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'' and enter the atmosphere, "scattering its radioactive fuel'' - even though the plutonium dioxide is in the form of a ceramic and a RTG case is designed to survive re-entry. The closest point of approach was at an altitude of 1,166 kilometres above the eastern South Pacific at 20:28 local time on 17 August (03:28 Universal Time on 18 August). Travelling at almost 20 kilometres per second, it presented a fleeting target for observers on Pitcairn and Easter Islands. The arrival time was within 0.6 second of the schedule, and the altitude was within 5 kilometres of nominal.
The magnetometer started sampling on 13 August, and its 11-metre boom was deployed on 15 August. The magnetopause was encountered at a range of about 10 Earth radii. The calibration of the mode in which the instrument will operate once in the Saturnian magnetosphere was verified,47 and useful data on the solar-terrestrial relationship was secured.48 The Radio and Plasma Wave Spectrometer was activated a month prior to the fly-by, while the spacecraft was some 600 Earth radii upstream in the solar wind.49,50,51,52 Particles and fields studies were coordinated with satellites in Earth orbit. These included the Polar spacecraft, in an elliptical orbit that enabled it to spend most of its time high over the north pole. Cassini's trajectory penetrated the afternoon plasmasphere, cutting through the outer reaches of the ionosphere, out
through the plasma sheet on the dawn side at 5.2 radii and on into the magnetotail. Cassini's Plasma Spectrometer determined the electron populations in this region on an unprecedented timescale, defining the boundaries of the plasma regions.53,54,55 The magnetometer detected the emergence from the magnetotail at a range of about 60 radii, after which there were multiple re-entries as the tail flapped in the gusty solar wind. As the spacecraft withdrew, it flew alongside the magnetotail.56 The geometry provided an opportunity for the Magnetospheric Imaging Instrument to investigate the magnetotail to 6,000 radii.57,58,59 The INCA imagery provided a significantly better spatial resolution than the 'picture' inferred from the Geotail spacecraft's data.60 The particles and fields phase of the Earth fly-by was concluded on 19 September.
While in the Earth's vicinity, the Radio and Plasma Wave Spectrometer made a coordinated study of the Jovian decametric radio emissions with the Wind spacecraft that was orbiting the Earth.61,62,63 The small but finite difference in viewing angles (a few degrees) meant that their data sets could be correlated to investigate 'beaming' characteristics. The beam width was 1.5 ( + 0.5) degrees and the instantaneous widths of the walls of the hollow conical radiation beams could be measured. The emissions from Io were confirmed to sweep around with the moon's rotation rate, rather than with the planet.
This fly-by provided an opportunity to test the imaging instruments, and verify their calibrations. C.C. Porco was delighted by the Imaging Science System, which was functioning ''beautifully''. A few scans of the Moon provided a useful check on the Visual and Infrared Mapping Spectrometer.64 And while passing over South America, the radar made scatterometry and radiometry measurements in coordination with remote-sensing satellites.65
The Earth slingshot accelerated Cassini by 5,477 metres per second, giving it sufficient energy to reach Jupiter, and the trajectory was refined on 31 August by a 6.7-metre-per-second burn.66 Once beyond the Earth's orbit, the Ultraviolet Imaging Spectrograph's absorption cell started a systematic study of hydrogen and helium in the interplanetary medium, its data placing a strong constraint on the distribution of the interstellar hydrogen and helium flowing through the Solar System.67 Heading swiftly out, Cassini crossed Mars's orbit on 25 September, and in November it adopted an off-Sun orientation to place the Cosmic Dust Analyser's aperture in the plane of the ecliptic, preparatory to entering the asteroid belt a few weeks later.
Meanwhile, one team of planners had developed the operational concept for the science observations to be undertaken during the fly-by through the Jovian system, and another team was making progress towards allocating time to the various instruments for the 'near encounter' period (that is, 30 minutes either side of the point of closest approach) of the Titan fly-bys. Additionally, the first meeting was held to define the Probe Relay Critical Sequence. The team's task was to review the interfaces between Cassini and Huygens, and an end-to-end test of the relay system was scheduled for February, at which time a Deep Space Network antenna would mimic the probe by transmitting to the relay receiver on Cassini. On 15 December 1999, 21 scientific papers were presented to an American Geophysical Union meeting
in San Francisco, reporting the preliminary results of Cassini's time spent in the inner Solar System.
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