Cassini Approach Phase

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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 order to validate the operating modes. It started with the Dual-Technique Magnetometer, Plasma Spectrometer and Magnetospheric Imaging Instrument monitoring the state of the solar wind heading for Saturn's magnetosphere while the Radio and Plasma Wave Spectrometer monitored radio emissions from the planet's auroral activity and the Hubble Space Telescope took imagery of the ultraviolet emission. Large shocks in the gusty solar wind on 17 and 25 January changed both the aurora and the radio emission. In the case of Earth an aurora lasts for several hours, but on Saturn it can last for days and differ in character day by day, often moving, other times stationary. Viewed from space, an auroral display is a ring of light around the magnetic pole. In the case of Saturn the ring shrinks in diameter when an aurora becomes brighter and more powerful, then expands when it dims, whereas on Earth the ring stays the same size as an aurora brightens and temporarily fills the central area.144,145,146 The light is emitted by atoms and molecules excited by electrons flowing in the magnetic field. In the case of Earth it is mostly from oxygen atoms and nitrogen molecules, but on Saturn it is from atomic and molecular hydrogen.

On 6 February Cassini began to take imagery of Saturn for optical navigation, and 3 days later returned the first of a sequence of weekly colour 'postcards' of the planet, which was then 66 million kilometres away. With the north pole tilted away from the

Sun and the spacecraft travelling in the plane of the ecliptic, the view was of the southern polar region and the illuminated 'underside' of the ring system. As it closed in, Cassini took imagery for colour movies to monitor the temporal variation of the atmosphere and measure the wind speeds. Imagery also enabled the ephemerides of the known moons to be updated, and facilitated a search for hitherto undiscovered moonlets. There were no plans to try to image the recently discovered satellites out beyond Phoebe. The composition of the rings was to be investigated by imaging in the mid-infrared, and the distribution of the atomic hydrogen inside the magnetosphere was to be mapped by ultraviolet imaging. On 23 February Cassini took imagery of the 'F' ring. The data had to be magnified and contrast enhanced in order to display the 50-kilometre-wide ring, but several 'clumps' were readily apparent.

Prior to 1993 it was believed that interplanetary dust must be either 'primitive' material originating in interstellar space that was passing through the Solar System, or silicate from collisions between small bodies such as asteroids. However, the Ulysses spacecraft found streams of dust originating from the Jovian system,147 and the Galileo mission later identified the source of this dust as volcanoes on the moon, Io.148 In early 2004, Cassini sporadically encountered 'bursts' of dust as it closed in on Saturn. An analysis found that the particles escaping from the Saturnian system were between 0.01 and 0.1 micron in size. It was reasoned that while larger grains would be confined by the planet's gravity field, smaller grains would be confined by its magnetosphere; therefore only those grains in this size range could escape. The dust was emerging at a fairly constant rate, but Saturn's magnetic field and the solar wind were concentrating it into streams, through which, from time to time, Cassini flew, giving the impression of bursts.149,150 Further analysis showed that the particles came from the outer part of the 'A' ring and were silicate, indicating that they are representative of the 'impurities' rather than the icy ring material.151

An inflight demonstration of the Huygens probe relay in late February exercised the commands that would be needed to receive the data from the probe when it entered the atmosphere of Titan. Prior to the test, all of Cassini's instruments were either switched off or muted - as they would be on the day. Continuous round-the-clock coverage was provided by the Goldstone, Madrid and Canberra stations of the Deep Space Network. The activity started with uplinking the sequences to the solid-state recorder. During the test, Cassini adopted the requisite relay attitude,

Two images taken 2 hours apart on 23 February 2004 contrast-adjusted to show the motion of clumps in the 'F' ring.

The upper frames were taken over an interval of 26 days, starting on 22 February 2004, and show two Saturnian storms in an anti-cyclonic shear zone near 36°S, in which the flow to the north is westward relative to the flow to the south, causing the northern storm to catch, engage in a counterclockwise dance, and merge with its southern counterpart on 19-20 March. The lower frames were taken on 19, 20, 21 and 22 March. The merged feature was elongated in the north-south direction with bright clouds on either end, but within 2 days it had adopted a more circular shape and the bright clouds had become circumferential.

recorded some 6 hours of simulated data, then turned its high-gain antenna to Earth for replay. The test ended with the delivery of the data to the Huygens Project Operations Centre at Darmstadt in Germany. The spacecraft then resumed its science activities.

In February, now 60 million kilometres distant, Cassini began to resolve discrete features in Saturn's atmosphere. On 19-20 March it was able to observe the merger of two storms in the southern hemisphere, each of which was about 1,000 kilometres in diameter. Both were drifting westwards, the more northerly one at twice the rate

The upper frames were taken over an interval of 26 days, starting on 22 February 2004, and show two Saturnian storms in an anti-cyclonic shear zone near 36°S, in which the flow to the north is westward relative to the flow to the south, causing the northern storm to catch, engage in a counterclockwise dance, and merge with its southern counterpart on 19-20 March. The lower frames were taken on 19, 20, 21 and 22 March. The merged feature was elongated in the north-south direction with bright clouds on either end, but within 2 days it had adopted a more circular shape and the bright clouds had become circumferential.

of the southerly one. As they met they spun around each other in a counterclockwise manner. The resulting storm was elongated in the north-south direction with bright clouds on each end, but within 2 days it had adopted a more circular shape and the bright clouds were in a halo forming its circumference. It was only the second time that a merger had been observed on Saturn. In addition, there was a distinctive 'dark' circular spot precisely at the south pole. This correlated with an observation by the Keck Observatory on 4 February showing the spot as a bright feature in the wavelength range 8 to 25 microns, indicating it to be warm.152 In fact, it was the warmest place on the planet. The mystery was not that the polar region was warm - it had, after all, been in continuous sunlight for several years, heating the polar vortex, which was a persistent weather pattern akin to a jet stream. But both the distinct boundary of the vortex flowing around the pole and the hot spot at its centre were unexpected. If the increased southern temperatures were the result of the seasonal variation in sunlight, they ought to increase smoothly towards the pole, but this was not the case - the tropospheric temperature increased abruptly from 88K to 89K near

On 10 March 2004 Cassini made its first sighting of the shepherding moonlets of the 'F' ring: Prometheus (interior) and Pandora (exterior).

70°S, and to 91K at the pole, and the stratospheric temperature rose even more abruptly from 146K to 150K at 70°S, and to 151K. If the abrupt change at 70°S was the result of a concentration of sunlight-absorbing particulates trapping heat in the upper atmosphere, this would explain why the 'hot spot' appeared dark in visible light.

Meanwhile, on 10 March Cassini caught its first sight of the 'F' ring shepherds Pandora and Prometheus. An image taken on 27 March from a range of 47.7 million kilometres was notable because the rings spanned the frame of the narrow-angle camera.

On 1 April the readiness review of the critical event sequence for the Saturn Orbit Insertion manoeuvre recommended several new contingency measures. A week later there was an operational readiness test, as a dry run of all the nominal events that were to be undertaken on 2-3 June, immediately after Cassini had achieved orbit. In April, Cassini began to image Titan at wavelengths that would show methane clouds and the surface, and by mid month the narrow-angle camera was resolving details that previously could be seen only by the use of adaptive optics on large terrestrial telescopes. Despite the 38-second exposure, there was no discernible smearing - the spacecraft was a remarkably stable platform.

Meanwhile, JPL ran an end-to-end rehearsal of Saturn Orbit Insertion by using the Integrated Test Laboratory as a stand-in for the spacecraft. The first act on 9 April was to load the critical sequence into the solid-state recorders, then simulate the final trajectory correction prior to the critical sequence for Saturn Orbit Insertion, ending on 1 May with a simulation of the first orbit trim manoeuvre designed to 'clean up' small residuals of the insertion burn. A second operational readiness test, completed on 9 May, demonstrated that the team could recover from an anomalous insertion burn by defining and executing a substantial corrective manoeuvre within 3 days. Meanwhile, on 5 May, at a range of 29.3 million kilometres, Cassini imaged Titan at a resolution exceeding that attainable by the

By 5 May 2004 Cassini's view of Titan had a resolution exceeding that attainable by the best terrestrial telescopes. The southern pole was in full daylight.

best terrestrial telescopes. Later in the month, the spacecraft began to take movies to enable wind speeds to be measured from the motions of the clouds - data that was required by the Huygens mission. In parallel, the Titan Monitoring Project at the Keck Observatory took frequent high-resolution imagery to: (1) determine the typical size, frequency and life time of the clouds in the south polar region; (2) search for meteorological activity at other latitudes; and (3) compile the most detailed map possible of the surface to assist with planning the observations to be made by the spacecraft during its fly-bys once it was safely in orbit around Saturn.

Just why the atmospheres of Jupiter and Saturn have an alternating pattern of east-west winds varying in direction with latitude was a matter of conjecture. In contrast to Earth, whose weather is driven primarily by sunlight, the giant planets are still in a state of gravitational collapse and so have an additional energy source in the form of the heat that leaks from their interiors. The challenge was to understand the role of these interior energy sources in sustaining the strong jet-stream winds. In one theory, circulation was driven by solar heating of a shallow layer at the top of the atmosphere. In another theory the winds extended deep into the interior and were driven by the energy that leaks out. Neither theory accounted for the maximum wind speed occurring at the equator. One test was to measure the long-term sensitivity of the winds to variations in the amount of sunlight resulting from seasonal and other influences. Studies had shown Jupiter's winds to be insensitive to seasonal changes, but the case of Saturn was difficult because its cloud structure was 'muted' visually. When the Voyagers passed by they saw sufficient detail to enable wind speeds to be measured, and established the equatorial jet stream to be an astonishingly fast 1,700 kilometres per hour. The Hubble Space Telescope could track sufficient detail to facilitate further studies. Observations in 1996-2001 found that despite the change in season and the location of the shadow cast by the ring system, the jets far from the equator had not changed, which suggested that they were deeply rooted.153 However, at just 990 kilometres per hour the equatorial jet stream was much weaker than in 1980-1981. One suggestion for the apparent change was that the structures observed by the Hubble Space Telescope were at higher altitudes, where the winds might well be slower. In mid-May 2004 Cassini's Composite Infrared Spectrometer made the first of a number of long integrations to measure the temperature fields across the planet's southern hemisphere.154 It produced a three-dimensional chart of the winds in the stratosphere that confirmed that the equatorial winds diminished rapidly with increasing altitude above the level of the ammonia cloud tops, the gradient being as steep as 500 kilometres per hour for an altitude increase of 300 kilometres. This suggested that the Hubble Space Telescope had indeed seen structures at a higher altitude than those observed by the Voyagers, and that the apparent slowing was due to storms projecting their 'tops' to higher altitudes, where they were more readily seen from Earth. Cassini provided a clue to what might be maintaining the energetic jets. At an earlier stage it had found the region near 35°S to be particularly active - so much so, in fact, that it was dubbed 'storm alley'. One imaging sequence showed dark spots emerge from the upper-level outflow of a convective circulation structure, thereby yielding their energy to the jets - suggesting that this was how the energy escaping from the interior sustained the horizontal winds of the upper atmosphere.

A trajectory correction on 27 May 2004 refined the forthcoming Phoebe fly-by to an altitude of 2,000 kilometres. The burn lasted 362 seconds for a delta-V of 34.71 metres per second. As the regulator for the helium pressurant for the propellant tanks had developed a leak soon after launch, a latch was being used to isolate this system to preclude it overpressurising the tanks between firings. This burn could have been performed in 'blowdown' mode, which used the residual pressure in the tanks, but because the helium system would be required for Saturn Orbit Insertion and this had not been exercised since the Deep Space Manoeuvre in 1998, it was decided to fully pressurise the tanks in order to determine the leak rate. The latch was opened 100 seconds prior to the burn, and closed shortly afterwards. The results confirmed that the rate of leakage was unlikely to overpressurise the tanks during the much longer Saturn Orbit Insertion burn. This trajectory correction marked a milestone since, as chief navigator Jeremy Jones put it, ''If the road to Saturn were a highway, Cassini would have passed the sign that says 'Saturn county line'.''

In the final month of the approach, the spacecraft took high-resolution imagery to search for moonlets that might pose a hazard to the planned ring-plane crossings. Two hitherto unknown moonlets 3 to 4 kilometres in size were located 194,000 and 211,000 kilometres from the planet's centre - between the orbits of Mimas and Enceladus.155 They were spotted by Sebastien Charnoz, a planetary dynamicist at the University of Paris in France, in imaging sequences taken on 1 June for the purpose. The International Astronomical Union labelled them S/2004S1 and S/2004S2, then later named them Methone and Pallene. Their discovery caught Charnoz by surprise, ''I had looked for such objects for weeks while at my office, and it was only on holiday using my laptop that my program detected them; this tells me that I should take more holidays!''

Operational readiness tests using the Integrated Test Laboratory - in some cases lasting 2.5 weeks - continued into early June. These explored different scenarios for Saturn Orbit Insertion. In one case the burn was interrupted several minutes short of the planned duration, requiring a correction 2 days later of 150 metres per second to achieve an acceptable capture orbit.

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Renewable Energy 101

Renewable Energy 101

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