Today on Galileo
June 29, 1999
During the Perijove Reduction Campaign, the spacecraft's Perijove distance, or closest distance to Jupiter for a given orbit, will be changed from one slightly higher than the orbit of Europa (9 Jupiter radii, 643,000 kilometers, or 400,000 miles) to a distance that allows Galileo to fly within the orbit of Io (5.5 Jupiter radii, 393,000 kilometers, or 244,000 miles). After the current flyby, the first two Callisto flybys will have reduced the spacecraft's Perijove distance from 9.4 Jupiter radii (672,000 kilometers, or 418,000 miles) to 7.3 Jupiter radii (522,000 kilometers, 324,000 miles). With the reduction in the Perijove distance comes an increase in the amount of radiation to which the spacecraft is exposed. Flight team members will be closely monitoring spacecraft performance during the upcoming months to diagnose and mitigate any radiation effects the spacecraft might exhibit.
This week's encounter spans the next 4-1/4 days, ending early morning on Saturday, July 3. During this time, the spacecraft will be approximately 778 million kilometers (483 million miles) from Earth. It will take radio signals just under 44 minutes to travel between the spacecraft and Earth. The encounter's closest approach to Callisto occurs on Wednesday, June 30, at about 00:47 am PDT (see note 1), at a distance of 1047 kilometers (651 miles) above Callisto's surface.
The start of the encounter marks the resumption of the magnetosphere survey performed by the Fields and Particles Instruments. During the survey, the instruments take measurements of plasma, dust, and electric and magnetic fields. These measurements, repeated from encounter to encounter, allow scientists to study long term variations of the inner portions of Jupiter's magnetosphere. The survey is scheduled to continue through Saturday.
The radio science team begins carefully tracking the changes in the frequency of Galileo's radio signal just prior to 3 pm PDT. The changes are caused by Callisto's gravitational pull on the spacecraft, and the resulting Doppler shift in Galileo's radio signals. The team will track these changes for 20 hours, centered on the point of closest approach to Callisto, and will use the measurements to refine models of Callisto's gravity field and internal structure.
Prior to starting any recorded observations, the spacecraft performs standard maintenance on its onboard tape recorder. There is only one recorded observation scheduled today. In it, the Near-Infrared Mapping Spectrometer obtains data of Callisto's surface at high spectral and spatial resolution. Spectral data contains information describing the light emitted from the different materials on Callisto's surface. Each material emits a unique spectra which allows scientists to identify the given material. In conjunction, spatial data will allow scientists to determine how these materials are distributed on Callisto's surface. The information obtained in this observation is designed to further the ongoing compositional studies that were started during Galileo's primary mission.
Note 1. All times listed correspond to the Pacific Time zone (currently daylight time) 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, Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian Time (GMT), and it takes radio signals 44 minutes to travel between the spacecraft and Earth.
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page.
Return to Project Galileo Homepage
| Comments and suggestions about this website may be directed to gllbug@jplpio.jpl.nasa.gov Last updated: June 29, 1999 |
 |