Equipment

Probe data was received by an articulated 1.1 m diameter L-band high-gain antenna on the orbiter, equipped with dual feeds for the redundant (parallel) L-band transmissions from the probe at 1387.0 and 1387.1 MHz. The two channels had opposite circular polarizations. Transmit RF power was 23 W. The receivers on the orbiter were specified to acquire the probe signal within 50 s, with a minimum acquisition signal strength of 31 dB Hz"1 and tracking threshold of 26 dB Hz"1.

The 1.25 m diameter entry shield (comprising a carbon-phenolic sphere-cone forebody of 152 kg and a hemispherical nylon phenolic aft cover) had a mass of 220 kg. The shield thickness varied from 14.6 cm at the nose to about 5 cm at the edge (e.g. Green and Davy, 1981).

The dominant energy transfer from the atmosphere to the heat shield at these high speeds (48 km s"1) is radiative - the shock layer was expected to reach 14 000 K and heat loads at the nose to reach some 42 kW cm" 2.

Resistive sensors (called ARADs - Analog Resistance Ablation Detectors) embedded in the heat shield recorded its ablation - some 4.1 cm of material was removed at the nose, falling to about 2.5 cm at the edge, corresponding to a loss in mass of some 79 ± 4 kg (Milos, 1997; Milos et al., 1999a). It is believed the aft cover lost about 8.5 kg of material. Together with some pyrolysis loss (i.e. outgassing from unablated material) the total mass drop was some 88.9 kg.

The nominal energy requirement for the mission until entry +48 minutes was 16.3 A-hr, and a margin of 1.7 A-hr was carried. Some degradation during storage and cruise was expected and the total battery capacity at manufacture was 21 A-hr (about 730 W-hr, or 2.6 MJ). The battery was made with three modules of D-size lithium/sulphur dioxide (LiSO2) cells, each module with 13 cells with bypass diodes. Additionally four thermal batteries were carried for pyro actuation.

Note that unlike the Pioneer Venus probes the Galileo descent module was not a pressure vessel; individual units were protected with hermetically sealed housings as necessary. Views of the probe's interior are shown in Figure 22.1 and Figure 22.2, and the main characteristics of the experiments in Table 22.1.

The parachute system (deployed nominally at Mach 0.9 and a dynamic pressure of 6000 Pa) comprised a pilot parachute thrown through the wake at 30 m s"1 by a mortar; separation nuts then fired to release the aft cover, to which the pilot chute was attached. The cover pulled the main chute and stripped its bag, with full inflation completed 1.75 s after mortar actuation (Rodier et al. ,1981). Then, 10.25 s

Lithium battery modules

Lithium battery modules

Figure 22.1. Layout of the Galileo Probe equipment. Note that this view is inverted - the parachute and antenna are of course pointed upwards relative to the local Jovian gravity during descent.

Energetic particle instrument

Energetic particle instrument

Figure 22.2. Layout of the Galileo Probe experiments, showing their exterior access. Accommodating sensor requirements for field of view, exposure to airflow, etc. is a sometimes challenging task for the space probe designer.

after mortar firing, the aeroshell was allowed to fall away - 1.5 s later, with the aeroshell 30 m away, marked the 'official' start of the descent sequence. The sequence is shown in Figure 22.3.

Table 22.1. Galileo Probe science instruments and their main resource requirements

Mass

Power

Volume

Data rate

Instrument

(kg)

(W)

(l)

(bits s"1)

Atmospheric structure (ASI)

4.0

6.3

3.1

18

Nephelometer (NEP)

4.8

13.5

4.6

10

Helium abundance (HAD)

1.4

1.1

2.3

4

Net flux radiometer (NFR)

3.0

10.0

4.6

16

Neutral mass spectrometer (NMS)

12.3

29.3

8.6

32

Lightning and RF emissions/

2.5

2.3

2.9

8

energetic particles (LRD/EPI)

Total

28.0

62.5

26.1

88

separated

Figure 22.3. Parachute deployment sequence of the Galileo Probe.

separated

Figure 22.3. Parachute deployment sequence of the Galileo Probe.

Both parachutes were 20° conical ribbon chutes (chosen largely for attitude stability) made with Dacron. The pilot and main chutes had projected diameters of 0.74 and 2.5 m; the corresponding drag areas were 0.51 m2 and 4.97 m2, and fabric masses of 0.36 and 3.7 kg respectively. Kevlar was used for the main parachute riser and bridle.

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