Today On Galileo - July 20, 1998
The Europa-Orbit 16 encounter continues today with a significant increase in spacecraft activity. The increase in activity is prompted primarily by the spacecraft's close flyby of Europa, but Galileo's instruments also take a good look at Jupiter's atmosphere and at Io.
During the day, the spacecraft passes through closest approach to Jupiter, Callisto and Europa. Closest approach to Jupiter occurs at 5:18 pm PDT [See Note 1 below] at a distance of 633,000 kilometers (393,000 miles). Callisto follows at 8:55 pm PDT at an unremarkable distance of 1,250,000 kilometers (778,000 miles). Finally, the close flyby of Europa occurs at 10:04 pm PDT at an altitude of 1829 kilometers (1137 miles) and a speed of 6.2 kilometers per second (13,870 miles per hour).
The photopolarimeter radiometer instrument kicks off today's observing by taking a look at Io. The observation is designed to identify and gather information describing large, low temperature hot spots on Io's surface. The observation is expected to provide scientists with information on how rapidly Io's surface heats up and cools down.
The near-infrared spectrometer joins the observing activities with three observations of Jupiter's atmosphere. The first is designed to provide data that will allow scientists to track changes in temperature and composition from orbit to orbit. The remaining two will map a hot spot region at high spatial resolutions. Hot spots are areas where Jupiter's atmosphere is relatively cloud-free, allowing energy from the deeper, warmer layers of the atmosphere to escape into space. Observations of hot spots are the deepest the Galileo Orbiter will be able to see into Jupiter's atmosphere.
The photopolarimeter radiometer also looks at Jupiter's atmosphere today. Four of its observations are designed to search for subtle temperature variations in Jupiter's atmosphere. In addition, in a single observation, the instrument takes a look a the region where two white ovals have either merged or one has disappeared. White ovals are large, long lived storms, although they're not as big as Great Red Spot. They are typically found to roll around in the boundary regions between two opposing zonal jet streams. The merging or disappearance of one of these storms has not happened since observing of these features started, approximately 50 years ago.
The remainder of observations scheduled for today focus on the close flyby of Europa. Tied specifically to closest approach are observations performed by the fields and particles instruments and the radio science team. For about an hour surrounding the close flyby of Europa, the fields and particles instruments make high time resolution measurements of the dust, plasma, magnetic and electric field environment surrounding Europa. These measurements will be added to the growing repository of data that describes the interactions between Europa and Jupiter's magnetospheric environment. The radio science experiment, in contrast, starts 10 hours before and continues for 10 hours after the point of closest approach. During this time, the radio science team monitors Galileo's radio signal and measures changes in frequency caused by Europa's gravitational pull on the spacecraft. Using the Doppler effect, the team will be able to refine gravity field maps produced with measurements from previous orbits.
In the remote sensing arena, the photopolarimeter radiometer starts the observing of Europa. Throughout the day, the instrument completes three observations designed to characterize the thermal variations of Europa's surface and shed light on how the surface was formed, how old it might be and what it is made of. Two of the observations are performed on Europa's dark side and one looks at Europa's day side.
The spacecraft's camera gathers information on Europa's surface via nine observations taken at resolutions ranging from 19 meters (62 feet) to 240 meters (788 feet) per picture element. The regions captured in the observations include Agenor linea, the northern and southern portions of Thrace Macula, the Taliesin crater and Thynia linea. Also included in this image campaign is a region of pull-apart wedges, a region near Europa's terminator and a region of highly dissected terrain. The last of these is the one taken at the highest resolution of this encounter, 19 meters (62 feet) per picture element.
The near-infrared mapping spectrometer, together with the ultraviolet spectrometer, with four observations, also take a look at several different regions of Europa. Each region is characterized by distinct geological features associated with the formation of the Europan surface. The observations focus on regions containing contrasting bright and dark features, including a remarkable distribution of dark and bright materials in and around the Thrace Macula region. Information about the composition of these regions can be obtained by studying the contrast between the features.
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 35 to 50 minutes later, depending on the time of year (currently 36.5 minutes).