On the morning of Wednesday 15 October 1997, an ambitious voyage began with the launch of the joint NASA-ESA mission Cassini-Huygens from Cape Canaveral, Florida. This was the start of a seven-year journey to the planet Saturn and its moons.
The spacecraft consists of an orbiter – Cassini – that will circle the planet Saturn for at least three years and a probe – Huygens – that landed on Saturn's largest moon, Titan, in January 2005. On its way to Saturn the mission studied the interplanetary medium, the plasma environments of the Earth, Jupiter and Venus and the asteroid Masursky.
The journey to Saturn
From its launch in 1997 until it arrived in the Saturnian system in July 2004, the spacecraft measured interplanetary dust particles and observed the nature of the highly-ionised gas (plasma) that fills interplanetary space, in particular studying how energy is transferred from this plasma to the Earth, Venus and Jupiter. The spacecraft used the gravitational fields of Venus, Earth and Jupiter to change course and increase its speed on the way out to Saturn – this is known as the 'gravitational slingshot’ technique which reduces the amount of propellant needed for long missions.
Through the asteroid belt and on to Jupiter
In November 1999 Cassini entered the asteroid belt between Mars and Jupiter. Tolis Christou, a research student at Queen Mary College, realised that it would pass reasonably close to the asteroid 2685 Masursky.
Cassini was used to obtain images of the asteroid at a distance of 1.6 million km. These were used to determine its size, reflectivity, composition and rotation period. The results indicate that Masursky is between 15 and 20 km across.
Cassini left the asteroid belt in April 2000.
Cassini Jupiter flyby
Cassini passed through the Jupiter system between late 2000 and early 2001, sending back data that complemented the work of the Galileo probe that investigated the giant planet from 1995–2003. Cassini reached a minimum distance of 9.7 million km from Jupiter on 30 December 2000.
This color image of Jupiter was taken by the Cassini narrow angle camera from a distance of 81.3 million kilometers (about 50 million miles) from the planet. It is composed of images taken in the blue, green and red regions of the spectrum and is therefore close to the true color of Jupiter that one would see through an Earth-based telescope.
The image is strikingly similar to images taken by the Voyager 1 and 2 spacecraft in the early 1980s, illustrating the remarkable stability of Jupiter's weather patterns. The parallel dark and bright bands and many other large-scale features are quasi-permanent structures that survive despite the intense small-scale activity ongoing in the atmosphere. The longevity of the large-scale features is an intrinsic property of the atmospheric flows on a gaseous planet, like Jupiter, having no solid surface; but smaller features, like those in the dark bands to the north and south of the equator, are observed to form and disappear in a few days. Similar behavior was observed during the Voyager era.
Everything visible on the planet is a cloud. Unlike Earth, where only water condenses to form clouds, Jupiter has several cloud-forming substances in its atmosphere. The updrafts and downdrafts bring different mixtures of these substances up from below, leading to clouds of different colors. The bluish features just north of the equator are regions of reduced cloud cover, similar to the place where the Galileo atmospheric probe entered in 1995. They are called 'hot spots' because the reduced cloud cover allows heat to escape from warmer, deeper levels in the atmosphere.
The Galilean moon Europa is seen at the right, casting a shadow on the planet. It is this satellite of Jupiter which scientists believe holds promise of a liquid ocean beneath its surface.
Lightning storms in Jupiter's clouds
As Cassini flew past Jupiter it sent back some remarkable images of the giant planet.
They included closeups of lightning storms in the Jovian atmosphere and a new volcanic feature on Io. The images above were taken on 1 January 2001 and illustrate lightning flashes captured on the night side and their source regions seen on the day side approximately 2 hours earlier.
Cassini at Phoebe
As it made the final approach to Saturn in June 2004, Cassini passed 2100 km above the surface of the outermost moon, Phoebe. With a diameter of 220 km and an irregular shape, it has more in common with objects in the Kuiper belt of small bodies that orbit the Sun beyond Neptune.
Phoebe travels around the ringed planet in the opposite sense to all the other moons suggesting it may have been captured from solar orbit. Cassini's images of Phoebe showed it to be heavily cratered and irregular in shape. The highest resolution pictures revealed bright streaks and grooves running across a dark surface implying that Phoebe may be an ice-rich body with a dark coating.
Arrival at Saturn – the ring system in closeup
After a journey of seven years, the Cassini-Huygens probe entered orbit around Saturn on June 30 2004. As it swung past the planet, the probe passed through a gap in the ring system – the famous Cassini Division – where it could avoid collision with larger particles. Saturn's rings are made up of individual particles ranging in size from dust to boulders that each follow their own orbit around the planet.
Even in the Cassini Division, the spacecraft registered thousands of impacts with smaller pieces of debris but emerged unscathed. The close flyby allowed astronomers to obtain images of portions of the ring system in unprecedented detail. The highest-resolution images showed patterns of density waves in individual rings and a scalloped edge. These mark the points where particles clump together and then disperse.
Data from the probe shows the rings to be made of a mixture of water ice and darker 'dirt', supporting the idea that they formed from the remnants of a moon.
Huygens and Titan
One of the key targets for the Cassini-Huygens mission is Saturn's largest moon, Titan. The probe will study this object extensively until at least 2008, with 45 flybys planned.
Close encounters with Titan began in October 2004. So far the probe has flown to within 1200 km of the surface, studying it using visual light, infrared and radar imaging.
Unlike any other moons in the solar system, Titan has a thick atmosphere that mostly consists of nitrogen with a dash of methane and other hydrocarbon compounds. The temperature at the surface is around -180° Celsius (95 K) which is could allow methane (and ethane) to exist as solid, liquid and gas at the same time. There may even be oceans of methane and ethane lapping against more solid ground.
Titan may also have similar chemistry to that found on Earth before the development of life but the low temperature on the moon prevented any progress in that direction. This makes it a subject of intense interest to scientists evaluating the prospects for life elsewhere in the Universe.
Images and maps from Cassini-Huygens showed Titan has a complex range of dark and light areas with a variety of unexplained geological features. One light and relatively smooth area may be a lake of some kind. Streaks on the surface could be wind-blown material or moving liquid or ice and initial results confirmed that the surface is covered with organic material.
One of the highlights of the mission to Saturn was the success of the European Space Agency's Huygens probe, which separated from Cassini in December 2004. It cruised towards Titan over the next two weeks, before making a soft landing on its surface on 14 January 2005. This was the first time a spacecraft had landed on the moon of another planet.
Titan has a weak gravitational field compared with the Earth so its atmosphere extends far into space. The parachute deployed to slow Huygens down took full advantage of this, reducing the probe's speed to allow a leisurely 2.5 hours to descend to the surface.
During its descent the Descent Imager-Spectral Radiometer (DISR) sent back progressively sharper images of the moon, starting at an altitude of 150 km and culminating in the first pictures of the surface. DISR pictures show evidence for precipitation and erosion eerily reminiscent of Earth. The fluid driving these is not water but methane – the terrestrial gas exists as solid, liquid and gas in Titan's frigid surface temperature of -170°C.
On landing the penetrometer device attached to Huygens measured the consistency of the surface, which seems to have a hard crust sitting on softer material. This led one astronomer to describe it as being analogous to crème brûlée or wet sand.
Scientists were pleasantly surprised that Huygens transmitted for far longer from the ground than originally envisaged – the probe worked for several hours rather than the anticipated three minutes. It sent back pictures of what are probably blocks of super-hard water ice set on a desert-like landscape beneath an orange Titanian sky.
Saturn's other moons
Iapetus is 1400 km across. It is a two-tone moon – the leading hemisphere is as dark as tarmac on a road and the trailing hemisphere is as white as snow.
At the end of 2004 Cassini passed Iapetus at a distance of 123,000 km. The images from the probe show a ridge that runs along the equator and bulges 20 km above the surface, giving the moon a lumpy appearance. Dark streaks run across the boundary between the dark and light hemispheres and suggest that material fell on to the moon. Scientists are still divided as to whether this came from inside the moon or from an external source.
Underground lakes on Enceladus?
Cassini encountered Enceladus in February, March and July 2005, passing by at a distance of just 175 km.
Enceladus is one of the most reflective objects in the solar system, with a surface of water ice. The region near the south pole is relatively young and has few impact craters, but large blocks of ice cover the ground. All this implies that an internal heat source has reshaped the surface and erased older features.
In March 2006 Cassini mission scientists presented images of plumes of material erupting into the space around Enceladus. These are thought to be jets from pools of liquid water just beneath the moon’s surface. Other moons like Jupiter’s Europa are thought to have subsurface oceans but they are sealed beneath layers of ice tens of kilometres thick. The discovery of liquid water makes Enceladus another possible refuge for extraterrestrial life and a prime target for future missions.
Hyperion – Saturn’s spongy moon
Battered Hyperion is a moon with a very strange appearance. Cassini made its first close approach in September 2005 and sent back images of an irregularly shaped body pummelled by numerous impacts.
The spongy terrain is thought to result from thermal erosion, where dark materials building up on crater floors are warmed by sunlight and melt deeper into the moon’s surface. Surrounding ice then sublimes away.
Cassini made a close flyby of the moon Dione in October 2005, passing just 500 km above its surface. Like most of the other moons, it has a surface largely made up of water ice.
Images show a heavily cratered surface, with cracks and fractures on one side of the moon. These are the youngest features and are evidence for early geological activity.
Cassini made a close flyby of Mimas on 2 August 2005, the first visit by a spacecraft since the Voyager 2 spacecraft sent back the first images in 1980. The most dramatic feature is Herschel crater, which has a diameter of 140 km. Mimas is just 397 km across so if the impact that created Herschel had been any larger, the moon would probably have split apart.
In several 2005 encounters, Cassini mapped most of Mimas, finding a heavily-cratered surface and kilometre-deep grooves that run for as much as 100 km. They may also be related to the colossal impact that formed Herschel.
Tethys is an icy moon with a diameter of 1060 km. The surface is dominated by a 400-km wide impact crater, Odysseus, and the 2000-km valley, Ithaca Chasma.
Cassini flew past Tethys in September 2005, passing at a distance of just 1500 km. The most detailed images show numerous small craters within Odysseus and soft edges on Ithaca Chasma – in both cases evidence for the extreme age of these features.
Rhea, Saturn's second-largest moon, has a diameter of 1500 km. Most of its volume is water ice, which surrounds a small rocky core.
Cassini made a close encounter with Rhea in November 2005, passing just 500 km above the surface. Images from this date and other flybys show a moon with two distinct hemispheres. The side that leads as Rhea moves around Saturn is bright and heavily cratered. This is distinctly different from the trailing side – it has light streaks that may be ice cliffs set against a darker surface.
UK scientists have been particularly successful in the selection process for this international mission, with involvement in seven experiments on the Cassini Orbiter and two on the Huygens Probe.
The Open University provided the Principal Investigator for the ESA Surface Science Package on the Huygens probe, which was the first spacecraft with UK involvement to land on another planetary body (other than the unsuccessful Beagle 2 lander). Imperial College London provided the Principal Investigator for the Cassini Dual Technique Magnetometer.
The other UK groups are at the Mullard Space Science Laboratory, Oxford University, Queen Mary College, Sheffield University and the Rutherford Appleton Laboratory. This degree of involvement will give UK space scientists a substantial scientific return over a major part of the coming two decades.