Table 1

Europa encounter geometry and gravity results. The location of the spacecraft's closest approach is given in the first three rows, where longitude is measured west of the Europa-Jupiter direction and altitude is referenced to a sphere of radiusR = 1565 km (4). The SEP angle is the elongation between the sun and Jupiter. For SEP angles greater than 90°, the minimum amount of phase noise is introduced into the S-band (2.3 GHz) radio wave as it propagates through solar plasma (19). The next three rows give the direction cosines of the Doppler line of sight for the Europa flyby trajectory at closest approach. The cross-track component is aligned with the Europa-spacecraft direction, the along-track component is aligned with the spacecraft's Europa-centered velocity vector, and the normal component is aligned with the spacecraft's Europa-centered orbital angular momentum vector. The quality of the available radio Doppler data depends on whether the data were coherent with hydrogen maser frequency standards at the DSN complexes. Coherency is achieved only when the spacecraft radio system is locked to a signal from a DSN station by means of its S-band transponder. Otherwise, the data are referenced to the spacecraft's crystal oscillator with its relatively poor frequency stability, unknown frequency bias, and unknown frequency drift. In fitting E4 and E6 data, we included the bias and drift as parameters in the model. These two parameters are unnecessary for the coherent E11 and E12 data. Other factors, particularly the continuity of the data and the location of data gaps with respect to the closest approach time, also affect the data quality. The last four rows give the results of the data analysis (7), with each flyby analyzed independently. In accordance with equilibrium theory (6), the coefficient J 2 (the negative of C 20) has been constrained a priori to 10/3 ofC 22. The last row represents an estimate of the axial moment of inertia normalized to MR 2 from Radau-Darwin equilibrium theory (6). It is not an independent parameter of the model but is calculated from the inferred value of C 22. For a sphere of constant density,C/MR 2 = 0.4.

E4 E6 E11 E12
Latitude (°) –1.7–17.025.7–8.7
Longitude (°)36.8324.7140.6225.0
Altitude (km)6975912048205
SEP (°)24.625.289.054.6
Direction cosines for line of sight
Cross-track0.7720.656–0.744–0.985
Along-track0.6340.7290.5690.068
Normal–0.035–0.1940.350–0.160
Coherent Doppler?NoNoYesYes
J2(10–6)215 ± 102438 ± 45442 ± 28438 ± 9
C22(10–6)65 ± 31132 ± 13133 ± 8 132 ± 2
μ0.9990.9950.9860.847
C/MR 2 0.264 ± 0.0410.347 ± 0.0140.349 ± 0.0090.348 ± 0.002