Fine structure

The mechanical stability of the ring system had been studied mathematically by P.S. Laplace, who had proposed that it is a multiplicity of extremely thin ringlets. During a lengthy study of the system under a range of illuminations, William Herschel found little supporting evidence. In fact, his only suspicion of fine structure was reported five years before Laplace advanced his theory on four nights in 1780, when the rings were wide open and he could trace them all the way around he had noted...

Capture Orbit Apoapsis

There was a hiatus from 5 to 12 July 2004 for solar conjunction with the Sun within 3 degrees of the line of sight, during which Cassini maintained its high-gain antenna facing Earth and the Deep Space Network uplinked sequences of 'no op' commands for periods of 5 minutes 10 to 20 times per day in order to accumulate statistics on the reliability of the uplink in such conditions. Meanwhile, the spacecraft recorded magnetospheric and plasma science observations. On 13 July Cassini emerged back...

Shrinking rings

Struve announced a detailed analysis of the rings. After correlating all the available observations dating back to those of Huygens, he concluded that the inner edge of the 'B' ring was shrinking by about 1 second of arc per century. This meant, he argued, that the material of which the ring was composed was spiralling in towards the planet at a rate of 100 kilometres per year. At this rate, the planet would accrete the ring system in a few hundred years. This startling result...

Acknowledgements

I would like to acknowledge the help of - in no particular order - Roger D. Launius, formerly Chief Historian at NASA Headquarters in Washington, now at the National Air and Space Museum Robert W. Carlson, Ellis D. Miner, Marc D. Rayman and David Seidel of NASA's Jet Propulsion Laboratory in Pasadena Ken Glover William K. Hartmann of the Planetary Science Institute, Tucson, Arizona Michael Hanlon Alex R. Blackwell of the University of Hawaii Harald Kucharek Ralph D. Lorenz of the Applied...

Info

The Huygens probe's descent profile through Titan's atmosphere. Cover by fiberglass struts with pyrotechnic mechanisms. The parachutes are carried on the Top Platform. The upper and lower surfaces of the Equipment Platform accommodate the five lithium sulphide batteries of the Electrical Power Subsystem and the six science experiments. As Titan's atmosphere is so deep, the first contact will be at an altitude of about 1,250 kilometres, travelling at 6,150 metres per second. The timer will...

Co

The top set were taken at ranges between 800,000 and 400,000 kilometres, and show the giant crater in the centre of the leading hemisphere. North is towards the top. The bottom set were taken at ranges between 300,000 and 127,000 kilometres as the spacecraft passed the moon, showing the heavily cratered south polar region. North is to the left in this sequence, and the south pole is on the terminator on the rightmost image. intensely cratered. The closest approach...

Enceladus

Although Voyager 1 did not approach Enceladus closely, it confirmed the telescopic inference that the moon has a very highly reflective surface. In fact, with a geometric albedo of 95 per cent, it is more reflective than a field of fresh snow. Voyager 2 was able to provide imagery of the northern portion of the trailing hemisphere at a resolution of several kilometres per pixel. At 500 kilometres in diameter, Enceladus is only slightly larger than Mimas, and it had been thought that as they...

Geysers On Enceladus

A re-examination of imagery taken on 16 January 2005 from a range of 209,000 kilometres showed Enceladus as a crescent with a glow around its south polar region shining in forward-scattered sunlight. It was decided to make further observations, and the non-targeted fly-by on 27 November 2005 showed there to be jets from many sources that extended 500 kilometres into space. It was a discovery on a par with the erupting volcanoes on Jupiter's moon Io. As C.C. Porco put it, ''For planetary...

Huygens Reaches Titan

On the original schedule, the Master Timing Unit would have powered up Huygens about 45 minutes before the predicted moment of contact with Titan's atmosphere, but the schedule had been revised to do this with 4 hours remaining, to enable the radio system to warm up and further reduce the Doppler problem. With 90 minutes to go, Cassini started the command sequence by which it was to turn to aim its high-gain antenna towards the probe, record the transmission, and finally turn back to Earth to...

Hints Of Outer And Inner Rings

Although the ring-plane crossing of 17 April 1907 was unobservable as Saturn was too close to the Sun in the sky, the crossings of 4 October 1907 and 7 January 1908 were more favourable, and W.W. Campbell using the 36-inch refractor at the Lick Observatory, Percival Lowell using his 26-inch refractor, and E.E. Barnard using the 40-inch refractor at the Yerkes Observatory made independent studies of the bright clumps in the edge-on system, noting that one was in the 'B' ring close to Cassini's...

Planning for Saturn

The dilemma for the Saturn-encounter planners lay in chosing the most appropriate ., Pioneer 11 was able to 'double back' on a By passing under Jupiter's south polar region, trajectory steeply inclined to the ecliptic. ., Pioneer 11 was able to 'double back' on a After Jupiter, Pioneer 11 flew high above the ecliptic on a five-year cruise to Saturn, on After Jupiter, Pioneer 11 flew high above the ecliptic on a five-year cruise to Saturn, on trajectory for the spacecraft to fly through the...

A third ring

On 15 November 1850, with the rings wide open, W.C. Bond, perceived with the Harvard College Observatory's 15-inch refractor a dusky inner ring which extended half way in towards the planet as a continuation of the 'B' ring. Two weeks later, W.R. Dawes observed the same feature using a 6-inch refractor at his observatory at Wateringbury in England. When William Lassell paid Dawes a visit on 3 December, Dawes said it was ''like a crepe veil covering a part of the sky within the inner ring''....

Encounter With Saturn

While JPL was determined to preserve the 'option' (as yet unfunded) of flying the Grand Tour, by sending Voyager 1 deep into the Jovian magnetosphere the resulting slingshot had accelerated it so much that it would reach Saturn too early for onward routeing. The two vehicles had been dispatched a few weeks apart - Voyager 2 first -and had arrived at Jupiter a few months apart - Voyager 1 leading - but by not approaching the giant planet so closely Voyager 2 had set off on a slower trajectory to...

General Considerations

Using masses of the Saturnian moons derived from a study of their orbits, and an estimate of Titan's diameter, in 1931 Georg Struve estimated the sizes of the others by assuming that they all had the same albedo. Given the masses and the sizes of the moons, he was able to estimate their densities (Table 4.1). Not surprisingly, however, Struve's assumption that all the satellites had similar albedos was eventually proved false. As spectroscopic studies identified water frost on their...

Interrelationships

With its system of rings and more moons than any other planet, Saturn continued to fascinate mathematically-minded astronomers. In the 1880s M.W. Meyer amplified Kirkwood's analysis of the ring system in terms of resonances with the satellites. In fact, the relationships between the moons themselves showed that the entire system is tightly integrated. Considered in terms of their distance from the planet, several 'groups' of moons were perceived Mimas, Enceladus, Tethys, Dione, Rhea

Callito

A polar projection of Voyager 2's route through the Jovian system. Three months later, Voyager 2 made its approach. When it sensed the bow shock on 2 July 1979, it became evident that the solar wind had abated slightly, and the magnetosphere had re-inflated.20 The spacecraft's computer had been programmed to follow-up its predecessor's discoveries. This time, most of the satellites were encountered on the way in.21 First was Callisto on 8 July at 215,000 kilometres, then Ganymede on 9 July at...

Voyager

In contrast to the Pioneers, which were commanded from Earth, the Voyagers would have sophisticated computers that could be uploaded periodically with sequences of activities that they would execute autonomously, and they would have a high degree of fault tolerance to enable them to look after themselves. While they were to study particles and fields, they were also to be capable of remotely sensing a planet or a moon, and so could stabilise themselves. On the interplanetary cruise, and while...

Ring Division

Cassini made the momentous discovery in 1675 of a thin dark line on Saturn's ring. It was presumed that this was a mark on the ring which divided the broad bright inner section from the narrower outer section. A year later, he noticed a 'belt' on the planet's disk, just south of the equator. Such latitudinal banding had been discovered on Jupiter in 1630 by Francesco Fontana of Naples, but Saturn's version was more subdued in character. In 1705 J.J. Cassini (G.D. Cassini's son, and...

Huygens Tests

On 2 November 2004 a 10-day end-to-end test of Huygens using the Integrated Test Laboratory at JPL began to exercise the command sequences planned for the interval 15 December 2004 to 16 January 2005 to perform the probe's independent mission. On 15 November the Titan Atmosphere Model Working Group convened at JPL to discuss the results of the Ta fly-by. Its analysis was delivered to the Huygens team on 23 November to steer the final planning review. Huygens had flown the interplanetary cruise...

Herschels Discoveries

Born in Hanover in 1738, Friedrich Wilhelm Herschel anglicised his forenames to Frederick William upon relocating to England in 1757. As an accomplished musician, he took the post of organist at the Octagon Chapel in Bath in 1767. His passion was astronomy though, and a few years later he was routinely giving eight music lessons during the day and then observing the sky at night. In 1774, after several frustrating years of making and using long and unwieldy refractors, he switched to reflectors...

Thinking

In 1970, following up the trajectory work by G.A. Flandro, NASA's Jet Propulsion Laboratory began to design a sophisticated 'Mark 2' version of its highly successful Mariner series of planetary probes. The plan was to dispatch two pairs of vehicles to investigate the outer Solar System. The first pair would employ Jovian slingshots to reach Saturn, whereupon they would be deflected on to distant Pluto. The second pair would exploit Jupiter to visit first Uranus and then Neptune. Although...

Does Titan Have An Internal Ocean

The T11 fly-by on 27 February 2006 was at an altitude of 1,813 kilometres, and this time the highlight was the monitoring of the radio signal to measure the 'low degree coefficients' of Titan's gravity field, to enable a team led by Arvydas Kliore of JPL to investigate whether there is a subsurface liquid layer. In fact, this was the start of a campaign of four fly-bys - this one, T22 in December 2006, T33 in June 2007 and T38 in December 2007 - timed to examine the gravity field with the moon...

Other Praxis books by David M Harland

The Mir Space Station - Precursor to space colonization The Space Shuttle - Roles, missions and accomplishments Exploring the Moon - The Apollo Expeditions Jupiter Odyssey - The Story of NASA's Galileo Mission The Earth in Context - A Guide to the Solar System Mission to Saturn - Cassini and the Huygens Probe The Big Bang - A View from the 21st Century The Story of the Space Shuttle How NASA Learned to Fly in Space - The Gemini Missions The Story of Space Station Mir Water and the Search for...

The Astronomical Unit

In 1767, at the age of 18, Pierre Simon Laplace was appointed as professor of mathematics in Paris, where he devoted the majority of his attention to a comprehensive study of the manner in which the planets perturb one another gravitationally, with a view to determining whether the observed arrangement of the Solar System was stable. He concluded that while the eccentricities of the individual orbits would vary over time, the system would adjust to compensate, and was therefore stable. In...

The Xanadu Continent On Titan

Revolution 23 began with apoapsis at 68.3 planetary radii on 9 April 2006. The T13 fly-by on 30 April was at an altitude of 1,855 kilometres. The radar undertook altimetry, radiometry, scatterometry and imaging. It showed Xanadu to stand above the surrounding dark terrain, making it, indeed, a continent. Although the dark area to the west was a sea of dunes, these were absent on the dark terrain to the east. The elevated ground was hilly, and deeply incised by river channels. As Steve Wall, the...

Nomenclature

After five Saturnian satellites had been discovered, a scheme was introduced in which they were referred to numerically in accordance with their distance from the planet. This was satisfactory until William Herschel discovered two additional satellites orbiting closer in. In 1847, John Herschel, who had just returned from South Africa where he had made a thorough study of the southern sky to extend his father's study in the north, decided to rectify the situation by naming Saturn's moons. In...

View Of Tethys

On 28 October 2004 Cassini flew by Tethys at 250,000 kilometres. The trajectory provided a view of the trailing hemisphere, which was almost saturated with craters. At 1.5 kilometres per pixel the resolution was better than that of the Voyagers. An image taken on 15 December from 564,000 kilometres showed a new perspective on Ithaca Chasma. Two views of Tethys (left) the trailing hemisphere (north to the top) on 28 October 2004 showing the cratered surface and part of Ithaca Chasma, and (right)...

Chapter Saturn from afar

Saturn as drawn by Galileo Galilei in 1610 3 As Saturn appeared to Galileo in Early sketches of Saturn's Christiaan Huygens using an 'arial' A flat ring, nowhere The shadow cast on Saturn by the G.D. Cassini discovered a division on the ring 11 J.F. Encke's division in the 'A' Warren de la Rue drew fine subdivisions on the rings 20 E.L. Trouvelot's drawing of the translucent 'C' ring 21 J.E. Keeler's thin 'A' ring A light curve of Iapetus passing through the 'C' ring's shadow 24 E.M....

Titan Land Of A Thousand Lakes

Revolution 25 began with apoapsis at 68.3 planetary radii on 10 June 2006. On 1 July Cassini reached the half-way point in its 4-year primary tour. The T15 fly-by on 2 July was at an altitude of 1,906 kilometres. Revolution 26 began at 48.4 planetary radii on 12 July. Cassini was to begin the process of rotating the apoapsis of its orbit from down-Sun to up-Sun in order to switch from investigating the magnetotail to imaging Saturn in full sunlight. Rather than perform this rotation within the...

Glimpse Of Titan

Titan is the only member of Saturn's retinue to show a disk. Early efforts to directly measure its apparent diameter were subjective, and produced a wide range of values. E.E. Barnard and Percival Lowell independently measured it using micrometers, and calculated its diameter as 4,150 kilometres. As recently as the dawning of the Space Age, its diameter was given as 5,680 kilometres. In this range, it was clearly a close rival to Ganymede, the largest of Jupiter's Galileans. Given an estimate...

Planning

The Saturnian system was visited by robotic explorers from the planet Earth in three successive years 1979, 1980 and 1981. In contrast to the expected bland frozen realm, the system has turned out to be incredibly rich in diversity Enceladus has been comprehensively resurfaced, and might still be undergoing cryovolcanic activity with geysers venting jets of water into space Titan has a dense reducing atmosphere that may well be in a prebiotic state Iapetus has its enigmatic dark hemisphere and...

Features Named On Titan

Revolution 19 began with apoapsis at 53.6 planetary radii on 11 December 2005. On 12 December the altitudes for the remaining Titan fly-bys of the primary mission were decided. Features on Titan had initially a been referred to by nicknames, but in August the International Astronomical Union had announced several official names. The problem was classifying features that were poorly glimpsed through the haze, and whose character remained to be determined. The scheme was extended to include, for...

White Dots On Iapetus

On 17 October 2004 Cassini passed within 1.1 million kilometres of Iapetus, and saw a chain of 'white dots' on western Cassini Regio. In fact, these were hinted at in the Voyager imagery.178 What was surprising, was that the line seemed to continue across Cassini Regio as a dark linear feature. The 'white dots' appeared to be peaks some 10 to 20 kilometres in height.179 An image taken on 22 October was exposed for 180 seconds in order to view the Saturn-facing hemisphere lit by 'Saturnshine',...

Earth

The second Venus encounter had accelerated Cassini by 6,690 metres per second and deflected it towards the Earth. Trajectory correction manoeuvres (of 43.5 metres per second on 6 July 5.1 metres per second on 19 July 36.3 metres per second on 2 August and 12.26 metres per second on 11 August) refined the Earth encounter. Despite JPL's confidence in its interplanetary navigators, the anti-nuke community was vocal in expressing its concern that the spacecraft would somehow spin out of control''...

Iapetus

Iapetus was the most enigmatic of the Saturnian moons for the early telescopic astronomers. Soon after discovering it, G.D. Cassini realised that the moon's brightness was varying by fully two visual magnitudes in a systematic manner as it moved around its orbit. It was brightest at western elongation and faintest at eastern elongation. The fact that this pattern remained fixed implied that the moon's rotation was synchronised with its orbital period. For some reason, the leading hemisphere was...

The Missing Moon

In 1815, Harvard College began to consider the erection of an observatory and W.C. Bond was sent to Europe to inspect the workings of similar institutions. Upon his return, he submitted his report and promptly established his own observatory at Dorchester in order to develop instruments and methods. When the Harvard College Observatory was completed in 1844 with a fine 15-inch refractor made in Germany by Merz-Mahler, Bond was appointed as its first director. Noting that the distribution of...

Discovering Polydeuces

An image of Iapetus lit by 'Saturnshine' on 22 October 2004, showing the 'halo' to the east of Cassini Regio. Six narrow-angle camera frames taken over a 3-hour period on 21 October 2004 led to the discovery of S 2004S5.180'181 A search of earlier Cassini images traced it back six months.182 Subsequently named Polydeuces, it is about 5 kilometres in size and is a Trojan of Dione. Trojans occupy the gravitationally stable Lagrange points 60 degrees ahead of, or in trail of, a body. Three Trojan...

Dione

In terms of size, Dione is the near-twin of Tethys. A bulk density of 1.43 g cm3 and an overall albedo of about 50 per cent indicated a predominantly icy body with an exposed icy surface. Telescopic studies noted a 0.6-magnitude variation around its orbit.43 The Voyager imagery revealed why there is dark mottling with an albedo of about 20 per cent on the trailing hemisphere, with a pattern of bright streaks with 70 per cent albedo superimposed upon it making a striking contrast.44,45 A...

Phoebe

With the exception of Iapetus and Phoebe, Saturn's satellites all travel in circular prograde orbits coplanar with both the planet's equator and the ring system. Phoebe has an elliptical, inclined and retrograde orbit. Voyager 2's best imagery from a range of 2.2 million kilometres had a resolution of 20 kilometres per pixel, just enough to establish that this moon is spheroidal with a diameter of about 220 kilometres, rotates in 9.5 hours in a prograde manner, is cratered, and that while its...

Iapetus Revealed

A meeting at JPL on 20 December 2004 reviewed the science plan for the fly-by of Iapetus. The sequence was uplinked on 30 December for execution the next day, and as Cassini passed the moon at a range of 123,400 kilometres it took imagery at a resolution of 1 kilometre per pixel. The dark linear feature detected in October was not only seen to span the width of Cassini Regio but, because it was visible on the limb, it was also identifiable as a ridge.206 It was basically continuous, and simply...

Acce

The externally mounted ACC-E sensor of the Surface Science Package measured the force as the Huygens probe made contact and settled. The internally located ACC-I sensor measured the 'g' of the deceleration. The externally mounted ACC-E sensor of the Surface Science Package measured the force as the Huygens probe made contact and settled. The internally located ACC-I sensor measured the 'g' of the deceleration. provided a source of heat. In addition, the inlet of the Gas Chromatograph Mass...

Cd

A section of the synthetic-aperture radar track taken on 26 October 2004 centred at 82 W, 50 N, at a resolution of 300 metres per pixel. It suggested a complex surface geology. A section of the synthetic-aperture radar track taken on 26 October 2004 centred at 30 W, 45 N covering an area about 150 kilometres square, showing what might be a cryovolcanic flow. across the Solar System, and our one big fear was that Titan might look like Jupiter's moon Callisto, which is an old dead world covered...

Dunes On Titan

Revolution 17 began with apoapsis at 41 planetary radii on 20 October 2005. The T8 fly-by on 28 October was at an altitude of 1,353 kilometres. This time, the radar imaged the dark Belet terrain to the west of Xanadu, which was revealed to contain a field of 100-metre-tall dunes. The individual dunes ran for hundreds of kilometres, and the field extended at least 1,000 kilometres along the equatorial zone.268,269 The 'cat scratches' of T3 were seen to be smaller dune fields. ''It's bizarre,''...

Janus And Epimetheus

On 21 January 2006 Janus and Epimetheus swapped orbits. Janus will occupy the low position until 2010. There is no danger of a collision during such exchanges, as the bodies can approach no closer than 15,000 kilometres. Revolution 21 began on 5 February with apoapsis at 68.2 planetary radii, which was the furthest that Cassini had been from Saturn since the conclusion of revolution 'a' in November 2004 at 78 planetary radii. Janus and Epimetheus imaged on 25 December 2005 shortly before the...

Approaching Saturn

Weak, long-wavelength (kilometric) radio busts had been detected from Saturn, so it was presumed to have a magnetic field. However, there was debate over whether this would be sufficiently strong to ward off the solar wind and form a magnetosphere of trapped radiation. Those researchers who thought it could, expected Pioneer 11 to find the bow shock somewhere between 50 and 20 radii from the planet. On 30 August, as the spacecraft crossed the 50-radii line, it was still in the solar wind. The...

Origin Of The Rings

Laplace realised in 1785 that if Saturn's rings were a solid body, then the 'tidal' effects of the planet's gravitational field would have disrupted them.1 Instead, he argued that the rings were composed of a multiplicity of very narrow ringlets. Interestingly, the Edinburgh maker of reflecting telescopes, James Short, had remarked to J.J. Lalande several decades earlier that he had once seen many divisions on the rings, but no one else had reported such finely...

B

Voyager 1 's view of the fully-illuminated trailing hemisphere of Tethys from a range of 2 million kilometres (left) showed a strange circular albedo feature. A view of the Saturn-facing hemisphere from 1.2 million kilometres revealed a tremendous canyon system, and later views of the terminator from half that range revealed the cratered terrain immediately to the west of the canyon (whose southern section is visible on the terminator). North is towards the top in all cases. kilometres wide...

Blurred Vision

Imagery of the star Spica (alpha Virginis) taken in March 2001 in order to refine the boresighting of the Imaging Science Subsystem's narrow-angle camera with the field of view of the Visual and Infrared Mapping Spectrometer revealed an excess of light in the camera's field. This was presumed to be a contaminant that had coated the lens during the Jovian fly-by, possibly thruster efflux during the manoeuvring while the reaction wheel system was inoperable. In July the camera returned its first...

Look At Dione

Cassini's non-targeted fly-by of Dione on 14 December 2004 at a range of 72,500 kilometres provided a view of the trailing hemisphere containing the bright 'wispy' features. The 'quick look' data was the gossip of the 5-9 December meeting of the American Geophysical Union in San Francisco, California. Surprisingly, instead of strips of ice that had been extruded from fissures, the bright streaks were found to be grooves whose cliff-like walls of exposed ice were brighter than the 'contaminated'...

More Enceladus Discoveries

Revolution 10 began with apoapsis at 40.7 planetary radii on 17 June 2005. There were non-targeted fly-bys of Titan on 22 June at a range of 920,703 kilometres and on 26 June at 874,627 kilometres. Revolution 11 began on 5 July. While there were no encounters with Titan this time, Cassini returned to Enceladus on 14 July, with its point of closest approach over the southern hemisphere providing the first view of the south pole, recently discovered to be marked by arcuate features dubbed 'tiger...

Titans Seasonal Rains

The T18 fly-by on 23 September 2006 was at an altitude of 960 kilometres. On this occasion, the Visual and Infrared Mapping Spectrometer showed a vast tropospheric cloud occupying the observed range of longitudes (10 to 190 W) and latitudes (51 to 68 N) that appeared to be composed of ethane.287 It is believed to be the result of the concentration of organic haze in the descending north polar vortex. Ethane raining into the lakes of methane would dissolve. If the temperature dipped low enough...

Launch

The 'launch window' that opened on 6 October 1997 would last until the end of the month. The hope of dispatching the mission on the first day was foiled when an air conditioner damaged a piece of the Huygens probe's thermal insulation, so the launch was rescheduled for 13 October, on which date the window ran from 04 55 to 07 15 local time. However, unacceptably high winds at altitude prompted a 'scrub' and a two-day recycle. Watched by a large number of project members, including several...

Saturns Lightning Storms

At the time of the Voyager fly-bys, the sporadic radio emissions from Saturn were inferred to be due to electrostatic discharges in storms in the equatorial region, which was then in the shadow of the ring system. Cassini's Radio and Plasma Wave Spectrometer first detected the discharges in July 2003, at a distance of 161 million kilometres. On its Earth fly-by, the instrument had detected lightning at a range of 89,200 kilometres, measured from Earth's surface. The mystery was why Saturn's...

Through the ring plane

The crossing of the ring plane at 2.87 planetary radii, just outside the visible edge of the 'A' ring', was predicted for 09 02 on 1 September. In fact, because the system is so incredibly thin, the spacecraft darted through in a fraction of a second. With the planet on the far side of the Solar System, the spacecraft's signal took 86 minutes to reach the Earth, so the nail-biting moment for Ames was 1028. As the final minute

Saturn By Ringlight

On 23 January 2006, while Cassini was deep in Saturn's magnetotail, the Radio and Plasma Wave Spectrometer detected a strong burst of emission of the type known to be produced by lightning discharges. The spacecraft's position prevented it viewing the daylit hemisphere, but astronomers in France reported that a storm had appeared in 'storm alley' in the southern hemisphere. Cassini was told to watch for the storm on the night-time hemisphere by viewing in the illumination of sunlight reflected...

Tethys

At 1,060 kilometres in diameter, Tethys is twice the size of Enceladus, but its bulk density is comparable. Its surface is uniformly bright, but it is not quite as reflective as Enceladus. It received fairly comprehensive imaging coverage, with only a section of the southern hemisphere being missed. A physiographic study identified several terrains.37 The oldest region is a hilly cratered terrain that is characterised by rugged topography. It is densely cratered, but most of the larger craters...

A flat ring nowhere connected

Narrow arms projecting to either side of the planetary disk. In early 1656, using a refractor with a 2.5-inch lens, a 23-foot focal length and a magnification of 100, he noted that the ansae had disappeared. However, the narrow arms reappeared in October, and using an 'arial' refractor with a focal length of 136 feet, Huygens later noticed that a shadow had been cast on the planet - an observation which showed that the ansae were not merely crescent-forms 'alongside' the globe, but formed a...

Second Flyby Of Titan

The Ta fly-by had revealed there to be a layer of detached haze 150-200 kilometres higher than the expected maximum.201 ''The change in the detached haze over the 25 years since Voyager implied either the photochemical processes that make the haze, or the atmospheric circulation that distributes it around the planet, may change with the seasons,'' noted Robert West of JPL. ''It will be a challenge for models to be able to predict how and where these detached hazes occur.'' This prompted concern...

Pathfinding

It was standard procedure at that time to build two identical spacecraft in order to provide a degree of redundancy in case one was lost at launch, or failed thereafter. If only one survived dispatch, it would report on the solar wind beyond the orbit of Mars, pass through the asteroid belt, make a fly-by of Jupiter as a reconnaissance of its magnetosphere, then fly on out of the Solar System. If the first achieved its primary objective, there would be an option of expanding the backup's...

Saturns Interior

By the 1930s, thanks largely to Harold Jeffreys, the idea that the giant planets were 'failed' stars had finally died off. In 1938 Rupert Wildt posited a detailed model for their internal structure. Arguing by analogy with the manner in which the Earth was presumed to have gravitationally separated, he proposed that these planets should have undergone a similar process, and have formed rocky cores. As their masses are sufficient to retain even the most lightweight of elements, they must have...

Glimpse Of Iapetus

On 3 July 2004 Cassini turned its attention to Iapetus. The range was 3 million kilometres, but the imagery was welcome since it provided the first glimpse of detail on Cassini Regio, the dark area on the leading hemisphere that was featureless in the Voyager imagery, showing a number of large impact basins the presence of which implied that the crust dated to an early period of heavy bombardment. The ratio of the diameter of the largest basin to the diameter of the satellite was about 0.3, A...

Saving The Huygens Mission

In December 2000, the Huygens Communications Link Enquiry Board announced that while Cassini would receive the probe's tracking signal, the Doppler shift would degrade the telemetry subcarrier to barely 10 per cent of its intended strength. The problem was traced to an omission in the specification for the hardware.111 As originally planned, two days after releasing the Huygens probe on 6 November 2004, Cassini would have deflected the inbound leg of its 'capture orbit' to make a close fly-by...

Designing The Orbital Tour

Much had been learned from the process by which the Galileo spacecraft's orbits of Jupiter had been planned on a just-in-time basis.68 In the case of the Jovian system, there were four moons sufficiently massive for gravity-assists to enable a spacecraft to contrive an evolving trajectory for a specific orbital tour. Cassini would be able to use only Titan. An early decision was to set up a fly-by of the enigmatic outer moon Phoebe on the way into the system. For the 1997 launch window, this...

Giant Moon

Christiaan Huygens was born in 1629 into a notable Dutch family with a tradition of diplomatic service to the House of Orange. He studied law and mathematics, and his father provided a stipend to enable him to devote himself completely to his chosen profession the study of nature. Huygens first worked on mathematical problems, but soon expanded his activities to experiments in mechanics and optics. In 1655, upon hearing an enthusiastic report by Johannes Hevelius, who had an observatory in...

Phoebe Flyby

Phoebe is some 220 kilometres in diameter, and slightly irregular. It has an elliptical retrograde orbit of 18 months that is more or less in the plane of the ecliptic. A study during December 2003 and January 2004 in support of Cassini precisely measured the repeating 0.12-magnitude variation to determine the rotational period as 9.2735 hours ( + 0.0015), which was an improvement over the previous measures by a factor of 10.156 Although Cassini would have only one opportunity to inspect...

Hot Spot On Titan

The T5 fly-by on 16 April 2005 at an altitude of 1,026 kilometres was the first 'low pass' of the tour, although for safety it was made a little higher than the originally planned 950 kilometres. In addition to offering improved data on any intrinsic magnetic field and the moon's interaction with its environment, this fly-by was to enable the Ion and Neutral Mass Spectrometer to gather more data to study the composition of the thermosphere and ionosphere and the thermal structure at various...

First Impressions Of Titan

On 2 July 2004, some 30 hours into the capture orbit, designated revolution '0', Cassini passed Titan at a range of 339,120 kilometres. Although non-targeted, this fly-by, referred to as T0, provided a welcome 'first look' prior to the close encounter scheduled for October. At closest approach, the frame of the narrow angle camera was tighter than the moon's disk, and because the spacecraft was south of the plane of the Saturnian system it offered a view of the illuminated south polar region....

Closest Telescope Position To Spin Axis

Ultraviolet Photometer and Infrared Radiometer to scan Jupiter. However, imaging is difficult from a rotating spacecraft. In interplanetary space this would not matter as there would be nothing specific to look at, but it was unthinkable to send a vehicle to Jupiter and not take pictures of it. In the absence of a stabilised scan platform, Ames added a narrow-angle scanning Photopolarimeter and developed a method of joining a succession of scan strips to assemble an image. This 'spin-scan'...

The Discovery Of Janus

Saturn was at opposition on 19 September 1966. The first of a series of ring-plane crossing occurred on 2 April 1966, but Saturn was close to the Sun in the sky then, and by the time astronomers were able to make observations the rings had begun to open up again. However, between the crossings on 29 October and 17 December astronomers were able to view the southern side of the system which, because it was not illuminated, meant that the rings were virtually invisible. When Saturn's rings were...

Cassini Resumes Its Tour

A few days before Cassini reached apoapsis at 59.3 planetary radii on 1 February it performed a main engine burn of 120.1 seconds for a delta-V of 18.68 metres per second to put it on course for revolution 3 of the nominal tour, with some revisions to the timings on the T18-5 plan, the T3 fly-by of Titan was to have been at an altitude A prediction of the areal coverage of the down-facing imager on the Huygens probe during its descent superimposed on an image of the intended landing site,...

Early Views Of Hyperion

At various times during its early orbits, Cassini imaged Hyperion from distances ranging between 1.3 and 1.6 million kilometres at spatial resolutions exceeding that of the Voyagers, but these were merely tasters. Imagery taken between 9 and 11 June 2005 as the range decreased from 815,000 to 168,000 kilometres made a movie that strikingly demonstrated the moon's irregular shape. There were craters A view of the south polar region of Titan taken on 6 June 2005 showing clouds and a dark surface...

Themis

On 28 April 1905 Pickering reported the discovery of another satellite, which he named Themis. On the basis of several sightings he inferred that it orbited between Titan and Iapetus, in the region occupied by Hyperion, with about the same orbital period. Interestingly, he concluded that the plane of its orbit was inclined at the same angle as Hyperion's, but in the opposite sense. However, when confirmation was not forthcoming Themis was dismissed as a 'false alarm'. In fact, the...

Enceladus Discoveries

The revision of the orbital tour in order to recover from the Huygens relay problem gave a bonus in the form of a non-targeted encounter with Enceladus on 17 February 2005 at a range of 1,264 kilometres. The imagery of the southern part of the trailing hemisphere at a resolution of about 100 metres per pixel revealed the smooth terrain to bear faults, fractures, folds and troughs with vertical relief of about 1 kilometre. Tracking by the Deep Space Network suggested that the rock ice ratio of...

Saturnus Triformis

First Picture Saturn

In the same year that Kepler announced the laws that governed the movements of the planets, a paradigm shift revolutionised the study of astronomy. Galileo Galilei, the son of a musician, was born in Pisa, Italy, in 1564. Although he attended the University of Pisa as a medical student his passion was mechanics, and in pursuing this interest he became the first real experimental physicist since Archimedes of Syracuse, almost two thousand years earlier. He is reputed to have dropped differently...

Continuing Ring Studies

In April 2005 Cassini began to use a succession of Titan encounters to steepen the inclination of its orbit. Revolution 7 began with apoapsis on 23 April at a distance of 40.6 planetary radii. On 28 April Cassini made a main engine burn of 131.6 seconds for a delta-V of 20.5 metres per second in order to revise the altitude of the fly-by of Enceladus in July down to 175 kilometres. There was a non-targeted fly-by of Titan on 4 May at a range of 864,305 kilometres. Six frames over a 16-minute...

Pioneer

The Pioneers heading for Jupiter coordinated with their predecessors in the inner Solar System to study the solar wind. the distance from the Sun, this would not be practicable in the outer Solar System. The only option was to use Radioisotope Thermo-electric Generator (RTG) power cells that produce power by the natural radioactive decay of non-weapons-grade plutonium dioxide, transforming it into electricity using solid-state thermoelectric converters. It had two, mounted on short booms,...

Recent Studies Of Titan

The first solid hint of meteorological activity on Titan was reported in 1998 by a team led by C.A. Griffith, then based at Northern Arizona University in Flagstaff, in which whole-disk brightening in near-infrared data taken in 1995 was attributed to the presence of a methane cloud in the troposphere at an indeterminate position.122 A re-examination of contemporary observations obtained by the Hubble Space Telescope was supportive.123 Later observations by Griffith indicated such activity to...

Cassini Approach Phase

As 2004 began, astronomers set up a program of imagery and spectroscopy using both the Hubble Space Telescope and ground-based telescopes to determine the state of Saturn's atmosphere as a prelude to Cassini's arrival. On 9 January the spacecraft initiated its own program. There had been occasional opportunities for science on the interplanetary cruise, but the approach phase was to involve science observations at a level of activity representative of that planned for the primary mission, in...

Timely Paradigm Shift

''We long to see the actual texture of the rings,'' wrote the astronomer and popular author R.S. Ball in 1886 in his best-selling The Story of the Heavens. But how could this ever be achieved When G.D. Cassini was appointed to the directorship of the Paris Observatory, one enthusiastic optician had proposed developing a 1,000-foot-long arial refractor with which - it was calculated - it should be possible to see the inhabitants of the Moon. Of course, this telescope was never built. It was even...

S

Highlights of Cassini's interplanetary cruise to Saturn. released the Huygens probe this would enter Titan's atmosphere, and the spacecraft would have to deflect its trajectory to allow it to pass close by the moon as the probe was transmitting its data. The timing was constrained by the requirement for the probe to enter on the dayside hemisphere to enable its optical instruments to function. For a specific approach trajectory and entry angle, it would be possible to land only along a certain...

Beyond Jupiter

''After Jupiter, we're in completely uncharted territory,'' observed Andrew Coates of the CAPS instrument's electron spectrometer, as ''particle measurements beyond Jupiter were poor on Pioneer and Voyager''. On 26 November Cassini began the second major campaign of its interplanetary cruise, this time seeking evidence of 'gravitational waves' passing through the Solar System. Albert Einstein had posited the existence of 'ripples' in the fabric of spacetime that propagate across the Universe in...

Jovian Surprises

Voyager 1 was launched on 5 September 1977 upon a Titan III launch vehicle with a Centaur escape stage, the most powerful combination available and, in fact, the final vehicle of this type to run off the production line as NASA phased out 'expendable' rockets in favour of the then-favoured Shuttle-only policy. As Voyager 1 started its interplanetary cruise, it deployed its various booms and activated its scan platform, whose action was impaired by tiny fragments of debris trapped in the...

Space Particles And Fields

On 31 January 1958 America's first satellite, Explorer 1, was sent into an elliptical orbit ranging out to an altitude of 2,000 kilometres. The Geiger-Muller counter that it carried revealed that electrically charged particles circulate in the Earth's magnetic field. The instrument's principal investigator was J.A. Van Allen of the University of Iowa, and this radiation became known as the Van Allen belt. In 1962, as Mariner 2 departed the Earth's vicinity to make a fly-by of Venus, it found...

Lake Ontario On Titan

Revolution 9 began with apoapsis at 40.6 planetary radii on 30 May 2005, and on 6 June provided a non-targeted fly-by of Titan at a range of 425,552 kilometres. This was only the second opportunity to view the south pole at a high angle of inspection, the first having been T0. Imagery taken to make a movie of the clouds near 80 S revealed an oval patch that was considerably darker than anything else in its vicinity and had smooth boundaries suggestive of a shoreline. A.S. McEwen of the Lunar...

The discovery of Phoebe

When the 24-inch astrograph funded by Catherine Bruce (and therefore referred to as the Bruce Telescope) was installed at Arequipa in 1897, Pickering renewed his search again. In April 1899, he found indications of a new satellite on plates taken on 16, 17 and 18 August of the previous year. It was at the surprisingly large distance of 12.8 million kilometres from Saturn. He named it Phoebe, and estimated its orbital period as 490 days. When confirmation was not forthcoming, he ''began to...

Cruising On

On 3 April 2002, in the first burn since the trajectory refinement after the Jovian fly-by, Cassini fired its main engine for 9.85 seconds. This was routine maintenance, as the engineers did not wish to allow more than 400 days to pass without firing the engine to clear its propellant lines. On this occasion it had been decided to test new Manoeuvre Automation Software designed to simplify the process of drawing up the command sequence for a manoeuvre. As another test, the sequence began and...

Nakedeye Astronomers

From his home on the island of Rhodes in the Aegean, Hipparchus, the greatest of the ancient Greek astronomers, drew up a catalogue of the positions and motions of the objects in the sky. He interpreted the observations as meaning that the Earth was at the centre of everything, and that the planets revolved around the Earth in circles. Claudius Ptolemaeus (more usually called simply Ptolemy), a Greek living in Alexandria in Egypt, observed that the planets did not precisely follow their...

Saturns Deep Clouds

Starting with periapsis on 17 February 2005, Cassini's Visual and Infrared Mapping Spectrometer began a campaign in which it imaged Saturn at 5 microns to utilise the heat continuously leaking from the deep interior to illuminate the cloud structures in silhouette. Previously the deep clouds had been sought by imaging in sunlight, but the view had been obscured by the upper level hazes and clouds. At 5 microns it was possible to map both the day and night sides, and there was a mass of...

Target Of Opportunity

The initial science budget had not included investigating 'targets of opportunity' in the asteroid belt. However, a graduate student of Carl Murray's at the University of London, Tolis Christou, realised that Cassini would fly close to 2685 Masursky, an asteroid that had been named in memory of Harold Masursky, a renowned planetary geologist who died in 1990. As one scientist put it on a mission of exploration was it realistic to fly past an object without bothering to switch on the cameras...

Saturn Orbit Insertion

An early proposal to fit Cassini with a medium-gain steerable antenna to enable it to maintain communications during manoeuvres that required the high-gain antenna on the spacecraft's axis to be pointed away from Earth had been precluded by mass and cost constraints. The plan had therefore been to make the Saturn Orbit Insertion manoeuvre 'in the blind'. However, following the loss of the Mars Polar Lander and CONTOUR missions while manoeuvring in this mode (with the result that there was no...

Cassini Spacecraft

Cassini Instrumentation

The Galileo spacecraft had incorporated a spin-bearing assembly so that one part of the vehicle could be three-axis stabilised while the remainder rotated like the Pioneers to optimise particles and fields measurements. Spinning communication satellites in geostationary orbit use similar spin bearings to hold their antennas facing the Earth. However, it was decided not to utilise such a complex mechanism on Cassini, therefore the spacecraft will have to stabilise itself to perform aimed...

Encounter With The Giant

In the final week of January 2000, project scientists congregated at JPL to further refine plans for the Jovian encounter and the Titan fly-bys. The first workshop was held to plan the fly-bys of Saturn's icy satellites, and planning began for a workshop that would be held at Oxford University in England to maximise synergy between the Cassini and Huygens teams. In February, Cassini and Galileo undertook a Conjunction Experiment to observe the Jovian radio emissions stereoscopically, as Cassini...