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 telemetry from which to infer how they were lost) NASA directed that a means be devised to monitor the Saturn Orbit Insertion burn in order that, in the event of the mission being lost, it would be possible to determine whether this derived from an onboard or external cause. One option was to hold the high-gain antenna pointing at Earth throughout the manoeuvre, but the off-path angle of the thrust vector, which would vary as the burn progressed, would impose a tremendous penalty in terms of propellant - so much so, in fact, that it would use up almost all the margin that had been built up in the hope of extending the mission beyond the 4-year primary tour of the Saturnian system. Furthermore, since such an off-path burn would last longer, it would curtail the time available for science activities while north of the ring plane. In any case, there would still be gaps in the telemetry when Cassini was occulted by the rings, particularly the 'B' ring, which is the densest part of the system. Also, the fact that the reporting cycle was 62 seconds meant that the onset of a problem that led to a catastrophic failure would likely be missed. Because even a particle the size of a grain of sand could be disabling, Cassini was to face its 4-metre-diameter high-gain antenna forward to serve as a shield during the ring-plane crossings. Another option was to turn after the ascending crossing to briefly point the antenna at Earth to report the status of the spacecraft prior to adopting the optimal attitude for the imminent burn; if Cassini were to be lost during Saturn Orbit Insertion, this might establish whether damage suffered crossing the ring plane was a contributing factor. The constraint on the science activities would be less severe. In November 2002 it was decided to adhere to the original plan, but have one of the low-gain antennas transmit a strong carrier signal prior to, during, and for some time after the Saturn Orbit Insertion burn. However, because Saturn and Earth would be on opposite sides of the Sun, this signal would not only be weak but also noisy, and hence be unable to carry even low data-rate telemetry. The best that could be done was for the Deep Space Network to monitor the Doppler on the signal, to provide real-time indications of the start, progress and termination of the manoeuvre. Immediately after the burn, the spacecraft was to turn to issue a status report. The procedure for using a low-gain antenna in this way was demonstrated on the trajectory refinement on 27 May.
As project manager Robert Mitchell told reporters on the day before Saturn Orbit Insertion, ''Everything has to go just right. The burn must run for all 96 minutes, the turns must be at the right time, the computer must keep the critical sequence going even in the event of something unexpected happening - it has been programmed to continue even in the event of an emergency. With a one-way light-time of 1 hour 24 minutes, we had to teach the spacecraft to take care of itself. We don't want it to call home if a problem arises, we want it to keep going. This is precisely what we've told it: 'Don't stop, keep going until you've put in all 96 minutes of burn'.'' At the start of the burn the spacecraft's velocity relative to Saturn would be 24.26 kilometres per second and, although the burn would slow it by 626 metres per second, at shutdown its speed would be 30.53 kilometres per second. Asked by a veteran space journalist why the spacecraft's velocity would be higher at the end of the 'braking' burn than at the start, Mitchell explained, ''Orbit insertion is sort of like applying your brakes while driving your car down a hill. Although you have got your foot on the brakes, you still pick up speed as the steep gravity well draws you in.'' The objective of the burn was to slow the spacecraft by just enough to enable it to be captured instead of making a slingshot.
Since the Saturnian system was tilted at 26.75 degrees to its orbit around the Sun, which was itself inclined at 2.5 degrees to the plane of the ecliptic in which Cassini was travelling, and Cassini's trajectory would take it within a planetocentric distance
of 1.3 radii, it, like Pioneer 11 and Voyager 2, was to cross the ring plane by passing through the 30,000-kilometre-wide gap between the 'F' and 'G' rings at 2.627 radii. This had been examined for hazards by terrestrial telescopes, and by Cassini during the approach phase, and if any threatening debris had been detected this would have resulted in a switch to a back-up plan in which the crossing point would be relocated outside the orbit of Mimas - resulting in a less scientifically satisfying orbital tour. The decision to proceed as planned had been taken on 19 May, a week ahead of the trajectory refinement that set up the crossing points.
At 23:47 Universal Time on 30 June, Cassini turned its high-gain antenna away from Earth to face forward to act as a shield. For JPL the loss of the signal became evident at 18:11.166 One hour later, the spacecraft crossed the ring plane northbound at a point some 19,000 kilometres beyond the 'F' ring. The Radio and Plasma Wave Spectrometer detected the 'puffs' of plasma created as the spacecraft was struck by micron-sized grains at a relative speed of 22 kilometres per second. The impact rate started to increase dramatically 2 minutes prior to the crossing, peaked at more than 1,000 hits per second at crossing, then returned to the previous level 2 minutes later.
At 19:30 Cassini adopted the attitude for the Saturn Orbit Insertion manoeuvre. Its bipropellant propulsion system had demonstrated its performance on the Deep Space Manoeuvre, which was only slightly shorter than the nominal duration of the Saturn Orbit Insertion burn. The most efficient burn in terms of propellant would be centred on the moment of closest approach to Saturn, but in late 2002 it had been decided to advance the start by 30 minutes in order to provide a more favourable geometry for observing the ring system over a period of 75 minutes while north of the ring plane. This was significantly longer than for a 'centred' burn, and although the cost was 28 metres per second, advancing the manoeuvre would also provide time in which to rectify any issues that might arise. For example, if engine 'A' were to malfunction, the computer would shut it down and start engine 'B' to continue the burn, with the switch-over taking 10 minutes. However, as project scientist D.L. Matson pointed out, ''Getting the spacecraft into orbit is the priority; getting the science is a bonus.'' A wall display in the control facility at JPL showed a chart of the Doppler on the transmission from Cassini's low-gain antenna. At 19:36 the horizontal line deflected downwards, indicating both that the burn had started on time and was progressing as planned. The spacecraft pivoted at 0.008 degree per second through a total 'steering angle' of 46 degrees in order to maintain the thrust vector aligned with the Saturn-relative velocity, to increase the efficiency of the burn and use less propellant than if the direction had remained fixed. Particles and fields measurements were to be made prior to, during, and after the manoeuvre. The Dual-Technique Magnetometer was to measure the strength and direction of the magnetic field, as
irregularities in the field close to the planet would provide clues to the structure of its deep interior.167 The Radio and Plasma Wave Spectrometer sampled the ionosphere and listened for radio bursts produced by lightning in the atmosphere. At 21:02 Cassini reached its closest point of approach, 20,000 kilometres from the cloud tops and 80,230 kilometres or 1.3 radii from the centre of the planet. Precisely on time at 21:12, the trace on the Doppler plot ended its downward slide, indicating that the burn had been nominal. Cassini then turned to point its high-gain antenna at Earth to make a 1-minute status report.
After a journey through interplanetary space of 3.5 billion kilometres lasting 6.7 years, Cassini was in a highly elliptical 'capture orbit' inclined at 17 degrees to the equatorial plane of the Saturnian system. At 21:31, while 16,000 kilometres 'above' the ring system, it started its remote-sensing observations. This sequence required it to adopt two attitudes for oblique viewing in order to maximise its coverage during the limited time available to investigate the structure, composition and temperature of the rings from this vantage point; this was the only time in the primary mission that Cassini would be in such close proximity to the rings, and the observations were a priority scientific objective. While south of the ring plane, Cassini had viewed the illuminated face, but now, on the shadowed side, the system was seen in silhouette. Meanwhile, at 21:57 Cassini passed behind Saturn's limb as viewed from Earth, and 3 minutes later entered the planet's shadow. At 22:32, as it passed back into sunlight and ended its Earth occultation, Cassini aimed its high-gain antenna forward for the descending ring-plane crossing at 22:58, then turned for remote-sensing of Saturn's southern hemisphere. At 24:00 Cassini again aimed its high-gain antenna at Earth in order to downlink the accumulated engineering and scientific data - a task that took almost 20 hours.
''I have been working on this mission for 14 years, and I shouldn't be surprised, but it is remarkable how startling it is to view these images,'' pointed out C.C. Porco on first seeing some of the high-resolution imagery taken north of the ring plane. ''It is mind-boggling, just mind-boggling.'' An image of a part of the 'A' ring illustrated wave patterns produced by the gravitational perturbations of nearby moons causing the particles within the rings to jostle one another in their individual orbits. The ring system had changed little since the Voyager era, but the relative brightnesses of the major discrete ringlets of the 'D' ring had changed significantly, and one had migrated 200 kilometres closer to the planet. In fact, the 'D' ring would be revealed to be much more complex and dynamic than believed, and observations from various phase angles would establish the presence of more than one population of particles in this
region.168 Data taken by the Magnetospheric Imaging Instrument during the Saturn Orbit Insertion period revealed the presence of a hitherto unsuspected radiation belt between the 'D' ring and top of the planet's atmosphere. Although the spacecraft did not fly through this region, the instrument was able to make the discovery because it sensed charged particles by their electromagnetic emissions rather than by directly sampling them.169 The Ultraviolet Imaging Spectrograph documented compositional differences in the 'A', 'B' and 'C' rings.170 The 'A' ring was 'dirty' near the inside and icier outside. The Encke Division was also dirty. There was no ice in Cassini's Division, the 4,700-kilometre-wide gap that separates the 'A' and 'B' rings. The 'B' ring was mainly ice. The 'C' ring was dirty towards the inside. There were also thin ringlets of dirt embedded throughout the system. Similar indications were noted by the Visual and Infrared Mapping Spectrometer.171 It found Cassini's Division to contain a non-ice material with spectral characteristics similar to the dark material that was present on Phoebe. The 'F' ring seemed to be more dirt than ice. The Composite Infrared Spectrometer gave the most detailed temperatures to date for the rings, showing the temperature on the shadowed side to vary. The opaque parts, most notably the outer part of the 'A' ring and the entire 'B' ring, were cooler, while the translucent parts, such as Cassini's Division and the 'C' ring, were warmer. The 'A' ring was 90K, the outer 'B' ring was 70K, the inner 'B' ring was 90K, and the 'C' ring was 110K. This could be explained in terms of sunlight penetrating the sparsely filled rings to warm the material on the 'far side'.172 The Plasma Spectrometer found a hitherto unknown low-energy plasma trapped by magnetic field lines that threaded through Cassini's Division. In one image, C.D. Murray of the University of London in England found a hitherto unknown ringlet, which the International Astronomical Union designated R/2004S1.173 Estimated at 300 kilometres wide, this ringlet was 138,000 kilometres from Saturn's centre, just beyond the 'A' ring and coincident with the orbit of Atlas, implying a relationship between the moonlet and the ring.
The gravitational interactions between the small moonlets that orbit in and just beyond the outer part of the ring system transfer angular momentum in a manner that causes the moons to move outwards and the rings to sag towards Saturn, which implied that the rings had a life time of several hundred million years. However, if there was a collisional
This view of the 325-kilometre-wide cascade at work in which lar§e moons stmck
Encke Division taken shortly after by asteroids and comets were broken into Cassini had passed back south of the smaller moons, and so on, the result would be ring plane shows ringlets within the a supply of particles to sustain the system gap and the 'scalloped' nature of its beyond the time in which it would otherwise inner edge. fade. Imagery taken north of the ring plane
showed the inner edge of Encke's Division to be scalloped. This was later found to be caused by the fact that the moonlet Pan - responsible for the 325-kilometre-wide gap - was in an eccentric orbit. The pattern enabled the mass of the moonlet to be calculated, which in turn led to a density of 0.5 g/cm3. Observations of the perturbations of Atlas gave a similar value. As solid water-ice is 0.93 g/cm3, this meant these objects were porous, which was consistent with their being reaccretions of ice. The ongoing reaccretion of ring material would greatly extend the life time of the system.174 In total, the material is equivalent to an icy moon with a diameter of 200 kilometres - less than half that of Mimas. The debate about whether the rings were material that never accreted as a moon, or the debris of a moon that was shattered, was therefore seen to have been futile, since the system is in an essentially steady state, with different incarnations of moonlets and rings present at different times. As L.W. Esposito of the University of Colorado at Boulder put it, ''The individual rings and moons we see are ephemeral, but the phenomenon persists for billions of years.''
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