This Week on Galileo - March 17-23, 1997
Playback plans for this week start with the second pass through the data stored on Galileo's tape recorder during its previous encounter with the Jupiter system back in February. This second opportunity allows scientists to return new data that they initially thought they might not have enough time to return. Other data can fill in small gaps in already played-back data caused by transmission and reception problems. The second pass also allows scientists to re-play parts of observations that have been identified as being interesting enough for a second look - in particular , observations of Jupiter's atmosphere, White Ovals for this orbit. But why would a re-play of the data be required for a second look?
Given the low data rates Galileo transmits to earth, compression algorithms play a big part in determining the total amount of data that can be gathered from a particular tapeload of observations. Higher compression can transmit more data. Unfortunately, high compression can produce what are know as compression artifacts. You may have noticed some blocky looking regions in some of the images that have been posted here on our home page. These are compression artifacts . These compresion artifacts can and do sometimes hide some of the more interesting features and information that can be extracted from some of the observations. This type of compression can also be known as "lossy" compression because it creates "loss" of some data.
Once a region has been identified as being of significant scientific value, the data recorded on the tape recorder can be transmitted again to earth with lower compression ratios or with a different compression algorithm that does not create compression artifacts. This would be known as "lossless" compression. The lossless compression is only used sparingly as it would take too much time to transmit the complete observations. This preview / re-play plan has been successful in allowing scientists to confirm the existence of features that were believed to exist since the Voyager flybys, but had not been seen directly due to insufficient resolution in the observations.
In a similar manner, the second pass allows scientists to use lossy compression data return to locate small objects, like the minor satellites - whose orbits are not so well known, in an observation. Once the object is located, the relevant data is transmitted with lossless compression. This method prevents having to return a lot of empty sky at lower compression ratios which would wast valuable data return time. The minor satellite observation returned during this week is a single image of Thebe.
Other data returned during this week are NIMS (Near Infrared Mapping Spectrometer) and PPR (Photopolarimeter Radiometer) observations of White Ovals, Io observations by SSI (Solid State Imaging camera) and NIMS, Callisto observations by NIMS and PPR, and Europa observations by NIMS, SSI and PPR. Next week is the last week of playback prior to Galileo's next encounter on April 4th with Ganymede.