Since pre–historic times man has viewed the relative movement of the Sun, Moon, and
the five visible planets against the backdrop of the stars. Saturn is the furthest
planet visible with the naked eye from Earth, and appears as a bright yellow
disk. The planet’s movement in the sky was first recorded in Mesopotamia
in the mid–seventh century BC.
Further observations occurred in the classical world, which the Greeks used to try to work out the shape of the orbits of the planets. Later the Hellenistic astronomer Claudius Ptolemaeus (AD c.100– c.170) made various observations of Saturn. Saturn’s position continued to be recorded into the late eighth century and early ninth century AD by Islamic astronomers in observatories at Damascus and Baghdad. Techniques in observations changed and improved over time, but celestial bodies were still only seen with the naked eye in the sixteenth century by astronomers such as Tycho Brahe (1546–1601). New information on the physical nature of Saturn could not be gathered until astronomers developed a new piece of equipment.
All astronomical observations until the early seventeenth century had to be performed with the unaided eye. Though Saturn’s motion could be followed through the sky and its future positions predicted, Saturn’s disk could not be resolved. But in July 1610 AD the mathematician Galileo Galilei (1564–1642) looked at Saturn through two lenses encased in a tube. Galileo’s telescope could magnify Saturn by only 32 times, and imperfections in the telescope’s lenses meant that Saturn’s Ring appeared as two satellites. During subsequent observations Galileo saw Saturn as a single disk and then with two great handles attached to the planet. In bringing to light the strange appearance of Saturn, Galileo presented astronomers with a problem that took them almost half a century to solve.
Gian Domenico Cassini, also known as Jean–Dominique Cassini, was born in
Pernaldo, Italy on 8th June 1625. In 1648 he began working at the Panzano
observatory near Bologna, and in 1650 he was appointed to the principle chair of
astronomy at the University of Bologna. In 1653 Cassini designed and
commissioned a new and larger meridian for the church of San Petronio of
Bologna. During the period 1656 to 1662 he produced a number of publications
based on observations using his innovative and precise meridian. In 1664
Cassini obtained lenses from the famous optical lens makers Giuseppe Campani and
Eustachio Divini. He used these in celestial telescopes to make a number of
important discoveries relating to the periods of rotation of Jupiter (1664),
Mars (1666), and Venus (1667). In 1667 Cassini was invited to join the Academie
Royale des Sciences and also to assist in the creation of the Observatoire
Royale in Paris. Before leaving for Paris in 1669 Cassini published
Emphermerides Bononienses mediceorem siderum (1668): a table of the
movements of the satellites of Jupiter.
Once in Paris, Cassini set about establishing the observatory and began numerous observations and discoveries, including Japetus (VII) (a second satellite of Saturn) in 1671, Rhea (V) (a third satellite of Saturn) in 1672, and finally two more satellites of Saturn; Tethys (III) and Dione (V) (March 1684). It was during this time that, using his highly developed observational talents, Cassini discerned a band on the surface of Saturn and proposed that it was ring around the planet. Furthermore, he discovered that this ring was subdivided in two thinner rings separated by a narrow band, now known as Cassini’s division. At the beginning of the eighteenth century Cassini’s astronomical activities declined and his son, Jacques, gradually replaced him. Cassini died in Paris on 14 September 1712.
The Cassini orbiter is named after this famous Italian-French astronomer for his work involving Saturn.
Christiaan Huygens was born in The Hague, the Netherlands, on 14th April 1629 into
a prominent and intellectual Dutch family. Huygens received the best possible
education, having been taught by, amongst others, the French philosopher Rene
Descartes. In 1645 Huygens began studying mathematics and law at the
University of Leiden. By 1651 he had published his first work on geometry,
entitled Theoremata de quadratura hyperboles, ellypsis et circuli.
When Huygens visited Paris in 1655, he made contact with various lens makers
with the ultimately successful aim of improving the design and techniques of his
own lenses. Returning to The Hague he made several observational discoveries
that would make him famous, including the discovery of Titan
(a satellite of Saturn) in 1655 which he published in De Saturni luna
observation nova (1656). Furthermore, he observed the ’arms’ of
hypothesised that the planet was surrounded by a ring in
Systema Saturnium (1659).
Between 1660 and 1663, Huygens travelled back and forth between Paris and The Hague – working on lenses, pendulum clocks, mathematics and other activities. In 1663 he was appointed a founding member of the Academie Royale Des Sciences by Louis XIV. From 1666 to 1681 Huygens worked in Paris, mainly on pendulum clocks and in 1673 he published his seminal work Horologium oscillatorium. During this period he also developed a wave theory of light, which he discussed in Traite de la lumiere (1678). In 1681 Huygens returned to The Hague, where he worked on optics and clocks until his death in 1695.
The Huygens probe was named after this Dutch astronomer for his work involving Saturn and Titan.
The Cassini–Huygens Space Mission formally began in 1982
– 15 years before it launched and 22 years before it arrived at Saturn.
By 1985 a joint European Space Agency (ESA) and National Aeronautics and Space
Administration (NASA) assessment of a Saturn orbiter and Titan probe was
completed, and in 1986 ESA’s Science Program Committee gave approval to
start the first stages of research on the Saturn orbiter (now named Cassini).
In 1987 the Titan probe was renamed Huygens by the ESA.
At this time NASA and ESA announced the opportunity for scientists to propose scientific investigations for the mission.
Then in 1992 a funding cap was placed on the project. The Cassini–Huygens mission had to be restructured to cut costs and to simplify the mechanical design of the spacecraft. The designs of Cassini and Huygens were based around creating the most durable, reliable, and sophisticated spacecraft to date, while reducing cost and mass. The largest effect of these design choices was on the large number of moving parts that had to be removed from the original design, where functions could be performed without them.
Cassini–Huygens was an international collaboration between three space agencies. Seventeen nations contributed to building the spacecraft. The Cassini orbiter was built and managed by NASA/CalTech’s Jet Propulsion Laboratory. The Huygens probe was built by the European Space Agency. The Italian Space Agency provided Cassini’s high–gain communication antenna. On 15th October 1997, the Cassini–Huygens Spacecraft was launched sending it on its way to a meeting with Saturn in July 2004 and Titan in January 2004.
The Cassini–Huygens spacecraft consisted of two main elements: the Cassini orbiter, named after the Italian–French astronomer Giovanni Domenico Cassini, and the Huygens probe, named after the Dutch astronomer Christiaan Huygens. The spacecraft was launched on 15th October 1997 and entered Saturn’s orbit on 1st July 2004. On 25th December 2004, the probe separated from the orbiter at approximately 02:00 UTC. The probe reached Saturn’s moon Titan on 14th January 2005, where it made an atmospheric descent to the surface.
It has been planned that the Cassini orbiter will orbit Saturn and its moons for four years. Cassini will conduct various investigations including observations of the atmospheres of Titan and Saturn, and the measurement of the gravitational fields of the planet and its satellites. The Cassini spacecraft, including the orbiter and the Huygens probe, is the largest, heaviest, and most complex interplanetary spacecraft built to date.
The Huygens probe scrutinised the clouds, atmosphere, and surface of
Saturn’s moon Titan in its descent on 15th January 2005. It was designed to
enter and brake in Titan’s atmosphere and parachute down to the surface.
The Huygens probe system consisted of
the probe itself, which descended to Titan, and the Probe Support Equipment
(PSE) on Cassini, which remained attached to the orbiting spacecraft. The PSE
included the electronics necessary to track the probe, to recover the data
gathered during its descent, and to process and deliver the data to the orbiter,
from which it was transmitted to Earth.
The Huygens probe had six complex instruments aboard, including the Surface–Science Package (SSP), that took in a wide range of scientific data after the probe descended into Titan’s atmosphere. The SSP contained a number of sensors designed to determine the physical properties of Titan’s surface at the point of impact, whether the surface was solid or liquid. During descent, measurements of the speed of sound gave information on atmospheric composition and temperature, and an accelerometer recorded the deceleration profile at impact, indicating the hardness and structure of the surface. If the surface had been liquid, other sensors would also have measured its density, temperature and light reflecting properties, thermal conductivity, heat capacity, and electrical permittivity.
Huygens was expected to transmit valuable data from the surface of Titan only for between three and thirty minutes. However the Huygens power source lasted much longer than anticipated, and data continued to be transmitted to Cassini for one hour and twenty minutes. Even after that Huygens was still functioning, but Cassini then moved out of communications range and the data could no longer be transmitted to Earth.
Scientists are still busy analysing the 474 Megabits of data they retrieved including 350 images.
On 1st July 2004, the spacecraft flew through a gap in the thin outermost area of Saturn’s rings and achieved orbit, after a seven year voyage. It is the first spacecraft to ever orbit Saturn.
Cassini had its first distant flyby of Saturn’s largest moon, Titan, on
2nd July 2004, only a day after orbit insertion, when it approached to within
339,000 kilometres (211,000 miles) of Titan and provided the best look at the
moon’s surface to date. Images taken through special filters showed south
polar clouds thought to be composed of methane and surface features with widely
differing brightness. On 27th October 2004 the spacecraft executed the first of
the 45 planned close flybys of Titan when it flew a mere 1,200 kilometres above
the moon. Almost four gigabytes (4GB) of data were collected and transmitted to Earth,
including the first radar images of the moon’s haze–enshrouded
surface. Radar imagery observed no conclusive evidence of lakes of liquid
hydrocarbons, though it did not dismiss the possibility such lakes could exist.
It also revealed the surface of Titan (at least the area covered by radar) to be
relatively flat, with topography reaching no more than about 50 meters in
Cassini released the Huygens probe on 25th December 2004. It entered the atmosphere of Titan on 14th January 2005.
Huygens was designed to enter and brake in Titan’s atmosphere and parachute a fully instrumented robotic laboratory down to the surface. When the mission was planned, it was not yet certain whether the landing site would be a mountain range, a flat plain, an ocean, or something in between. It was hoped that analysis of data from Cassini would help to answer these questions. The Huygens probe was designed to survive the impact with Titan’s surface and for several minutes send back data on the conditions there. The spacecraft had no more than three hours of battery life, most of which was planned to be taken up by the descent. Engineers only expected to get at best 30 minutes of data from the surface.
The preliminary findings confirm that the targeted region is near the shoreline of a liquid ocean. The photos indicate the existence of drainage channels near the mainland and what appears to be a methane sea complete with islands and a mist–shrouded coastline. There are indications of chunks of water ice scattered over an orange surface, the majority of which is covered by a thin haze of methane. The instruments revealed "a dense cloud or thick haze approximately 18–20 kilometres (11–12 miles) from the surface" which is likely to be due to the reservoir of methane on the surface. The surface itself appears to be clay–like "material which might have a thin crust followed by a region of relative uniform consistency."
What exists beyond our world has always been a fascination of mankind throughout
the centuries. Christiaan Huygens was no exception, he speculated on what might
be discovered on other worlds and their moons in his book The Celestial Worlds
Discovered (1698). Huygens assumed that the moons of other planets including
Saturn were probably similar to our own moon:
"But this we may venture to say, without fear, that all the Attendants of ...Saturn are of the same nature with our Moon."
Huygens considered the nature of the Earth’s moon and drew conclusions, which he then applied to Saturn’s moons. Huygens thought the mountains and valleys visible on the moon might "easily be occasioned by natural causes" , rather than made by any inhabitants. Nor does Huygens believe that any sea, river, water or clouds had been observed on the Earth’s moon:
"’Tis certain moreover, that the Moon has no Air or Atmosphere surrounding it as we have."
When referring directly to the moons of Saturn, Huygens thought that it would be odd if all those celestial bodies were barren of life:
"What! and must all these Moons round...Saturn be condemned to the same uselesness? I do not know what to think of it, because I know of nothing like them to found a conjecture upon. And yet ’tis not improbable that those great and noble Bodies have somewhat or other growing and living upon them, tho very different from what we see and enjoy here. Perhaps their Plants and Animals may have another sort of Nourishment there. Perhaps the moisture of the Earth there is but just sufficient to cause a Mist or Dew, which may be very suitable to the growth of their herbs."
These expectations of Huygens were not so very different to contemporary hopes of alien environments and life.
Preconceptions of Titan before the landing of Cassini–Huygens were more
informed and detailed due to flybys of Titan by recent spacecraft, which gave
scientists a more accurate picture of its atmosphere
and composition. The scientists at NASA and ESA envisioned a moon that was
similar to planet Earth in its infancy, but because of the conditions found at
1.5 billion kilometres (900 million miles) from the Sun it would be frozen.
Many thought that the entire surface of the Titan, or at least large portions
of it, would be covered in an ocean of ethane, acetylene, propane, and other
hydrocarbons, that could be as deep as 1 kilometre (~2/3 mile). However from
ground–based studies on Earth, it was surmised that there would be lakes
and seas, and erosion caused by the movement of hydrocarbons, but no global
It was expected that Titan would have winds and be covered in ice, as solid as rock on Earth, like many other bodies in that area of the solar system. But it was not possible to determine either of these things from the Earth, and so it was up to Cassini–Huygens mission to provide the answers to these questions.
Space exploration is one area of science that has great appeal to the general
public. The discoveries from voyages into space are chronicled in a way that
few other scientific endeavours are by the media. The Cassini–Huygens
mission is no exception to this rule.
Over the course of the Cassini–Huygens journey, over 374,000 pages of information has been generated on the Internet concerning all aspect of the mission. The coverage that Cassini–Huygens has received ranges greatly. Some articles debate the scientific purpose of the mission, while others unveil the spectacular images captured on the mission. The coverage includes the people behind the mission, the larger questions of life and humanity that may arise from the research, and the transmission of facts, achievements, and milestones, by which we as the public may judge Cassini–Huygen’s success.
On this site are samples of the different types of material that were published on the Cassini–Huygens mission in the media.
This exhibit was designed and organised by the students of the M.Sc. in
History of Science: Instruments, Museums, Science, Technology for 2004-2005.
Those students are Polly Basak, Elizabeth Bruton, Charlotte Edwards and Thad
Parsons. We, the students, would like to thank and acknowledge the help of
the staff at the Museum of History of Science, Oxford.
This website was designed, implemented and tested by Elizabeth Bruton.
It has been tested with Mozilla Firefox 1.0 (Win) and Internet Explorer 6 (Win).
It can be viewed at a screen size of 640 x 480 or above.