Joi And The Lost Io Flyby

The only Io fly-by during the nominal Galileo mission took place shortly before Jupiter orbit insertion (JOI). The fly-by occurred at an altitude of 897 km over 8.5° south latitude, 101.1 ° west longitude (Anderson et al., 1996). This in-bound fly-by would allow for remote-sensing observations of the anti-Jovian hemisphere, as well as investigations of Io's possible magnetic field and interactions between Io and Jupiter's magnetic field. Then, with just 2 months to go until arrival at Jupiter, the planned

Table 3.2. Galileo orbits and Io activities.

Orbit

Fly-by

Date of

Closest

Notable Io activities

satellite

main fly-by in orbit

approach distance to Io (km)

JO

Io

12/7/1995

897

Close fly-by: fields and particles observations, no remote sensing. Gravitational detection of Io's core

Gl

Ganymede

6/27/1996

697,000

Distant observations of surface changes since Voyager, NIMS dayside and nightside maps, high-phase imaging, first eclipse images

G2

Ganymede

9/6/1996

441,000

Color imaging of anti-Jupiter hemisphere, NIMS maps

C3

Callisto

11/4/1996

244,000

Topography of anti-Jupiter hemisphere, high-phase imaging of the sodium cloud

E4

Europa

12/19/1996

321,000

Global color imaging

5

None

E6

Europa

2/20/1997

401,000

Eclipse imaging

G7

Ganymede

4/5/1997

531,000

NIMS observations of Loki

G8

Ganymede

5/7/1997

956,000

Eclipse imaging, auroral emissions

C9

Callisto

6/25/1997

607,000

Discovery of Pillan eruption

ClO

Callisto

9/17/1997

319,000

Dark Pillan deposits first seen

Ell

Europa

11/6/1997

780,000

SSI plume inventory

El2

Europa

12/16/1997

485,000

NIMS spectral maps

l3

None

438,000

El4

Europa

3/29/1998

252,000

Multi-spectral color of anti-Jupiter hemisphere

El5

Europa

5/31/1998

312,000

Best UVS observation, color eclipse imaging

El6

Europa

7/21/1998

702,000

NIMS spectral maps

El7

Europa

9/26/1998

800,000

El8

Europa

11/22/1998

996,000

El9

Europa

2/1/1999

856,000

C2O

Callisto

5/5/1999

789,000

NIMS spectral maps

C2l

Callisto

6/30/1999

127,000

Best SSI resolution yet on the anti-Jupiter hemisphere

C22

Callisto

8/14/1999

737,000

Distant plume monitoring, NIMS maps

C23

Callisto

9/16/1999

448,000

I24

Io

10/11/1999

611

First close-up remote sensing

I25

Io

11/26/1999

301

Tvashtar eruption images (SSI, NIMS)

E26

Europa

1/03/2000

340,000

Color imaging of Loki-Daedaldus region, NIMS maps

I27

Io

2/22/2000

198

Trouble-free fly-by, high-resolution remote sensing

G28

Ganymede

5/20/2000

379,000

G29

Ganymede

12/28/2000

963,000

Distant imaging: Tvashtar plume, NIMS maps

C3O

Callisto

5/25/2001

342,000

NIMS dayside, nightside maps

I3l

Io

8/6/2001

194

Discovery of Thor eruption (NIMS and SSI), high-resolution remote sensing

I32

Io

10/16/2001

184

Trouble-free fly-by, high-resolution remote sensing

I33

Io

1/17/2002

102

Almost all remote sensing lost

A34

Amalthea

11/7/2002

45,800

Trailing hemisphere observations scrapped due to budget constraints

J35

Jupiter

9/21/2003

impact

science observations at Io had to be drastically curtailed as a result of a serious tape recorder anomaly. Following the first image of the Jupiter system, the commanded tape recorder rewind failed. Telemetry showed the recorder was running but the tape was not moving. The tape recorder was the key to the LGA mission for storage of high-rate data, including the Probe data, remote sensing, and high-rate fields and particles data.

Investigation suggested that the tape was sticking to one of the heads and was slipping on the capstan. A spacecraft test on 20 October 1995 moved the tape forward for a few seconds and a comprehensive program was begun to characterize tape motion and derive a set of operating rules to minimize the chance of subsequent sticking. It was believed that the motor would always have sufficient authority in the forward direction to break the tape free. After JOI, flight software was augmented to directly control some of the recorder functions and to detect a stuck tape and stop the recorder in that event. These precautions worked well for the next 6 years, until April 2002 when the tape stuck again.

As the tape recorder investigation proceeded, project leaders had to make a painful decision regarding science observations inbound to Jupiter. Only the slowest tape speed had been demonstrated to be safe and would be sufficient to capture the one-time only Probe data acquisition as the Orbiter overflew the descending Probe. Unique fields and particles data at Io, and in the Io torus, were also benign and those observations were added to the revised science plan at little risk. Unfortunately, inbound remote sensing of Jupiter, Europa, and Io, and other high-rate data had to be eliminated from the arrival sequence. This change would maximize the chances of securing the Probe data and preserving the recorder for its now virtually essential role in Galileo's orbital tour.

The magnetometer and plasma measurements during the fly-by were preserved in this adjusted science data acquisition plan, however, at a lower resolution than desired. As a result, the magnetometer measurements at JOI were inconclusive regarding the presence of an internal field, and at odds with the results of the plasma instrument (Kivelson et al., 1996). The question of whether Io had an internally generated magnetic field, or whether the magnetic signal could be entirely explained by currents driven in its extended ionosphere had to wait for the acquisition of additional data. Studies of the torus using plasma waves generated in the region had to be abandoned because of the tape recorder restrictions. Sufficient data were acquired that questions of charge exchange and ion pickup between the ionosphere and the torus could be addressed in the ensuing months (Huddleston et al., 1998). The intensity of current along the Io auroral footprint field lines was a surprise. Intense electron beams were found to be streaming along the Io flux tube, the cylindrical volume created by the magnetic field lines that connect Io with Jupiter. The electron beams were found to be aligned with the magnetic field and moving both up and down the field lines. Analysis showed that the electrons were accelerated in the region of Io's flux tube just above Jupiter's ionosphere and form the downward current portion of the current system associated with Io's interaction with Jupiter's co-rotating plasma. This current system is illustrated in Figure 3.1.

Ideal Alfven wing model

Figure 3.1. Side view and the front view of the ideal Alfven wing model applied to Io. In the side view, Jupiter is located behind Io, in the front view Jupiter is to the far left. Magnetic field lines are bold with arrows, current flow lines are dashed with arrows, the boundaries of the current tubes are solid. Currents leaving the near-field region are connected along Alfven characteristics to the Alfven current tubes closing in the far-field region (after Saur et al., 2004).

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