Today on Galileo - November 5, 1997
The fourth of the eight days in this encounter period sees a dramatic increase in the number and types of science observations. Many of today's observations focus on Jupiter as it gets larger in the view fields of the instruments. Observations of Io's torus also continue today and more observations of Europa are added to the schedule. Today, the spacecraft reaches distances from Jupiter of 20 Jupiter Radii, 1.4 million kilometers (890,000 miles), and 15 Jupiter Radii, 1.1 million kilometers (670,000 miles), at about 9 AM and 9 PM PST, respectively. To prepare for the completion of the first set of encounter instructions tomorrow, the flight team transmits the second set of instructions to the spacecraft today.
Throughout the day, initially together with the Extreme Ultraviolet spectrometer (EUV) and later by itself, the Ultraviolet Spectrometer (UVS) continues observations for orbit-to-orbit tracking of the shape, size and activity level of Io's plasma torus. The UVS instrument also performs a split observation (a couple of hours in the morning and a few hours in the afternoon) of Jupiter's darkside. Like yesterday's brightside observation, this observation has been repeated almost every orbit to provide a record of changes in Jupiter's atmosphere. This observation also provides information on the distribution of hydrogen and, subsequently, energy in Jupiter's atmosphere.
After the morning's observation of the Io torus, the EUV and UVS instruments continue to work together in an observation of Jupiter's northern auroral region. These observations are complemented later in the day by "time-lapse" observations of the northern aurora performed by the Solid-State Imaging (SSI) camera. SSI will take a series of four observations at 46 minute intervals as the auroral phenomena appears out from Jupiter's darkside and moves toward the terminator (line dividing the sun-lit side of a target from the dark side). Also performed in the morning is single observation of Callisto performed by the SSI instrument. This observation is taken at low solar phase angles and will provide photometric (light intensity) data of one of Callisto's hemisphere. The observed hemisphere is the opposite hemisphere from that seen in a similar observation during the Ganymede - Orbit 2 encounter period.
The first set of observations from a campaign to study the features in Jupiter's atmosphere known as brown barges is also performed today. A repeat feature from the Ganymede - Orbit 7 (G7) encounter, brown barges were originally detected in data obtained during the Voyager spacecraft encounters with Jupiter. Their name comes from their unique color ratio of blue and red. The color is thought to represent the presence of a unique "chemical brew". A similar cloud feature was detected during the G7 campaign, but it was found at a higher latitude than expected. Experts argue that what was seen in G7 may not have been a "classical" brown barge as was seen in the Voyager data, because it did not have the same shape characteristics. It did have some of the same color characteristics as the classical brown barges. This encounter's series of observations are coordinated amongst different instruments and the Hubble Space Telescope. These data will give scientists the chance to perform comparative studies of brown barges and perhaps solve this mystery. Today's contribution to the observation campaign is provided by the SSI instrument and the Near Infrared Mapping Spectrometer (NIMS). The UVS instrument and the Photopolarimeter Radiometer (PPR) obtain data by "riding along" (collecting data at the same time but not in a separate observation) with the NIMS observations.
As NIMS continues observing Jupiter throughout the afternoon and late into the night, it performs six observations designed to build a large cylindrical map of Jupiter's equatorial region, from about -30 degrees latitude to about +30 degrees latitude. The resolution of this map is designed to be about 630 kilometers (390 miles) per NIMS map element. Interspersed with this series of observations are observations of Europa performed by UVS and NIMS at a relatively far distance from Europa (15 to 20 hours before closest approach) to avoid "noisy data". The noise in the data is thought to be caused by an increase in radiation as the spacecraft gets closer to Jupiter. These data are expected to provide information on some of the minor components of the material on Europa's surface.