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 that the planned minimum altitude of 950 kilometres was too low. The problem was that if the spacecraft were to encounter significant drag and had to use its thrusters to maintain the requisite attitude, and were to be overwhelmed in doing so, it would be obliged to 'safe' itself, which would terminate all science activities. Going low for better data at the point of closest approach therefore had to be traded against the risk of losing the post-encounter science. In late November it was decided to raise the altitude of the T5 fly-by to 1,025 kilometres, as a compromise between the 1,174 kilometres of Ta and the nominal minimum in order to reassess the risk. As a side-effect of this revision, the altitude of the T4 fly-by was lowered by 1,000 kilometres to 1,500 kilometres.
The redesign of the trajectory to relay the Huygens data cost about 95 metres per second of delta-V, which used up about 40 per cent of the margin predicted (prior to that) for the entire mission. Nevertheless, the remaining reserves would probably be sufficient to accommodate likely contingencies and still facilitate an extended mission. But if Huygens were to fail to release at its first deployment opportunity, it would cost about 25 metres per second to set up the backup date. If it were to prove impossible to release the probe, it would cost another 55 metres per second to carry it around for the remainder of the mission. One benefit of redesigning the early orbits of the tour was a fly-by of Iapetus on 1 January 2005 at a range of 60,000 kilometres. However, the way in which Huygens was to reach Titan meant it had to be released prior to apoapsis on revolution 'b', and follow a ballistic arc through apoapsis to encounter Titan 21 days later, inbound on revolution 'c'. As initially planned, the probe would have undertaken its independent mission rather earlier, and Iapetus would not have been in the vicinity when it was in transit. To address concern about how Iapetus might perturb the free-falling probe, it was decided on 4 October to revise the Probe Targeting Manoeuvre on 16 December, release the probe on 25 December, and perform the Orbiter Deflection Manoeuvre on 27 December in order to advance the fly-by of Iapetus to 31 December, thereby opening the range of the closest approach by the probe to 121,128 kilometres. This would degrade Cassini's imaging of Iapetus, but the opportunity would still be a considerable bonus because, on the original tour, the closest the spacecraft would have approached this satellite in advance of its targeted encounter in September 2007 would have been 415,000 kilometres. A further bonus of revising the tour for the Huygens relay was a close encounter with Enceladus on 17 February 2005. The downstream consequence of the revision designed to avoid Iapetus was that the altitude of the Tb fly-by was reduced by 1,000 kilometres, which required that the science sequences be redefined to suit the revised geometry.
For two days on the way in to the Tb fly-by on 13 December 2004, Cassini shot a movie of cloud motions. The mid-latitude clouds that were absent for the Ta fly-by had returned. The fact that one of the mid-latitude clouds appeared to have been drawn out by the prevailing wind into a streak strengthened the possibility that it was associated with a surface feature.202 The fact that it was moving at only several metres per second suggested that it was at low altitude. However, H.G. Roe noted that if the cloud had formed in the wake of a mountain, it might 'stand' in the same manner as cloud banks stand offshore on Earth, in which case its motion would not provide a measure of the local wind speed.
One key task was to determine whether the albedo variations correlated with surface topography and, if so, how. The Ta and Tb fly-bys were at similar 'phase angles' (i.e. the angle between the Sun, Titan and Cassini) with the Sun 'behind' the spacecraft. While this was ideal for albedo studies, there were no shadows to reveal topographic relief. In fact, even under oblique illumination there might not be shadows because the haze was so efficient at scattering sunlight as to render the illumination at the surface more or less uniform. However, as K.H. Baines of the Visual and Infrared Mapping Spectrometer noted, ''By the time you get out to 2 microns the sky is clear. We believe we'll see shadows at 2.0 and 2.7 microns, so there is a good chance that we could find some topography.'' On the other hand, it would be difficult to interpret near-infrared shadows or stereoscopic imaging at wavelengths
where the theoretical resolution was degraded by the haze. Altimetry was the only unambiguous measure of topography, but this had to be correlated with surface albedo to interpret the results. However, for economic reasons Cassini had been built without a scan platform for its remote-sensing instruments, and with the high-gain antenna on the main axis and the pallet of instruments affixed to the side, it was impossible to make simultaneous remote-sensing and radar observations of a target. Although both had been assigned time during the Ta fly-by, there were to be no radar observations on Tb, and no observations would be possible on Tc because the high-gain antenna would be required to receive the probe's transmission.203 To correlate optical albedo with radar imagery and altimetry it would be necessary to combine data taken during many fly-bys, but this would not only take time, there would be no guarantee of being able to make follow-up observations of an interesting feature seen by one instrument by the use of another instrument. To make a start, it was decided to take altimetry across near-infrared light-dark boundaries on T3 to definitively establish their relative elevations, and take optical imagery on T4 of the areas that were imaged by the radar on Ta to see what some of those features actually looked like.
As Cassini encountered Titan on the Tb fly-by at essentially the same position as on Ta, the best imagery was again of the Xanadu area. In some places the boundary of this infrared-bright feature was quite sharp, and in others it was fuzzy; in some places it was fairly straight, and in others it was irregular. Because the bright and dark areas were very similar when viewed in the half-dozen near-infrared 'windows' available to the Visual and Infrared Mapping Spectrometer, there was still no good differentiation in terms of their compositions. Nevertheless, it was presumed that the dark areas were organic haze fall-out, perhaps windblown, that had accumulated in low-lying regions. There was still no unambiguous evidence for impact craters, but there was a possible multiple-ring form. When Cassini made its closest approach at an altitude of 1,192 kilometres, it imaged the Huygens target for stereoscopic mapping in combination with the imagery taken on the Ta fly-by. Outbound, the Visual and Infrared Mapping Spectrometer observed the north polar region, which had been in continuous darkness for a number of years. The Ultraviolet Imaging Spectrograph observed occultations of lambda Scorpii (Shaula) on Titan's southern limb and alpha Virginis (Spica) on the northern limb during egress to profile its atmosphere in the altitude range 450 to 1,600 kilometres. The Spica observation was made using thrusters to control instrument pointing, and was marred towards the end of the sequence by drift off axis. The Shaula observation was controlled using the reaction wheel system, which accurately pointed throughout. Six species were identified: methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), diacetylene (C4H2) and hydrogen cyanide (HCN). The abundances of complex hydrocarbons and hydrogen cyanide peaked sharply at about 800 kilometres, and decreased in the 750- to 600-kilometre range; the only detectable organic below that level was methane.204 The asymptotic kinetic temperature at the top of the atmosphere was 151K. The temperature profile of methane placed the mesopause at 615 kilometres, where the temperature was 114K. The results confirmed the atmospheric model not only in the entry trajectory of the Huygens probe, but also in the design of its heat shield and parachute systems, and were fed into the calculation to refine the minimum altitude for future Cassini fly-bys.
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