbon lakes on the surface of Titan.
  • Enceladus: the discovery the moon has a thin, ionised water vapour atmosphere, fed by geysers of water, carbon dioxide and various hydrocarbons erupting at the moon’s south polar regions; the first measurements of elevated temperatures in some surface features; the first direct evidence of a large salty, global ocean of liquid water beneath the moon’s icy crust.
  • Rhea: indications that the moon may not have a rocky core, but might comprise ice and slush beneath its ice crust, or may have a liquid water ocean surrounding a small solid core.
  • The discovery of five never-before imaged moons of Saturn: Methone and Polydeuces (2004), Daphnis (2005), Anthe (2007) and Aegaeon (2008). A sixth “new” moon, Pallene, imaged in 2004 by Cassini was subsequently found to have been imaged by Voyager 2 in 1981, but hadn’t been recognised as a moon of Saturn.
  • Possibly the first images of a moon being “born” (2014) within the material of Saturn’s A ring.
  • A detailed analysis of the structure of Saturn’s rings using radio occultation over a four-month period (May-August 2005) and a detailed, decade-long study of the ring system and its complex dynamics which has helped with understanding the formation of planets and moons.
  • The first images of the vertical structure of Saturn’s rings.
A stunning composite image in true colour created by Gordan Ugarkovic. Using multiple images and data captured by Cassini during passes over Saturn’s north pole, it reveals the planet, its northern hexagon and the intricate structure of the entire ring system. Credit: NASA/JPL / Space Science Institute / G. Ugarkovic via National Geographic
  • The first real-time study of an extraterrestrial hurricane. In November 2006, Cassini imaged a storm at the south pole of Saturn with a distinct eyewall previously only seen in terrestrial hurricanes. Stationary above the pole, the storm is 8,000 km (5,000 mi) across, and 70 km (43 mi) high, with winds blowing at 560 km/h (350 mph).
  • The first up-close study of Saturn’s Great White Spot (also known as the Northern Electrostatic Disturbance): a periodic storm in Saturn’s northern latitudes that recurs roughly every 30 years. Cassini revealed the storm included a massive discharge which caused temperature spike in the stratosphere of Saturn 83 K (83 °C; 149 °F) above normal, making it the largest, hottest stratospheric vortex ever detected in the solar system. It coincided with a huge increase in ethylene gas, normally highly uncommon in Saturn’s atmosphere, more than 100 times greater than was previously thought possible for the planet. The storm is thought to be a combination of seasonal cooling of the atmosphere above Saturn’s northern hemisphere in winter, leading to a complex cycle of cooling, raining and contraction and heating resulting in density and thermal instabilities within the layers of the atmosphere, which due to the sheer size of Saturn’s atmosphere take a long time to build up, before erupting as violent storms during the northern latitude’s summer season.
Saturn’s Great White Spot, 2010-2011. Credit: NASA/JPL / Space Science Institute
  • The first detailed studies of Saturn’s Northern Hexagon.
  • The discovery that Saturn’s radio emissions are not linked to its rotational period. We don’t actually know how fast Saturn is rotating about its axis, as there are no visual means to observe this. Instead, the repetition of Saturn’s radio emissions (known as Saturn Kilometric Radiation) has been used in an attempts to measure Saturn’s possible rotational period. However, Cassini revealed that the radio rotational period has changed since it was first measured in 1980 by Voyager 1  and appears to different in the northern and southern hemispheres. It also suggests the two hemispheres have actually swapped rates, with each appearing to have seasonal variations. This is regarded as an important discovery, even if it still leaves Saturn’s length of day unknown.

Since April 2017, Cassini has been engaged in the last phase of its mission, the Grand Finale, a process that actually started in December 2016, when a flyby of Titan was used to significantly change the probe’s orbit around Saturn swinging through the outer edges of the F ring. On April 22nd, 2017, a final flyby of Titan further altered Cassini’s flight path such that it would pass between Saturn and its rings – something no other space probe has done. Since then, and around once a week, Cassini has made one complete orbit of Saturn, slipping between planet and rings each time, providing us with yet more information on both, and on the complex interactions between planet and rings, planet and moons, and moons and rings.

The Day the Earth Smiled, July 19th, 2013 – A true-colour mosaic of 323 images captured by Cassini as it comes round the “night” side of Saturn to reveal the sunlit rings out as far as the normally faint F ring (deliberately over-exposed to reveal it) and, arrowed, shining like a single star and some 1.445858 billion km (898.414 million mi) away, the Earth and Moon. Credit: NASA/JPL / Space Science Institute

However, on Friday, September 15th, things will be different. With the propellants for its manoeuvring systems (which allow the vehicle to orient itself for data gathering and images its study subject and for communicating with Earth) all but expended, Cassini’s final orbit will not simply take it between Saturn and its rings – it will carry the probe into Saturn’s upper atmosphere to break up / burn up. The reason for doing this is to avoid any future risk of the vehicle impacting one of Titan’s moons and possibly contaminating it with radioactive debris from the three radioisotope thermoelectric generators (RTGs) used to provide it with electrical power, or the risk of any hardy little Earth microbes which may have survived deep within the spacecraft also contaminating a moon.

“The end of Cassini’s mission will be a poignant moment, but a fitting and very necessary completion of an astonishing journey,” said Earl Maize, the mission’s project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “The Grand Finale represents the culmination of a seven-year plan to use the spacecraft’s remaining resources in the most scientifically productive way possible. By safely disposing of the spacecraft in Saturn’s atmosphere, we avoid any possibility Cassini could impact one of Saturn’s moons somewhere down the road, keeping them pristine for future exploration.”

Cassini will enter Saturn’s atmosphere at around 11:00 UTC (05:00 PDT; 08:00 EDT; 12:00 noon BST; 13:00 CST). The spacecraft will continue to collect and transmit scientific data as it does so, however, the mission team do not anticipate getting much data back one the craft is within Saturn’s atmosphere, as atmospheric drag will quickly cause the spacecraft to tumble, severing the connection with Earth.