Flight performance

The probe had been designed for a 3-year interplanetary cruise, but endured one twice as long. A probe checkout was conducted on 15 March 1995, verifying battery performance. On 12 April an 8 cm s-1 trajectory correction was applied to line the probe up with the entry corridor. Formal release readiness reviews were conducted, and the probe activated on 5 July. The umbilical was severed by explosive guillotine on 11 July, ready for spin-up the following day (the entire orbiter, which had a dual-spin architecture wherein part usually remained three-axis stabilised, spun at 10.5 rpm) and on 13 July, the probe was released by the firing of explosive bolts. Separation 1V of 0.3 m s-1 was introduced by springs. The orbiter performed a 61 m s-1 deflection manoeuvre on July 27.

The probe was powered up by timer 6 h before entry, the entry interface being defined as 450 km above the 1 bar level. About 3 h prior to entry, at 5 Jupiter radii (and within the Io plasma torus) the probe collected data on energetic particles (which penetrated through the heat shield). Data was stored in solid-state memory for subsequent transmission during the descent.

The entry site was constrained to a low latitude, to maximise the reduction in entry speed due to Jupiter's rotation; the latitude requirement was 1 to 6.6° (the low-latitude limit was invoked to avoid flying the probe through Jupiter's ring). Entry occurred at 6.57°N, at a speed of some 48 km s-1.

The spin rate of the probe was recorded about 1 h prior to entry (from the magnetic field sensor in the lightning and radio emissions detector instrument -Lanzerotti et al., 1998) at 10.4 rpm. Post-entry measurements show that the spin rate decayed from 33.5 rpm 4.9 minutes after entry to 14.2 rpm at the end of the mission 45 minutes later (interestingly within 5% of the terminal spin rate in a drop test on Earth). Evidently there was significant spin-up of the probe during entry, presumably from asymmetric ablation in the heat shield. The decay of the probe spin was lessened by the presence of three spin vanes, although the torque due to these vanes was less than the parachute swivel torque.

Frequency analysis of the Doppler shift of the telemetry signal shows motions (with an amplitude of 0.5 m s- in line-of-sight velocity) with a period of 20-25 s, and a higher frequency component with a period of 5 s or so. The latter is attributed to pendulum motion under the parachute.

Two significant anomalies occurred during the descent. First, data acquisition and transmission began at a rather greater depth than anticipated. Descent measurements began at a pressure of 0.35 bar some 53 s later than the planned altitude of 0.1 bar, 50 km above the 1 bar level. This delay has been determined to be due to a wiring error - the wires of the two g-switches were crossed: specifically G1 was to go 'high' at 6 g and down at 4.5 g, while G2 would trigger at 25 g and reset at 20 g. The expected sequence would have G1 switching on first, then G2, then G2 off, then G1 off. Telemetry from the probe determined that in fact G2 had triggered first.

A second issue which caused considerable difficulty in the scientific interpretation of the telemetered experiment data is that the probe temperatures became uncomfortably high. This appears in part to be due to more rapid heat

Figure 22.4. Schematic of the Galileo Probe's mission timeline.

transfer in the probe than was expected; hot gas appears to have circulated inside the probe. The experiments therefore were operating at temperatures at which they had not been calibrated (for example, the atmospheric structure instrument recorded temperatures from 35 K colder to 65 K warmer than the calibration range, and rapid temperature changes of 7.3 K min~ ). That said, while the probe specification called for its operation to a depth of 10 bar after nominally 33 minutes of descent, the last transmissions were received from some 23 bar (at an ambient temperature of 425 K), 57 minutes after the start of descent (in fact 61.4 minutes after the entry interface). A schematic overview of the probe's entry and descent is shown in Figure 22.4.

Scientifically, another challenge was that the probe entered in an 'atypical' region, specifically a '5 mm hotspot', a region of meteorological downwelling, where the air had been dried. This, however, can hardly be attributable to the engineering design of the probe itself but is rather a matter of mission goals - any single probe is likely to suffer this problem on a complex planet. The multiple vehicle approach of the 1970s has advantages other than simple reliability through redundancy.

Many pre-mission analyses of the aerothermodynamic environment of Jovian entry and the design of appropriate thermal protection are discussed in two volumes of the series Progress in Astronautics and Aeronautics, namely vol. 56 Thermo-physics of Spacecraft and Outer Planet Entry Probes (1976) and vol. 64 Outer Planet Entry Heating and Thermal Protection (1979), both published by AIAA.

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