Today on Galileo - September 25, 1998
The sixth encounter of the Galileo Europa Mission started last night at 9:00 pm PDT [See note 1]. The encounter features a close flyby of Europa that occurs at 8:54 pm PDT, today, at an altitude of 3582 kilometers (2226 miles). The encounter period lasts through Sunday, but the bulk of science observations occur today, coincident with the Europa flyby. During the encounter the spacecraft is 598 million kilometers (372 million miles) from Earth. At that distance, it will take radio signals approximately 33 minutes to travel between the spacecraft and Earth.
Observations of the Jupiter system started last night when the fields and particles instruments began collecting data on the inner portion of Jupiter's magnetosphere. This observation is scheduled to last until 7:00 pm PDT on Saturday. One instrument, the Dust Detector, has been collecting data for the past five days on small particles streaming outward from Jupiter, and will continue to make measurements until five days after closest approach to Jupiter.
Three other observations were performed last night, one by the ultraviolet spectrometer and two by the near-infrared mapping spectrometer. The ultraviolet observation looked at aurora near Jupiter's south pole. The observation will allow scientists to better understand the long-term interactions between Jupiter's upper atmosphere and its magnetosphere. One of the near-infrared observations also looked at Jupiter and will provide information to track changes in the makeup and temperature of the atmosphere. The second near-infrared observation took a distant look at Europa with special emphasis on detecting the non-ice components of the surface.
Today's activity not only includes the close flyby of Europa, but also this orbit's closest approach to Io and Ganymede. Closest approach to Io occurs at just after 3 am PDT at a distance of 799,000 kilometers (497,000 miles) from Io's surface. Later in the day, just after 8 pm PDT, closest approach to Ganymede occurs at a distance of 1.7 million kilometers (1 million miles) from the surface.
Today's remote sensing observations are focused on Europa, but also include observations of Jupiter and its rings, and a single look at Io. The observation of Io is performed by the ultraviolet spectrometer and is designed to locate regions of thick sulfur dioxide gas that can be associated with volcanic plumes. The observations of Jupiter's rings are performed by the spacecraft camera under different lighting conditions (or solar phase angles). Their data will provide information on the sizes of the different particles that make up the rings.
Jupiter is observed by all four remote sensing instruments. Most of the observation campaign is focused on gathering information on a newly created white oval. The white oval was formed in the Spring of 1998, when two other white ovals merged. The spacecraft camera is scheduled to take two sets of images of the feature, while the near-infrared mapping spectrometer takes three spectral maps of the region. Finally the ultraviolet spectrometer takes two observations of the region. The participation of multiple instruments on this observing campaign will provide scientists with a good characterization of the white oval.
The near-infrared spectrometer performs two more Jupiter observations today. Like last night's observation, they will be useful for tracking changes in the composition and temperature of Jupiter's atmosphere. The photopolarimeter radiometer completes today's round of Jupiter observations. In two separate observations, the instrument looks at Jupiter's Southern equatorial, tropical and temperate belts with the objective of characterizing the temperature structure and thermal properties of the region. This will be the first time the instrument observes this region of Jupiter.
In the morning, the radio science team will begin to measure changes in the frequency of Galileo's radio signal. These measurements are taken for 20 hours,centered at the point of closest approach to Europa. Using the Doppler effect, they will allow scientists to improve their knowledge of the gravity field produced by Europa.
More observations of Europa are spread throughout the day and are made, again, by all four remote sensing instruments. The ultraviolet spectrometer starts the day off with two observations of Europa's tenuous atmosphere designed to monitor emissions and outgassing. Later, the instrument looks at Europa's surface to provide information that might lead to understanding how external sources (meteoroid bombardment, magnetosphere interaction, etc) may be affecting Europa's surface characteristics.
The ultraviolet spectrometer also gathers data concurrently with most of the observations of Europa performed by the near-infrared mapping spectrometer. After a global scale observation, the two instruments, via four observations, focus on obtaining data describing the surface characteristics of four different regions of Europa. The first two look at regions characterized by the presence of non-ice materials near the equator and high southern latitudes. The third observation looks at a high latitude region on Europa's leading side, the side that is away from the bombardment of particles contained in Jupiter's magnetosphere. And the fourth looks at a region of diffuse dark material.
Late in the afternoon the photopolarimeter radiometer instrument starts taking its observations of Europa. Throughout the day, the instrument takes four observations, each contributing to the general objective of obtaining data on thermal variations across Europa's surface. This data will allow scientists to increase their knowledge of how the surface was formed, how old it might be and what it is made of. All four observations capture Europa's darkside, with one of them focusing on Europa's polar regions.
The spacecraft camera's observation campaign of Europa is by far the most extensive of the encounter. In sixteen observations, the camera observes a variety of terrains at a variety of resolutions. The observing starts with a global scale observation and is closely followed by two regional maps taken near Europa's terminator (or line separating Europa's sunlit side from its dark side). The maps cut across a variety of terrains that are believed to provide the best evidence of the existence of a sub-surface liquid layer on Europa. Additional regional mapping, on the opposite side of Europa, is performed after closest approach.
High resolution images of various regions on Europa are also on the camera's observation schedule. Three sets of images of the Agenor Linea region, some in color, will provide information on this extremely bright feature. Its brightness is believed to be an indication that it is fairly young. An observation of Thrace Macula is expected to clarify the nature of a large dark area that may be similar to the regions containing iceberg-like fragments of older crust seen in observations taken during Galileo's primary mission. The camera also looks at the Rhiannon crater, which is one of only a handful of large impact craters on Europa. These craters have a variety of forms, which is believed to be related to the thickness of Europa's crust in the region of the impacts and/or at the time of the impacts. Comparing Rhiannon's form to those other craters should help to clarify this relationship. Other regions observed today include Libya Linea, Thynia Linea, South pole terrains, pull-apart wedges, dissected terrain, and a strike-slip fault.
Note 1. All times listed correspond to the Pacific Time zone (currently daylight saving) and spacecraft event time. Radio signals indicating that an event has occurred on the spacecraft reach the Earth 33 to 50 minutes later, depending on the time of year (currently 33 minutes).